I think every person has observed an interesting dependency concerning the influence of the surrounding temperature on hunger and the choice of food. Well, it occurs that when it's cold outside, and we happen to freeze, the feeling of hunger is stronger and more difficult to satiate then when the temperature outside is high. This effect is particularly visible during summer heat. Why does it happen?
The mechanisms of hunger control
It's obvious that the mechanisms of food intake are only partly dependent on our will. Our bodies have many instruments which force us to provide nutrients to the system. The center of hunger regulation can be found in the part of brain called hypothalamus, to be precise, in the lateral part (the center of hunger) and ventromedial (the satiety center). The following areas also take part in some processes connected with food intake: vision, olfaction and taste, found in cerebral cortex. In the hypothalamus the addition and processing of different signals, which are the transmitters of information concerning the nutrition of the body and fullness of digestive tract, take place. These signals can be divided into:
motile – connected with filling the stomach, perceived by special receptors and transmitted to the hypothalamus by the vagus nerve and the stomach nerves,
metabolic – connected with the presence of nutrients in circulatory system (to be precise: glucose, amino acids, fatty acids) – these signals are perceived by highly specialized hypothalamus neurons,
hormonal – connected with the activity of hormones, including insulin, leptin or intestinal hormones (e.g. cholecystokinin).
The influence of temperature
Apart from the motile, metabolic and hormonal factors mentioned above, thermal signals also have significant influence on the process of hunger control. Human body has special receptors which make it possible to know how the temperature changes. These receptors are placed within skin (they read the surrounding temperature) and inside the body. Thermal changes are the factors which influence many parameters in the body, including the energy reserves mobilization.
For example, when we feel cold, the energy supply is bigger because of thermogenesis, which is the production of heat. The body wants to make even the temperature by increasing food intake – that's why we feel hungry. When it's hot, the situation is opposite – the feeling of hunger gets smaller, the body wants to decrease the expenditure of energy connected with thermogenesis, and because this process is connected with consuming food (there exists the thermal effect of food), smaller appetite is one of the remedies. Besides, increase of temperature is sometimes the signal of good nourishment of the body (as I mentioned, thermogenesis increases after meals) and this is another explanation of feeling less hungry during heat.
Decreased hunger during hot days is mostly connected with the activity of thermoreceptors and their influence on the mechanisms of food intake control. People who, for some reason, would like to break these limits (say, because of the need of increasing muscle mass), can experiment with cold baths before eating meals.