What Happens to Your Body When You Go on an Extreme Diet

Extreme diets yield extreme results – but not always in the way you’d like.

In the short term, extreme diets can help move the needle down on your scale, but they’ll likely also make you feel sluggish, moody, nauseated and achy. In the short term, some of these issues may be concerning to your health. In the long term, they can set you up for rebound weight gain.

Your immediate “success” on a crash diet is just an illusion, as any pounds lost likely come from water rather than fat, says Dr. Louis J. Aronne, an internal medicine and obesity specialist with the Comprehensive Weight Control Center at Weill Cornell Medicine in New York City.

That’s because when you restrict calories, carbohydrates or both, the first source of energy your body burns – long before fat – is glycogen, he says. Glycogen is a form of carbohydrate stored in the liver and muscles, and 3 grams of water are attached to every gram of carbohydrate.

So when you burn through all of your glycogen, the adjoining water exits the body. “On a lot of these three-day diets, people end up losing so much water they get dehydrated,” he says. Symptoms include headache, fatigue and dizziness.

Depending on the diet you’re following, you don’t get a steady supply of carbohydrates, protein, fat and fiber throughout the day to keep your blood sugar and insulin levels steady, he says. And if you’re following a detox or cleanse diet, the regular influx of high-sugar, low-fiber juice can cause your blood sugar levels to spike and quickly drop, which may cause you to feel extremely hungry a few hours later.

Also, since extreme diets are associated with yo-yoing, or gaining back all of the weight that was lost on the diet, there is some research to suggest that these fluctuations in weight may contribute to insulin resistance and potentially Type 2 diabetes. However, there’s no definitive data as research has yielded mixed results in both human and animal studies.

During slow-and-steady weight loss – losing no more than about 1% of your body weight per week – 75% of pounds lost are from fat, and 25% are from a combination of water and muscle, Aronne says. Take a more aggressive approach by cutting calories below about 1,000 and 1,200, based on your personal needs, and your body breaks down your muscle proteins for energy, he says.

Research shows that when you lose weight fast, you lose about three times more muscle than you would if you took things slowly. So, even if you’re losing weight, your physique probably isn’t looking much better.

Meanwhile, if weight loss is extremely fast, the muscles of the heart can atrophy. Extreme low-calorie liquid diets, for instance, have been linked to ventricular arrhythmias and death.

With less muscle, your metabolic rate (the number of calories you burn at rest) automatically drops. That means you burn fewer calories walking, talking and even exercising. Your metabolism eventually drops low enough that you stop losing weight and, when you go off your diet, your metabolism will always be more sluggish than it was before you went on your crash diet, Aronne says.

Meanwhile, your brain, thinking that food is in short supply, triggers your body to cling to calories to prevent you from starving to death, says Kristin Kirkpatrick, a registered dietitian and manager of wellness nutrition services at the Cleveland Clinic in Ohio.