Nutrition in the heat

Heat changes not only well-being but also the way 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 competitions in hot weather, drawing on consensus recommendations and research on the physiology of heat.

What heat changes in the body

In hot weather the body has to supply blood at the same time to the working muscles and to the skin, through which heat is given off. The volume of blood is limited, so 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 falls. The problem worsens in humid air, where sweat evaporates worse and cools less.

The consensus recommendations on training and competing in the heat (Racinais et al., 2015) emphasise that the most important protection is heat acclimatisation — 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 acclimatisation, but it can substantially ease work in the heat: support energy stores, hydration, electrolyte balance and the comfort of the digestive system. It is to these aspects that this article is devoted.

Carbohydrates and energy in hot weather

The review by Febbraio (2001) describes that during physical exertion in the heat, muscles use up more glycogen than at the same intensity in cool conditions. Metabolism shifts towards 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. Too concentrated solutions, for example juices or fizzy drinks without dilution, stay 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 decreases. Undereating during summer camps or tournaments is a common problem that leads to weight loss, fatigue and worsened recovery.

Sodium, electrolytes and drinks

Sodium is the main electrolyte lost with sweat. Its deficiency together with large volumes of plain water increases the risk of hyponatremia, while a sufficient 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, fruit 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 linked to the loss of fluid and salt, but modern data also point to the role of neuromuscular fatigue. Sodium can 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 part of the heat from the body. This makes them a useful cooling tool.

Ice slurry — a mixture of fine ice and liquid — proved especially effective. The transition of ice from a solid to a liquid state absorbs a significant amount of heat. In the study by Siegel and colleagues (2010), consuming ice slurry before running in the heat lowered body temperature and prolonged running 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 tested in training: in some people it causes a cold-induced headache or discomfort in the stomach.
• External cooling — cooling vests, towels with ice — can be combined with internal.

Cooling before the start is no reason to ignore the body's signals during a competition. Dizziness, confusion, cessation of sweating or nausea can be signs of heatstroke. The NATA position stand (Casa et al., 2015) emphasises that heatstroke is an emergency condition that requires 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 kholodnyk or gazpacho, fruit with a high water content.

During exertion in the heat, gastrointestinal symptoms arise more often: nausea, cramps, diarrhoea. The systematic review by Costa and colleagues (2017) describes "exercise-induced gastrointestinal syndrome", which is promoted by a decrease in the blood supply to the intestine and a high body temperature.

To reduce the risk, before training in the heat it is worth limiting fatty, spicy food and products with a high fibre content, and also training the intestine — gradually accustoming it to consuming carbohydrates during exertion.

A separate aspect is food safety. In the heat bacteria in products 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 with a reduced appetite and careful adherence to food safety rules.

Nutrition does not replace heat acclimatisation 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 familiarising yourself with our materials about how much water to drink during training, about isotonic drinks and about the glycemic index for the 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.