Endurance athletes experience periods of high-volume training that can cause fatigue and muscle damage, affecting performance. To overcome this, athletes rely on nutritional information and supplements to enhance performance, maintain energy for their sport, and to aid in recovery (1). Competitive endurance athletes rely on health professionals to gain information about the current scientific research regarding the safe and efficacious use of nutritional supplements. The purpose of this review is to share recent developments in the scientific literature regarding the optimal timing, dosing, potential side effects, and sport-specific benefits of nutritional supplements for endurance athletes. In addition, we will discuss new research on the effects of energy availability, macronutrient consumption, micronutrients in the diet, and fluid intake on exercise performance.
We based our choice of possible nutritional supplements from the Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals published by the National Institutes of Health in June 2017 (2). The fact sheet included antioxidants (vitamin C, vitamin E, and coenzyme Q10), arginine, beetroot, beta-alanine, beta-hydroxy-beta-methylbutyrate (HMB), betaine, branched chain amino acids (leucine, isoleucine and valine), caffeine, citrulline, creatine, deer antler velvet, dehydroepiandrosterone (DHEA), ginseng, glutamine, iron, protein, quercetin, ribose, sodium bicarbonate, tart cherry juice, and tribulus terrestris. We then searched Pubmed for studies published in 2017 using the listed supplements combined with performance and time trial as keywords. Studies were included in our review if they were written in English and placebo-controlled, crossover studies, using healthy, competitive, endurance athletes under normal temperature, and sea level conditions. Endurance events were defined as exercises longer than 2 min, because this is when mainly oxidative energy systems are used. Articles also could only examine one ingredient and use time trials to assess performance. We chose not to use studies with time to exhaustion as that is not a typical competitive activity for athletes. The only nutritional supplements that had articles that fit our criteria and that had new publications in 2017 were beetroot and caffeine. However, we did include ideal dosing, timing, event duration, and possible side effects for supplements that have scientific evidence of performance enhancement (Table 1). We also included new research from 2017 on energy availability, macronutrient intake (carbohydrate, fat and protein), micronutrient intake (calcium and vitamin D), and fluid intake as keywords combined with performance and athletes, because these are important for exercise performance (9).
Health professionals should be mindful when giving sports nutrition advice because energy and macronutrient requirements fluctuate. Periodized dietary recommendations should be developed based on individual characteristics and preferences, the energy systems used, and the muscle fiber types recruited for the sporting event, time of season, training volume and intensity, and the goal of the training block (fat loss, fat-free mass gain, strength and power improvements, or increased endurance capacity) (1,9). Maintaining adequate energy availability (energy intake minus exercise energy expenditure relative to fat-free mass) is vital for optimal health and exercise performance (1,10). Measuring energy needs can be difficult. For more information on how to estimate overall energy expenditure, see an excellent review by Bytomski (1) and Thomas et al. (9).
Recommended caloric intake for staying in energy balance is approximately 45 kcal·kg−1 FFM per day (9). Low-energy availability (energy availability, < 30 kcal·kg−1 FFM per day) can result in depressed hypothalamic-pituitary axis function in an attempt to conserve energy and thus, reduced reproductive hormone levels, endocrine function, bone health, resting metabolic rate, peak catecholamine levels, and exercise performance (10,11). Heikura et al. (10) examined energy availability in male and female world-class distance runners and race walkers during a high-load precompetition training block. Thirty-seven percent of the female athletes were amenorrheic, and 40% of the male athletes had low testosterone levels. The female athletes also had lower bone mineral density, whereas both sexes had low thyroid hormone levels and a 4.5 times greater incidence of bone injuries compared with those with normal reproductive function. The study found that risk assessment tools (Triad Cumulative Risk Assessment Tool, Relative Energy Deficiency in Sport Assessment Tool, and the Low Energy Availability in Females Questionnaire) were better predictors of overall health status of individual athletes than trying to assess energy intake and expenditure through diet and training logs.
