Why do some people regain weight after losing it?

Why we shouldn’t oversimplify weight gain

Most weight loss methods lead to some initial weight loss, but keeping the weight off can be difficult, even when people continue with the same diet and exercise routine.¹

This is because your body responds to weight loss by slowing down how many calories it burns and increasing hunger, making it harder to maintain the weight loss.

When this happens however, it is so often the individual that is blamed for being ‘weak’ or not trying ‘hard enough’ and it can potentially lead to harmful cycles of self loathing or guilt, which feed into the problem.

Do food calories matter?

Calories measure the energy you get from food. For more than a century, they have been used to help explain weight loss, based on the idea that eating fewer calories than your body burns leads to weight loss.

This is known as energy balance. While the principle is correct, it doesn't mean that how much energy you eat and burn is entirely within your conscious control. Your body has complex biological systems that influence both.

What calories don't tell us

Calories also don't tell us everything about the food we eat. Two foods with the same number of calories can contain very different amounts of protein, fibre, vitamins, and minerals.

Foods rich in protein, fibre, and healthy fats can help you feel fuller for longer, while some ultra processed foods may be less filling, making it easier to eat more.

Your body doesn't absorb all calories in the same way, either. How food is prepared, cooked, and processed can affect how much energy you absorb. The other foods you eat alongside it can also make a difference.

Calories are a useful way to measure the energy in food, but they don't tell the whole story. They don't fully explain how much energy your body absorbs, how satisfied you feel after eating, or the nutrients you get from your food.

What about the calories we burn?

We often think we only burn calories when we exercise, but around 60-70% of the energy we use each day goes towards keeping us alive.²

This includes breathing, circulating blood, and regulating body temperature, which happen automatically as part of our basal metabolic rate.

Our body also uses energy to digest and process food. This accounts for around 10% of the energy we burn each day.³

Physical activity accounts for the remaining energy we use. This includes planned exercise, such as running, but also everyday activities such as walking, cleaning, shopping, and climbing stairs.⁴

Why does weight loss change how much energy we burn?

Although exercise affects how much energy we burn, our bodies control much of this automatically.

When you lose weight, your body may start using less energy to conserve it. This is called adaptive thermogenesis.

Your biology also affects how hungry you feel and how much you eat, meaning weight management is more complicated than simply balancing calories in and calories out.

How your body regulates your weight

Your body constantly works to maintain stability - a process known as homeostasis. This helps regulate essential functions such as body temperature, fluid balance, and body weight.

Your brain and body communicate through hormones - and other chemical signals - to help control how much you eat and how much energy you use.

When your energy stores fall, your brain can increase hunger and reduce feelings of fullness. Your body may also adapt by using less energy.

To understand how this works, we need to look at how body fat and the gut communicate with your brain.

Fat isn't just energy storage

For many years, body fat was seen simply as a way to store energy. We now know that fat tissue also produces hormones and chemical signals that communicate with your brain and the rest of the body.

One important hormone is leptin, which is released by fat cells and helps your brain track how much energy your body has stored.

When fat stores increase, leptin levels usually rise, helping reduce hunger. When fat stores fall during weight loss, leptin levels drop, which can increase hunger and lower the amount of energy your body burns.

However, leptin is just one part of the complex system that controls appetite and body weight.

The gut-brain connection

The gut also communicates with your brain to help regulate appetite. Ghrelin - a hormone released mainly by the stomach - signals hunger, particularly when your stomach is empty.

After you eat, hormones such as GLP-1, peptide YY (PYY), and cholecystokinin (CCK) help signal fullness and influence how much you eat.

Your brain combines these signals with information from the body's energy stores to help regulate hunger, fullness, and satisfaction after eating.

This means appetite isn't simply a matter of willpower or choosing whether to eat. It is strongly influenced by biological signals from your gut and the rest of your body.

This becomes especially important when you lose weight, as changes in these signals can increase hunger and make it harder to keep the weight off.

What happens when you lose weight?

When you lose weight, your energy stores fall. In response, your brain receives hormonal signals that can increase hunger and encourage your body to conserve the energy it has left.

Research has found that after weight loss, levels of leptin and hormones that help us feel full can fall by up to 65%, while levels of ghrelin - which stimulates hunger - can rise. People may also feel hungrier.⁵

These changes can persist for a year or more, even when weight loss is maintained.

This helps explain why keeping weight off can be difficult. After weight loss, your body may respond by increasing hunger and using less energy, making weight regain more likely. Some people may even regain more weight than they originally lost.

Does our body have a set point?

When we lose weight, the body can respond by increasing hunger and using less energy. This raises an important question: does our body try to maintain a certain weight?

Set point theory

In 1953, British physiologist Gordon Kennedy proposed that body fat sends signals to the brain to help regulate energy stores.⁶

This idea, known as lipostasis, gained support decades later when scientists discovered leptin, a hormone released by fat cells that helps the brain monitor energy stores. This helped form the basis of body weight set point theory.

The theory suggests that our bodies may try to defend a certain weight, even if that weight isn't ideal for our health.

When you lose weight, your body may respond by increasing hunger and using less energy, making it harder to keep the weight off. These changes can continue long after the initial weight loss.

However, this doesn't mean that everyone has one fixed weight programmed into their brain. Set point is still a theory, and some researchers believe the body may instead defend a range of weights.

What we do know is that the body can respond to weight loss in ways that make maintaining it more difficult.

But if our weight is regulated, why has obesity become more common?

Our biology doesn't operate in isolation. The body weight you reach and defend can be influenced by a multitude of internal and external factors such as your genetics, life stage, certain medical conditions, and the environment in which you live.

Over recent decades, our environment has changed dramatically: energy-dense highly-palatable foods are readily available, many of us move less during everyday life, and disrupted sleep, stress, medications, and hormonal changes can all interact with the biological systems controlling appetite, energy expenditure, and fat storage.

This may help explain why the same powerful systems that once protected us during periods of food scarcity can make maintaining a healthy weight particularly challenging in our modern environment.

Where weight-loss medications fit in

Weight loss medications such as Mounjaro and Wegovy have shone a light on the role our biology plays in controlling body weight.

They also offer benefits beyond weight loss, including improved mobility and a lower chance of certain serious health conditions.

These medications can reduce physical hunger, cravings, and food noise - the persistent thoughts about food that can make eating less difficult. As people lose weight, movement may also become easier, helping improve fitness and strength.

Weight loss medications aren't an easy way out or a form of cheating. They don't automatically lead to healthier food choices or habits, but they can make these changes easier by helping manage the biological signals that can make losing weight and keeping it off difficult.