One of the main focuses in swimming training is high volume training ³ that includes 1-3 daily sessions. Training sessions are performed both in the pool and on land with strength training, core, or cross training ¹⁴.

Occasionally, the high training volume results in below adequate energy intake for the swimmer to maintain adequate energy balance, with intake that can reach the 2400 kcal/day on extreme training periods ¹⁵. It is worth mentioning here, that energy deficits maintained for long periods of time, could result in decreased basal metabolic rate, cause alterations in hormonal functions, disrupt menstrual cycles in women, as well as increase the risk of injury and disease ^16,17. It has recently been found that high level female swimmers who experience menstrual dysfunctions show a decrease in thyroid hormones and growth factors, coupled with a decrease in speed in the 400-m test after a 12-week training period ¹⁸.

Regarding carbohydrate intake, it is necessary to point out that a 10% reduction in daily carbohydrate intake has been shown to lower performance in a 365-m freestyle test in swimmers, while an increase of 10% led to improvements in both tests of 91.5 and 365 m ¹⁹. Although these results may suggest a need to follow a diet rich in carbohydrates, it is worth highlighting another study that did not observe any differences in performance among a group of swimmers who followed a diet of 6 g/kg/day compared to a 12 g/kg/day carbohydrate diet intake, after following a moderate training volume ²⁰. Carbohydrate requirements should be periodized according to training demands ¹⁴. Beginning a training session with low carbohydrate availability (due to low carbohydrate intake since the last training session), causes a higher metabolic stress that can optimize training adaptations ^3,16, as long as the training sessions are of moderate intensity.

Therefore, for athletes who follow a moderate- to high-intensity training program, carbohydrate intake should be kept in ranges of 6-8 g/kg/day on days when training sessions are of high volume and low intensity, or high intensity and very low volume. On days of moderate or high volume with high intensity, carbohydrate intake should be 10-12 g/kg/day ²¹. It is important to point out that when engaging in high intensity sessions with the recommended amount of carbohydrate intake, it will keep the immune system in a better state through an increase in the salivary immunoglobulin response ²², decreasing the risk of respiratory tract infection ²³. However, these general recommendations should be fine-tuned with individual consideration of total energy needs, specific training needs, and feedback from training performance

Recommendations for carbohydrate timing are the same as in other sport modalities, except before high-intensity sessions, where training sessions require high CHO-availability ³. Following the recommendations of other sports modalities, it could be recommended that 200- 300 g ²⁴ or 1-2 g/kg be consumed 3-4 hours prior to the training sessions ²⁵. Carbohydrates, especially high on the glycemic index must not be ingested 45 minutes prior to exertion, to avoid reactive hypoglycemia ²⁶.

Carbohydrate intake during exercise favors glycogen oxidation in the muscle, keeping blood sugar levels stable, while preserving glycogen stores during longer periods of time, and avoiding protein catabolism ²⁷. Therefore, carbohydrate intake during high-intensity training sessions should be favored with intakes up to 90 g/hour ²⁸ in a 2:1 ratio of glucose and fructose ²⁹.

Studies have shown that open water competitions have the capacity to deplete muscle glycogen storages ⁹. Because intake of carbohydrates during competition is difficult, athletes are recommended to eat a super-compensation carbohydrate diet ¹⁰. Athletes should consume 10-12 g/kg/day of carbohydrates 48 hours prior to a competition, the same amounts as recommended prior to high-intensity interval sessions ³⁰. Then, 4 hours before a competition the intake should be at 1-4 g/kg/ day.

Protein requirements for the athletes range between 1.2-2 g/kg/day ³¹. Due to the strength requirements in swimming, the recommended intakes should be on the higher limit that is 2/g/kg/day. The dietary protein intake, which only provides a very small amount to the energy metabolism, especially in situations where glycogen stores are depleted ^3,16, is necessary for the synthesis of new myofibrillar tissue, as a response to strength training, as well as sarcoplasmic, and mitochondrial proteins in response to endurance, and interval training ³².