Woods et al. (12) found a 5% decrease in resting metabolic rate, an 18% decrease in fat mass, increased perceived fatigue, and reduced 5-km time trial performance in elite male and female rowers studied before and after 4 wk of intensified training. These athletes did not change their habitual caloric or carbohydrate intake despite a 21% increase in training load, likely resulting in low energy availability. Schaal et al. (13) examined energy availability, endocrine markers of energy conservation, and perceived fatigue in elite synchronized swimmers, before and after 4 wk of intensified training. Despite a 27% increase in energy expenditure, the swimmers decreased their overall energy intake compared with baseline. As a consequence, energy availability, fat mass, and leptin (indicator of adequate energy availability) concentrations decreased, while ghrelin concentration (indictor of low energy) increased significantly from baseline. After intensified training, 400-m swim speed decreased by approximately 1% (14). These studies did not evaluate the athletes after a reduction in training load (taper period) and did not show individual responses for the time trial results in relation to energy availability. It is possible that some of the athletes matched energy intake to expenditure and had a positive training response. It is recommended that athletes stay in energy balance for most of the competitive season, save body fat loss for the off season and limit caloric deficits to 250 to 500 kcal·d−1 from their periodized energy needs for optimal health and performance (9).
Adequate glycogen stores are important for endurance athletes to offset symptoms of fatigue and for maintaining a high work rate (1,9). Experts no longer recommend habitual high-carbohydrate diets for competitive athletes, but instead suggest periodizing carbohydrate intake depending on the intensity and duration of the training session and training block goals (Table 2) (15). If fat loss, improved fat utilization, or increased endurance capacity through long slow duration training is the goal, then lower overall carbohydrate intake can be recommended or by altering the timing of intake (training in a fasted state or undertaking a second session of exercise without adequately refueling) (9). If the training session/block goal is to increase power output or speed through high-intensity training, then higher carbohydrate intake will be required to support the use of the glycolytic energy pathways and the use of fast twitch muscle fibers. Recommendations for glycogen super compensation before a competition (Table 2) no longer suggest glycogen-depleting exercise and one full week of carbohydrate loading (15). Alternatively, incorporating a higher carbohydrate intake of 8 to 12 g·kg−1 for 36 to 48 h before the competition, in combination with an exercise taper should suffice (15).
Recommendations for preexercise carbohydrate intake are in Table 2. Current guidelines state that there is little extra value in ingesting low glycemic index carbohydrates before exercise unless the exercise is prolonged with no supplementation (9). Baur et al. (18) confirmed this by examining the effects of ingesting iso-caloric, iso-carbohydrate (75 g) beverages containing either a low glycemic index, hydrothermally modified waxy maize starch (UCAN), a high glycemic index sucrose and glucose-based sport supplement or a noncaloric placebo 30 min before exercise, on 5-km running time trial performance in competitive male runners (18). Runners had fasted overnight before arriving at the lab. The time trial was performed after 60 min of running at 60% to 75% V˙O2max and the total running time was about 1.5 h. The study found no difference in time trial performance between treatments.
The current recommendations for carbohydrate ingestion during training or competition are listed in Table 2. Carbohydrate ingestion for exercise durations <60 min is not needed (9). This was confirmed by Shei et al. (19), who examined trained male cyclists (>6 h after their last meal) while randomly performing three 4-km cycling time trials, separated by 15 min of active recovery, on four separate occasions. Carbohydrate (80 g) was given either before time trial 1, before time trial 2, before time trial 3 or not at all. Placebo was given before the other time trials. Mean power output and time to completion were not different between treatments, indicating that carbohydrate ingestion did not improve repeated high-intensity cycling bouts for a duration less than 1 h.
In contrast, studies have shown that frequent contact of carbohydrate-containing fluids with the mouth and oral cavity, through a mouth rinse, during short, high-intensity work bouts, can stimulate parts of the brain and central nervous system and reduce perceived exertion and increase work intensity (20). To test this idea, James et al. (16) examined competitive male cyclists after an overnight fast that were randomly assigned three trials where they mouth rinsed for 5 s with either a 7% or 14% maltodextrin solution, or a taste-matched placebo, every 12.5% of total exercise duration. Cyclists completed the time trial faster (P < 0.001) during the 7% (57.3 ± 4.5 min) and 14% (57.4 ± 4.1 min) carbohydrate rinsing treatments, compared with placebo (59.5 ± 4.9 min).
