Version July 2025
INTRODUCTION —
Endurance activities such as running and bicycling are among the most popular forms of recreational exercise and competitive sport among adults. Endurance exercise is characterized by continuous activity performed for 30 minutes or longer, which creates specific nutritional needs for participants.
The nutritional demands posed by endurance exercise and beneficial ways to meet those demands are reviewed here. Other aspects of nutrition and exercise for adults are discussed separately. (See "Healthy diet in adults" and "Exercise for adults: Terminology, patient assessment, and medical clearance" and "Exercise prescription and guidance for adults".)
TERMINOLOGY AND FUNDAMENTAL CONCEPTS —
The term "endurance sport" has different meanings for different people. For some, the Ironman triathlon is the quintessential endurance event; for others, a 5 km park run is a notable endurance activity. Endurance exercise can be performed while swimming, cycling, rowing, cross-country skiing, participating in a triathlon, or doing many other sporting activities. We define endurance sport or exercise as follows:
●Endurance exercise or sport – Endurance exercise is characterized by continuous activity carried out for 30 minutes or longer [1]. During such activities, muscle fuel is provided primarily by aerobic (oxygen-requiring) production of energy from metabolism of fat and carbohydrates. (See "Energy metabolism in muscle".)
Although "endurance" conjures notions of survival over a prolonged period, endurance sport may involve races requiring speed, power, skill, and tactics performed under environmental and mental pressure. Importantly, the same event may have different characteristics for different competitors. As an example, completion times and physiologic demands vary widely among participants in a big city marathon. Elite competitors finish the race within two to two and a half hours, exercising at approximately 85 to 95 of their maximal aerobic capacity [2]. Age group competitors may strive for a personal best, finishing within three to four and a half hours, exercising at 60 to 75 percent of their aerobic capacity. Recreational runners who simply desire to complete the course, may take five to eight hours of "stop-start" or slower running to cover the distance.
TRAINING DIET
Energy intake — Energy from food provides the fuel for exercise and maintenance of bodily functions and health, and underpins changes in body mass and composition. It also affects the availability of macronutrients and micronutrients.
Estimating energy requirements — The energy requirements of an endurance athlete vary depending upon their sex, age, stage of growth, lifestyle or work-related activity, and training and competition schedules. For some nonelite endurance athletes, training may entail daily or near-daily training of several hours (eg, age-group participant preparing for Ironman triathlon), making it a major determinant of energy requirements, while for casual recreational athletes, training may be done for only a few hours each week.
Detailed explanations of the calculations used to determine precise individual energy needs are beyond the scope of this topic. Sports nutritionists use a range of prediction equations involving age, sex, total body mass, and lean body mass to approximate basal energy requirements. These calculations may be adjusted further based on the athlete's training program and other lifestyle-related exercise [3].
Many technologies and "wearables" (eg, power meters on bicycles, personal monitors or watches) are widely used among elite and recreational athletes to provide estimates of energy expenditure during exercise or totals over the day. Calculations from such monitors involve some degree of error [4]. In addition, even if athletes obtain some reliable information about the energy cost of their training, they may fail to consider the costs of other daily activities and either under- or over-estimate these demands. As an example, athletes who cycle to work or perform undemanding manual labor (eg, gardening) may not include these activities in estimates of their total energy expenditure.
Some athletes naturally follow a diet that meets their energy requirements, but others may require assessment by a dietitian or sports nutritionist to adjust their diet to meet their needs. This is often the case when there is a sudden change in exercise intensity, for example, when injury or the off-season reduce energy demands or preparation for an upcoming competition increases them. Athletes who wish to change their body composition (eg, lose body fat or gain muscle) may also benefit from expert guidance. (See 'Achieving optimal physique for endurance sport' below.)
Estimating macro and micro nutrient requirements — While overall energy intake provides a framework for the athlete's diet, individuals must also ensure that their food choices contribute appropriate amounts of macronutrients (eg, fat, carbohydrate, and protein) and micronutrients (vitamins, minerals, and phytochemicals) within the allotted calories. Particularly for those restricting energy intake, it is important to choose nutrient-dense foods (table 1 and table 2).
A basic principle of modern training is periodization, which is the gradual accumulation of different types of exercise stimuli over micro-cycles (usually weekly) and longer macro-cycles to develop the technical, tactical, physiologic, and mental characteristics needed for success in a specific competition. The athlete's training program should combine and balance different types of training (eg, longer moderate intensity endurance sessions, strength/resistance training, interval/repetition training, tempo/higher intensity sessions, race practice) in a series of progressive phases that prepare the athlete to peak for a targeted competition. Energy expenditure and nutritional demands often change with each training phase.