It must be emphasized, that the protein food must be of high bioavailable value, and protein timing must be prescribed in relation to the exertion effort ¹⁶. With the aim of optimizing protein synthesis in swimmers, an intake of 0.3 g/kg of high bioavailable protein must be consumed after finishing a competition, strength training, or interval training session ¹⁴.

It should be noted, that amounts higher than 0.3 g/kg do not increase the protein synthesis rate ³³. Additionally, athletes should be encouraged to ingest this amount of protein in 4-5 times during the day ³⁴. These recommendations are based on a study that showed a higher protein muscle synthesis response when athletes consumed 4 intakes of 20 g of high-quality protein in 3-hour intervals after training, compared to 2 intakes of 40 g or 8 intakes of 10 g in similar conditions ³⁴. However, it is important to note that these studies were not in aquatics sports.

Regarding nutrient timing, especially carbohydrate intake, it must be noted that after 2-4 hours of training, a higher muscular glycogen resynthesis potential exists, thanks to a higher sensitivity to insulin action during this period, together with a higher activity of the glycogen synthase enzyme activation ³⁵. The activation occurs because of the increment in the presence of calcium from the successive action potentials that take place during the effort ³⁶. The higher glycogen synthesis rate takes place when carbohydrates intake reaches 1 g/kg/hour ^3,16. When the goal between training sessions is to recover muscle glycogen stores, as well as favor protein synthesis, 1 g/kg of carbohydrate together with 20-25 g of protein should be ingested ³⁶. On the contrary, when the goal of the training sessions is to achieve maximal efficacy of fat and carbohydrate metabolism, 0.3 g/kg of protein alone should be consumed. Lower amounts of carbohydrates (6-8 g/kg/day versus 10-12 g/kg/day) should also be consumed, once the period of glycogen synthesis is promoted, which will cause incomplete muscle glycogen stores. The state of incomplete muscle glycogen storage is optimal for lower intensity sessions ³⁷, due to the fact that the transcription of genes that underlie the trainings gets favored, with the objective of improving fat and carbohydrate metabolism efficiency ³⁸.

Regarding fat intake, we must consider that these, besides their energetic function, play an important immunologic role ³⁹. In this way, a diet with a low-fat content, can increase inflammatory cytokine levels, decrease antioxidant potential and negatively affect blood lipoprotein levels ⁴⁰. The fat content in the diet is adjusted once the protein and carbohydrate content has been adjusted, although, the fat intake should represent at least a 20-25% of the energy intake ¹³, since, it would be difficult for diets with lower fat content to reach the essential fatty acids and liposoluble vitamin requirements ⁴¹. Furthermore, as it has been proved an association between the injury rates in women athletes and the lipid intake ⁴², it is possible that a deficient fat intake could cause sport injuries, although these data correspond in female runners and non-swimmers.

A diet higher in fat content during periods of high volume and low-intensity training could maximize the contribution and efficiency of fatty acid metabolism during exercise ^31,43. During low-intensity training session, it would be recommended to increase the intake of fats, up to 35% of daily energy intake. With respect to the type of fatty acids, saturated fatty acids should not exceed 10% of daily energy intake ³¹, while the intake of essential and monounsaturated fatty acids should be encouraged ⁴⁴ (Table I).

Although dehydration during water activities is lower than in other outdoor sports ⁴⁵, due to thermoregulation improvement of different mechanisms from sweat as convection and conduction ⁴⁶, the hydration needs of swimmers is higher than of the sedentary population ⁴⁷. The higher the water temperature is, the higher the water loss in sweat will be ⁴⁶.

The first goal with regard to hydration is starting both training sessions and competitions in a euhydration state, which requires the athlete to hydrate in a rate of 5-7 ml/kg 4 hours prior to training, in addition to 3-5 ml/kg in case the athlete does not urinate during that time, or if the color of the urine is dark ⁴⁸. The beverage should be cold, because cold beverage intake favors thermoregulation during swimming, especially when the temperature is high ⁴⁹.

