Fast or slow metabolism: what it really explains

A fast metabolism burns more calories at rest and during activity, so you need to eat more to maintain your weight. A slow metabolism burns fewer, and the same person has to eat less to avoid gaining weight. That is the practical difference, and it explains why some people eat a lot without putting on weight while others do not. It is not, however, an explanation for everything — and the genetic part is largely outside your control.

Worth knowing from the outset: the idea that your metabolism is "broken" and that there is a quick way to fix it has no support in the available evidence. What does have support is duller and more useful, and that is what this article is about.

What actually separates a fast metabolism from a slow one

Basal metabolic rate — the energy your body spends simply existing — varies a great deal from person to person. There is no "normal" value. Everyone has their own, and scientists can only give averages: about 1,696 calories a day for the average man and about 1,410 for the average woman. If your number does not match these, that does not mean something is wrong.

What influences that rate? Body size, the amount of muscle mass, the amount of body fat, sex, age and genetics. And there are temporary factors that alter it: skipping meals or fasting, room temperature, thyroid hormone levels, illness or injury, and stimulants such as caffeine and nicotine. Notice that almost all of these factors are either beyond your control or only partly within it.

There is also a finding that contradicts much of the chat on this subject. A study using doubly labelled water, published in 2021, showed that total energy expenditure adjusted for fat-free mass stays stable throughout adulthood, between the ages of 20 and 60, including during pregnancy, and only declines at older ages. In other words: the idea that your metabolism "slows down" at 35 just because you are 35 is not supported by this evidence.

Why genetics explains less than you might think

Metabolism is partly genetic. That is true and not up for debate. But look at the other half of the sentence: genes do not change at the speed at which obesity has risen in recent decades. If the problem were mainly an inheritance of slow metabolisms, we would not have seen the population-level shift we did. Changes in diet and lack of exercise are far more likely candidates.

None of this means genetics does not count. It means that attributing your weight to an inherited metabolism is a partial explanation used as if it were the whole story. When someone tells you they have "a slow metabolism", they are often describing a mix of factors — some real, some behavioural — without separating them.

What you can change this week

The available evidence is clear about the most reliable starting point: improving the type of food you eat and increasing physical activity. This is not a motivational slogan; it is the conclusion of people who have looked at the problem. If you want to start today, start there, not by looking for ways to speed up your metabolic rate.

And do not expect immediate results. A week is long enough to know whether you can keep the routine; it is not long enough to judge whether it worked.

On trying to change the metabolic rate itself, the evidence is less encouraging. It is generally quite difficult to alter it significantly. If your goal is weight, chasing your metabolism as the main lever is choosing the wrong lever.

What tends not to help

There is a set of popular advice that promises more than it delivers. Eating few times a day to "speed up your metabolism", for example: skipping meals or fasting appears in the literature as something that temporarily alters metabolic rate, not as a strategy for raising it in any lasting way. Caffeine and nicotine appear on the same list of temporary factors — nicotine, in particular, is not a health option, and caffeine produces a change that does not last.

You will not find a list of signs of a slow metabolism here either. This is where we have to be honest: there is no reliable list of signs that lets you conclude you have a slow metabolism. What does exist is the clinical description of an underactive thyroid, which is a different thing and is not for self-diagnosis — we come back to that below.

And supplements? Where they honestly fit

They fit very little, and it is better to say so plainly. Green tea is the ingredient you may find in products in this category, including Active Metabolism Gummies, but there is no authorised claim of benefit we can make about the product here.

If you still want to try it, read the label and follow it. Active Metabolism Gummies are for adults, at a dose of 2 gummies a day, after a meal, and never on an empty stomach. They should not be taken by pregnant or breastfeeding women or by anyone under 18, nor by anyone consuming other products containing green tea on the same day. If you are ill, especially with liver disease, or if you take medication, speak to a healthcare professional first. And note that a supplement is no substitute for a varied and balanced diet and a healthy lifestyle — nor is it, in any circumstances, a treatment.

What you should not expect from any supplement is that it solves the problem on its own. If your question is about weight, the lever with the best support remains diet and physical activity.

When this stops being a conversation and becomes an appointment

There is one concrete situation that justifies going to the doctor: suspecting an underactive thyroid. Symptoms include extreme tiredness and weight gain, but they can be mild and easy to miss. Many of them are common and can have other causes. If you think you may have an underactive thyroid, book an appointment with your GP; the diagnosis is made with blood tests for thyroid-stimulating hormone (TSH) and thyroxine (T4).

Do not try to infer this from your weight or from how easily you gain or lose kilos. An overactive thyroid, which speeds up metabolism, tends to cause unintentional weight loss; an underactive thyroid lowers the metabolic rate. These are clinical signs, assessed in a consultation, not in mirrors or on scales.

If you take one idea from this article, let it be this: your metabolism is real, it is partly yours, and it is far less mouldable than the industry suggests. Start with what can be changed — what you eat and how much you move — give it months rather than days, and take any suspicion of a thyroid problem to a doctor instead of trying to solve it with a bottle. The rest is chatter.