Health Debunked: Can You Safely Lose Weight Quickly?

There are many fad diets that promise a quick fix for weight loss. Perhaps you’ve seen the ads featuring people who lost a lot of weight or dropped a few pant sizes in just 10 days. But is this type of quick weight loss realistic? And is it safe for your overall health?

What is fast weight loss?

A realistic weight-loss goal is losing a maximum of 2 lbs per week, according to the CDC. At this pace, you may learn habits grounded in nutritious eating and healthy exercise. And you may start to add smart, sustainable changes to your lifestyle. 

You’re more likely to stay at a weight that feels right for you if you make lifestyle changes rather than trying a fad diet.

Fast weight loss is different. It’s common for fad diets to promise accelerated weight loss of up to 10 lbs or 15 lbs per week. These fast-fix diets are often accompanied by: 

• Very low-calorie eating plans

• Diet pills

• Weight-loss drinks

• Detox juices

• Fasting regimes

Diets like these can fall short of essential nutrients and don’t teach long-term, sustainable, healthy habits. 

Claim: Losing weight fast can be healthy

Many fad diets claim that they can deliver fast weight loss. Some of these diets are very low in calories (less than 800 calories per day). And some recommend pills or weight-loss drinks that promote water loss through frequent urination or diarrhea. 

So, are these a healthy choice when it comes to making sustainable weight changes?

What does the science say about losing weight quickly?

Fast weight-loss diets aim to reduce calorie intake and to burn more calories. Eating less and moving more can affect your weight. But it’s not the whole picture. Your weight is also related to many other factors, including:

• Food accessibility and affordability

• Medical conditions

• Hormone levels

• Medications

• Cooking skills

• Level of education

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Eating less and moving more is still the only thing that many clinical diet studies focus on. Fast weight-loss plans may “work” in the short term, since they can help you lose weight. But the pounds that you drop are mostly from water and muscle, rather than fat. True weight loss involves shrinking fat cells, not just eliminating water for a temporary fix. 

Fast vs. slow weight loss

One study looked at fast weight loss versus slow weight loss. In the study, fast weight loss was considered 5% weight loss in 5 weeks. And slow weight loss was 5% weight loss in 15 weeks. 

Both approaches led to lost pounds. But with slow weight loss, people lost more fat and less muscle. So the changes in their body composition was more favorable. Both fast and slow weight loss improved lipid and blood sugar levels. 

However, the study didn’t evaluate what happened long term, and that’s a major problem. Studies show that fast weight loss is usually temporary. And weight regain is common. 

Plus, if you follow a diet of eating less and moving more, your body will make adjustments, like to your metabolism and hormone levels. These changes are your body’s way of resisting weight loss. So the fast weight-loss diet won’t lead to real results in the end.

Low-calorie diets 

Low-calorie diets with less than 800 calories a day aren’t recommended, except in limited circumstances, such as before surgery. Eating such a small amount of food can result in:

• Nutrient deficiencies

• Dehydration

• Gallstones

• Ongoing slow metabolism 

Supplements for weight loss

Some supplements are promoted for fast weight loss, such as caffeine or green tea extract. These can be labeled as “fat burners.” And they’re said to increase weight loss by increasing your fat metabolism. 

However, there’s not enough evidence on these supplements to make these claims. So, don’t trust the marketing saying otherwise. 

What do the experts say about slow vs. rapid weight loss?

“Quick weight loss results in water and muscle loss, not true fat loss,” explained dietitian Lisa Andrews, who works at Soundbites Nutrition in Cincinnati, Ohio. 

Andrews explained that cutting calories too much by dramatically reducing your food intake may lead to electrolyte abnormalities. And the reduction in muscle mass can negatively impact metabolism in the long term.

“Slow and steady weight loss is much healthier and easier on the body long term,” said Andrews. 

Hormone changes

Meanwhile, fast weight loss can negatively impact the hormones that are involved in appetite and body weight, including leptin and ghrelin, said dietitian Michele Phulsuksombati of Food Frame of Mind in Frederick, Maryland.

“In theory, leptin suppresses appetite, and ghrelin increases appetite,” said Phulsuksombati. “If you lose 10 lbs in a week, the body thinks it needs to eat, as the leptin levels are low and ghrelin levels rise.” 

This can cause weight gain, rather than weight loss.

Muscle loss, medical problems

“Weight loss is not a race,” added dietitian Melissa Mitri of Zen Master Wellness. “When you lose weight too quickly, it puts you at risk for muscle loss, nutrient deficiencies, and medical problems such as gallstones. It can actually slow your metabolism over time.” 

How can I healthily lose weight?

Weight loss is complicated and different for everyone. And finding a comfortable body weight for you depends on your personal goals. But there are some general tips you can lean on to lose weight in a healthy way. 

The Academy of Nutrition and Dietetics recommends lifestyle intervention, which includes:

• Eating nutritious foods

• Moving more

• Addressing eating behaviors

• Building a healthy sleep routine

Work with a dietitian or healthcare professional to figure out an eating and moving plan that works for your needs. This plan should be sustainable, enjoyable, and accessible — as well as affordable. For more information on weight loss, check out our guide here.

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Written by:

Cara Rosenbloom, RD

Cara Rosenbloom, RD, has been a registered dietitian for 22 years. She began her career working as a dietitian at Toronto's Hospital for Sick Children (SickKids).

Edited by:

Katie E. Golden, MD

Katie E. Golden, MD, is a board-certified emergency medicine physician and a medical editor at GoodRx.

Reviewed by:

Karen Hovav, MD, FAAP

Karen Hovav, MD, FAAP, has more than 15 years of experience as an attending pediatrician. She has worked in a large academic center in an urban city, a small community hospital, a private practice, and an urgent care clinic.

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