More than 70% of American adults have obesity or are overweight, according to the Centers for Disease Control and Prevention. Those wanting to lose weight as fast as possible face an important question: How can you lose that weight quickly and safely?

“The fastest way to lose weight is by creating a consistent calorie deficit with balanced meals and physical activity,” says Gaby Vaca-Flores, a registered dietitian and founder of Glow+Greens, a nutrition and skin care consultancy based in Santa Monica, California.

While it’s certainly possible to restrict calories enough to lose lots of weight in a week, Vaca-Flores says it’s not a good idea to choose the most extreme diet. “Although you may initially see fast results, a weight loss plateau may be waiting right around the corner due to the slow-down of your metabolism,” she says.

This slow-down, also known as metabolic adaptation and sometimes called “starvation mode,” happens when rapid weight loss, aka a crash diet, tricks your body into thinking you’re starving.

“The body requires energy – calories – to power basic biological functions, including digestion, breathing (and) pumping blood through the body, along with the activity you do throughout the day,” explains Mia Syn, a registered dietitian based in Charleston, South Carolina, and author of “Mostly Plant-Based.” “When you don’t consume enough energy from food for these functions, you can put your health at risk.”

To compensate in a restricted calorie environment, the body adapts to using fewer calories each day to function. This leads to a slowing in metabolism, which makes it even more difficult to lose weight and ultimately leads to rebound weight gain.

Cutting calories too drastically also can lead to weight loss via muscle loss, which further compounds the problem. Muscle is more metabolically active than fat, which means it burns more calories at rest than fat does. If your body starts breaking down muscle to fuel its basic functions, that can further slow your metabolism and make losing weight and keeping it off long term difficult.

Quick Weight Loss Can Lead to Rebound Weight Gain

If you’ve followed dieting advice and experienced significant weight loss only to have it return, you’re not alone. It’s also not your fault. The human body is designed to maintain equilibrium as much as possible; your metabolism adjusts as you lose weight because the system thinks food scarcity has become a threat to survival.

Because the body wants to preserve weight, a slow approach to losing extra pounds is healthier than drastically slashing calories. “It’s better to pace your weight loss by eating just enough calories to keep you at a deficit while still meeting most of your body’s energy needs,” Vaca-Flores says. “This will help keep your metabolism working efficiently.”

Determining the number of calories you need to eat to meet that rate of weight loss will depend on several factors, including your age, starting weight, activity level and sex. Candace Pumper, a staff dietitian with the Ohio State University Wexner Medical Center in Columbus, says, “The Academy of Nutrition and Dietetics position statement on adult weight management recommends 1,200 to 1,500 calories per day for women and 1,500 to 1,800 calories per day for men to support weight loss.”

She notes, however, those guidelines are “not meant to substitute for professional nutritional advice or treatment. It’s recommended individuals follow the advice of their dietitian’s guidance regarding appropriate calorie intake, as many factors affect calorie needs.”

In addition, “if you lose weight too quickly, you’ll probably see dips in your energy levels and even headaches, which can make it harder to stay on your weight loss journey,” Vaca-Flores says. “This typically happens when you’re not eating enough or are over-exercising.”

Rapid weight loss may only be appropriate in rare circumstances. “A very low-calorie diet is sometimes recommended for adults who have obesity and need to lose weight for health reasons or before weight-loss surgery,” Syn says. In these cases, the diet “is only followed for a short period of time and under the guidance of a doctor or dietitian.” If you’re looking to lose weight quickly, she urges you to “work with and be supervised by a health care provider.”

Other Weight Loss Options

While many people want to lose weight fast without exercise, it’s important to note that exercise can help you jumpstart weight loss and sustain it over the longer term; remember, muscle is more metabolically active than fat and can help you maintain your weight. As a 2021 review study in the journal iScience found, exercise is more important for longevity and health benefits than losing weight.

Bariatric or weight loss surgery and other types of medically supervised weight loss can help those with a lot of weight to safely and permanently shed pounds. These approaches aren’t always super-fast paths to success, though, and may require patience.

Some weight loss medications can help adjust the hormones affecting your rate of weight loss. Wegovy, for example, is an injectable medication that was recently approved by the FDA to support weight loss. Wegovy works on the GLP-1 hormone secreted in the gut, causing the stomach to slow its typical rate of emptying so you feel fuller longer after eating. It also suppresses appetite. Metformin, typically administered as a pill or in liquid form, is used to treat Type 2 diabetes and may offer dieters additional support. This medication works by increasing the body’s sensitivity to insulin and leptin, two hormones that can affect how full you feel and help you feel less hungry.

These are options you can discuss with your doctor. It’s important to note, however, that going on a powerful medication isn’t a quick fix to help you lose a lot of weight in two weeks to get ready for that upcoming high school reunion. Instead, it should be one piece of an overall, long-term lifestyle shift that can help you get to a healthy weight at a pace that’s safe for your body. Your doctor can help connect you with the right weight loss program for your needs and goals.

The Best Way to Lose Weight

Syn says that when looking to lose weight, “it’s important to set realistic goals for yourself.” Going too fast can set you up for a weight rebound and potential health complications later.

To avoid the all-too-common yo-yo dieting fate, Vaca-Flores recommends aiming for a modest weight loss of about half a pound per week, depending on how much weight you want and need to lose. “Taking a slower route can help prevent burnout or a frustrating weight loss plateau,” she says.

As exciting as shedding weight fast might seem, “it’s usually short-lived” because the extreme measures used to achieve fast results aren’t sustainable, “which can result in burnout and gaining all of the weight back,” Vaca-Flores adds. “Taking a slow and steady approach is key for seeing consistent results that last.”

Scaling back portion sizes, swapping in more nutrient-dense food options and getting plenty of sleep and exercise are all good strategies for healthy and long-lasting weight loss.

“Weight loss takes lifestyle change, not just a quick fix diet,” Syn says. “In order to stay healthy and look your best long term, you have to make many small changes.”