Few things can be less appetising in 30C weather than the idea of a big bowl of stew, or any other hot and heavy meal, but what is the biological process that causes this?
The diet-suppressing effect of high temperatures can make eating a full meal difficult as our feeling of hunger can often disappear in hot weather. The energy-burning systems in our bodies responsible for this complex process have been studied by scientists for decades as they hold the key to understanding how and when our body turns fat into energy and increases our metabolism.
The centre of this process is the hypothalamus, the part of the brain responsible for regulating both body temperature and feelings of hunger, as well as a myriad of other hormonal functions. This brain structure keeps our bodies in a stable state that biologists call homeostasis, a biological balancing act that regulates our energy input and output.
As digesting food and burning calories are both heat-generating biological functions, when our core body temperature starts to rise, the hypothalamus slows our digestive processes to regulate body temperature and maintain homeostasis.
Nutritionist Kerry Torrens, speaking to BBC GoodFood, explained the effect of this in hot weather: “Seasonal changes including temperature and the number of daylight hours are thought to influence our appetite. In the summer our appetite tends to be reduced, especially when we’re feeling hot.
“One reason for this is that the body tries to regulate our body temperature by cutting down on heat-generating functions like the digestion of food.”
A study published in the British Journal of Nutrition found that men from a variety of backgrounds and nationalities typically consumed around 300 fewer calories after two hours in 30C heat. Though researchers were looking for temperature-related weight management, their findings hold true for anyone who has baulked at the idea of a big lunch on a hot day.
If weight loss is your aim, researchers suggest spending time in “heated rooms, clothing, sauna, hot baths or outdoors” could be a helpful way to suppress your feelings of hunger.
Scientists looking further into how homeostasis regulates our temperature and hunger, have also found that our stomach plays a key role in communicating this information to our hypothalamus. Using mice, researchers found that high temperatures lower our gut’s production of the hormone ghrelin, which sends hungry signals to our brains.
The research team found that high temperatures induced lower ghrelin levels, while also inducing a sense of “torpor” or lethargy. Meaning that their bodies generated less heat through both movement and calorie-burning.
So, if you are feeling the heat but not the hunger, or a bit tired after an hour in the sun, it’s a sign that your body’s self-regulating system is working.