Exercise durations >60 min can benefit from carbohydrate intake (9,21) (Table 2). Glucose and fructose are absorbed by the intestine via different transporters (SGLT1 and GLUT5, respectively), and their combination in ingestible sports products allow for greater absorption of carbohydrate, resulting in higher rates of oxidation (22). Interestingly, the ideal ratio of glucose to fructose (~1.0) is found naturally in most fruits and vegetables (22). Nutrition for competition should be practiced in training to match individual needs, preferences, and gastrointestinal comfort. The gut can be trained by increasing the exposure to carbohydrate, which increases the number and activity of sodium/glucose transporters, resulting in greater absorption and oxidation of carbohydrate, less gut discomfort and improved exercise performance (23). Alcohol should be limited after exercise, because it can inhibit glycogen storage (15).
Fat is an essential component of an athlete’s diet, and should not be less than 20% of the caloric intake (1). N-3 PUFA (n-3) polyunsaturated fatty acids have recently been suggested as a supplement to improve exercise performance and recovery, by reducing inflammation. To date, there is no scientific evidence to support N-3 PUFA (n-3) polyunsaturated fatty acids for improved performance in athletes (24).
Metabolic flexibility and greater use of fat as an energy source to spare limited glycogen stores can improve exercise performance in ultra-endurance events (>3 h) (23). Improved fat oxidation can be achieved through training (e.g., long, slow duration exercise) or through dietary manipulation such as fasting, acute preexercise intake of fat/ketones, and by high-fat, low-carbohydrate diets (1). While these strategies improve performance for very long-distance events, most competitive events or high-quality training sessions are performed at high intensities of exercise and are impaired by low glycogen levels or by down-regulation of glycogenolysis imposed by low-carbohydrate diets (9,23). Very low-carbohydrate diets usually lead to ketosis, where the liver oxidizes nonesterified fatty acids into ketone bodies, including 3-hydroxybutyrate, acetoacetate, and acetone (25). With low blood glucose levels induced by low-carbohydrate diets, the brain and muscle can metabolize ketones as a fuel source. Supplementation with ketones (acetoacetate and Β-hydroxybutyrate) before exercise has been promoted as a way to increase fat utilization without having to commit to a long term low-carbohydrate/high-fat diet (26). Pinckaers et al. (26) published a review on ketone supplementation and concluded that there is currently no evidence to support the use of ketone bodies as an ergogenic aid.
Increased ability to use fat with low-carbohydrate/high-fat diets (1.5 g·min−1 vs 1.0 g·min−1 in traditional diets) has been suggested to improve long duration exercise performance by preserving limited muscle glycogen (25). To test this hypothesis, Burke et al. (27) investigated the effects of adaptation to a ketogenic low-carbohydrate/high-fat diet during 3 wk of intensified training on performance in world-class endurance athletes. The groups receiving the 3 dietary conditions were well matched and all consumed identical calories and protein (~40 kcal·kg−1 FFM per day, 2.2 g·kg−1·d−1). The groups consumed either constant high carbohydrate (8.6 g·kg−1·d−1); periodized high carbohydrate (8.6 g·kg−1·d−1), or a low-carbohydrate/high-fat diet (<50 g·d−1 carbohydrate and 78% fat). The low-carbohydrate/high-fat diet was associated with increased whole-body fat oxidation, but also increased the oxygen cost at a speed approximating a 20-km race. Both high-carbohydrate diets improved 10-km race walk times (~42 to 45.5 min) by 5% to 7%, after intensified training, but there was no improvement (−1.6%) for the low-carbohydrate/high-fat group.
