Most of us have a friend that can eat whatever they want, whenever they want, and still somehow stay slim. They may say they’ve been blessed with a fast metabolism, but is that really the case?

The metabolism is the process by which our bodies turns food and drink into the energy we need for life. Often, scientists talk about metabolism in the context of the body’s energy use over the course of a day, which is called the metabolic rate. The more energy expended, the higher the metabolic rate. It follows, then, that people with fast metabolisms are those that burn through a lot of energy in one day.

The majority of the energy we get from food is used by the body simply to keep us alive. Living is costly: we need energy to keep us warm in winter, to fight off invading pathogens, to pump blood through our arteries, and to breathe in and out.

The energy requirements of these fundamental tasks are often referred to as our basal metabolic rate (BMR), and BMR accounts for 60 to 80 per cent of our total energy expenditure in a given day. For most of us, physical activity accounts for 15 to 30 per cent of our daily energy expenditure. A small percentage of energy is put towards energy harvesting itself, powering our digestive system.

One of the biggest components of BMR is the energy demands of muscle cells. Compared to other cell types, muscles are much more active, needing energy to contract and relax, and to repair themselves against daily wear and tear. The more muscle a person has, the higher their BMR.

This brings us back to our skinny friend who can eat whatever their heart desires: they will generally have less muscle mass, and so their BMR will be lower, giving them a slower metabolism. People who weigh more will have more cells of all types, including muscle cells, making their body’s daily energy demands higher and giving them a higher metabolic rate, or fast metabolism.

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But weight on its own isn’t a reliable indicator for a metabolic rate, because two people who both weigh the same can have vastly different muscle mass. Physical appearance is no better metric – there are some visibly skinny people who will have more muscle mass and very little body fat, like Olympic medallists and professional dancers, and thus have a higher BMR overall.

So why is our friend so slim? It could be inherited – genetics has a small impact on BMR – or they could be leading an active lifestyle. An unbalanced diet could be contributing to their weight loss, or they may be eating less when they’re away from your company.

Whatever the reason, you’re now armed with a retort when they say, “I’m so lucky to have a fast metabolism!”

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