You’ve probably heard the “calories in, calories out” principle – also known as the energy balance equation – by now. If you want to lose weight, so the theory goes, you need to be in a calorie deficit, either by decreasing your calorie intake (i.e. by eating less) or increasing your calorie expenditure (get moving).
But this has been largely debunked as an oversimplification, as it doesn’t take into account individual factors like hormones, insulin resistance, gut health, and the all-powerful metabolism. Often envisioned as a kind of gas pedal that controls how fast you burn calories, your metabolism is actually far more complex, both highly individualised and resistant to quick hacking.
What does it mean to speed up your metabolism?
Metabolism is the process by which your body converts fuel (i.e. food and drink) into energy. Your basal metabolic rate measures how many calories your body needs to perform its most basic tasks – to keep you alive, essentially. This rate is different for everyone: some people have naturally fast metabolic rates, meaning they burn calories more efficiently, so their body requires more calories to maintain its current weight. Others have a naturally slower metabolism, meaning their body burns calories at a more leisurely rate, and they can wind up in a state of calorie surplus. Because the body tends to store those excess calories as fat, those with slow metabolisms can, in many cases, end up gaining weight more easily.
Lots of things can influence your metabolic rate – including age, gender, physical activity levels (exercising more can speed it up), and muscle-to-body-fat ratio (having more muscle speeds it up). But what about changing your diet to increase calories burned? Could adjusting what foods you eat speed up your metabolism, thereby increasing the number of calories your body burns, even when you’re not doing anything?
The short answer is yes. The long answer paints a slightly more complicated picture.
Which foods can speed up my metabolism, and how does this work?
According to professor Gary Frost, head of the Section for Nutrition Research at Imperial College London, eating high-protein foods (like meat, fish, legumes, eggs and dairy) can increase your metabolic rate. This is because your body processes protein quite inefficiently compared to other macronutrients, so it uses up more calories to do this processing.
But! There’s a catch. Or actually, there are a few. Catch number one is that only around 10% of your body’s calorie expenditure comes from digesting food, and so changing your diet to incorporate more protein can likely only increase the number of calories your body will burn in a day by two to three per cent. If you’re an average-sized man, this equates to about 75 calories, or a slice of bread.
That’s not nothing, and over time, in combination with increased exercise and reduced calorie consumption, it could help make a difference, says Frost. But, another catch: “The long-term studies really haven’t shown any impact,” he adds. “There is a small rise in energy expenditure [from increasing protein intake]. But it’s tiny, and often what happens is that you get an increase in intake to compensate for the increase in energy expenditure, so things remain the same. Shifting that is very, very, very difficult to do.”
The other thing protein does do, says Frost, is decrease appetite, and there’s a little more evidence that this can make a difference to attempted weight-loss. But that still leaves us with the problem that, as Frost says, “in many Western societies, we already consume more protein than we need, and we’ve still got a problem with increasing bodyweight gain.”