McSwiney et al. (28) examined endurance-trained athletes who self-selected into a habitually high-carbohydrate (% carbohydrate/protein/fat, 65/14/20), or a low-carbohydrate/ketogenic diet (% carbohydrate/protein/fat, 6/17/77), combined with an identical 12-wk training intervention. During postintervention testing the high-carbohydrate group consumed 30 to 60 g·h−1 carbohydrate, whereas the low-carbohydrate group consumed water and electrolytes. The low-carbohydrate group experienced a significantly greater decrease in body mass (−5.9 kg vs −0.8 kg) and body fat percentage (−5.2% vs −0.7%). There was a trend toward faster cycling 100-km time trial times (2.5 to 3 h) after training in the low-carbohydrate group (−4.07 min vs −1.13 min, P = 0.057). Fat oxidation in the low-carbohydrate group was significantly greater throughout the 100-km time trial. The differing results in these studies probably relates to the length of the diet (3 wk vs 12 wk), the length of the time trial (~45 min vs 2.5 to 3 h), and the preexercise glycogen levels. Longer durations of diet adaptation (>4 wk) and longer duration events (>2 h) where glycogen levels are reduced, especially if they are conducted in a low glycogen state, could result in improved performance after low-carbohydrate/high-fat diets.
A new International Society of Sports Nutrition Position Stand states that protein requirements are higher for athletes and increase with high training volumes in order to maintain energy balance, protein balance, and muscle mass (Table 2) (17). Better responses have been reported from spreading high-quality (high leucine content) protein intake throughout the day (~0.3 g·kg−1 every 3 to 5 h) (1,17). Protein requirements should consider the individual goals and preferences of the athlete and can increase with age, in those trying to lose body fat while maintaining or increasing fat-free mass, with hypocaloric and low-carbohydrate diets and with high volumes of training (17). Adding protein to a carbohydrate beverage/gel during exhaustive endurance exercise suppresses markers of muscle damage 12 to 24 h postexercise and decreases muscular soreness (17). Therefore, the position statement recommended 0.25 g of protein·kg−1·h−1 along with 30 to 60 g·h−1 carbohydrate for endurance activities longer than 1 h (17). Research has shown no harmful effects of protein ingestion up to 3.3 g·kg−1·d−1 in healthy athletes (17). Consuming casein protein (30 to 40 g) before sleep can increase muscle protein synthesis (17). Adequate carbohydrate and caloric consumption are important so amino acid intake can be directed towards repair and synthesis rather than being used as an energy source.
An optimal athletic diet requires good hydration for thermoregulation and exercise performance (Table 1). Since there is a lot of variation in fluid and electrolyte loss with individuals, monitoring urine color (pale yellow optimal) and weighing before and after exercise (~1% to 2% weight loss optimal) are good ways to ensure adequate fluid intake with training (29). Sweat loss causes loss of electrolytes. Therefore, athletes should drink fluids combined with electrolytes or by adding salts through food. Relying on ad libitum fluid intake instead of prescribed fluid intake during prolonged endurance training may lead to impaired exercise performance. Bardis et al. (29) studied heat-acclimatized male cyclists with two cycling tests consisting of three sets of 5-km cycling at 50% of maximum power output followed by 5-km time trial at 3% grade (total 30 km). During the tests, athletes either consumed as much water as they desired or ingested water every 1 km at a rate to match 100% of fluid lost via sweating. The mean cycling speed for the third bout of the 5-km time trial was greater in the prescribed drinking trial (30.2 ± 2.4 km·h−1) compared with the ad libitum trial (28.8 ± 2.6 km·h−1).
A well-balanced whole food diet with a variety of fruits, vegetables, whole grains, dairy, and lean protein sources should provide adequate micronutrients for most athletes (Table 3). Vegetarian or vegan athletes may require vitamin B12, iron, calcium, vitamin D, riboflavin, and zinc supplementation (1). Micronutrients are vital to optimal health, but have not been shown to improve endurance exercise performance unless athletes are deficient (1,31). See Table 4 for recommended values in athletes experiencing deficiency.
