What Happens to Your Body When You Lose Weight Fast

Commercials disrupt your video streaming to push fat-burning diet programs, while social media ads flood your news feeds with pills, powders and teas promising split-second weight loss. You watch celebrities sashay across the red carpet after dropping 13 pounds in two weeks and hear influencers share their best-kept weight-loss secrets. It’s no wonder so many people feel intense pressure to lose weight—and lose it swiftly. But are there consequences to rapid weight loss?

People who lose weight fast end up regaining it—and then some—compared to people who lose weight slowly with reasonable, sustainable diet changes and physical activity.

Why the rush to lose weight so quickly? For starters, in today’s world, you can get a myriad of things in an incredibly short amount of time. Binge-watch entire seasons of your favorite shows, order a hot meal to your doorstep within the hour or ship household items to your home in a jiff—all with a push of a button on your phone. With instant gratification within easy reach, it makes sense that you’re drawn to a quick fix.

Everyone has their reasons for wanting to lose weight, but rapid weight-loss efforts can backfire. And while there’s nothing wrong with wanting to lose weight, it’s crucial to your health that you do so safely—and that you are reasonable about the amount of weight you want to lose and keep off.

So, what happens to your body when you lose weight too fast? Let’s just say you may be getting more than lower numbers on the scale. Here are the possible outcomes of rapid weight loss—and why healthy, slow-and-steady weight loss is the way to go.

You May Lose Muscle

The top two strategies adults choose when trying to lose weight are eating less and exercising more. These popular strategies create a calorie deficit, the driver of weight loss. In a calorie deficit, your body switches gears and goes into breakdown mode, looking for tissues in the body that can be broken down to use as energy. It can break down fat tissue, but it can also break down muscle tissue—which happens on very low-calorie diets, including when you use a GLP-1 weight-loss medication.

“When you lose weight too quickly, much of this weight loss is muscle and water weight, not fat,” says Melissa Mitri, M.S., RD, a registered dietitian.

Muscle loss impacts much more than your ability to lift heavy things and to have proper balance. It can be problematic to many parts of your body that use muscle to function—like your heart pumping blood to your organs (because your heart is a muscle) or the expansion of your lungs for breathing. Strong muscles and ligaments help protect your joints, reducing your risk of injury. Muscle also plays a role in using glucose and helping to maintain stable blood sugar levels—among many other functions.

Research suggests that strength training can help preserve muscle mass during weight loss. It’s important to note that you might not lose weight as quickly as you’d like when you strength-train. In fact, you might even gain weight—but the weight you’re keeping is important.

Preserve your muscle mass, strength and health by treating your weight loss as a journey and not a sprint—and include strength training as part of your journey.

You Might Regain the Weight You Lost

Muscle losses continue to impact your body by causing your resting metabolic rate to drop. RMR measures how quickly your body can burn calories while at rest. That’s why bodybuilders need lots of calories, protein and nutrition to maintain their physiques; because the more muscle they gain, the more they rev up their metabolism to burn more calories. And the calories are necessary to maintain that muscle mass. But you don’t have to be a bodybuilder to need enough calories to maintain muscle mass.

Weight regain can be a reality when your body tries to adapt to sudden calorie restrictions. “The body’s natural response [to rapid weight loss] is to fight back with a powerful mechanism called survival mode, governed by the primitive part of the brain,” says Johna Burdeos, RD, health writer and nutrition expert. “This will lower your metabolism, and your body will hoard every consumed calorie, causing weight gain when you go off the fad diet.”

Think of it this way: When you drastically cut calories, your body still has to perform its normal functions—like making sure your heart beats, your lungs take in air, your brain is receiving glucose to function and innumerable other functions going on inside your body. These are what make up your basal metabolic rate—the calories needed for you to simply survive if you did nothing.

But then there are the necessary additional calories needed to function at an optimal level—whatever that level is for you. Simply getting up and dressed in the morning uses calories beyond your BMR. And any activity you do, from work to play, uses calories. When you drastically reduce the calories you’re taking in, your body thinks you’re starving and goes into that survival mode, slowing your metabolism down to conserve energy. If you suddenly increase your calorie intake again, your metabolism is still low and burning calories at a slower rate. What calories your body can’t use, it stores up as fat to use at a later time when calories are reduced again and it needs to pull from the stores for energy.

Because of this, we recommend opting for slow and steady weight-loss tweaks that are healthy and sustainable to maintain muscle mass and a healthy metabolism.

