Your friend’s trying a new diet where you lose 10 lbs in a week, she’s a bit overweight but not obese and wants to look good for her wedding. Here’s why she won’t lose 10 lbs in a week.
When you dramatically cut your calories (which is necessary to lose weight quickly), your body does two things: firstly it starts burn stored energy (glycogen, fat, and muscle); secondly, it downregulates to conserve energy (decrease in thermic effect of food, decrease in non-exercise activity thermogenesis, decreased energy levels, decreased body temperature, etc.).
Drastically cutting calories doesn’t allow your body to slowly adapt to its new set point (the weight at which your body tries to keep you at), so it rapidly tries to adjust your metabolism to keep you at that bodyweight.
“But once I lost 10 lbs in a week” – you lost water weight. When you eat carbs, your body breaks it down into glucose which binds with water to form glycogen which is stored in your muscles and liver. The more carbs = the more water weight. When you’re eating less, your insulin levels decrease which causes your body to get rid of glycogen stores, and also a lot of water. In fact, it can be up to 4.4 pounds in 24 hours according to a study by Passmore et al.
You also break down muscle mass, which is responsible for some of the weight loss; which slows down your metabolism, making a plateau and future weight gain more likely.
When you lose weight gradually a few things happen: your appetite decreases slowly. Rather than jumping to a low-calorie diet and feeling hungry, when you slowly decrease calories your appetite hormones slowly decrease as your stomach shrinks. You burn fat with extra muscle and your body adjusts to its new set point.
Numerous studies show that losing weight slower is easier to maintain. A study by the University of Florida found that the group who lost weight faster (1.5 lbs a week) regained it 5 times faster than the slower group (0.5 lbs a week) after a year and a half.