Digesting a heavy meal generates internal heat that can worsen the lethargy many Britons already feel during a heatwave, a registered nutritionist has warned. As temperatures climb, the body diverts blood flow from the digestive tract to the skin to cool itself, making digestion slower and more taxing. Meals that are rich, greasy or high in protein require extra energy to break down, adding to the metabolic heat burden and leaving people feeling more sluggish.
The heat of digestion
Lily Keeling, a registered nutritionist with the Association for Nutrition who holds a degree in Food Science and Nutrition from the University of Leeds, explained that heavy meals can compound the fatigue caused by hot weather because the digestion process itself raises body temperature. “Heavy meals can sometimes add to that sluggish feeling because digestion itself generates heat within the body,” she said. The body’s natural response to heat includes hormonal changes that lower appetite – levels of ghrelin, the hunger hormone, drop – and a reduction in blood flow to the gut, which can exacerbate existing digestive issues. Keeling, who has spent more than a decade in the food industry, including roles at Premier Foods and Bakkavor before joining Green Chef in 2020, recommended switching to lighter fare. Meals centred on vegetables, lean proteins and wholegrains are far easier for the body to process when the mercury rises. “Things like salads with protein, grain bowls, wraps or yoghurt-based breakfasts can all work really well,” she advised. Spreading smaller portions throughout the day rather than consuming large meals can help people feel considerably more comfortable.
Highly processed and salty foods such as crisps and ready meals can leave people feeling bloated and parched, while heavy dairy products like cheese and full-fat milk can be uncomfortable. Spicy foods, which can increase body temperature and stimulate sweating, may also contribute to dehydration. Caffeinated drinks such as coffee, tea and energy drinks have a diuretic effect that can increase fluid loss. Equally, extremely cold drinks can cause stomach discomfort, so tepid or room-temperature fluids may be better tolerated.
Hydrating from within
Staying properly hydrated is critical during a heatwave, but Keeling stressed that hydration is not solely about drinking water. “During hot weather, we naturally lose more fluids and electrolytes through sweating as the body tries to cool itself down,” she said. “Hydration isn’t just about drinking water, though – foods like cucumber, watermelon, strawberries, tomatoes and lettuce can also help support hydration levels because of their high water content.” Cucumber and celery each contain roughly 95% water, while watermelon is over 91% water. These water-rich foods offer an additional means of replenishing lost fluids without overloading the system. Keeling recommended drinking small amounts of fluid regularly – a “little and often” approach – rather than chugging large volumes, because the body absorbs fluids more efficiently that way. She advised not waiting until thirst sets in, as thirst is a sign that dehydration has already begun. Checking urine colour can be a simple guide: pale yellow indicates good hydration, while dark urine signals a need for more fluids.
Pairing fluids with food can help maintain electrolyte balance. While water is the primary source, Keeling noted that diluted squash, coconut water, herbal teas (such as peppermint or hibiscus), low-fat milk and low-sugar sports or rehydration drinks are all helpful. She cautioned against excessive consumption of plain water without adequate mineral intake, as this can dilute electrolyte levels and potentially lead to hyponatraemia. Fruit-infused water offers a flavoured alternative without added sugar.
The nutritionist specifically warned against relying on sugary snacks and fizzy drinks for a quick energy boost. “One of the biggest mistakes people make is relying on sugary snacks and drinks for quick energy. While they might give a temporary boost, they can often leave energy levels crashing later on,” she said. High-sugar items such as fruit juice, smoothies and sugary soft drinks can also dehydrate the body because of their sugar content.
Alcohol presents particular risks during extreme heat. Keeling explained that alcohol is a diuretic, increasing fluid loss and impairing the body’s ability to regulate temperature properly. Even moderate consumption can cause dizziness and confusion, and there is no safe level of alcohol intake in a heatwave, as it can contribute to dehydration, heat exhaustion and heatstroke.
Replenishing lost nutrients
After extended periods outdoors in the heat, replenishing lost nutrients becomes essential alongside rehydration. “Foods containing potassium, magnesium and sodium can help support hydration balance and muscle function,” Keeling advised. Bananas are rich in potassium; yoghurt provides cooling nutrients and electrolytes; leafy greens such as spinach supply magnesium; and beans and legumes offer a combination of minerals. Broths and lightly salted meals can also help restore sodium lost through sweat.
Frozen treats offer a refreshing way to contribute to fluid intake while cooling the body. Keeling suggested frozen grapes, yoghurt bark and homemade fruit lollies or ice pops as delightful options. Frozen strawberries and a watermelon-and-strawberry slushie are also effective. These alternatives to shop-bought ice creams avoid the high sugar and additive content of many commercial products.
The UK’s climate and housing stock make heatwaves particularly challenging. Buildings are often designed to retain heat, and air conditioning is not widespread. Humidity means sweat evaporates less quickly, impairing the body’s natural cooling mechanism. Temperature fluctuations can also prevent the body from fully adapting to prolonged heat. Keeling’s advice on food storage is equally important: foods with high water content – berries, salad leaves, chopped fruit, vegetables and soft herbs – spoil quickly in warm weather if not stored correctly. Maintaining fridge temperatures between 3°C and 5°C and freezers at -18°C, and avoiding overfilling, is crucial to prevent spoilage and bacterial growth.
Heatwaves can also disrupt sleep – a UK study found 67% of participants reported poorer sleep quality – and poor sleep can lead to cravings for sugary, ultra-processed foods the next day. Balanced meals containing protein, fibre and slow-release carbohydrates are far more effective for sustaining steady energy throughout the day. Keeling’s single piece of overarching guidance was consistency. “The biggest thing is consistency. Rather than waiting until you feel dehydrated or overly hungry, it’s better to eat and drink regularly throughout the day,” she said. Keeping meals straightforward while ensuring adequate protein and fibre intake will help Britons weather the heatwave successfully.