Some population-based nutrition guidelines provide general targets for macronutrients as a percentage of energy intake (eg, protein, fat, and carbohydrate should contribute 15 to 20 percent, 25 to 35 percent, and 50 to 60 percent of total energy intake, respectively). Although such publications provide general guidance, they should not be considered prescriptive for all. Importantly, energy intake can change independently of protein and carbohydrate targets. For athletes with lower energy requirements or deliberate restriction, a larger percentage of energy intake may be comprised of protein and carbohydrate. In addition, energy and carbohydrate intake should fluctuate to accommodate daily and phase-based changes in training loads.
A convenient way to coordinate diet and training is to focus on the specific nutrient support needed for each training session, including presession fueling, within-session fueling and hydration, and postexercise recovery nutrition (protein, carbohydrate, fluid, and electrolytes). Energy, carbohydrate, and protein intake will increase and decrease with changing training needs.
Achieving optimal physique for endurance sport — Body mass and composition play an important role in the performance of many endurance sports. The ratio of power to body weight partially determines speed and economy of movement. Traditionally, coaches and athletes have emphasized the body mass side of this equation, in particular, reducing body fat, and underemphasized power and strength. We believe that endurance athletes should work towards achieving a healthy balance of lean muscle mass and body fat. Furthermore, we believe that athletes should not aim to achieve minimal levels of body fat and body mass, and that recreational athletes should not aspire to achieve the physique of elite competitors if this is not their habitual, healthy physique.
A systematic review performed by members of the International Olympic Committee of 29 studies investigating the effect of physique on performance reported that an increase in lean muscle mass was more consistently associated with improved performance across a range of endurance sports than a reduction in body fat [5]. Resources for increasing muscle mass are found separately. (See "Practical guidelines for implementing a strength training program for adults" and "Strength training for health in adults: Terminology, principles, benefits, and risks".)
The prototypical physical characteristic of endurance athletes is leanness (low body fat), often coupled with slight stature (picture 1). Such a physique is common among elite distance runners and road cyclists, especially those who excel in events involving hilly terrain or hot weather. In addition to the reduced energy cost of movement, a small frame and its higher ratio of body surface area to volume aids heat dissipation. However, among a group of endurance athletes of similar caliber, there is generally no correlation between physique and performance, suggesting it is only one of several factors underpinning success.
While some endurance athletes arrive at their desired physique through genetics and training, others deliberately reduce their energy intake to achieve low body fat. Few high-quality studies have investigated the effect of manipulating body fat on endurance performance. Although intuitively a leaner body is lighter and faster, most evidence of benefit is anecdotal [6,7]. Furthermore, the potentially harmful effects of common methods used to achieve fat loss must be considered. Although some athletes achieve an initial performance boost from a fat loss program, it is often followed by harms associated with the magnitude of the weight loss and the unhealthy practices used to achieve it. Harms may include reduced muscle fuel and poor quality of training, loss of lean muscle, and reduced quality of life [8].
Ideally, elite athletes should maintain their body composition within a narrow range, across and between training cycles. This means achieving "race weight" for only brief periods around the time of competitions, and training with a slightly higher body mass and body fat composition to increase resilience and nutritional support [9]. It should be emphasized that there is no "ideal" body fat concentration or "ideal" body composition. Recreational athletes should maintain adequate lean muscle mass and body fat necessary for good health.
In some endurance events (eg, time-trial cycling, ocean rowing), the importance of power is well recognized, and athletes devote considerable effort to building muscle through resistance exercise. However, many endurance athletes continue to emphasize reducing body fat concentration over increasing muscle mass. An obsession with achieving low body mass and body fat at the elite levels of some endurance sports contributes to the widespread problem of low energy availability (ie, relative energy deficiency in sport [REDs]), including well-documented accounts of pathologic weight loss strategies [6], and body shaming [10]. Many athletic and sports medicine organizations have produced guidelines to address these issues [5,11,12]. In addition, the methods commonly used to determine body composition have limitations. We recommend that body composition assessment be performed only for highly trained athletes over 18 years competing at a national level or higher [5].
Although this topic is intended primarily for nonelite athletes, it is helpful for clinicians to be aware of the culture and issues involved in high-level sport, as recreational athletes often emulate diet and training programs used by elite athletes. The results can range from misguided attempts at weight loss to formal eating disorders, and can manifest in different ways [12-14]. As examples, individuals with disordered eating may embark upon endurance training as an adjunct or to mask their eating, while recreational athletes may adopt extreme eating strategies in the belief that this demonstrates commitment to their sporting goals. (See "Eating disorders: Overview of epidemiology, clinical features, and diagnosis".)
