What Happens to Your Body When You Lose Weight

The benefits, the downsides and everything in between!

Reviewed by Dietitian Maria Laura Haddad-Garcia

The global weight-loss market (including products and services) is massive. According to a 2021 report from Research and Markets, this market is expected to grow from $254.9 billion in 2021 to $377.3 billion by 2026. Even more so, a 2021 report released by Ipsos, a market research company, shows that 45% of adults worldwide say they are trying to lose weight.

While some may try weight loss through exercise, changing their eating patterns, or any of the myriad available weight-loss products and programs, it's clear that many people are attempting to lose weight. Whatever your reasons for pursuing weight loss, understanding the benefits and risks is important. Learn more about fat and muscle loss physiology, as well as physical, mental and emotional changes, and safer ways to lose weight.

Related: The #1 Mistake People Make When Starting a New Diet, According to a Dietitian

Where Does Fat Go When You Lose Weight?

Many people are unaware of how fat cells work, the process of "burning fat," or where fat goes when you lose weight.Here's a little physiology, per a 2019 article published in the Bulletin of the National Research Centre: The energy your body doesn't use is stored as fat (in the form of triglycerides) inside your fat cells (called adipose tissue). The term "burning fat" really describes a complex hormonal and enzymatic pathway where triglycerides are released into the bloodstream to be transported anywhere in your body to be used as energy. When losing fat, your fat cells per se don't go anywhere—they stay where they are, under the skin and on top of muscles. Instead, you're really losing the stored fat inside your fat cells, which "shrink" as a result, making them smaller. Just as your fat cells can shrink as you lose fat, your adipose tissue can also grow as the stored fat in your body increases.

Losing Body Fat vs. Muscle Mass

When you lose weight, you're not just losing fat. "A common misconception is that we can target fat and lose fat in certain areas. That's not how our physiology works," says Jasmine Hormati, M.S., RD, CDN, a New York City-based dietitian who specializes in eating disorder recovery, "If you aren't eating enough to meet your energy needs, your body will turn to its glycogen reserves, then your muscles, and then your fat [to use as energy]."

Consequently, a percentage of fat-free mass (also known as lean mass), which can include water, skeletal muscle or even bone mass, usually accompanies the weight loss. The percentage of fat mass versus fat-free mass lost depends on different factors. A small 2018 study published in Medicine & Science in Sports & Exercise found that participants who lost weight due to calorie restriction had a reduction of their lower-extremity lean mass. In contrast, those who achieved weight loss due to exercise maintained their fat-free mass. Additionally, some groups of people may be at particular risk of losing fat-free mass during weight loss, including post-menopausal people, older people, those with metabolic disease and athletes, per a 2021 article in Nutrients.

It's important to incorporate physical activity and avoid eliminating complete food groups to prevent reducing your muscle mass when trying to lose weight. Working with a registered dietitian when embarking on a weight-loss journey can help ensure a healthy and sustainable weight loss.

What Happens When You Lose Weight

You May Improve Your Metabolic Health

Research shows that losing a small percentage of your weight (5% to 10%) can improve blood pressure, cholesterol and blood sugar levels, per the Centers for Disease Control and Prevention. The improved blood markers can help to decrease your risk of or better manage chronic and metabolic diseases, such as cardiovascular disease and diabetes. "The medical community often focuses on changes in weight and BMI to determine a person's overall health. What really helps us determine health are these blood markers. BMI only tells you a person's height in relation to their weight and was never meant to be used as a health indicator," says Hormati.

You May Feel Better and More Confident

"Because a diet can be easy to pick up and [people] may see some body changes, they start to feel more confident as they are 'succeeding' and getting compliments and validation from family, friends and co-workers," says Hormati. A 2018 study published in Obesity showed that initial weight loss during high-intensity intervention led to participants reporting a higher quality of life, including increased self-esteem and improved physical function, work and sex life and less public distress.

