If losing weight were simply a matter of eating less and moving more, why do so many people regain the weight they lose? Millions of Americans attempt to lose weight each year, yet long-term success remains surprisingly uncommon. Weight loss is often portrayed as a test of willpower, but research suggests that biology, behavior, environment, and lifestyle all play important roles.

In this section, we will examine common weight-loss strategies and what the scientific evidence says about their effectiveness. Successful weight loss is generally defined as intentionally losing at least 10 percent of body weight and maintaining that loss for at least one year.^[1] Unfortunately, studies show that long-term success is difficult to achieve. In a large study of more than 14,000 adults, only about one in six individuals with overweight or obesity lost weight and kept it off for at least one year.^[2]

Why is long-term weight loss so challenging? Much of the answer lies in understanding how the body responds to weight loss and the biological mechanisms that work to defend body weight.

Biology of Weight Loss

As discussed earlier, if you eat more calories than you burn, you gain weight. If you burn more calories than you eat, you lose weight. This concept, known as energy balance, provides the foundation for weight management.

However, the body is more complex than a simple calorie equation. Your body constantly works to maintain a relatively stable weight. In the past, this helped humans survive periods of food scarcity. Today, however, we live in an environment where high-calorie foods are widely available and physical activity is often limited. As a result, maintaining a healthy weight can be challenging.

When people begin a weight-loss program, they often lose weight fairly quickly. Over time, however, weight loss usually slows. This happens because the body responds to weight loss by trying to conserve energy and restore lost weight.^[3]

As body weight decreases, fewer calories are needed to support basic body functions and to move a lighter body. In addition, the body often lowers its metabolic rate in response to calorie restriction, making further weight loss more difficult. In other words, the body interprets weight loss as a potential threat and responds by becoming more energy efficient.

Researchers have also discovered that some people appear to have a naturally “thrifty” metabolism, while others have a more “spendthrift” metabolism. Individuals with a thrifty metabolism reduce their calorie expenditure more during fasting or calorie restriction, making weight loss more difficult. Those with a spendthrift metabolism burn more calories under the same conditions, which may make weight loss somewhat easier.^[4] These findings suggest that some people may need to work harder than others to achieve the same weight-loss results.

Together, these biological adaptations help explain why long-term weight loss can be so challenging. The video below explores these factors in greater detail.

Changes in hormone levels may also make it difficult to maintain weight loss. As body weight decreases, levels of thyroid hormones often decline, causing the body to burn fewer calories. At the same time, shrinking fat cells produce less leptin, a hormone that helps regulate appetite and energy balance. Lower leptin levels can increase hunger and slow energy expenditure. Together, these hormonal changes encourage the body to regain lost weight and make long-term weight loss more challenging.^[5]

These biological adaptations help explain why losing weight is often easier than keeping it off. The video below explores these factors and their influence on body weight in greater detail.

Components of Weight Loss Program

Evidence shows that successful weight loss and long-term weight maintenance are possible, even with the body’s natural biological adaptations that can make weight loss challenging. Because individuals differ in their preferences, health status, and environment, no single approach works for everyone.

However, most effective weight management strategies include three key components:

• Reducing calorie intake, particularly from foods high in fat and added sugars
• Increasing physical activity to support energy balance and overall health
• Using behavioral strategies, such as goal setting, self-monitoring, and building sustainable habits

Dietary Considerations for Weight Loss

Although following a lower-calorie healthy eating plan is often the first approach to weight loss, research shows that no single dietary strategy is superior to others.^9,10   For example, a recent trial, called the DIETFITS study, followed participants on either a low-fat or a low-carbohydrate diet for 1 year and found no significant difference in weight loss between the groups. ^[6]  Both dietary strategies produced a range of weight loss results. Some participants lost more than 60 pounds, while others gained 20 pounds over the year, suggesting that what works for one individual may have varying effects on others.

To learn more about the DIETFITS study, check out the following video.

National Weight Control Registry

The National Weight Control Registry (NWCR) has tracked more than 10,000 people who have successfully lost 30 pounds and maintained this weight loss for at least one year. Their research findings show that 98 percent of participants in the registry modified their food intake, and 94 percent increased their physical activity, mainly by walking. ^[7]

There was a wide variety of approaches NWCR members used to achieve successful weight loss. Most involved a low-calorie, low-fat diet while exercising. (Most got one hour of exercise per day.) Moreover, most members eat breakfast every day, watch fewer than ten hours of television per week, and weigh themselves at least once per week. About half of them lost weight independently, and the other half used a weight-loss program.

In most scientific studies, successful weight loss is accomplished only by changing the diet while increasing physical activity. Doing one without the other limits the amount of weight loss and the duration of sustained weight loss. ^[8]

 GLP-1 Medications

New medications such as semaglutide and tirzepatide help some individuals lose weight by slowing stomach emptying, increasing feelings of fullness, and reducing appetite. These medications can be effective for some people, but they are not appropriate for everyone and work best when combined with healthy eating and physical activity.

Diek,” from TED, 12:30 minutes.

attributions

This section is an adaptation of “Best Practices for Weight Management” in Nutrition: Science and Everyday Application, v. 1.0 by Alice Callahan, PhD; Heather Leonard, MEd, RDN; and Tamberly Powell, MS, RDN, licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.