Low energy availability (LEA) and relative energy deficiency in sport (REDs) — The term energy availability (EA) refers to the amount of energy available to the body to perform metabolic functions and maintain health. Mathematically, EA is calculated by subtracting the energy cost of an athlete's exercise program from their dietary energy intake, and normalizing it to their fat-free mass, as this represents the body's most active tissue type:
●EA = Energy Intake (kcal) - Exercise Energy Expenditure (kcal)/FFM [Fat-Free Mass (kg) per day]
It is expressed as kcal x kg FFM/day
LEA is caused by an energy mismatch, which occurs for one of the following reasons:
●Increase in energy expenditure (eg, increase in training volume)
●Decrease in energy intake (eg, calorie-restricted diet)
●Some combination of the two
A range of scenarios commonly seen in endurance athletes can be associated with LEA [15]. These can be classified as follows:
●Pathologic causes (eg, eating disorder, systemic illness)
●Deliberate but potentially mismanaged causes (eg, weight loss plan, increased training load, strenuous competition program)
●Unintentional causes (eg, reduced appetite, poor food availability, limited knowledge of nutrition)
The adaptive response to energy scarcity involves a transient shunting of energy away from nonessential body functions. The effects of such transient LEA are generally benign and reversible. Most athletes can tolerate short-term manipulation of body composition and high training volumes, which may promote sporting success without health costs. However, periods of prolonged or extreme LEA may contribute to a set of inter-related problems termed REDs.
The REDs syndrome can produce a range of health and performance impairments in athletes regardless of their sex, sporting event, or level of competition [16]. The clinical manifestations of REDs may stem from other causes and an appropriate workup is necessary before the diagnosis can be established.
REDs expands on concepts originally described as the Female and Male Athlete Triads, which focused on bone and reproductive health [17-19]. Low bone mineral density, increased risk of bone stress injuries, impairments in menstrual function (females), and decreased libido (males) are the issues most clearly associated with LEA [16]. (See "Functional hypothalamic amenorrhea: Pathophysiology and clinical manifestations".)
Carbohydrate needs for endurance sport — Many athletes alter the type, volume, and intensity of training sessions on a near-daily basis and throughout training cycles. Dietary carbohydrate should be manipulated to meet these changing needs. The phrase "fuel for the work required" summarizes this concept [20,21].
We provide two tables to assist these determinations. The first provides targets for carbohydrate intake, with explanations of how these should be adjusted based on consideration of total energy needs, training needs, and feedback from training performance (table 1). The second summarizes different approaches to carbohydrate intake over a week according to the training volume and goals of different endurance athletes (table 2).
In some circumstances, explicit strategies are needed to ensure adequate carbohydrate is available to meet demands. Such circumstances include:
●Prolonged training sessions (>90 minutes) involving submaximal exercise.
●Periods of high-intensity work, such as interval training.
●Preparation for some race-related activities, such as breaking away from an opponent or sprinting to the finish line. Although these are addressed in nutrition strategies for competition, they must also be considered in the everyday diet to support training goals.
The 2010 consensus meeting of the International Olympic Committee on Nutrition for Sport updated the concept of the high carbohydrate diet by defining daily carbohydrate intake in relation to the calorie costs of training and competition, rather than recommending a predetermined amount. "High carbohydrate availability" was defined as a dietary practice that matches acute carbohydrate intake to the immediate (eg, daily) fuel cost of exercise. "Low carbohydrate availability" described scenarios in which body carbohydrate stores become depleted or are insufficient for the exercise program [22]. Immediate performance is enhanced when exercise is undertaken with high carbohydrate availability.
Exercise undertaken with depleted muscle glycogen stores creates an upregulation of cellular signaling that may enhance the training response [23,24]. This phenomenon is likely to occur naturally in the high-volume training programs of elite athletes, as there may be insufficient time for complete refueling between multiple training sessions undertaken within a 24-hour period. There is emerging interest in the intentional manipulation of training and carbohydrate intake to perform some training sessions with low carbohydrate availability [25]. Programs that incorporate low carbohydrate availability are more appropriate for high-level endurance athletes, rather than recreational athletes, and are best undertaken under the guidance of a sports dietitian.
Training with low carbohydrate availability can be done by training in a fasted state (water only permitted during the session) or by performing a glycogen-depleting training session and then consuming carbohydrate-restricted meals until the next session. Observational studies suggest that integrating this approach may produce greater improvements in performance than training with consistently high carbohydrate availability [26-28], although the best methods for doing so remain a subject of research [29-31].
The importance of carbohydrate as an exercise fuel has been appreciated for many decades [32]. Features of body carbohydrate stores of interest to endurance athletes include the following [33]:
●Carbohydrate is an economical oxidative fuel. It produces adenosine triphosphate (ATP) via pathways involving oxygen, with the capacity to produce 4 to 5 percent more ATP from the same amount of oxygen than fat-based fuels.
●Carbohydrate can produce ATP via nonoxidative (glycolytic) pathways when a more rapid energy supply is needed in the absence of sufficient oxygen.
●Carbohydrate is an important source of fuel for the central nervous system.
●Carbohydrate is stored in the body as glycogen in muscle and the liver, and as blood glucose. It is stored in much smaller amounts than fat and must be restocked continually based on fuel needs.
