What Happens to Your Body When You Lose Weight Fast

Quick weight loss is often sought after in our society, but is it sustainable—or safe?

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Photo: Design elements: Getty Images. Collage: Cassie Basford.

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 many people feel pressure to lose weight swiftly. According to a 2017 article from Michigan Medicine, the academic medical center of the University of Michigan, about 90% of people who lose weight fast end up regaining it after all their hard work.

Everyone has their own reasons for losing weight; however, rapid weight-loss efforts can backfire. What's the rush to lose weight? Nowadays, in the 2020s, you can get a myriad of things in a short time—you can 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. With instant gratification within easy reach, it makes sense that you're drawn to drop a pants size in a day.

There's nothing wrong with wanting to reduce your weight, but it's crucial to your health to do it safely. So, what happens to your body when you lose weight too fast? You may be getting more than lower numbers on the scale.

Learn more to find out why healthy, slow-and-steady weight loss is the way to go.

You're Likely to Lose Muscle

According to the Centers for Disease Control and Prevention, the top two strategies adults choose when wanting 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 gets into breakdown mode, looking for different tissues to break down. It can break down fat tissue, but it can also break down muscle tissue, which may be more than you bargained for, per a 2018 publication in Medicine & Science in Sports & Exercise.

Melissa Mitri, M.S., RD, a registered dietitian at Melissa Mitri Nutrition, says, "When you lose weight too quickly, much of this weight loss is muscle and water weight, not fat." 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, or the expansion of your lungs for breathing.

Loss of muscle also causes weakness, making you more prone to injuries. "One study showed losing weight quickly can result in up to six times more muscle loss than a more gradual weight loss plan," says Mitri. The study Mitri refers to was published in 2016 in Obesity.

Preserve your muscle, strength and health by treating your weight loss as a journey and not a sprint.

You May Gain the Weight Back

According to a 2020 article published in the British Journal of Nutrition, 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.

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

Health professionals often advise a 5% to 10% decrease in body weight to promote health. A modest drop of 5% in weight has been shown to improve metabolism, per a 2016 study published in Cell Metabolism.

Opt for weight-loss tweaks that are healthy and sustainable to maintain a healthy metabolism.

Your Body May Miss Out on Must-Have Nutrients

These drastic weight-loss tactics may work to your scale's advantage; however, they can rob your body of getting the nutrients it needs for optimal health. With the typical Western diet, meeting the recommended dietary guidelines is challenging, even without pursuing weight loss.

A 2018 study published in Nutrients which analyzed the nutrient profiles of three commercial diets found deficits in vitamin D, B12 and calcium compared to recommendations. Nutrient deficiencies can lead to hair loss, anemia, fatigue and weak bones.

Eating a variety of nutrient-rich foods, especially many plant-based foods, offers your body balanced nutrition from many nutrients.

You Could Increase Your Chances of Gallstones

Lisa Andrews, M.Ed., RD, a dietitian and owner of Sound Bites Nutrition, states, "Losing weight too fast can increase the risk of gallstones. 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, per MedlinePlus.

According to the National Institute of Diabetes and Digestive and Kidney Diseases, very low-calorie diets also may promote the development of gallstones because of the vicious cycle of losing and regaining weight. 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 for gallstones, focus on a healthy weight loss. According to Merve Ceylan, RDN, a registered dietitian nutritionist, "1 to 2 pounds per week is ideal."

You Might Exhaust Your Energy

With hopes to meet their weight-loss goals in the short term, many people may seriously restrict calories to witness steep declines on the scale.

"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 dietitian Colleen Christensen, RD.

A 2020 review of 14 studies, published in Nutrients, evaluated the effects of rapid weight loss on athletes. The authors 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.

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

You May Harm Your Mental Health

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. When the weight creeps back up again, you may experience feelings of failure, shame, guilt or low self-esteem.

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

The Bottom Line

Christensen says, "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. My advice is not to focus on weight but on how you feel."

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

  • Is the short-term benefit, weighed against the long-term risks to my physical and mental health, 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?

Always consult your health care provider before starting a new weight-loss journey.

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