When the losses of sweat are higher than 2% of body weight, and the amount of time in between training is less than 6-8 hours, it is necessary to establish a specific post training rehydration program ^3,16. With the goal of optimizing hydration, and water retention (considering that some part will be excreted via diuresis), the amount of water should be equivalent to around 150% of fluid lost during training. Adding sodium to the beverage ^3,16 could favor hydric retention, even if the co-ingested with carbohydrates.

In addition to adhering to an adequate nutrition plan, the intake of some nutritional supplements could favor both the athletes’ health status, as well as performance ¹³. Owing to the effect of supplement changes depending on the type of effort and sport modality ⁵⁰, some supplements could have an ergogenic effect or show an improvement in performance for water sports. Swimmers are regularly reported to have high rates of supplement use. Even when there is an evidence base for the use of such products, the swimmer is advised to be aware of the appropriate situations and protocols of use, and to take into account the “cost” of products in terms of financial outlay and the small but real risk of an inadvertent doping outcome ⁵¹. The supplements and their effects are presented in Table 2.

Caffeine (1,3,7, trimethylxanthine) is an alkaloid from the family of the methylated xanthines, antagonist of the adenosine receptor ⁵². Caffeine intake in the athlete’s diet is common, as it is found in a wide variety of foods, such as coffee, tea, chocolate, energy drinks, and gels. Caffeine is quickly absorbed at the intestinal levels ⁵³, detecting high concentration levels 15-45 minutes post intake, and maximum levels in the blood an hour post intake ⁵⁴.

Although studies, like the one published by Costill et al. show how supplementing with caffeine stimulates the rise of fatty acids in the blood, and thereby delaying period within which glycogen storage depletion occurs ⁵⁵, there is no sufficient evidence to justify this hypothesis at the moment ⁵⁶.

Nevertheless, caffeine consumption can cause other side effects that could improve sport performance, as it stimulates the central nervous system due to the adenosine antagonistic effect ⁵³, increases catecholamine levels ⁵⁷, improves muscle contractility ⁵⁸, improves action potential ⁵⁹, as well as decreases the perceived exertion ⁶⁰. Nowadays, caffeine is actually considered a supplement with ergogenic effects both in cardiorespiratory endurance, as well as in strength ⁶¹.

Although there are few studies found on the consumption of caffeine in swimmers, there is evidence that prerace caffeine supplementation may enhance swimming performance ^62,63, but further studies into swimming sports are required to know the dose and timing of prerace caffeine intake that are effective for swimming events, since recent studies suggest that the ergogenic benefit of taking caffeine alone for repeated 200-m swimming performance appears limited ⁶⁴.

There is some evidence from endurance sports protocols that small to moderate doses of caffeine (1-3 mg/kg) are as effective as larger doses (5-6 mg/kg) in achieving benefits and may reduce risk of side effects ⁵¹. Recently, Goods et al. had concluded that, a moderate dose of caffeine (3 mg/kg) ingested 1 h before a repeat sprint freestyle set significantly improves mean sprint time in elite swimmers, and the combination of at least a moderate dose of caffeine (>3 mg/kg) with trained athletes appears the most likely to result in ergogenic benefit to anaerobic exercise performance ⁶⁵.

Caffeine may be consumed in drinks or as an ingredient in some sport products (e.g., some gels). In this way, a recent study had concluded that a caffeinated energy drink increased some aspects of swimming performance in competitive sprinters, whereas the side effects derived from the intake of this beverage were marginal at this dosage (3 mg/kg) ⁶⁶.

Further research is required to determine the precise mechanism(s) responsible for caffeine’s ergogenic potential for anaerobic exercise performance.

The purpose of supplementing with phosphocreatine is to increase the concentrations of muscular creatine, with the goal of maximizing the resynthesis of ATP through the phosphagen system ⁸. This process depends on the availability of muscular creatine ⁶⁷. The metabolic pathway will have a great impact on performance, like high-intensity training carried out in most sports. During high-intensity training, creatine supplementation allows for higher training loads, which leads to higher training adaptations ⁸.