Fat adaptation for endurance sport — There is widespread interest in carbohydrate-restricted diets (eg, "keto" diet). Even among lean endurance athletes, the body's relatively large fat stores provide a sizeable source of fuel for prolonged aerobic exercise and a possible alternative or adjunct to diets with high carbohydrate availability. (See "Healthy diet in adults", section on 'Low-carbohydrate diet'.)
However, although more research is needed to investigate the long-term effects of ketogenic low carbohydrate, high fat (LCHF) diets on health and metabolism, evidence from performance-focused studies suggest only limited utility for endurance athletes. LCHF diets may be useful for a small range of sporting events in which rates of energy production are low enough to accommodate the additional oxygen required for fat oxidation, or when the athlete is unwilling or unable to follow optimal carbohydrate use [34]. Suitable sporting events may include some ultraendurance competitions.
Endurance training increases muscle's capacity to use fat as a fuel. This capacity can be doubled in as little as five to ten days of adaptation to an LCHF diet. Adaptation occurs regardless of whether carbohydrate is reduced (15 to 20 percent of energy) or severely restricted (<50 g/day) to achieve sustained ketosis (ketogenic LCHF diet), and involves an increase in fatty acid transport, uptake, and utilization in the mitochondria. Although there is considerable variability in the response, some individuals following a ketogenic LCHF diet are able to maintain capacity during submaximal (approximately 60 to 65 percent of VO2 max) exercise [35].
Proponents of LCHF for endurance sport appeal to testimonials of elite athletes who follow this approach. However, controlled, prospective studies report that the LCHF diet is associated with impaired performance during higher-intensity endurance competition [30,36]. Strategies to integrate enhanced fat oxidation with other fuel sources, including restoration of glycogen with prerace carbohydrate loading [37,38] or consumption of ketone ester supplements [39], have failed to prevent such impairment. Laboratory investigations have found that strategies to enhance fat oxidation in muscle also reduce its capacity to break down available glycogen and slow its entry into the Krebs cycle, which is involved in the oxidative production of ATP. (See "Energy metabolism in muscle".)
Success in endurance sport is determined by high-intensity aerobic performance maintained throughout the event or at its critical stages. Greater ATP production from optimized carbohydrate availability and oxidation is best suited to achieving the highest sustainable rates of muscle energy turnover [34]. (See 'Carbohydrate needs for endurance sport' above.)
Protein for endurance sport
Protein requirements — Historically, protein needs have been emphasized for participants in resistance training and strength sports (eg, weightlifting) but not for endurance athletes. However, all athletes who undertake strenuous training programs, whether resistance, endurance, or team sport-based, have increased protein requirements. Dietary protein is needed to support exercise-induced skeletal muscle remodeling and repair and to optimize body composition (table 3).
During periods of heavy training, endurance athletes should consume 1.2 to 1.6 grams of protein/kilograms body weight each day, roughly double the standard recommended daily allowance for adults [40,41]. The definition of "heavy training" varies by sport. For elite middle- and long-distance runners, 10 to 15 hours per week constitutes a heavy training load. For elite rowers and cyclists, heavy training may involve upwards of 20 hours per week.
Across the spectrum of endurance athletes, this equates to a daily protein amount of between approximately 70 grams (eg, for a small, female runner) and 150 grams (eg, for a large, male rower). The daily protein requirements of the average recreational endurance athlete who exercises every second day for 30 to 40 minutes are unlikely to exceed 1 gram of protein/kilogram body weight. The additional energy requirements associated with training allow an increased intake of everyday foods, and surveys of athletes across a range of levels typically show that protein intake exceeds the targets outlined here.
Although evidence is more limited, some suggest that optimal protein intake may be higher (up to 2.2 to 2.4 grams/kilograms body weight per day) in some circumstances, such as when reducing body fat [41] or unable to train due to injury [42]. In such settings, increased protein intake is intended to reduce the risk of losing muscle mass.
Studies assessing protein needs for endurance sport have used multiple outcomes, including markers of acute change in muscle protein synthesis and recovery indices, such as muscle soreness and function. Interventions have included modifications in protein amount, type, formulation, timing, and frequency of intake [41,43,44]. Fewer studies have involved longer-term follow-up to assess whether acute changes are associated with chronic improvements in body composition or performance.
Timing of protein intake — When protein intake is at the higher end of the recommended range, maximal muscle remodeling and recovery generally occurs regardless of other consumption characteristics, such as timing and type of protein. When protein intake is at the lower end of the recommended range, or during periods of strenuous training or competition, there may be advantages to ingesting small amounts of protein throughout the day. This can consist of four to six meals and snacks, each containing 20 to 40 grams of protein. In particular, protein consumed soon after exercise and before sleep may enhance muscle protein synthesis and functional outcomes [43,45].
