Metabolism is one of those words that we hear time and again in relation to exercise and dieting, but do you really know what it means? It comes from the Greek ‘metabole’, meaning ‘change’. It refers to the thousands of chemical reactions taking place every second in your body to keep you alive, and all this activity is powered by either nutrients from food you’ve eaten recently, or from your fat reserves.
Like a car needs petrol to drive, your body needs fuel to function. You use the energy you get from this fuel-burning process to simply keep warm, keep your heart beating, brain working, cells replacing, blood pumping and digestion working. The energy expended by these very basic processes of life is known as your basal metabolic rate, and it accounts for 60-70% of all the calories you use each day (your total energy expenditure).
Contrary to popular myth, the higher your weight, the higher your basal metabolic rate (BMR) will be, because a larger body burns more energy to simply keep all these basic functions ticking over. You can think of it in terms of cars again – a Rolls Royce needs a bigger engine and a larger fuel tank than a Mini, because it’s heavier to move.
It’s not just your weight that affects your metabolic rate, either – how your body is made up (body composition) is also important. Pound for pound, muscle uses around seven times more energy than fat, so the greater the quantity of muscle, the faster your metabolic rate will be. This explains why, on average, men have a higher BMR than women – it’s simply because a greater proportion of their body mass is muscle.
Exercise and metabolism
The remainder of your daily energy expenditure, around 30-40%, is accounted for by physical activity – this means anything more energetic than complete rest, and that includes everything from what we’d typically class as ‘exercise’, such as jogging, cycling, playing football or swimming, to incidental activity like standing up, typing, gardening and even fidgeting. These ‘non-sport’ activities are sometimes called NEAT – non-exercise activity thermogenesis – and it’s surprising how much effect they can cumulatively have on your overall daily energy expenditure.
No amount of weight loss will ‘damage’ your metabolism. What happens when you first start a diet, in the first one to two weeks, is that your BMR drops by around 15% as your body uses your fat reserves as fuel. Then, when you stop dieting, it goes back up by 15% – but remember that will be 15% of your new, lower weight, which will be less than 15% of your larger original weight. As a rule of thumb, for every pound you lose, your BMR drops
by 7 kcal.
Imagine a 35-year-old woman of 5ft 5in, who weighs 14st. Her BMR is roughly 1,585 kcal per day. If she loses 3st her BMR will reduce to 1,427 kcal. This doesn’t mean that her metabolic rate has been damaged – even though her metabolic rate has gone down, it will still be the same as another woman of the same size and lifestyle.
This brings us back to our car analogy. Driving around in a Rolls Royce body uses lots of fuel (calories), but when you lose weight you’re essentially turning your Rolls-Royce sized body into a Mini, and obviously your fuel consumption goes down. This doesn’t mean your metabolism has stopped working properly – you just need less fuel to transport and power the new, lighter you.