Studies conducted specifically with the swimming population, showed that the supplementation of 20 g/day over 6 days, did not achieve any improvement in a group of trained swimmers, in a test of 6 x 50 m ⁶⁸, or with a 25 g/kg supplementation during 4 days in a test of 10 x 50 m ⁶⁹. A literature review therefore concluded that there was not enough evidence to recommend supplementing with creatine to improve performance in swimming tests, although it could improve performance during interval session in the swimming pool ⁷⁰. The most likely benefits for swimmers come from using creatine in the training phase to enhance training adaptations to interval and resistance training. However, while there are studies that report an enhancement of the performance of such sessions in highly trained swimmers ⁷¹, there is some controversy about it at the moment ⁷².

Nevertheless, because a higher quality in training means higher adaptation, creatine could be useful for water sports, especially during periods of high intensity training. The recommended dosage is 3 g/day, since it showed the same effects in 4 weeks with a load protocol of 5 days (20-30 g/day), followed by maintenance dosage of 3 g/day, compared to continued intake of 3 g/day ⁷³, although this should be studied in swimmers.

The increase in H+ derived from lactic acid dissociation, leads to a decrease in blood and muscle pH ⁷⁴, disturbing the glycolytic rate ⁷⁵, the uptake and reuptake of calcium in the sarcoplasmic reticulum ⁷⁶, decreasing the contractile speed, while at the same time, causing perceived exertion to increase ⁷⁷. These mechanisms aim to decrease the glycolytic contribution to metabolism, most likely as a defense mechanism facing a hypothetic extreme acidosis situation. Supplementing with sodium bicarbonate allows the release of H+ ions into the bloodstream, avoiding a situation of extreme acidosis in the intracellular compartment, which could inhibit muscular contractions ⁷⁸, thus considering it as the main regulating agent of the acid-base balance at the extracellular level ⁵¹. It has been shown that supplementing swimmers with sodium bicarbonate, increases levels of sodium bicarbonate and pH in the blood, increases performance in 50-m swim tests ⁷⁹, 200 m ⁸⁰, successive sets of freestyle 100 m ⁸¹, and increases the average time in a 4-repetition set of freestyle 50-m swim ⁷⁹. Since supplementing with sodium bicarbonate can increase performance of repeated sprints ^81,82, it could also improve the quality of trainings, allowing the body to adapt ³.

The acute use of bicarbonate to increase extracellular buffering capacity (300 mg/kg 1-2 hour prerace) might enhance the performance of 200-800 m swimming events via increased tolerance to the production of H+ ions via anaerobic glycolysis ⁵¹. Further field studies are needed with high-level swimmers since the few available studies show controversies ^{64, 83,84}.

The main purpose of supplementing with b-alanine is to increase muscular carnosine levels. The synthesis of carnosine depends on the bioavailability of the dietary intake of b-alanine ⁸⁵. Carnosine, just like sodium bicarbonate, plays an important role with pH regulation, but acts differently than sodium bicarbonate, in that it acts at an intracellular level ⁸⁶. In addition to its acid base regulation effect, carnosine has also shown to improve performance, through improving the Myosin ATPase activation ⁸⁷, as well as improve calcium sensibility on muscular fibers ⁸⁸ because of its antioxidant function ⁸⁹.

In swimmers, chronic supplementation with B-alanine to increase muscle carnosine, an intracellular buffer, may provide an alternative or additive strategy to support training adaptations and race performance ⁵¹. In a 4-week study performed on swimmers, the effect of 300 mg/kg of sodium bicarbonate, and 4.8 g/day of b-alanine in two tests of 100-m freestyle showed, that b-alanine alone did not affect performance, while the supplementation of b-alanine with sodium bicarbonate did improve performance on the second set of 100 m ⁸¹. It should be noted that even though b-alanine did not show an increase in performance, the recommended dosage is 6.4 g/day, for a period of 4-6 weeks ⁹⁰, which is higher than what was used during the study. In relation to this, another study with highly trained swimmers who were supplemented with 6.4 g/day of b-alanine for 5 weeks, showed positive effects on performance in 100 m, and a tendency to improve in 200 m freestyle ⁹¹.

It is, therefore, recommended to supplement 6.4 g/day of b-alanine for 4 or 6 weeks taken in 4-8 dosages of 0/8-1/6 g, with the aim of avoiding the main secondary effect from the supplement (paresthesia) ⁹⁰.