You May Find Community

Hormati says, "Clients have shared with me that being on a diet [to lose weight] initially feels good. They have clear rules to follow and are in community with others who are also doing the diet—that belonging feels good!" Studies, including a 2022 one published in Digital Health, have shown that weight-loss groups, especially on the internet, can create a strong social bond and sense of identity for people pursuing similar health goals. These internet interactions can provide motivation and encouragement to continue a person's weight-loss journey.

You Might Find It Hard to Maintain Your Weight Loss

"While diets are easy to pick up, the hard part is maintaining these diets over a long period of time," says Hormati—especially fad diets, which are usually pretty restrictive. "When you eat less, your body responds by increasing your appetite and cravings." In addition, a 2019 review published in Medical Clinics of North America notes that some long-term weight-loss studies have shown that attempts to keep lost weight off often fail. In 29 long-term studies, more than half of the lost weight was regained within two years, and more than 80% was regained by five years. The key to sustainable weight loss is to ditch restriction and focus on a balanced and nutritious eating pattern that's easy to adapt.

You Might Increase Your Risk of Weight Cycling

While restrictive dieting usually leads to short-term weight loss, it doesn't work long-term. Maintaining long-term weight loss is extremely challenging because of biological, behavioral and environmental factors, per the 2019 review mentioned above. "Dieting to promote short-term weight loss can have harmful effects like weight cycling, which can be more dangerous to a person's health than the weight itself," says Hormati. "It can lead people to have challenging relationships with food and their body [and feelings of shame and guilt around the "loss of control"], which increases the risk of disordered eating and eating disorders," adds Hormati.

Related: How Disordered Eating Can Impact Brain Health, According to Science

How to Eat for a Healthy Body Weight

Rather than focusing on losing weight, Hormati recommends a holistic approach: "When I work with my clients, we focus on behavior change and build up to their goals over time. This allows for us to incorporate the behavior so that they can sustain it long-term because that can impact their health." Here are some of her top tips for a healthy relationship with food and maintaining a healthy weight.

Make Sure You Eat Enough Food

Eating too little can actually get in the way of your weight-loss goal. "Our bodies are really smart and hardwired for survival. When you restrict food or overexercise without properly refueling, your body thinks food is scarce," says Hormati. "This actually turns on the hormones to tell you to seek out food and to reserve as much energy as possible in case of famine." That's why you may experience increased cravings and decreased metabolism when you eat too little.

Avoid Long Periods Without Food

Eating at regular intervals can help boost your metabolism and support healthy and regular digestion. "When my clients don't eat enough throughout the day, they notice that they come to the next meal very hungry. Sometimes that leads them to overeat because they may be eating fast or think they need more food to satisfy their hunger, which can lead to bloating and feeling overly full, which can be uncomfortable," says Hormati.

Eat Satisfying Meals

Meals that include all the food groups—protein, fat and carbohydrates (including high-fiber foods)—can help you feel full and satisfied. Hormati says, "Having a combination of carbs, fat and protein is so important for satisfaction. They send different signals in our body to help it feel fullness. Combining these elements can also help create more interesting meals with great flavor and texture."

Learn to Eat Intuitively

Intuitive eating is a methodology that ditches diets and helps eaters turn inward through body autonomy and learning to eat with attunement. "Intuitive eating is a helpful tool to get people to reconnect to their bodies and not let food dictate their lives. It's not a diet with rigid rules (at least not in the way I use it in my practice). It's a way of eating that allows for flexibility so that each person can tailor it to their life. Recently, it's been co-opted by places like Noom for weight loss; but intuitive eating isn't about that. Instead, it encourages us to respect our bodies," says Hormati.

The Bottom Line

Many people want to lose weight, and many attempt to do so. If you're looking to lose weight, understanding the possible benefits and risks can help you take the right approach. Incorporating sustainable healthy eating habits not only may help you achieve your weight-loss goals but also will promote overall health. Working with a qualified health professional, such as a registered dietitian, can also support you in your weight-loss journey. However, Hormati states, "We have no idea how these behaviors will influence their body size, but we can see how it impacts their overall well-being and health."

Related: The #1 Food to Help You Lose Weight, According to a Dietitian