We concur with the general recommendation that endurance athletes choose an eating pattern that distributes their intake of high-quality protein throughout the day. This approach generally provides the full complement of amino acids and maintains blood concentrations of leucine above the threshold needed for optimal muscle protein synthesis [40]. Leucine is an important amino acid for muscle synthesis.
Sources of protein — The quality of a protein is determined by its amino acid composition and rate of digestion. Proteins with more essential amino acids that are more rapidly digestible are considered higher quality. They lead to a more rapid rise in the rate of muscle synthesis than lower-quality protein. Foods high in leucine may be especially important, as higher leucine concentrations may promote muscle protein synthesis [46,47].
Endurance athletes usually can obtain adequate high-quality protein by consuming standard portions from animal sources (meats, poultry, fish, eggs, dairy products). Plant-based protein sources are receiving scrutiny due to the growing popularity of vegetarian and vegan diets among athletes. Potential problems when restricted to plant sources include differences in digestibility and amino acid composition. Studies of muscle protein synthesis that compare isolated protein sources (eg, whey or egg protein versus soy, pea, or wheat protein) often report inferior outcomes with plant sources [41,43,44]. However, for real-world applications and in longer-term studies, plant-based diets have been shown to meet sports nutrition goals when protein is consumed as part of a meal that also provides carbohydrate and fat and is at the higher end of the recommended protein range [48,49].
Iron, calcium, and vitamin D deficiencies — For most adults, a varied diet consisting of nutrient-rich foods meets any potential increases in micronutrient needs associated with strenuous endurance exercise. However, some athletes may consume inadequate amounts of iron and calcium and require supplementation.
Iron deficiency — Iron plays numerous roles in metabolism, particularly as a cofactor for enzymes, and in endurance athletes plays a crucial role in oxygen transport in the blood (hemoglobin) and muscle (myoglobin) [50]. The causes, clinical presentation, and diagnostic evaluation of iron deficiency are reviewed separately; issues of special importance to endurance athletes are discussed here. (See "Diagnosis of iron deficiency and iron deficiency anemia in adults".)
●Causes in endurance athletes – Endurance athletes may be at greater risk of iron deficiency due to increased turnover and losses. Possible mechanisms include footstrike hemolysis (damage to red blood cells from feet repeatedly hitting hard surfaces when running), losses in sweat, and occult gastrointestinal bleeding [50]. Athletes who participate in longer endurance events (eg, marathon runners) may develop occult gastrointestinal bleeding leading to iron deficiency. (See "Exercise-related gastrointestinal disorders", section on 'Gastrointestinal bleeding'.)
In addition, exercise affects the hormone hepcidin, which helps to regulate iron stores by reducing absorption of iron from the gut and recycling of iron from damaged red blood cells [51]. Hepcidin release is reduced when iron stores are low (eg, low blood ferritin concentrations). Hepcidin release increases several hours following any strenuous exercise that depletes body carbohydrate stores. (See "Regulation of iron balance", section on 'Hepcidin'.)
●Presentation – Iron deficiency anemia and many cases of iron deficiency without anemia are associated with fatigue, weakness, reduced exercise capacity, poor athletic performance, and impairments in immune system and neurologic function. (See "Diagnosis of iron deficiency and iron deficiency anemia in adults".)
●Dietary iron – Dietary iron is consumed in two forms. Heme iron is found primarily in red meats, darker cuts of poultry, and fish. Nonheme iron is found exclusively in plant foods, such as fortified cereals, legumes, and green leafy vegetables. The bioavailability of nonheme iron can be increased when consumed in combination with foods containing vitamin C, peptides found in meats, fermented food, and organic acids (eg, malate or citrate). It is reduced when consumed with phytates, oxalates, polyphenols (present in tea or coffee), and calcium. (See "Overview of dietary trace elements", section on 'Iron'.)
Proper timing for the intake of iron-rich foods or iron supplements can help to avoid periods of low iron absorption. Endurance athletes may want to consume iron early in the day before hepcidin release increases and avoid consumption during the period approximately three to six hours after strenuous exercise when hepcidin concentrations are high [50,51].
●Treatment with oral supplements – Endurance athletes at risk of iron deficiency may want to seek guidance from a sports nutritionist about iron sources, combinations of iron-rich foods in meals, timing of consumption, and the use of supplements to maximize iron stores [52]. This is especially true for vegetarian and vegan athletes limited to nonheme dietary sources.
It is reasonable to treat endurance athletes with suboptimal iron stores before anemia develops, as well as those with iron deficiency anemia. Although 50 ng/mL is the typical threshold for a low serum ferritin concentration in sedentary adults, for athletes performing strenuous training, a threshold of 35 ng/mL is used. This lower threshold provides a greater margin of safety for preserving training adaptations and avoiding decrements in training capacity.
When iron supplementation is needed to prevent or treat iron deficiency, ferrous salts are the supplement used most often, but several effective oral formulations are available (table 4) [50,53]. In general, standard treatment approaches (eg, doses) are appropriate for recreational endurance athletes. Treatment of iron deficiency is discussed in detail separately. (See "Treatment of iron deficiency anemia in adults".)
