“Drop six sizes in 90 days!” “Lose 7 pounds in 7 days!” “How to lose weight in 3 days!”

Although we may be drawn to the allure of rapid weight loss advertising, health experts have traditionally recommended a slow-and-steady method.

“Half a pound to two pounds per week is what’s universally considered a safe and sustainable,” says Jessica Crandall Snyder, registered dietitian and spokeswoman for the Academy of Nutrition and Dietetics.

So, what happens if those programs really do follow through with their “overnight” promise?

“Losing weight too quickly, especially through starvation techniques, can result in a number of side effects, some more downright dangerous than others,” says registered dietitian and Trifecta Nutrition Director Emmie Satrazemis, CSSD.

“Perhaps most notably: When people lose weight too quickly, they’re often not able to successfully keep it off.”

In fact, research has found that after losing weight, about two-thirds of dieters gain more than they initially dropped.

That’s just one of the ways rapid weight loss can backfire, though. Below are six other ways that slimming down too quickly can do more harm to your health than good.

1. You could be missing out on important nutrients

“Many [quick] diets and eating plans cut out whole food groups, which means you could be missing out on key nutrients, vitamins, and minerals that you need to stay healthy,” says Bonnie Taub-Dix, registered dietitian nutritionist, spokesperson for the California Avocado Commission, and author of “Read It Before You Eat It — Taking You from Label to Table.”

Snyder brings up how a dairy-free diet could result in a calcium deficiency while a diet that cuts carbs could mean you’re not getting enough fiber. Even on a lower-calorie diet, it’s important to get a range of nutrients including calcium, vitamin D, vitamin B-12, folate, and iron.

Potential consequences of nutritional deficiencies

  • decreased energy
  • brittle hair and nails
  • hair loss
  • extreme fatigue
  • compromised immune system
  • weakened bones and osteoporosis

In more extreme cases, malnutrition can result in a host of symptoms like decreased energy, generalized fatigue, anemia, brittle hair, and constipation.

Diets are especially dangerous for children In 2012, CBS Seattle reported that the Keep It Real campaign found that 80 percent of 10-year-old girls have at least gone on one diet. Research also shows that more than half of girls and one-third of boys start to want “thinner bodies” by ages 6 to 8.

Choose the right plan, not the fast one

When in doubt, pick an eating plan that includes all of the key macronutrients — fat, carbohydrates, protein — or work with an expert to pick a plan tailored to your needs and food allergies or restrictions.

“The goal is to think about your plan as a lifestyle, not a diet. A diet is something you go on and something you go off. There is no start and end date,” reminds Keri Gans, registered dietitian nutritionist, certified yoga instructor, and owner of Keri Gans Nutrition.

If you’re a parent, figure out what your child’s goals are and if they’re rooted in culture or genuine concern for health. There’s always a more productive, healthier option than rapid weight loss.

2. Your metabolism can get slower

Rapid weight loss usually occurs from extreme calorie deprivation, for example, people who go from eating 3,000 to 1,200 calories a day, says Gans.

Trouble is, our body recognizes this as a sign of limited food supply and goes into starvation mode. Kristina Alai, a personal trainer at The Bay Club Company, highlights the trouble with this: “When your body goes into starvation mode, your metabolism will slow down to help you conserve energy and your body will hang onto more fat.”

In fact, a recent study tracked “The Biggest Loser” contestants and found that the more pounds they lost, the more their metabolisms slowed. Ultimately, this led to many of the participants to gain more weight than when they’d started the show.

Don’t cut more than 500 calories

You don’t have to cut your calories in an extreme way.

“Most people will lose at least a pound a week if they consume 500 calories less a day through a combination of diet and exercise,” says Gans. “This approach may not offer the same instant gratification, but you’ll actually transform your body in the long run.”

3. You might be losing muscle, instead of fat

“When we lose weight, we want to get rid of true adipose tissue. Not muscle mass. I’ve never met someone who complained about having a higher percentage of body muscle,” says Snyder.

But if you cut calories too quickly, muscle tone will seriously suffer.

“Calorie restrictive diets may cause your body to break down the muscle for energy and fuel,” says Satrazemis.

In addition to waving goodbye to your shapely guns and rear, a loss in muscle mass can slow your metabolism.

“Muscle is more metabolically active than fat. That means one pound of muscle burns more calories a day than one pound of fat. So, a loss of muscle means you’ll burn less calories a day,” says Snyder.

Keep protein part of your plan

How to boost metabolism

  • eat protein at every meal
  • lift heavy weights
  • incorporate high intensity interval training
  • eat enough calories

“Eating a high-protein diet and participating in regular strength training while dieting can help preserve your lean mass and help you build more muscle to rev up your metabolism,” says Satrazemis.

Plus, the added strength can help you push yourself during the last bit of your HIIT or cycle class.

