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Question: Why do people get fat by not eating too much but by eating junk all day?
Answer: Let me start by rephrasing your question to make it more accurate. Why do people get fatter by not eating much rather than by eating more? The key word here is the metabolic rate. It is the rate at which your body uses oxygen to produce energy that is required for basic physiological functions like respiration, digestion and so forth. An increase in activity increases the metabolic rate to keep up with the energy demand. The body relies on nutrients from the diet to produce energy and maintain body tissues. In addition, the way we consume our diet dictates the fate of the nutrients. The sugar in the food (known as carbohydrates) is a major constituent of blood and fuels muscles with glycogen (a storage form of carbohydrates that provide fast energy). Excess sugar is converted into fat and stored in the adipose tissue (the fat cells). Proteins are the building blocks of body tissues including muscles. We constantly lose body proteins that need to be replenished daily by the diet, in a way to keep those tissues in their best shape. People who work out regularly require higher amount of protein in their diet to help with muscle building. Like sugars, excessive intake of protein ends up as fat in the adipose tissue. In addition, dietary fat has some body functions; however the most prominent is storage form of energy. Thus, many small portions of a balanced diet would be primarily used for the above-mentioned physiological processes. Few large portions of the same diet would do the same except that the excessive unused calories will end up in your fat cells. In essence, it is wise to spread out the caloric intake through out the day to keep the body using the nutrients rather than storing them as fat. So let's tackle the issue of significantly reducing caloric intake. When people go on a caloric restricted diet, the body feels that the nutrients are scarce and it assumes a 'starvation' response. It shifts its efficient state of energy production to a super efficient energy conservation state. Although people might experience weight loss in the first couple of days, it is no more than water lost from broken-down glycogen. After few days, the dieter, out of shape and energy, resumes eating (mostly overeats), with the body energy conservation ability at its maximum. The dieter ends up regaining the lost pounds plus some more because of the reduced metabolic rate. If the extra pounds are not lost within days the body shifts to a new set point, the higher body weight, and the metabolic rate is adjusted accordingly. This is the basis of the yo-yo dieting and this explains why people who diet often end up in the long run gaining weight rather than losing it. So back to your question, people who do not eat enough to maintain an optimal metabolic rate will end up gaining weight and efficiently keeping it, than those who consume higher caloric intake that is widely spread through out the day
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