Hepcidin increases approximately three to six hours after endurance exercise. Thus, when taking iron supplements, the most effective absorption opportunities appear to be in the morning, prior to exercise, or immediately following an endurance workout [54].
Gastrointestinal side effects are extremely common with oral iron supplementation. These may include metallic taste, nausea, flatulence, constipation, diarrhea, epigastric pain, and vomiting. Dosing adjustments and other strategies for reducing side effects are discussed separately. (See "Treatment of iron deficiency anemia in adults", section on 'Strategies to improve tolerability'.)
Serum ferritin concentrations (ie, iron status) interact with exercise effects to alter hepcidin concentrations [55]. The phase of the menstrual cycle may also affect hepcidin concentrations [56].
Calcium
●Role in bone health – Calcium is used in a wide range of physiologic processes, including muscle contraction, nerve transmission, hormone secretion, and intracellular signaling [57]. However, it is most closely associated with bone metabolism. Bone metabolism is discussed in detail separately. (See "Normal skeletal development and regulation of bone formation and resorption" and "Bone physiology and biochemical markers of bone turnover".)
Bone health is a major concern for endurance athletes. Low bone density is a relatively common problem among endurance athletes that predisposes to bone stress injuries, causing setbacks in training and competition [58-60]. Inadequate calcium intake is an important contributor to poor bone health. Additional contributing factors include inadequate vitamin D and low energy availability [57]. (See "Overview of bone stress injuries and stress fractures".)
●Dietary intake – The recommended dietary intake for calcium varies among countries and populations but ranges from 700 mg to 1300 mg per day. The lower end of the range applies to most adults, while the higher end applies primarily to adolescents and postmenopausal females, who have higher requirements [57]. Amenorrheic female athletes should also ingest higher amounts to compensate for their hormonal disturbances, while underlying causes are diagnosed and managed. (See "Bone health and calcium requirements in adolescents" and "Calcium and vitamin D supplementation in osteoporosis" and "Functional hypothalamic amenorrhea: Evaluation and management".)
Dairy foods (eg, milk, cheese, yogurt) are the major dietary source of calcium in Western diets (table 5 and figure 1). Other sources include fish eaten with bones (eg, sardines), green leafy vegetables (eg, kale, broccoli, spinach), and calcium-fortified dairy substitutes [57]. Calcium-containing supplements are available if necessary (table 6).
●Effects of exercise – Although regular bone-loading exercise is associated with long-term stimulation of bone formation, observational studies report acute reductions in blood ionic calcium concentration and increases in parathyroid hormone concentrations leading to bone resorption during exercise [61-63]. A reduction in such bone resorption may be achieved by consuming calcium supplements or calcium-rich foods in the pre-exercise meal [64,65]. While further study is needed to clarify the chronic effects of such a strategy on bone health, it may be useful for endurance athletes involved primarily or exclusively in non-weight-bearing exercise (eg, cycling, swimming).
Vitamin D deficiency
●Role, synthesis, risks for deficiency – Vitamin D is associated with bone health via its role in calcium metabolism and plays other important roles in muscle and immune function, and in the modulation of many genes. Vitamin D is a fat-soluble vitamin contained in several food sources, both naturally and via fortification. Nevertheless, 80 to 90 percent of vitamin D is synthesized in the skin following exposure to ultraviolet B (UVB) radiation from sunlight exposure [66]. Exposure of arms, legs, and torso to sunlight in the middle of the day, two to three times per week, for 5 minutes (fair skin) to 30 minutes (dark skin) is generally sufficient for adequate vitamin D synthesis [67].
Conversely, the main risk factor for vitamin D deficiency is inadequate exposure to sunlight. This can occur in individuals who are predominantly indoor dwelling or who consistently prevent exposure to UVB with sunscreen or whole-body clothing, as well as those with high melanin content in their skin (darker skin tone). Time of day and latitude also affect exposure. During early morning and late afternoon, and during winter at latitudes above 35 degrees north or south, there is insufficient UVB exposure to activate vitamin D synthesis [68]. In addition, endurance athletes living in hot climates may restrict outdoor training to early or late hours to avoid extreme heat and humidity.
Endurance athletes who may have inadequate exposure to sunlight should be screened. Studies of Vitamin D in endurance athletes often reveal groups with a high prevalence of insufficiency [57].
●Assessment – Vitamin D status is typically assessed by measuring circulating vitamin D (25-OHD) in the blood. Definitions of optimal and insufficient concentrations vary among authorities, as do recommendations for daily intake, which typically range from 200 to 600 international units per day [69]. However, some groups argue for larger amounts to address all aspects of health rather than solely bone health [57]. (See "Overview of vitamin D".)