A recent review by Dominguez et al. (3) examined the effects of beetroot on endurance exercise performance. Beetroot’s bioactivity is thought to be due to high concentrations of nitrates and various phytochemicals, including betalains. Both of these compounds are found in concentrated beetroot juice (BRJ), which has been shown to decrease oxygen consumption during submaximal exercise and improve running and cycling time trial performance (3). Most of the effects of BRJ are attributed to its high nitrate content, which is reduced to nitric oxide when ingested to improve muscle blood flow and oxygenation (3). Peak concentration in blood is obtained within 2 to 3 h of beetroot supplementation and the performance benefits are seen 150 min after ingestion (3). Oral antiseptic rinses should not be taken with beetroot supplementation, as these can prevent the conversion to nitrite. The majority of studies show ergogenic effects of beetroot at a supplementation dose of 6 to 8 mmol, but high-performance athletes might require a slightly higher dose (3). Shannon et al. (4) had trained male runners or triathletes complete four running time trials of 1.5 or 10 km. Time trials occurred 3 h after the supplementation with either 140 mL concentrated nitrate-rich (~12.5 mmol nitrate) BRJ or nitrate-deplete (~0.01 mmol nitrate) BRJ. Performance in the 1.5-km time trial was significantly faster with BRJ (319.6 ± 36.2 s vs 325.7 ± 38.8 s). While there was no significant difference in overall 10 km time trial performance between conditions (2643.1 ± 324.1 s vs 2649.9 ± 319.8 s), subjects had a significantly faster running speed (14.0 ± 1.6 km·h−1 vs 13.7 ± 1.5 km·h−1) and quicker time (1287.5 ± 153 s vs 1317.6 ± 149.9 s) in the first 5 km with BRJ compared with placebo. The authors hypothesized that better performance results with BRJ supplementation was seen with the 5-km time trial (<30 min), where higher intensities require recruitment of type IIa muscle fibers and improved oxygen delivery would be of greater benefit.
Many studies have seen improved performance with chronic supplementation (6 to 8 d) combined with supplementation 2 to 3 h before exercise. McQuillan et al. (32) used a double-blind crossover-design with highly trained cyclists who ingested 140 mL of nitrate-rich BRJ (~8.0 mmol nitrate), or placebo, for 7 d. Participants completed a cycling time trial on days 3 and 6 (4 km) and on days 4 and 7 (1 km) of supplementation. There was no significant difference in performance. The same authors repeated the exact same protocol, but with 70 mL of nitrate-rich BRJ (~4.0 mmol nitrate), or placebo, for 8 d in trained cyclists and showed improved 4 km time trial time (−0.7% ± 0.9%) and mean power output (2.4% ± 2.5%) (33). Differing results between these studies most likely was from the timing of supplementation just before the time trial. Peak plasma levels of nitrite are reached 2.5 to 3 h after supplementation and positive results only seen with exercise 2.5 h after supplementation (3).
Recent research also has shown that the betalains found in beetroots improve running time trial performance, possibly from reduced exercise related muscle cell damage (5). Montenegro et al. (5), using a randomized, double blind, crossover design, had 22 competitive male and female triathletes ingest 100 mg beetroot concentrate (25 mg betalains) 6 d before each test day, and 50 mg 2 h before exercise, and saw improved 10 km running time trial performance compared to placebo (49.5 ± 8.9 min vs 50.8 ± 10.3 min, P = 0.03). The triathletes also improved their 5-km time trial performance 24 h after the 10-km time trial (faster in 17 of 22 participants) (23.2 ± 4.4 vs 23.9 ± 4.7 min, P = 0.003).
