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What happens to your body when you go on an extreme diet?

U.S. News & World Report logo U.S. News & World Report 23/06/2015 K. Aleisha Fetters

If you follow an unbalanced or calorically low diet long enough for your body to adapt to a malnourished state, it can be dangerous to abruptly resume your normal diet.: Pea and measuring tape on a plate © (iStockPhoto) Pea and measuring tape on a plate Extreme diets yield extreme results – but not always in the way you’d like.

While in the short term they 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. Here’s a breakdown of the changes that happen in your body when you boycott carbs, drink every meal or deprive your body of the calories it needs.

1. You Become Dehydrated

Your immediate “success” on a crash diet is just an illusion, as any pounds lost likely come from water rather than fat, says Louis J. Aronne, an internal medicine and obesity specialist at New York-Presbyterian University Hospital of Columbia and Cornell. 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 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,” he says. Symptoms include headache, fatigue and dizziness.

2. Your Blood Sugar Runs Amok 

A diabetic person is checking her blood sugar level with a glucometer. © GARO/PHANIE/REX A diabetic person is checking her blood sugar level with a glucometer. 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 levels to spike and quickly drop, which, over time, can contribute to insulin resistance.

Also, since extreme diets are associated with yo-yoing, or gaining back all of the weight that was lost on the diet, they contribute to insulin resistance and potentially Type 2 diabetes, per a 2013 Diabetes study. They may even raise your insulin resistance more than if you hadn’t gone on the diet in the first place, according to researchers.

3. Your Muscles Break Down

Research shows that when you lose weight fast, you lose about three times more muscle than you would if you took things slowly. © Tim Tadder/Corbis Research shows that when you lose weight fast, you lose about three times more muscle than you would if you took things slowly. 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. 

4. Your Metabolism Slows

With a drop in your metabolism rate, you will burn fewer calories even after exercising. © WestEnd61/Rex Features With a drop in your metabolism rate, you will burn fewer calories even after exercising. 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 cave man 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."

5. Malnutrition Begins

© Sipa Press/Rex Features “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 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.

6. Your Brain Suffers

Any crash diet increases your brain’s levels of the stress hormone corticosterone, making your brain more susceptible to depression. © Ghislain & Marie David de Lo Any crash diet increases your brain’s levels of the stress hormone corticosterone, making your brain more susceptible to depression. 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.

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