People who lose weight in the summer
Humans are homeothermic animals and always maintain their core body temperature at 37℃. Various biochemical enzymes are required for metabolism in the human body, and since these enzymes are activated at temperatures around 37℃, the body temperature cannot be maintained at a constant level.
In order to maintain a constant body temperature, the amount of heat generated within the body and the amount of heat entering and leaving the body must be constant. However, the energy intake of our body varies every day, and the degree to which body temperature is emitted changes frequently depending on the external temperature or humidity. Therefore, the body temperature control system must actively operate in accordance with this to control body temperature.
Energy homeostasis is achieved through the balance of energy intake and expenditure, and the most important factor related to energy intake is appetite. In the process of consuming energy, the resting metabolic rate (basal metabolism) accounts for 60-75% of total energy consumption, the energy used for digestion and absorption (adaptive thermogenesis) accounts for 5-10%, and physical activity accounts for 15-30%. Therefore, it can be said that controlling appetite and resting metabolic rate is the most effective in the active body temperature control system.
 Energy Balace (출처=Cancer Epidemiol Biomarkers Prev 2012) |
A hormone called leptin plays an important role in regulating energy homeostasis (Lancet 2005). As its name, derived from the Greek word leptos meaning “thin,” is known to suppress appetite, reduce food intake, and stimulate the sympathetic nervous system to induce lipolysis, thereby reducing weight (Nature 2015).
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When the weather gets hot, there is less need to produce additional heat and the energy required decreases, so leptin is secreted to maintain energy balance. That is why we lose our appetite and lose weight in the summer. Elderly people say that they should eat no matter what even if they have no appetite to be healthy, but in fact, it is quite natural to lose their appetite when it is hot. On the contrary, if you have a good appetite all year round like me, that may be more of a problem. Let's take a look at why.
People with good appetite all year round
Many people think that body fat only stores energy, but in fact, it is an important endocrine organ that actively controls energy metabolism in the body by producing and secreting various signaling substances, hormones, and vascular regulators. Therefore, in order to maintain homeostasis in the human body, body fat must be maintained at an appropriate level. However, excessive amounts are also a problem.
There are two types of adipose tissue: white adipose tissue, which stores energy in the form of neutral fat, and brown adipose tissue, which consumes energy and generates heat. Brown adipose tissue has less fat than white adipose tissue and has more mitochondria that produce energy (=heat), so it is brown.
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| White adipocytes (left) and brown adipocytes (right) that make up each adipose tissue (Alexandra Paul, 2018) |
White adipose tissue is white because it has a large amount of fat and mainly plays a role in storing energy, but it also secretes hormones called adipokines. One of them, leptin, is also secreted from white adipose tissue (Nature, 1994) and acts on the hypothalamus, which is the center of appetite control (Nat Metab. 2019).
Until now, it has been thought that leptin stimulates the heat production of brown adipose tissue and increases energy consumption, thereby causing weight loss, but recent studies have shown that leptin is not related to the activation of brown adipose tissue or an increase in energy consumption. Leptin does not stimulate the heat production of brown adipose tissue, but indirectly regulates body temperature by changing the set point of body temperature. It is true that leptin affects the growth and activity of the sympathetic nerves to promote lipolysis, but the main reason for leptin's body fat reduction effect is that it reduces food intake due to appetite suppression (Endocr Rev. 2020).
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In normal conditions, when body fat increases more than usual or when additional heat production is not needed due to hot weather, leptin is secreted to suppress appetite, reduce body fat, and regulate energy homeostasis. If leptin functioned properly, there would be no fat people.
In fact, biological brakes such as leptin secretion do not work well in real life. In an environment that promotes obesity, modern people mostly eat when they want to eat (pleasure) rather than when they are hungry (instinct). In addition, since eating is a habit, there are many cases where people eat when the time comes even when they have no appetite and are not hungry.
In particular, the fatter the person, the more hungry they are and the more they tend to eat. Of course, fat people also secrete leptin. No, rather, they secrete more than people of normal weight. This is because leptin increases in proportion to body fat (Nat Med. 1995). This is called hyperleptinemia, and high leptin levels themselves cause leptin resistance, which means the body doesn't respond very well to leptin.
The fatter you are, the more you want to eat, and the thinner you are, the smaller your mouth is. This is because of body fat and leptin resistance. Whether the weather is hot or cold, you always want to eat something, and once you start eating, it is hard to put down the spoon, and if you eat the same as others but feel like you are gaining weight, and if you feel colder than others in the winter, you can see that you have leptin resistance. If you are obese, this is even more true.
So what should we do?
In animal models, it has been reported that reducing sugar intake and exercising regularly increases leptin sensitivity (Gerontology 2011), and repeating fasting and refeeding improves leptin resistance (J Kor Endocr Soc., 2008). Eating less and having a healthy diet and lifestyle, which everyone knows, and intermittent fasting may help reduce leptin resistance.