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Exercise for weight loss: Does it work?

Robbie Puddick
Written by

Robbie Puddick

Medically reviewed by

Fiona Moncrieff

11 min read
Last updated June 2024

Jump to: You tend to eat more to compensate for exercise | You can’t outrun a bad diet | Energy expenditure is constrained | Take home message

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Does exercise help you lose weight? Not really. 

Surprisingly, human clinical trials have shown that exercise doesn’t tend to lead to significant weight loss without changes to diet. This is despite the many positive effects of exercise on our health.

There seem to be three main reasons for this:

  1. You tend to eat more to compensate for exercise: Your body responds to exercise by encouraging you to eat to replenish what you’ve burnt. So, people compensate for their exercise by eating more to maintain body weight.
  2. You can’t outrun a bad diet: While being fit and active is positive whatever your diet looks like, if you’ve got a diet based on ultra-processed foods and low in protein, fibre, and essential micronutrients, exercise alone doesn’t seem enough to compensate for this in the long run.
  3. Energy expenditure is ‘constrained’: We previously believed the calories burnt from exercise to be ‘additive’, so the more you burnt, the more likely you were to lose weight. But new research suggests that when you burn more calories through exercise, your brain turns down other processes that use energy to compensate.

However, this doesn’t mean that exercise can’t support weight loss or that you shouldn’t do it. It’s more to say that this is a complex issue, and we might have to be more clever about how we approach it.

That starts with developing a more in-depth understanding of how the human body works.

Moving beyond ‘eat less, move more’

While disappointing, the research around exercise and weight loss suggest the importance of a healthy diet when weight loss is your goal.

It also highlights how complex the human body is and how simple maths equations can’t predict weight loss success.

If it sounds too good to be true, it probably is, and that’s the case with weight loss. It may be a ‘simple’ equation of burning more calories than you consume to be in a calorie deficit to lose weight, but the complexity comes from what influences both sides of the equation.

Our brain is constantly working to protect us and ensure our survival so we can reproduce. At our core, that is what we have evolved to do: survive and reproduce.

So, it makes sense that the brain would try and protect our energy stores when we exercise more and that it would encourage us to eat more to compensate for what we’ve just burnt.

The brain doesn’t care about your health or weight. It cares about whether you’re going to survive long enough to reproduce.

Exercise for health

Too much focus is often on how many calories you burn through exercise. You see videos on YouTube titled ‘30-minute calorie burner!’ or ‘Belly fat burning workout!’.

The truth is the health benefits of exercise go way beyond the calorie burn. Exercise (whether it’s aerobic/cardio, HIIT, or strength training) is possibly the most significant health and longevity tool at our disposal, alongside sleep, mental health, and nutrition.

Research has consistently shown that the fitter you are and the higher your muscle mass, the lower your risk of death from all causes and the risk of chronic diseases like type 2 diabetes, heart disease, and cancer.

There’s also strong evidence that exercise promotes a healthier brain. It seems to promote the ability of the brain to produce new neural connections (as well as strengthen existing ones) to support learning and memory, as well as productivity and creativity.

Additionally, exercise and physical activity are strongly linked to weight loss maintenance. Research on people who’ve lost weight in clinical trials shows that physically active individuals are much more likely to maintain weight loss compared to those who don’t.

Long-term exercisers seem to have improved satiety (hunger and fullness) signalling compared to sedentary individuals. Regular exercise training may improve your body’s ability to self-regulate your calorie intake.

So, while diet will likely predict your chances of weight loss success, exercise seems to be the other habit that will significantly improve your chances of keeping the weight off for good.

Essentially, there doesn’t seem to be one element of the body that isn’t improved in some way by regular exercise.

Research suggests a combination of resistance training, moderate-intensity cardio, and high-intensity interval training will help to maximise its positive effects.

So, while exercise might not be the magic cure for weight loss – it will help you live a longer, healthier, and happier life.

At Second Nature, you’re provided with the support of a registered dietitian or nutritionist trained in all areas of behavioural and health science.

They’re online five days a week to support you in making the changes you need to help you lose weight, get fit, and live a longer, happier life.