●Treatment – Treatment of Vitamin D deficiency may require large weekly or daily doses at the commencement of therapy [57]. However, thereafter levels can be maintained with a daily intake of 1500 to 2000 international units in most cases. (See "Vitamin D deficiency in adults: Definition, clinical manifestations, and treatment".)
Multivitamin — Many endurance athletes take a multivitamin and mineral supplement [70]. These are often used as a safeguard against possible dietary deficiencies, poor eating practices, or lack of confidence in the available food supply. Such supplementation is often unnecessary, as most athletes who consume a wide variety of nutrient-rich foods are able to meet their micronutrient needs, including any increased requirements stemming from strenuous exercise [71].
Athletes at risk of deficiencies include those who severely restrict their energy intake or food variety for prolonged periods [71]. The period necessary to develop signs of deficiency will vary depending on the severity of the dietary restrictions and the training demands of the athlete, among other factors, but may be months or longer. This group may include athletes with eating disorders and well-meaning athletes who embrace diets that restrict multiple food groups. Some restricted diets (eg, vegan, vegetarian, low carbohydrate-high fat) can be managed to achieve micronutrient goals. However, athletes may lack sufficient knowledge or food availability to replace the nutrient content of the proscribed foods. These athletes may benefit from a multivitamin and mineral supplement. In addition, they may benefit from consulting a sports nutritionist who can assess diet adequacy and nutrient status and can work with them to optimize their nutrition.
PRE- AND POST-EXERCISE NUTRITION
Preparation for endurance exercise sessions consists primarily of ensuring adequate hydration and carbohydrate stores. In the two to four hours before exercise, a reasonable approach is to drink approximately 5 to 10 mL/kg body weight of fluid. This provides adequate hydration and allows sufficient time for excess fluid to be voided. In the hour or so prior to exercise, a reasonable target for athletes is to ingest 1 to 4 g/kg body weight of carbohydrate.
Recovery following a session of endurance exercise has two goals:
●Replenishment of losses (eg, fluids, electrolytes, energy). This restores capacity for subsequent training.
●Adaptation to the stresses imposed by training. Such adaptations gradually increase the body's capacities to manage these specific types of exercise-related stress and enable improvements in performance.
Several tables summarizing nutrition for recovery are provided. These include guidance about what to eat, particularly when recovery is important (eg, after intense training), and when devoted recovery efforts are unnecessary (table 7 and table 3). For the recreational athlete who has plenty of time between training sessions, special nutritional efforts are usually unnecessary, and recovery is readily addressed with the next meal. In addition, light workouts may not create major nutritional demands.
The nutritional requirements for recovery depend on the physiologic stresses sustained during a particular workout. Sessions differ in how much they cause athletes to sweat, deplete muscle energy stores, stimulate protein synthesis, and damage tissues. According to the stresses of the session, a complex array of metabolic processes occur that enable replenishment and adaptation. These include restoration of muscle and liver glycogen stores ("refueling"), replacement of fluid and electrolytes lost in sweat ("rehydration"), and protein synthesis for repair and adaptation ("rebuilding").
Responses of other systems, such as the immune and antioxidant systems, also help the athlete to recover and remain healthy but are less well understood.
Therefore, each training session warrants its own recovery eating plan, which may differ from athlete to athlete. Misguided approaches to recovery eating may entail unnecessary expense and possibly lead to more significant problems, such as weight gain.
Depending upon the volume and intensity of training or competition, different types and amounts of nutrients are needed to restore normal nutritional status and achieve goals. Elite and recreational endurance athletes preparing for demanding events (eg, marathon, Ironman triathlon) often perform one or more workouts daily, allowing 4 to 24 hours for recovery between sessions of prolonged or high-intensity exercise. During a period of strenuous training, the nutritional goals of recovery may shift between adaptation to training and restoration of nutritional status, which permits the athlete to continue training hard. The relative importance of the speed with which particular nutrients are consumed depends on whether the nutrients are handled differently following exercise and the time needed to achieve restoration [72].
SUPPLEMENTS AND SPORTS FOODS —
The use of sports supplements has grown rapidly over the past decades and includes a wide range of substances associated with numerous claims about improved performance [73-78]. We suggest that athletes limit their use of supplements and sports foods to those supported by sound evidence and whenever possible do so in consultation with a sports medicine physician or sports nutritionist. Issues around sports supplements and the benefits and risks of particular substances are discussed in detail separately. (See "Nutritional and non-medication supplements permitted for performance enhancement" and "Prescription and non-prescription medications permitted for performance enhancement" and "Use of androgens and other hormones by athletes" and "Overview of herbal medicine and dietary supplements".)
Sports supplements are often unnecessarily expensive, and manufacturing of many products is not well regulated. Some ingredients may pose health risks. While particular supplements may help to address a nutrient deficiency or otherwise be of value to endurance athletes, they should be viewed as part of a holistic nutrition plan, including careful assessment of the cause for any inadequate nutrient intake, excessive losses, or increased requirements.
