In a perfect world our bodies would burn what we eat to power the processes necessary to survive and they would replenish any fat supplies we need for a ‘rainy day’. In that perfect world we’d never overeat and our body would maintain a perfect weight throughout our life.

We know this is not what happens to us. But we didn’t know why until very recently. We thought that our metabolism, the ‘furnace’ that powers our muscles, maintains our body heat and helps every cell in our body do its work, wound down as we got older and we get a little bit overweight and slow. We believed that the ageing process affects the way our body burns food which is why we need to watch our diet.

We believed all that because it is hard to study the metabolism of a live body. As living systems, we are complex and always changing. And even though we have evolved to find a dynamic equilibrium, called homeostasis, this can be a different physical, mental and psychological state from person to person and from one moment to the next, in the same person.

Recent advances in science have provided us with tools that allow us to closely examine the way our body uses and stores energy and, as it turns out, there are some myths we need to bust, but first let’s define what we are actually talking about when we say “metabolism”.

We tend to think of metabolism as some kind of ‘place’ inside us which can then be dialed up or dialed down like a thermostat. Because of this we also believe that some foods like chilly peppers and green tea will increase it and others, like alcohol or fruit juice, will slow it down. None of this is true, though there is a small element of truth to some of it which we shall get to once we start our myth-busting.

Metabolism is a chemical process in the body that takes place in every little cell. Imagine that the ‘furnace’ we talked about at the start of this article, is present inside each of the approximately 30 trillion cells[1] that make up the human body. So there are 30 trillion furnaces burning inside us. But that analogy is also incorrect. Metabolism or our basal or resting metabolic rate, as it is often called, is a chemical process that is itself made up of two different processes.

The first of these processes, called Anabolism, is a process of synthesis. The body’s cells use the process of anabolism to create complex molecules from simple ones. Building muscle is an anabolic process and it has lent its name to the anabolic steroids that some body builders take to accelerate the growth of muscle tissue. Anabolism takes a lot of energy to happen. That energy is supplied by its other half called Catabolism. Catabolism is the break down of large complex molecules in the body. Glycolysis, the process via which the body breaks down glucose and generates energy, and the hydrolysis of adenosine triphosphate (ATP) which powers the muscles are two perfect examples of catabolism. We need both the building up and breaking down process provided by our metabolism to stay alive and remain healthy.

Myths About Metabolism

When it comes to metabolism and how we perceive it there are three persistent myths that show up again and again. They are then shared in internet chatrooms and fitness forums and even appear in some fitness articles on metabolism that have failed to follow the latest research. These myths are unsupported by current science and are incorrect.

In no specific order the three most prevalent myths are:

Myth #1: There are some foods that speed up our metabolic rate. While there are some studies[2] that have shown that foods like spices and chilli[3] and drinks like coffee[4] and green tea[5] will temporarily boost up our metabolism for a short time, the increase is negligible and the length of time in which this happens is also too short to have a true impact on our waistline, which is what we are looking for when we take them.

Myth #2: With age our metabolic rate slows down which is why we put on weight. A recently published study[6] used data from 6,500 people from 29 different countries, including women who had given birth, to compare individual energy needs across cultures and ages found evidence that shows that the base metabolic rate of an infant shoots to almost double that of an adult shortly after birth but then begins to steadily decline until about the age of 20. Then from age 20 – 60 it remains the same and then it slowly begins to decline again after the age of 60 reaching a 20% reduction by age 95. The same study showed that 65% of our daily energy needs, in other words 65% of our base metabolic rate, is used up by the brain, kidneys, liver and heart which represent only five per cent of our body weight. As we shall see a little later on this has other implications when it comes to diet and exercise.

Myth #3: Skinny people have a higher metabolic rate which is why they don’t put weight on. There are many genetic and lifestyle factors that affect how excess weight is added to the body. What we do know now, however, is that skinny people burn the same amount of energy per pound of weight as people who are overweight. So the difference in weight has to be attributed to other factors, some of which we shall explore next as we look at some of the new studies on the subject.

Metabolism and Weight Loss

The first hint that weight loss is not linked to metabolism in the way we think came from a 2020 study that showed that childhood trauma and even really stressful experiences felt by parents can result in changes in how the human body handles food which can be passed to their children in what is known as intergenerational epigenetic influences on metabolism.[8]

Then, in 2022 a new study looked at the evidence on metabolism and exercise gathered during the NBC reality TV series The Biggest Loser. What they saw was that diet and exercise affect the metabolism which slows down. [9] Then, however, it remains slow even after the required muscle and weight composition is reached.

