Nutrition in the Heat

Heat changes not only how you feel but also how the body uses energy, water and nutrients. Appetite drops, the stomach tolerates food worse, and carbohydrate stores are used up faster. The editorial team examined how to adapt nutrition to training and competition in hot weather, drawing on consensus recommendations and studies of heat physiology.

What heat changes in the body

In hot weather the body has to supply blood simultaneously to the working muscles and to the skin, through which heat is released. Blood volume is limited, so the heart rate rises, and the blood supply to internal organs, in particular the intestine, decreases.

Increased sweating leads to the loss of water and sodium. If the losses are not compensated, plasma volume decreases, body temperature rises faster, and performance drops. The problem is aggravated in humid air, where sweat evaporates worse and cools less.

Consensus recommendations on training and competition in the heat (Racinais et al., 2015) emphasize that the most important protection is heat acclimatization — 1–2 weeks of gradual training in hot conditions. During it, plasma volume increases, sweating begins earlier, and sodium losses in sweat decrease.

Nutrition does not replace acclimatization, but it can significantly ease work in the heat: support energy stores, hydration, electrolyte balance and comfort of the digestive system. It is precisely these aspects that this article is devoted to.

Carbohydrates and energy in hot weather

A review by Febbraio (2001) describes that during physical exertion in the heat the muscles use up more glycogen than at the same intensity in cool conditions. Metabolism shifts toward carbohydrates, so their stores are depleted faster.

For an athlete this means that carbohydrates before and during prolonged exertion in the heat are even more important than usual. Sports drinks with carbohydrates and sodium make it possible to solve two tasks at once — to supply energy and fluid.

At the same time, the concentration of carbohydrates in a drink affects the rate of gastric emptying. Overly concentrated solutions, for example juices or carbonated drinks without dilution, are retained in the stomach longer and can cause discomfort. Classic sports drinks contain about 6–8% carbohydrates.

The daily energy requirement during training in the heat does not decrease, even if appetite is reduced. Undereating during periods of summer camps or tournaments is a common problem that leads to weight loss, fatigue and impaired recovery.

Sodium, electrolytes and drinks

Sodium is the main electrolyte lost with sweat. Its deficit together with large volumes of plain water increases the risk of hyponatremia, while an adequate amount of sodium helps retain fluid and supports thirst, which stimulates drinking.

The concentration of sodium in sweat is individual. Athletes who sweat heavily and see white salt marks on their clothes should pay special attention to sodium: add salt to food, use sports drinks or electrolyte tablets during prolonged exertion.

Potassium, magnesium and calcium are also lost with sweat, but in much smaller amounts, and an ordinary diet with vegetables, fruits and dairy products usually compensates for them. Special supplements of these minerals are mostly not needed.

As for muscle cramps in the heat — they are traditionally associated with the loss of fluid and salt, but modern data also point to the role of neuromuscular fatigue. Sodium may help some athletes, but there is no universal «recipe against cramps».

Cooling from within: cold drinks and ice slurry

The temperature of the fluid consumed matters. Cold drinks are not only more pleasant in the heat and stimulate drinking more, but also take away some heat from the body. This makes them a useful cooling tool.

Ice slurry — a mixture of finely crushed ice and liquid — turned out to be especially effective. The transition of ice from a solid to a liquid state absorbs a significant amount of heat. In a study by Siegel and colleagues (2010), consuming ice slurry before running in the heat reduced body temperature and prolonged the time to exhaustion compared with cold water.

• Ice slurry is usually consumed 30–60 minutes before the start, in small portions.
• It can be prepared from an isotonic drink in order to get carbohydrates and sodium at the same time.
• The strategy should be tried out in training: in some people it causes a headache from the cold or discomfort in the stomach.
• External cooling — cooling vests, ice towels — can be combined with internal cooling.

Cooling before the start is no reason to ignore the body's signals during competition. Dizziness, confusion, cessation of sweating or nausea may be signs of heatstroke. The NATA position statement (Casa et al., 2015) emphasizes that heatstroke is an emergency requiring immediate cooling and medical care.

Appetite, digestion and food safety

Heat suppresses appetite, especially immediately after training. So on hot days it is useful to have light and cool options on hand: fermented milk drinks, smoothies with fruit and protein, cold soups such as cold borscht or gazpacho, fruits with a high water content.

During exertion in the heat, gastrointestinal symptoms occur more often: nausea, cramps, diarrhea. A systematic review by Costa and colleagues (2017) describes «exercise-induced gastrointestinal syndrome», which is promoted by reduced intestinal blood supply and high body temperature.

To reduce the risk, before training in the heat it is worth limiting fatty, spicy food and products high in fiber, as well as training the gut — gradually accustoming it to consuming carbohydrates during exertion.

A separate aspect is food safety. In the heat, bacteria in food multiply much faster. Salads with mayonnaise, dairy products, meat and fish that have stood for a long time without refrigeration are especially dangerous to eat during competitions and camps. Food poisoning can destroy months of preparation.

Editorial conclusions

In the heat the body uses up glycogen faster, loses more fluid and sodium, and the digestive system works in more difficult conditions. Nutrition must take all these changes into account.

The key steps are enough carbohydrates and sodium during prolonged exertion, cool drinks and ice slurry for cooling, light dishes when appetite is reduced, and careful adherence to food safety rules.

Nutrition does not replace heat acclimatization and common sense: on very hot days it is advisable to shift training to the morning or evening and reduce the intensity.

The editorial team also recommends getting acquainted with our materials on how much water to drink during training, on isotonic drinks and on the glycemic index for an athlete.

References

1. Racinais S, Alonso JM, Coutts AJ, et al. Consensus recommendations on training and competing in the heat. Scand J Med Sci Sports. 2015;25 Suppl 1:6–19.
2. Febbraio MA. Alterations in energy metabolism during exercise and heat stress. Sports Med. 2001;31(1):47–59.
3. Siegel R, Maté J, Brearley MB, et al. Ice slurry ingestion increases core temperature capacity and running time in the heat. Med Sci Sports Exerc. 2010;42(4):717–725.
4. Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome — implications for health and intestinal disease. Aliment Pharmacol Ther. 2017;46(3):246–265.
5. Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers' Association position statement: exertional heat illnesses. J Athl Train. 2015;50(9):986–1000.
6. American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377–390.