Caffeine is a commonly used ergogenic aid for performance, due to its accessibility and availability in low cost beverages such as coffee and tea. Caffeine has multiple proposed mechanisms of action, including increased Na+/K+ ATPase activity, increased calcium release from the sarcoplasmic reticulum, a greater catecholamine response to endurance activity and the ability to serve as an antagonist to adenosine receptors in the central nervous system, contributing to reduced pain sensation and reductions in perceived fatigue (6). The consumption of 3 to 6 mg·kg−1 of caffeine, 60 min before exercise has been shown to enhance exercise performance (6). Clarke et al. (34), in a double-blind, randomized, crossover, placebo-controlled design, had 13 trained male runners complete a 1-mile race, 60 min after the ingestion of 90 mg·kg−1 coffee, 90 mg·kg−1 decaffeinated coffee, or a placebo. 1 mile times were 1.3% faster after the ingestion of coffee (04:35:37 ± 00:10:51 mm:ss) compared with decaffeinated coffee (04:39:14 ± 00:11:21 mm:ss) and 1.9% faster compared with placebo (04:41:00 ± 00:09:57 mm:ss). Nieman et al. (35) examined the effects of 2 wk of randomized, crossover ingestion of high chlorogenic acid (CQA) coffee (474 mg of caffeine per day)(has high antioxidant and anti-inflammatory properties) or placebo, with a 2-wk washout period, on 50-km cycling time trial performance in trained cyclists. CQA coffee provided 1,066 mg CQA and 474 mg caffeine versus 187 mg CQA and 33 mg caffeine for placebo. Fifty-km cycling time performance and power did not differ between trials, suggesting no performance enhancement with the chronic consumption with CQA coffee.
Epigenetic alterations to CYP1A2 activity have been noted from chronic caffeine use, leading Gonçalves et al. (36) to assess the effect of consuming 6 mg·kg−1 of caffeine or placebo on time trial performance in 40 trained male cyclists with varying habitual caffeine consumption. Participants were labeled as low consumers (58 ± 29 mg·d−1), moderate consumers (143 ± 25 mg·d−1), and high consumers (351 ± 139 mg·d−1). A significant 3.3% improvement in time trial performance was seen 60 min after 3 to 6 mg·kg−1 of caffeine consumption in all tiers of caffeine consumers compared with the control treatment (29.92 ± 2.18 min vs 30.81 ± 2.67 min). However, no significant effect between habitual consumption pattern and time trial performance from the caffeine was seen. One variant to caffeine in coffee or tea is Yerba Mate, a plant rich in antioxidant phenolics, saponins, and xanthines (caffeine and theobromine). Areta et al. (37) examined the effects of ingesting 5 g of either Yerba Mate (0.7 mg·kg−1 of caffeine) or maltodextrin for 5 d and 1 h before a cycling time trial in 9 well trained cyclists. The Yerba Mate trial elicited a small, but significantly faster time trial time (30.1 ± 1.8 to 29.4 ± 1.4 min). The authors suggested that the ergogenic effects of Yerba Mate could be explained through a synergism between CQAs and caffeine in stimulating the central nervous system.
Maintaining energy availability (~45 kcal·kg−1·FFM per day) is essential for the majority of the season for endurance athletes and should be assessed through diet logs and estimates of energy expenditure via equations for RMR multiplied by activity factors or through activity monitors (1). Questionnaires (Triad Cumulative Risk Assessment Tool, Relative Energy Deficiency in Sport Assessment Tool, and the Low Energy Availability in Females Questionnaire) can also be used to evaluate if athletes are suffering from any signs or symptoms of low energy availability (10).
Athletes should periodize their carbohydrate intake based on their training goal and sporting event (Table 2) to maximize exercise performance and recovery. During the baseline, endurance phase of training, athletes might benefit from a low-carbohydrate diet/high-fat diet to improve fat utilization and endurance capacity and for reducing body fat (28), but as the season progresses, higher carbohydrate diets should predominate as they allow for an improved ability to access the glycolytic energy pathways for the higher intensity efforts needed for most competitions. Endurance athletes require higher daily protein intake and protein intake during and after exercise to maximize exercise performance and recovery (Table 2).
Multiple supplements have demonstrated performance enhancing effects within specific margins of dosing and timing for endurance athletes (Table 1). Health professionals should remain educated on the efficacy, safety, and interindividual variability of common supplements as they remain a popular component of the endurance athlete’s diet. Many dietary supplements in the marketplace contain multiple ingredients, and therefore, it is difficult to know the performance effects and safety of these products. In addition, many of the products claiming performance benefits contain proprietary blends of ingredients with no information on the amount of each ingredient in the blend. Therefore, those products should be used with caution.
The authors declare no conflict of interest and do not have any financial disclosures.
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