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Dangerous Side Effects of Going on a Diet, According to Science

If you're too aggressive about losing weight, your body can suffer the consequences.

Choosing to go on a diet isn't a 100% healthy endeavor. After all, if you're cutting yourself off from certain nutritious foods or entire food groups, drastically limiting your calorie intake overall, fasting for prolonged periods of time, or signing up for a fad-like program that promises extreme results in short order, there's a good chance you're actually embarking on a path that your body may not actually respond favorably to. Whether you're trying the Ketogenic diet or intermittent fasting, going low carb or low fat, below are some common side effects of going on a super-strict dieting regimen you should be aware of. And for more healthy weight loss tips, don't miss this list of Sneaky Weight Loss Tricks That Totally Work, According to Experts.

You may feel sick.

Sick woman sitting on couch wrapped in blanket

If you're considering intermittent fasting, in which you restrict your food consumption for long stretches on certain days of the week, you'd be wise to consider some of the consequences. "Depending on the length of the fasting period," write the health experts at Harvard Medical School, "people may experience headaches, lethargy, crankiness, and constipation. To decrease some of these unwanted side effects, you may want to switch from alternate-day fasting to periodic fasting or a time-restricted eating plan that allows you to eat every day within a certain time period."

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You'll feel fatigued.

tired man

A simple fact: If you eat less food (also known as fuel), your body will have less energy to burn and you'll ultimately feel sluggish. One study, published in the Annals of Internal Medicine, found that cutting carbohydrates from your diet was associated with a greater risk of fatigue. Another study, which focused entirely on the Ketogenic diet and was published in the Journal of the American Dietetic Association, found that "low-carbohydrate diets enhance fatigability and can reduce the desire to exercises in free-living individuals."

Cutting carbs completely from your diet isn't your only path to lower energy levels. Other studies have linked diets that restrict nutrients such as vitamin B12, folate, and iron with fatigue, as well as anemia. For more weight loss tips, make sure you're aware of the 12 Foods That Drive the Most Weight Loss of All, Say Experts.

You'll slow down your metabolism.

scale measuring tape

A now-famous study published in the journal Obesity that analyzed the weight-loss efforts of contestants of NBC's wildly popular series The Biggest Loser, which was conducted by researchers at the National Institutes of Health, found that people who go on an extreme crash diet hobbled their metabolisms so profoundly that they never fully recovered. The chief reason, according to the researchers, was the influence of leptin, the body's hormone that tells you you're satiated, or no longer hungry. Over the course of the crash diet, the contestants' leptin levels essentially flatlined. The researchers also tracked their ghrelin levels—the hormone that tells you when you're hungry—and it had actually risen. In effect, the dieters had reprogrammed their bodies to be fat-storing, low-energy machines.

You'll lose your hair.

Man looking at hair

According to a study published in the journal Dermatology Practical & Conceptual, embarking on a low-calorie diet is associated with hair loss, as the lack of nutrients disrupts your hair follicles to function as intended. "Nutritional deficiency may impact both hair structure and hair growth," write the researchers. "Effects on hair growth include acute telogen effluvium (TE), a well-known effect of sudden weight loss or decreased protein intake, as well as the diffuse alopecia seen in niacin deficiency."

You'll lose muscle mass.

weight lifting man

For a study published in The American Journal of Clinical Nutrition, researchers from the University of Kiel in Germany took 32 non-obese males and slashed their calories by an average of 1,300 for a three-week period. On the whole, the subjects emerged from the experience having gained weight while seeing a dramatic decrease in muscle mass—roughly 5% across the board.

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You could develop kidney stones.

Doctor and patient

"A lack of proper nutrition due to a fad diet can actually strain your organs and muscles," says Ashlee Van Buskirk, a nurse, health and wellness coach with a BS in Dietetics and Clinical Nutrition Studies, and the founder of Whole Intent. "For instance, a high-protein diet can actually lead to dehydration, which may place a significant strain on your kidneys as you may be more prone to developing kidney stones."

You could put yourself at risk of depression.

Depressed young woman sitting on floor at home

"Most diets fail most of the time [and] repeated diet failure is a negative predictor for successful long term weight loss," writes Anna Guerdjikova, Ph.D., LISW, CCRC, director of administrative services at the Harold C. Schott Foundation Eating Disorders Program at the University of Cincinnati. "Chronic dieters consistently report guilt and self-blame, irritability, anxiety and depression, difficulty concentrating and fatigue. Their self-esteem is decreased by continuous feelings of failure related to 'messing my diet up again,' leading to feelings of lack of control over one's food choices and further … life in general. Dieting can be particularly problematic in adolescents."

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You could deprive yourself of essential nutrients.


"Fad diets are not always terrible, but people should understand the food groups and should try to ingest foods from all of them to keep vitamin and mineral balance," says Stephen Newhart, Ph.D., owner of Vigor Active. "Grains provide energy, fiber, iron, and help with constipation, dairy provides calcium and iron, fruits and vegetables provide vitamins and minerals and protein supports muscle mass. Always try and eat something from all the food groups to sustain health, just be sure to eliminate the sugars."

If you're older, you could lose too much weight.

healthy foods to lose weight weight loss

If you're older and you're trying intermittent fasting, you could be at risk of losing too much weight. "If you're already marginal as far as body weight goes, I'd be concerned that you'd lose too much weight, which can affect your bones, overall immune system, and energy level," Kathy McManus, RD, director of the Department of Nutrition at Brigham and Women's Hospital, told Harvard Medical School.

You could develop an eating disorder.

Thoughtful girl sitting on sill embracing knees looking at window, sad depressed teenager spending time alone at home, young upset pensive woman feeling lonely or frustrated thinking about problems

According to the International Journal of Eating Disorders, 35% of "normal dieters" may become pathological dieters, and 20 to 25% of those are prone to develop an eating disorder. "The onset of eating disorders has commonly been associated with following restrictive diets, as they become a way for individuals to exercise control, counting calories and fat grams, limiting types and amounts of food, and obsessing about a number on the scale," write the experts at Behavioral Nutrition.

You may actually gain weight.

woman on scale weight loss

As the Cleveland Clinic notes, many health experts believe that "80 to 95% of dieters gain weight back that they've worked so hard to lose." If that's an experience you're aware of, don't miss these tips for Losing Weight and Keeping it Off for Good.

William Mayle
William Mayle is a UK-based writer who specializes in science, health, fitness, and other lifestyle topics. Read more about William