According to Nutritionist & Physiologist, Ritesh Bawri, "There are several theories about why we feel less hungry in summers or more in winter. One theory suggests the amount of light stimulates hunger. It says that when there is less light our body naturally begins to crave food to store it. Your body is afraid it might not get more. Another theory suggests that our body has to work harder to fight the cold and, therefore, you feel hungrier."
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Generally, during summers, heat tends to evaporate the water content from the body, which is because the body is trying to regulate temperature by expelling water in the form of sweat. The part of the body that regulates the temperature is known as hypothalamus. This body part has two jobs, one is to keep you cool, and two, recognising hunger and promoting satisfaction. Now, when the body is trying to sweat it out, hypothalamus tends to pay lesser attention towards your hunger.
The digestive process also generates certain amount of heat, which is suppressed by hypothalamus in order to control its workload. This can be one of the reasons why we feel less hungry in summers; also a way to promote drinking more water; thanks to the sweltering heat.
Ritesh Bawri says, "It is your body mass index (BMR) that changes between winters and summers, i.e. your body has to work harder to keep you alive. Therefore, minor changes in your BMR requirement could appear to be a significant increase or decrease in hunger levels."
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It is your body mass index (BMR) that changes between winters and summers.