If you've ever struggled to lose weight, chances are you've blamed your metabolism – that mysterious phenomenon we’re told keeps some people effortlessly slim while making weight loss an unwinnable battle for others.
But that’s not really how the body works – it’s a warped version of the story that leaves out the parts that matter most for fat loss.
For starters, metabolism is more than just ‘how many calories you burn’. It’s the vast network of chemical reactions inside your cells and tissues that power everything you do.
Still, most of us shrink it down to a single number: the calories burned at any given time.
That’s where ‘metabolic rate’ comes in – the go-to measure of how ‘fast’ your metabolism is running. In simple terms, it’s the energy you use just lying still, the bare minimum your body needs to ‘keep the lights on’.
Many people think it’s slim individuals who have a ‘fast’ metabolism, which allows them to burn more calories without effort. In reality, it’s actually larger bodies that have the ‘faster’ metabolism.
Metabolic rate is mostly driven by body size – the more there is to maintain, the more energy the body uses.
That said, body weight alone is a very crude measure. Two people might weigh the same, but how that weight is made up – fat vs muscle – can make a big difference to their metabolic rate.
This is because it’s the fat-free (or lean) mass, that demands most of our energy.
And within that lean mass, your organs are the true powerhouses. The liver and brain account for around half of your body’s resting energy needs, and even the kidneys need close to 20 per cent.
Skeletal muscle is less metabolically active than your organs (about 20 times less, gram for gram), but because there’s so much of it, muscle still makes up a big slice of energy use at rest.
Because organ mass doesn’t vary much between people of the same size, it’s muscle and fat that explain most of the differences in metabolism.
This also helps account for differences between men and women: men usually have more muscle and less fat, giving them a higher metabolic rate at the same weight.
But once body composition and gender are accounted for, metabolic rates are surprisingly predictable – challenging the idea that some people’s metabolisms run dramatically ‘faster’ than others.
The slowdown myth
A common belief is that your metabolic rate slows with age. But in relative terms, this isn’t really true, especially across middle age.
When you account for body composition, metabolic rate stays steady right up until around 65. Any decline before then is down to changes in muscle and fat, not some mysterious ‘ageing metabolism’.
That said, some life changes can affect metabolism as we age, most notably menopause.
The hormonal shifts that come with it often promote fat gain, particularly around the abdomen, and may also lead to some loss of muscle. Both can reduce metabolic rate.
There’s also evidence that falling oestrogen levels may slow metabolism by altering heat-producing processes known as thermogenesis, which may explain hot flushes.
Beyond 65, a slowdown in energy use becomes more likely. This isn’t so much the metabolism ‘ageing’, but rather the knock-on effect of wider changes in the body.
Muscle mass tends to decline more rapidly, and both muscle and organs become less metabolically active, reducing overall calorie burn.
The good news is that staying active, eating well and working to preserve muscle mass can help manage the decline.
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Built-in balancing act
If metabolism is mostly predictable, does that mean it’s completely fixed? Not quite. Your body can nudge calorie burning up or down a bit, depending on circumstances.
Take someone who’s overweight. They naturally burn more calories because there’s more tissue to maintain, but studies also suggest their bodies may also turn the dial up a little further, almost as if trying to shed the surplus.
At the other end of the scale, underweight people often burn marginally fewer calories than expected, because they’ve dialled their metabolic rate down further, becoming super frugal with energy.
How is this possible? Some research suggests the body has ways to deliberately waste energy by releasing it as heat, a process known as adaptive (or facultative) thermogenesis.
This involves specialised fat cells called brown fat, and the activity of certain proteins in muscle and other tissues that essentially let more (or less) calories ‘leak’ away as heat, instead of being stored.
You don’t notice this heat leaking away as sweat or a fever – it’s a quiet, behind-the-scenes adjustment that fine-tunes energy balance.
Adaptive thermogenesis doesn’t mean you’re trapped at a fixed weight. But it does explain why dieting feels like swimming against the tide: cut calories and your body tends to push back by slowing its metabolism, making progress harder to sustain.
So the real question is: can you do anything to change your metabolism – and if so, what really works?
No magic menu
Whether it be spicy foods, chilli extracts, caffeine or polyphenols in green tea, you’ve probably heard that certain foods can ‘speed up’ your metabolism, increasing calorie burning through thermogenesis.
However, measured calorie increases from these ingredients are so small they are only just detectable, plus they’re short-lived, lasting minutes or hours at best.
Then there’s protein, which you might have heard being championed, such that eating more of it can help boost your metabolism.
The thinking here is that the calorie cost of digesting and absorbing protein is higher than that of carbs and fat, suggesting you may not gain as many calories. But, again, while this is true, any discrepancy in overall calorie burning is pretty negligible.
Some point out that eating protein helps preserve and build muscle, which – as we’ve seen – supports a higher metabolic rate. But muscle growth doesn’t come from protein alone.
It needs the stimulus of exercise, particularly resistance training, to trigger muscle repair and growth.
Indeed, as well as maintaining muscle mass, exercise or physical activity is a more guaranteed way of elevating your metabolism, in an obvious way, by increasing your body’s use of energy (calories, essentially).
But exercise contributes more than simply the calories burned during the movement itself. After exercise, your metabolism remains elevated as you recover, and your muscles adapt to the previous stress of exercise.
This is measured as your excess post-exercise oxygen consumption (EPOC), commonly referred to as the ‘afterburn’ effect of exercise.
This essentially increases fuel and calorie use, an effect that can persist for several hours after exercise (even as long as 48 hours) – particularly when that exercise requires you to repair and build muscle.
It’s not the shortcut many hope for, but when it comes to metabolism and fat loss, exercise – especially the kind that challenges and rebuilds muscle – is by far the most effective tool.
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