Losing weight quickly can sound ideal on the surface: In a few weeks you’ll hit your goal and move on with your life. “It seems like everyone who wants to lose weight wants to lose it fast,” says Cheryl Mussatto, R.D., a registered dietician and author of The Nourished Brain. But just because it’s possible to lose double-digit pounds in less than a month doesn’t mean you should. Losing weight that fast is usually the result of an extreme deprivation diet — something that can throw off your metabolism, zap your energy, and harm your overall health. On top of that, rapid weight loss can cause rapid muscle loss — probably not the end goal you had in mind. Here’s what to know about the dangers of losing weight too fast and how to embrace the slow-and-steady approach instead.t65y7
What is rapid weight loss?
Consistently losing more than two pounds per week is considered rapid weight loss. It’s a sign that you’re cutting too many calories from your diet, which can trigger muscle loss, nutrient deficiencies, and more. That said, don’t worry if at first you lose more than two pounds a week.
“In most cases, weight loss tends to be more rapid as people start a weight-loss program — and decreases in rate as they continue the plan,” says Dr. Holly Lofton, M.D., professor of medicine and surgery at NYU Grossman School of Medicine and the director of the Medical Weight Management Program at NYU Langone Health in New York City. “This can be due to the effect of water weight loss initially, after which primarily fat loss and some muscle mass loss tends to dominate.”
A quick review of the science of weight loss can help you see what she means. When you eat fewer calories, your body burns through glycogen stores, which leads you to lose water weight pretty quickly. Once those glycogen stores are used up, you start burning fat (and some muscle) for energy. If your new approach to eating is sustainable — a lifestyle, not a fad diet — then your weight loss will taper off into that one-to-two pounds per week rate. But if you are restricting your diet in an unsustainable way and keep losing weight too rapidly, you can be doing a lot of damage to your health.
6 downsides of rapid weight loss
A sped-up approach to losing weight can do a number on your mental and physical health.
- You might not get enough of key nutrients.
Rapid weight-loss diet plans often rely on fad diets that eliminate entire food groups or macronutrients, like carbohydrates. And this way of eating can cause you to miss out on essential vitamins and minerals that your body needs. After several weeks, this could place you at risk for things like electrolyte imbalances, hypoglycemia (low blood sugar), and muscle cramping, says Dr. Lofton. A lack of certain micronutrients can also negatively affect your immune system, energy, mood, and even your sleep patterns. - You could drastically slow your metabolism.
Research has found that when you restrict your calories too much to lose weight, your metabolic rate also slows down — it’s a self-care mechanism where your body conserves energy because you're consuming fewer calories. This slower metabolism makes it harder to maintain any weight you’ve lost. But if you lose weight more slowly and don’t make as dramatic a cut in your calorie intake, it will have less of an impact on your metabolism. - You’ll lose more muscle.
When you lose weight by restricting calories, your body goes into a catabolic state — in which it effectively gets its energy by breaking down both fat and muscle. This reduces muscle mass as well as muscle quality, causing you to lose strength. A slower approach to weight loss will still still result in some muscle loss, but not as much (and that can be minimized even more by upping your protein intake and doing physical activity including resistance training). - You probably won’t feel so great.
Extremely low-calorie diets — ones that encourage eating 1,000 calories per day or less — and restrictive eating plans that completely cut out food groups can bring on lightheadedness, constipation, headaches, insomnia, and irritability, according to Mussatto. They result from a combination of low blood sugar and dehydration, since you’re losing so much water weight. You’re also going to find yourself pretty hungry and low on energy, since you aren’t eating enough to keep your body functioning. A surprise to no one: This can bring on mood changes. A study published in the journal Emotion finds hunger can cause people to experience a negative, antisocial emotional state. - Over time, you could seriously harm your health.
In one study, researchers found very low-calorie diets can cause a deterioration in heart function in some people. After just a week, people on a diet of 800 calories a day had elevated levels of triglycerides, which can put stress on your heart. Your gallbladder can also be affected. “The gallbladder becomes less active during periods of rapid weight loss,” says Lofton. As a result, you’re at a higher risk of getting gallstones, which can cause severe abdominal pain, and even possibly get an inflamed gallbladder. In fact, “there have been reports of patients requiring cholecystectomy, or gallbladder removal surgery, during periods of rapid weight loss,” she says. You might also start losing your hair or notice changes to your menstrual cycle, since your body starts to reserve energy for more important bodily functions. - Any weight loss will be temporary.
Crash diets that cause you to lose weight quickly aren’t usually sustainable — either your metabolism slows down to a crawl, making it tough to maintain weight loss, or you simply (and understandably) can’t stick with it. “Most people who try quick weight-loss diets ultimately fail, and usually struggle to keep weight off long-term,” says Mussatto.
Healthy tips for long-term weight loss
Losing weight is less about the actual number on the scale and more about the lasting benefits that come with weight loss — things like better heart health and having more energy. And to see those benefits, you want to adopt lifestyle habits that feel sustainable — like the ones prioritized by the WeightWatchers program. Here are a few key things to keep in mind:
Lean on fiber
Instead of cutting carbs out of your life, include carbohydrates that are full of fiber, like whole grains and fresh fruits and veggies. Not only can fiber help with weight loss by helping you stay full, but one study published in The Journal of Nutrition found that eating plenty of fiber also helps sustain the results in the long run.
Rely on protein
Protein-rich foods, like lean meat, low-fat dairy, eggs, tofu, and beans, can keep your blood sugar stable and help build lean muscle. And research has found that eating a diet with adequate protein and increasing your physical activity—especially resistance training—can help you maintain muscle mass and improve muscle strength even while you lose weight.
Make time to exercise
Finding activities you love and regularly moving your body relieves stress, burns calories, and supports weight loss. More importantly, a consistent workout routine can help you maintain your weight long term. While any exercise you’re willing to stick with is good, a systematic review found that resistance training was the best for both increasing and maintaining muscle mass while you're trying to lose weight.
Healthy weight loss on a GLP-1
There’s one thing that can cause more rapid weight loss that hasn’t been mentioned yet: GLP-1 medications. Wegovy (semaglutide) and Zepbound (tirzepatide) can lead to speedy weight loss — with most people losing more than 5% of their starting weight in just three months — primarily due to the impact they have on appetite. While taking a GLP-1 does come with some of the same risks as other approaches to rapid weight loss — namely, gallstones and the loss of muscle mass — there are things you can do to soften the impact. This includes strength training regularly and prioritizing foods high in fiber and protein. You can get help sticking to those habits with the WeightWatchers GLP-1 Program—it’s designed specifically to support those taking a GLP-1 for weight loss and management.
It’s key to keep communication open with your healthcare provider and check in on whether or not your weight loss is too rapid. “GLP-1 doses can always be adjusted to slow down weight loss,” says Lofton. Since your dosage increases gradually over the course of several months, it’s easy for your doc to keep you at the same or slightly lower your dose in order to curb quick weight loss and reduce the risk for possible side effects. As a member of WeightWatchers Clinic, you’ll have a dedicated care team who is always available to answer questions and built-in monthly check-ins with your provider to make sure you’re losing weight at a safe pace.
The bottom line
Yes, those rapid weight loss plans you see online can look tempting, but going that route can impair your heart function, make you feel crummy, lead to gallbladder problems, and contribute to hair loss (among other concerns). For sustainable weight loss, stick to a program that helps you establish healthy habits in a way that feels like a new lifestyle—even if it takes you a bit longer to reach your goals.
This content is for informational purposes only and does not constitute medical advice, diagnosis or treatment. It should not be regarded as a substitute for guidance from your healthcare provider.