If you’d like to join over 150,000 others who’ve made losing weight feel second nature, click here to take our health quiz.

Otherwise, keep reading as we unravel the science of why exercise doesn’t always lead to significant weight loss in the long run.

1) Overcompensating for energy burnt

A systematic review and meta-analysis of human trials investigating the effects of different approaches to weight loss found that exercise interventions alone had no meaningful effect on weight loss.

In comparison, the diet and diet with medication interventions supported an average of 3-6 kg of weight loss after two years.

People often assume that exercise is all you need to do to lose weight. The pounds should drop by burning off more calories and continuing to eat the way you are.

Unfortunately, what tends to happen is that individuals increase their food intake to compensate for most of the energy they’ve burned through exercise – and you’ll experience little to no weight loss.

A recent study compared the impact of exercise for two or six days a week on overweight participants’ energy intake, hunger hormones, and weight loss.

The study showed that weight loss between the two groups was very modest and not clinically significant, with the six-days-a-week group losing 1.04kg and the two-days-a-week group losing 0.76kg.

Interestingly, despite differences in exercise intensity, time, and changes in the hunger hormone ghrelin – both groups compensated equally in relation to their exercise levels. So, the participants that exercised more simply ate a bit more.

The importance of diet

While exercise alone doesn’t seem to support significant weight loss, diet and exercise interventions are effective at supporting weight loss, which seems to be primarily driven by the change in eating habits.

A randomised controlled trial compared the impact of exercise alone, diet, or exercise and diet together on weight loss after 12 months in overweight participants.

The results showed that both the diet and diet and exercise groups lost a significant amount of weight (10% and 9%), while no weight loss was observed in the exercise group.

Interestingly, adding exercise alongside the diet intervention didn’t lead to more weight loss than diet alone – again highlighting that weight loss is more dependent on dietary changes than the influence of exercise.

Key points:

  • Large-scale human trials have failed to show the benefit of exercise on weight loss when used in isolation.
  • One reason exercise doesn’t seem to support weight loss is because people will overcompensate and eat a bit more to balance out the energy they burnt through exercise.
  • When trials have compared diet alone to diet and exercise, there’s often no difference in weight loss markers. Suggesting the main predictor of weight loss is diet.
  • Combining exercise with diet changes leads to more significant improvements in other health markers than exercise alone.

2) You can’t outrun a bad diet

Exercising regularly and having a high level of physical fitness is an independent predictor of health and longevity. So, if you choose to eat a healthy diet or not, exercise will still benefit your health.

However, if you want to lose weight and achieve optimum health, eating a poor diet while exercising will only get you so far.

The randomised controlled trial mentioned above also analysed a measure of cardiovascular fitness, which we call peak oxygen consumption, alongside weight loss outcomes.

The diet and exercise group showed a more significant improvement in peak oxygen consumption than exercise alone (17% vs 8%), despite both groups completing the same exercise intervention.

Peak oxygen consumption is not only a measure of fitness but also of your health. Research has shown that the higher your peak oxygen consumption, the lower your risk of all-cause and cardiovascular disease mortality.

Additionally, physical fitness may also influence our satiety (fullness) signalling and support weight loss and weight maintenance in the long run.

This influence of exercise on satiety might be explained by the impact of exercise on insulin sensitivity and other hormones that help to regulate our weight.

Insulin’s famously known for its role in blood glucose management. But it also influences satiety signalling, which helps to regulate our eating.

Insulin’s role in supporting blood glucose can also positively affect satiety, as low blood sugar can lead to hunger.

The relationship between fitness, insulin, and appetite was demonstrated in a recent clinical study where participants living with obesity completed a 12-week exercise and diet intervention.

Participants saw a significant improvement in insulin sensitivity (indicating improved insulin function), fitness levels measured by peak oxygen consumption (VO2 max), and increased levels of the satiety hormone PYY.

Increased levels of PYY over the long term may help to curb appetite and improve weight management, which has been observed in weight loss trials with long-term follow-up.

A study of women living with obesity found that a year after weight loss, only physically active women maintained their lower weight. In comparison, women who were sedentary or moderately active regained around 7kg on average after 12 months.

