Plenty of fad diets promise rapid weight loss by eliminating foods or entire food groups, or drastically restricting calories, but in doing so, they also eliminate important sources of nutrition.
Losing more than 2 lb a week over several weeks is considered rapid weight loss, and it is generally the result of eating too few calories.
That practice is not usually recommended unless you’re under the supervision of a healthcare professional.
“It depends on what weight you’re starting at and your age, but people between 150 and 300 lb should not be losing more than 2 to 5 lb a week at any given time,” says Singh. “Anything more than that on a sustained basis is not healthy.”
Quick weight loss can put stress on the body and alter your hormonal response.
The hunger hormones leptin and ghrelin, which tell your body when you’re full and when you’re hungry, respectively, can get out of sync, making you want to eat more often.
If you drop 10 lb during the first week of a new diet, for example, it may seem like a huge success, but the weight loss will likely slow down, and you may even gain that weight right back once you stop or relax the diet.
Your metabolism may get out of whack as well. Your body adjusts to the lower calorie intake by slowing down the rate at which it burns calories, in an effort to guard against starvation.
This is called “metabolic adaptation,” and it happens whenever you burn more calories than you take in.
Once you go back to eating regularly, your body won’t know what to do with the extra calories, and that’s where weight gain sets back in. Linda Anegawa, MD, a Las Vegas–based double-board-certified physician in internal medicine and obesity medicine, and the founder of Pinnacle Metabolic Medicine, says that when you regain weight, you mostly regain fat.
Rapid weight loss can have other unhealthy side effects too. In addition to losing muscle mass, water, and bone density, it can introduce health issues such as gallstones, gout, fatigue, constipation, diarrhea, and nausea.
Losing weight too quickly may be especially dangerous for people with underlying health conditions, especially diabetes or kidney or stomach diseases, Singh says. Dr. Anegawa adds that if you shed a lot of weight, the appropriate dosage of your medications may change, so it’s important to work closely with a doctor to adjust your regimen.