4. You could become really dehydrated

Thanks to water weight, it’s common to see slightly faster weight loss in the first two weeks. “Especially on low-carb or no-carb diets, folks will lose a lot of water weight,” says Taub-Dix. According to her, it’s one the reasons the ketogenic diet is often praised for quick weight loss.

Trouble is, rapid water loss can lead to dehydration and a host of unpleasant side effects like constipation, headache, muscle cramps, and low energy.

Make sure your weight loss plan emphasizes hydration

This generally isn’t a problem with diets like juices and cleanses — which are also unhealthy — however newer diets that put a focus on food may cause you to neglect your water intake. Keep track of your H2O intake and make sure you’re consuming enough electrolytes. Adding a sprinkle of Himalayan salt to your food can help.

Watch out for signs of dehydration, especially in the first two weeks.

Signs of dehydration

  • constipation
  • headache
  • muscle cramping
  • low energy
  • dark yellow or amber urine
  • feelings of thirst
  • irritability

If any of these symptoms persist, Dr. Eric Westman, director of the Duke University Lifestyle Medicine Clinic and HEALcare Chief Medical Officer, says you need to go to your healthcare provider.

“If an individual is taking medications for diabetes or high blood pressure, fast weight loss can lead to these medications becoming too strong, which may result in these unsavory symptoms.”

5. You could feel ravenous

When you go on quick-fix, low-cal diets, your levels of leptin — the hormone that controls hunger and satiety — get wonky, says Taub-Dix.

When leptin levels are normal, it tells your brain when your body has enough fat, which signals the brain that you’re full. But research has found that on very-low-calorie diets, unbalanced leptin levels can result in an obsession with food. You may be more ravenous, hangry, and likely to binge.

Stick to high-quality foods

Research has proven quality is more important than calories consumed for weight loss and can influence how much you eat. The study linked starches or refined carbs with weight gain. However, quality and quantity go hand in hand.

As mentioned in our guide to resetting eating habits, restriction has more negative effects on the body and mind. Changing your diet should never just be about losing weight — it’s also about nourishment and honoring your body.

6. Your mental health may take a hit

“If you lose weight very quickly, there can be psychological consequences,” says Taub-Dix. “If someone doesn’t have time to settle into their new body shape and weight, it can lead to things like body dysmorphia, anorexia, or bulimia.”

Taub-Dix also points out, “Many folks start a diet with an ‘if X, then Y’ mentality. As in, ‘if I lose weight, then I’ll be happy. Or then I’ll find love.”

So, after weight loss, when those things haven’t materialized, it can exaggerate preexisting mental health conditions or further promote body image issues.

Ask yourself: What’s your real goal?

If you see weight loss as a prerequisite to hitting a personal goal, such as finding a relationship, becoming healthy, being productive, or having self-control, take some time to write out your intentions and desires. Often, you’ll find that weight loss is a small factor and taking a shortcut won’t truly give the growth you’re looking for.

“There should a lot of thought that goes into your weight loss approach. It’s more than just picking up and jumping into the latest fad,” says Gans. You’ll be kinder to yourself if you choose a slower, more progressive route for weight loss.

Go for steady and healthy weight loss

Although slow and steady weight loss doesn’t sound as promising, it’s the best way to honor your body. It’s also way more effective in helping you keep off the weight and develop a healthy and intentional relationship with food.

“Weight maintenance depends on the person, but drastic weight loss measures are potentially harder to maintain,” reiterates Satrazemis.

So, what’s the best way to lose weight?

“Healthy, sustainable weight loss includes many factors: better food choices, more sleep, increased physical activity, reduced stress, and focus on mental wellbeing,” says Gans.

Make sure to also create moments of joy in your journey. If you don’t like high-intensity workouts, try hiking where there are slight inclines. It’s fine to have a piece of chocolate or small bag of chips.

Keep these in mind like a mantra:

  • eat lean protein
  • cut back on sugar and simple carbohydrates
  • emphasize healthy fat
  • get plenty of rest
  • manage stress levels
  • incorporate strength and high-intensity training

“Remember that weight loss needs to be a holistic lifestyle change that values the long run,” says Gans. While that means maintaining a balance, opting for moderation in your food choices, and exercising, it also means quitting diet culture and possibly resetting your relationship with yourself.

Before you start any weight loss journey, dig deep to find your real motivation behind your desires. You don’t want to fall into the trap of yo-yo dieting, which could hurt your heart.

If the reason is temporary, such as fitting into an old dress for an upcoming event, would getting a new outfit fit your budget instead? You might be surprised to find that your goal isn’t about weight at all.


Gabrielle Kassel is a rugby-playing, mud-running, protein-smoothie-blending, meal-prepping, CrossFitting, New York-based wellness writer. She’s become a morning person, tried the Whole30 challenge, and eaten, drank, brushed with, scrubbed with, and bathed with charcoal — all in the name of journalism. In her free time, she can be found reading self-help books, bench-pressing, or practicing hygge. Follow her on Instagram.