The first thing that occurs during a severe calorie shortage is a decrease in your metabolic rate. The lower your calories, the slower your metabolism becomes. Simply put; when you eat less, your body burns less. When you eat more, your body burns more.
In the book, "Everything you need to know about fat loss," Bodybuilding nutritionist Chris Aceto uses a great analogy to describe the way this mechanism works: He wrote, "If you're earning $4000 a month, but your boss suddenly cuts your pay to $2500 a month, you will try to live the same lifestyle on $2500 a month as you did on $4000 a month. After a while, you have to adjust and save money, and change your lifestyle. The same is true with a calorie intake that is simply too low. When calories are cut below basal metabolic needs, the body will accommodate and slow its metabolism, so it becomes difficult to lose fat even on low calories."
This metabolic slowdown is well documented. When calories are restricted, your metabolism decreases by at least 20-30%. With severe calorie restriction, some studies have shown that resting metabolism can become depressed by as much as 45%! That's the equivalent of having your daily energy expenditure drop from 3000 calories per day to only 1650 calories per day! This is why, after prolonged low calorie dieting, you can eat very little food and still not lose weight. This also explains why it is so difficult to lose those last 10 or 20 pounds.
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