The intake of 300-400 mg/day of nitrate, in the form of sodium nitrate ^92,93, or beetroot juice (6-8 mmol of nitrate) ⁹⁴, has demonstrated a doubling or tripling of nitrate plasma levels. The increase in nitrate plasma levels can lead to a simultaneous increase of nitric oxide levels, which regulates the muscle blood flow, as well as regulates the mitochondrial respiration ⁸². Supplementing with nitrate has shown to decrease VO2, for the same intensity effort ⁹². Supplementing with nitrate decreases the magnitude of the slow component of the VO2 ^95,96, increasing the time until exhaustion at submaximal intensities ⁹⁷.

Although no studies on nitrates on aquatic spot modalities exist, it is feasible to assume that for these athletes who require a lower VO2 when employing a specific power output level, this supplement could have an ergogenic effect. This can especially be of importance in aquatic sports where a higher time in apnea (synchronized swimming) or swimming tests of longer distances in swimming pools or open waters is required. It is, therefore, recommended to supplement with nitrate salts, or beetroot juice (6-8 mmol of nitrate) in the 150-180 minutes prior to training sessions ⁹⁸

Vitamin D has multiple functions related to sports performance, bone remodeling being one of them ⁹⁹, improving muscle contractions, synthesizing new muscle proteins, as well as enhancing immune function ¹⁰⁰. A study found that one third of a group of high performance swimmers had low serum levels of this vitamin, placing aquatic sports athletes at risk of developing hypovitaminosis ¹⁰⁰. It may be caused due to limited sun light exposure, as most of the training takes place in indoor facilities, and high-level athletes usually dedicate the rest of the day to sedentary activities to aid recovery. This may explain the lower levels of serum vitamin D particularly found in swimmers, compared other sport modalities that train outdoors ¹⁰¹.

Owing to the fact that optimal levels of vitamin D are linked with higher bone mineral density ¹⁰², it is assumed that this vitamin is important for optimal bone mineral density ⁹⁹; therefore, lower levels of this vitamin could place the athlete at risk of injuries ³⁹. To avoid the negative effects on performance that are associated with a deficit of this vitamin ¹⁰, swimmers should periodically check their serum levels in case supplementation is necessary if presented with suboptimal levels, and consume doses 10 μg/day approximately ^100-103.

Bovine colostrum is the milk produced by cows after they give birth to their calves, and it is known as a substance rich in immunoglobulins and growth factors ¹⁰⁴. Because aquatic sports athletes frequently suffer from respiratory tract infections, it has been proposed that supplementing with bovine colostrum could improve the immune system, decreasing the risk of immunosuppression, and respiratory tract infections ³⁹.

In some studies, supplementation with bovine colostrum has demonstrated improvements in performance in cardiorespiratory endurance tests ^105,106, as well as speed and strength ¹⁰⁷. On the contrary, other research could not prove positive effects supplementing with bovine colostrum on strength or speed performances ¹⁰⁸.

Improvements in performance study were observed in a 12-minute cycloergometer test after ingesting 20 g/ day of bovine colostrum compared to 60 g/day ¹⁰⁶, and thus, it is feasible to suggest supplementing with 20 g/day of bovine colostrum, with the goal being to improve the immune function, and possibly improving performance.

β-hydroxy-βmethylbutyrate (HMB), is a bioactive metabolite formed from the decomposition of leucine, an essential branched-chain amino acid ¹⁰⁹. Although the body has the ability to synthesize HMB, the amount of leucine necessary to synthesize 3 g of HMB is 60 g of leucine, which would amount to 600 g of high biological value protein ¹¹⁰. The most important function of HMB for athletes is its anti-catabolic effect, improving protein synthesis, and inhibiting proteolysis ¹¹¹. Research has demonstrated that supplementing with 3 g/day of HMB during high-intensity training sessions and/or during short periods of recovery in between training sessions, as could be in some aquatic sports modalities, can improve performance. The supplement has shown to improve health, performance, as well as increasing muscle strength ^111,112,113.