Key points:

  • Exercise is good for you, whatever the quality of your diet looks like.
  • However, focusing only on exercise can get you so far, and a poor diet will likely result in unsuccessful weight loss attempts, poorer health, and weight regain following weight loss.
  • Clinical trials have shown that health markers, such as peak oxygen consumption and insulin sensitivity, improve more when diet and exercise are combined compared to exercise alone.
  • Exercise seems to improve satiety signalling, which may help regulate eating habits in the long term.
  • Research has also shown that following weight loss, being active helps to maintain weight loss and avoid regain.

3) Energy expenditure is constrained

The number of calories you burn each day, which we call your energy expenditure (EE), is made up of four areas:

  1. Basal metabolic rate (BMR): This is the energy required to keep all of your essential functions running, such as the energy needed for your heart, liver, and brain. It contributes around 60-70% of your total EE.
  2. Exercise activity thermogenesis (EAT): The energy required to fuel your formal exercise habits, such as running, cycling, or walking the dog. The contribution to your total EE depends on your activity levels, anywhere between 10-30%.
  3. Non-exercise activity thermogenesis (NEAT): The energy that fuels your spontaneous movement, such as fidgeting or when you move your hands whilst presenting to a group. The contribution to total EE again depends; it could be anywhere from 5-15%. (NEAT is always spontaneous and non-conscious movement, if you consciously move your hand, that’s not NEAT, it would count towards exercise activity.)
  4. Thermic effect of food (TEF): The energy required to break down the food you digest. Protein has the highest effect on this, then carbohydrates and fat. This accounts for around 5-10% of your total EE.

It was previously believed that the exercise component of our energy expenditure was additive. So, the more exercise you do, the more energy you burn and the more weight you can lose.

However, recent research shows that if you increase the energy you burn through exercise, your brain downregulates other areas of your metabolism to conserve energy.

Much of this is done at the cellular level, where your body will slow down the energy available for specific processes within the body that are using up too much energy.

But some of this response seems to be behavioural in your daily activities. So, if you start exercising more, you might choose to sit rather than stand or take the elevator instead of the stairs.

This is another example of how the brain always works to conserve energy to promote survival and reproduction.

If the brain senses an increased demand for energy in one area (exercise), it will respond by signalling the body to downregulate the need for energy elsewhere.

Not a reason to avoid exercise

This doesn’t mean we shouldn’t exercise because it ‘doesn’t burn more calories’.

You might not be burning fewer calories than you expected despite exercising more, but the way that your body uses energy and how it functions will significantly improve through exercise.

For example, one advantage of exercise is that you’re regularly burning through your body’s existing fat and carbohydrate stores, promoting what’s known as a state of flux.

The state of energy flux promoted by exercise has been shown to improve insulin sensitivity and blood sugar levels and regulate inflammation levels in the body.

All these adaptations to exercise can improve your health and chances of maintaining a healthy weight in the long term.

Key points:

  • It was previously believed that the energy needed for exercise only added to your total energy expenditure.
  • But new research suggests your body downregulates other functions that require energy, so your energy expenditure becomes constrained.
  • This doesn’t mean you shouldn’t exercise, as exercise improves the way your body uses energy and how it functions, improving your health.

Take home message

The relationship between exercise and weight is a complicated one. On the one hand, you typically find that physically active individuals tend to be slimmer and maintain a lower weight.

But on the other hand, when people living with obesity are told to exercise more without making other changes, their weight doesn’t seem to budge.

Reasons for this seem to extend beyond the activity of exercise in isolation. People who exercise tend to sleep more, have less stress, and self-regulate their eating habits.

The brain’s also designed to conserve energy and protect its current body fat levels to increase survival chances.

It’s likely that for people currently living with obesity, the brain’s set-point is hardwired to protect your current level of body fat and will work to do so when you start exercising more, preventing meaningful weight loss.

However, combined with a healthy diet and regular training (like aerobic exercise and weight lifting) over the long term, exercise should help you achieve better health and maintain a lower weight.

This is one of the reasons we promote long-term thinking at Second Nature. It’s not about where you want to be for a wedding or the summer but what life you’d like to lead for the next 5 or 10 years and more that will inherently lead to a healthier body and improved weight control.

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