Know this, extreme diet yields extreme results. Depriving your body of the calorie it needs can have a dangerous side effect on your health. Dieting in the short term can help move the needle on your scale, they’ll likely make you feel sluggish, moody, nauseated and achy. Plus, in the long term, they can set you up for metabolism problems, rebound weight gain and life-threatening medical conditions.

Let’s look at what happens to your body when you take dieting to the extreme;

You Become Dehydrated

Glycogen is a form of carbohydrate stored in the liver and muscles, and attached to every gram is water. 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,” – Louis J. AronneSymptoms include headache, fatigue and dizziness.

Your Muscles Break Down

During slow-and-steady weight loss – losing no more than about 1 percent of your body weight per week – 75 percent of pounds lost are from fat, and 25 percent 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 presented at the 2014 European Congress on Obesity 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 are 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.

Your Metabolism Slows

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 of your diet, your metabolism will always be more sluggish than it was before you went on your crash diet, he 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.

“This is a survival mechanism that worked great in the caveman era when food may have been scarce,” she says. “But today, when a cheeseburger and fries are just a quick walk or drive away, it doesn’t work that well.”

Malnutrition Begins

“Calories should be viewed as fuel and if you view food in that manner, then drastically cutting calories means depriving your body of the fuel it needs to function,” Kirkpatrick says. For instance, if you strip all calories from fats from your diet, your body isn’t able to absorb fat-soluble vitamins such as A, D, E and K. These vitamins act as powerful antioxidants, fighting free radicals to prevent inflammation and disease, as well as supporting healthy gene expression, cell growth, immune system function, cognitive ability, and bone health.

Also if you follow an extremely unbalanced or calorically low diet long enough for your body to adapt to a malnourished state, abruptly resuming your normal diet can cause your phosphorus, magnesium and potassium levels to drop and lead to heart failure. Called refeeding syndrome, the potentially lethal condition is primarily seen among patients suffering from anorexia but has also occurred in men and women following the cessation of a crash and liquid diets. 

On the less-lethal side of things, if you cut your carb intake to under roughly 100 grams per day, your body will eventually break down fatty acids to produce ketones, carbon-containing compounds that will get the job done, but don’t work as well at fueling your body as do carbohydrates, Aronne says. Plus, they cause bad breath, nausea and, in too-high concentrations, can be toxic to the liver and kidneys.

Your Brain Suffers

Likewise, if your brain is running on ketones, it’s running below capacity. Brain fog during the diet is common, but an animal study published in the journal Pediatric Research suggests it can result in long-term impairments in visual-spatial memory and decreased brain growth. Meanwhile, any crash diet, even if it contains an adequate supply of carbohydrates, increases your brain’s levels of the stress hormone corticosterone, making your brain more susceptible to stress, increasing your risk of depression and predisposing you to future binge-eating behaviors, according to research published in The Journal of Neuroscience.