When this evidence is supplemented by the more extensive findings of the study involving 6,500 individuals across different ages[6] the picture that emerges is one of the body adapting to the weight it reaches and then trying to maintain it regardless if we think it’s ideal or not.

So, let’s say someone who puts on 100g of fat per month over a period of 10 years, manages to accumulate 12 extra kilograms. Because of the slow addition of weight the body thinks this is its normal weight. When we try to lose it and get back to an older, fitter version of us, we are fighting not just against the weight we have accumulated but also against our own body’s mechanism which is trying hard to stop us from losing that weight and which will work even harder to make us put it back on again.

As a matter of fact a study involving mice[10] that were made to exercise showed that as their energy needs rose because of exercise their metabolism slowed down to compensate for it so that they ended up burning approximately the same number of calories they did before they exercised.

It’s always a leap from mice to people, but mice are used because certain aspects of their biology, like metabolism, for instance, is very similar to ours. We need to remember that our body is ancient in its makeup. The need to exercise in order to keep fit is very recent for us, as is the ability to eat lots of calorie-dense foods without having to first physically work for it (like in hunting).

What we are seeing is that putting weight on is easy because, according to the body’s ancient logic, it helps us survive, and taking it off is hard and the body resists it because it thinks it will starve.

Certain foods like coffee, spices and green tea temporarily increase the body’s internal activity and raise its temperature. This is called non-exercise induced thermogenesis. It lasts very little and it burns a negligible amount of calories so while true, it is untrue that these foods will help us lose weight. The same applies to similar activities like cold showers or ice baths. While the body, indeed, afterwards, increases its calorific burn, it quickly drops down to below its usual basal metabolic rate, to balance things.

How Do We Stay Trim?

Given what we know we understand now that exercise, as we use it, is an energy management activity. We try to increase the number of calories we burn through exercise so that we increase the total amount of calories we burn which, we hope, will be less per day than the amount of calories we consume.

It is true that we also, usually, increase the muscle to fat ratio so in theory we should be burning more calories anyway. But that too, doesn’t work the way we thought it did. A 2014 study [11] that involved primates whose muscle-to-fat ratio is a lot higher than ours and whose daily activities tend to be more energetic than ours, showed that they burn a lot less energy than expected.

Basically, what happens, is that as we put more muscle on we also eat more calories to maintain it so the amount of food we eat (i.e. energy we consume) balances the weight we have reached. If we exercise casually and then have a largely sedentary day the exercise we do is not enough to compensate for our sedentary lifestyle. After exercise the body’s metabolism slows down and we end up using, in a day, the same amount of energy we’d have used if we hadn’t exercised.

To truly stay trim the best tactic is to avoid putting extra weight on in the first instance. If, however, we do have some extra weight then losing it has to be a slow, methodical process that incorporates exercise and an increase in daily physical activity. Things like doing ironing, housework, taking the stairs instead of the elevator, walking to the nearby shops instead of driving, increase the body’s caloric burn significantly. When combined with a sustained and sustainable exercise program they become the sure-fire method to maintaining our ideal weight.

The proviso here is that we also keep an eye on what we eat and do not automatically increase our food intake to compensate for the increased level of physical activity.

How Exercise Affects Our Weight

Bear in mind that when we talk about exercise and weight we basically are talking about the muscle-to-fat ratio in the body. Exercise adds muscle to the body which requires more energy to maintain but, at the same time, reduces the effort that is required when we do exercise. A study[7] that followed hunter-gatherers that, on average, walked more than 19,000 steps a day showed that their daily energy needs were the roughly the same as adults of a similar age living a more sedentary lifestyle. 

What this indicates is that exercise will make us stronger and fitter and less likely to experience illness but weight control is only possible over the long term by incorporating a lot of activity in our day, not just exercise, and avoiding consistent over-indulgence in food. 


Age only affects our metabolic rate at the age of 20 and then, again, at 60. If we put on weight between the ages of 20 and 60 it is because we don’t move enough and eat a little more than we need. Throughout our life our basal metabolic rate adjusts to fit in with whatever weight we are at, which the body then considers to be normal. It will then resists our efforts to change it. To achieve lasting change that will help us be fit, strong and healthy we need to combine exercise with increased physical activity and sensible eating habits in a way we can sustain long term. What weight we lose needs to be small, incremental and we need to feel that the combination of daily activity, exercise and food we get is something that does not put us under undue stress, so that changes we gain can be lasting. 


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