Important considerations for the endurance athlete considering a sport supplement include the following:
●Unsubstantiated claims – Many claims about performance improvement have not been subject to careful, controlled study and are frequently overstated. The decision to use a supplement or sports foods is a personal choice for which the athlete must bear responsibility for any disadvantageous or harmful outcomes.
●Caffeine – Caffeine may be helpful for some endurance athletes but is often misused. (See "Nutritional and non-medication supplements permitted for performance enhancement", section on 'Caffeine' and "Benefits and risks of caffeine and caffeinated beverages" and "Acute caffeine poisoning".)
●Unsafe ingredients – Unsafe ingredients or substances banned by the World Anti-Doping Authority (WADA) may be included in some supplements. Elite athletes covered by antidoping regulations should document their use of all sports foods and sports supplements, noting batch numbers for the latter.
●Manufacturers and testing – When supplements are indicated, athletes should limit use to products from reputable manufacturers. Furthermore, any products ingested should be "batch tested" for the absence of banned substances by accredited, independent third-party organizations.
SUMMARY AND RECOMMENDATIONS
●Terminology and key concepts – Endurance exercise is characterized by continuous activity carried out for 30 minutes or longer. During such activities, muscle fuel is provided primarily by aerobic (oxygen-requiring) production of energy from metabolism of carbohydrates and fat. Endurance exercise may involve running, swimming, cycling, rowing, cross-country skiing, participating in a triathlon, or other activities. (See 'Terminology and fundamental concepts' above.)
Energy requirements for endurance sport vary depending upon the athlete's sex, age, stage of growth, lifestyle or work-related activity, and training and competition schedules. The athlete's diet must provide appropriate amounts of macronutrients (eg, fat, carbohydrate, and protein) and micronutrients (vitamins, minerals, and phytochemicals) within the allotted calories. (See 'Training diet' above.)
The ratio of power to body weight partially determines speed and economy of movement. Traditionally, endurance coaches and athletes have emphasized reducing body fat and underemphasized developing power and strength. We believe that endurance athletes should work towards achieving a healthy balance of lean muscle mass and body fat. Furthermore, we believe that athletes should not aim to achieve minimal levels of body fat and body mass. (See 'Achieving optimal physique for endurance sport' above.)
●Carbohydrate needs – Dietary carbohydrate intake should be managed to meet the athlete's needs, which can vary based on the type, volume, and intensity of daily training sessions and throughout training cycles. The following table provides guidance to help meet these needs (table 1). Different approaches to carbohydrate intake over the course of a training week are provided in this table (table 2). (See 'Energy intake' above and 'Carbohydrate needs for endurance sport' above.)
●Protein needs – Dietary protein is needed to support exercise-induced skeletal muscle remodeling and repair and to optimize body composition. During periods of heavy training, endurance athletes should consume 1.2 to 1.6 grams of protein/kilograms body weight each day, roughly double the standard recommended daily allowance for adults. This equates to a daily protein amount of between approximately 70 grams (eg, for a small, female runner) and 150 grams (eg, for a large, male rower). Endurance athletes should distribute their intake of high-quality protein throughout the day. Guidance about intake and sources of protein is provided in the following table (table 3). (See 'Protein for endurance sport' above.)
●Iron, calcium, and vitamin D needs – A varied diet consisting of nutrient-rich foods meets any potential increases in micronutrient needs associated with strenuous endurance exercise. However, some athletes may consume inadequate amounts of iron and calcium and require supplementation. These issues are discussed in the text. (See 'Iron, calcium, and vitamin D deficiencies' above.)
Notable differences in the presentation and management of endurance athletes include the following:
•Consumption of iron-rich foods is best avoided during the three to six hours after strenuous exercise when hepcidin concentrations are high. (See 'Iron deficiency' above.)
•For endurance athletes performing strenuous training, the threshold for a low serum ferritin concentration is 35 ng/mL, rather than the 50 ng/mL used for sedentary adults.
•Low bone density is a common problem among endurance athletes and predisposes to bone stress injuries. During endurance exercise, acute reductions in blood calcium and increases in parathyroid hormone concentrations may occur leading to bone resorption. Consuming calcium supplements or calcium-rich foods in the pre-exercise meal reduces these risks. (See 'Calcium' above.)
•Endurance athletes who may have inadequate exposure to sunlight should be screened for vitamin D deficiency and supplementation taken as indicated. (See 'Vitamin D deficiency' above.)
●Pre- and post-exercise needs – Preparation for endurance exercise sessions consists primarily of ensuring adequate hydration and carbohydrate stores. In the two to four hours before exercise, a reasonable approach is to drink approximately 5 to 10 mL/kg body weight of fluid. The following tables provide guidance for post-exercise carbohydrate refueling and protein intake (table 7 and table 3). (See 'Pre- and Post-exercise Nutrition' above.)