You May Not Eat Enough Essential Nutrients

These drastic weight-loss tactics may work to lose weight in the short run, but they can rob your body of getting the nutrients it needs for optimal health. When you reduce your calories too much, it’s difficult to fit all the essential nutrients into what you are eating. The result can be nutrient deficiencies that lead to hair loss, sallow skin, anemia, fatigue and weak bones.

Eating a variety of nutrient-rich foods, including many plant-based foods, offers your body balanced nutrition and essential nutrients necessary for good health and healthy aging.

You Could Increase Your Chances of Getting Gallstones

“Losing weight too fast can increase the risk of gallstones,” says Lisa Andrews, M.Ed., RD, a registered dietitian. “When weight drops rapidly, there’s an uptick of cholesterol released into bile from the liver, which can develop into gallstones. Gallstones are stone-like deposits formed in your gallbladder, and they can be painful.”

Very low-calorie diets also may promote the development of gallstones because of the vicious cycle of losing and regaining weight—also known as yo-yo dieting or weight cycling. The greater the amount of weight lost and regained, the more likely you are to have gallstones. Some symptoms of gallstones are pain, vomiting, indigestion and fever.

To lessen your chances of gallstones, focus on a healthy, steady weight loss—which is about 1 to 2 pounds per week or 4 to 8 pounds per month.

You Could Be Really Tired

Calories are energy. But with hopes of meeting their weight-loss goals in the short term, many people seriously restrict calories to witness steep declines on the scale. This can send energy levels plummeting and bring on brain fog.

“Our bodies are likely to not get enough energy and, as a result, feel fatigued and lethargic. If you’re active, your workouts may suffer due to lack of fuel,” says Colleen Christensen, RD, a registered dietitian and certified Intuitive Eating Counselor

A review of 14 studies evaluating the effects of rapid weight loss on athletes found that fatigue significantly increased due to quick declines in weight. The review also suggested a possible link between quick weight loss and dehydration, which can cause mental fatigue.

And since glucose is the only fuel your brain uses, when you’re not eating enough carbohydrates, you will feel foggy and unable to focus or think clearly.

You can function at your best when you take steps that preserve and fuel your energy through healthy eating for gradual weight loss.

Your Mental Health Might Suffer

Obsession over the scale, fear of body fat and pressure to be a certain size can be detrimental to your mental health—and potentially create an unhealthy relationship with food and resentment of your body. And, in the worst-case scenario, it may cause a life-threatening eating disorder. And if you don’t immediately achieve your weight-loss goals, you may experience feelings of failure, shame, guilt or low self-esteem.

Rapid weight loss can affect your social life too. “When attempting to lose weight too quickly, you may find yourself isolating from social interactions as well as feeling anxious around certain foods,” says Maddie Alfiero, RD, a registered dietitian.

Socialization—feeling connected with and supported by other people—is an important aspect of mental and physical health, too. There is also evidence of a link between loneliness and social isolation with mental and physical disease risk. When you’re following a strict diet and isolating yourself by bowing out of social activities with family and friends, it may harm your mental health.

Whether you’re following a diet or not, include plenty of mood-boosting and brain-healthy foods in your meals and snacks.

The Bottom Line

Our bodies are complex—and weight loss is no exception. While it’s easy to try to reduce weight to calories-in, calories-out, many other factors come into play. Sleep, stress, physical activity, medications, medical conditions and genes are a few of these other factors.

“The reality is we can’t necessarily say exactly what weight we want to be, similar to how we can’t pick our shoe size or our height,” says Christensen. “My advice is not to focus on weight but on how you feel.” Because, remember—a specific weight doesn’t necessarily mean healthy and energetic.

Here’s a list of questions Burdeos recommends asking yourself before starting a weight-loss plan:

• When weighed against the potential long-term risks to my physical and mental health, is the short-term benefit—rapid weight loss and a lower number on the scale—worth it?
• What are the credentials of the person promoting this diet?
• Does this diet appeal more to vanity than to how I feel in my body and mind?
• Is weight the only marker I use to measure my health?
• Do I measure my self-worth by the number I see on the scale?
• Does this diet take into account my taste, cultural food preferences, time and budget?
• Can I eat this way for the rest of my life?

Only you can decide if the answers to these questions make a fad diet and rapid weight loss worth the potential side effects—including rapid regain of the lost weight when you go off the diet. Always consult your health care provider before starting any new weight-loss journey. It’s also a good idea to work with a registered dietitian. If repairing your relationship with food and your body is what you’re looking for, find a registered dietitian who is certified as an Intuitive Eating Counselor.