What is metabolism?
"Bodily functions such as digestion, the beating of your heart, the working of your lungs and all the activities you do in the course of a day all need energy. Metabolism is the process by which nutrients in the food we eat are broken down in our cells to produce energy for these functions," explains Dr Toni Steer, nutritionist at Medical Research Council Human Nutrition Research, Cambridge.
"But when people talk about metabolism what they usually mean is their basal metabolic rate or BMR, which is the number of calories (or units of energy) that your body burns over a set period of time."
What determines your metabolism?
Metabolism is controlled by the thyroid gland in the neck, which in turn is governed by a part of the brain called the hypothalamus. The speed of your metabolism depends on complex chemical messages, which are sent to your brain by your body, telling it how much it needs to keep going. "Your body size and composition determine how much food you need to produce energy. The heavier you are the more energy your body needs to keep going, so the higher your metabolic rate," says Dr Steer.
This means that, contrary to popular opinion, if you're overweight you'll have a faster metabolism than someone who is slimmer - a big body like a big car needs more fuel.
Find out how to boost your metabolism with our guide
Do men have a higher metabolism than women?
The make-up of body tissue is one of the key things that affects metabolism. "Lean body tissue or muscle is more 'metabolically active' than fat, which means it burns more energy, even when you are at rest," explains Dr Steer. This is why men, who naturally have a higher ratio of muscle to fat, tend to burn up what they eat faster, although a fat man may have a slower metabolism than a slim women with more muscle tissue.
Could an under-active thyroid slow your metabolism?
If the thyroid isn't producing enough hormones it can slow down metabolic rate, although only by a small amount (10 per cent or less)," says Dr Steer. But unless your thyroid is extremely underactive - in which case there would be other clues such as extreme fatigue, depression, dry skin and hair and a loss of sex drive - it's unlikely to be to blame for weight gain. And once you get any underactivity treated it will return to normal.
Does age affect metabolism?
Metabolism slows down slightly with age, as a result of us having less muscle tissue as we get older, but only marginally. So a 70 kg man who stayed the same weight throughout life would need four to five per cent less energy at 50 to maintain his weight and 10 per cent less by the age of 70, which is why you need to eat slightly less or exercise slightly more as you get older," says Dr Steer.
Does exercise speed up metabolism?
"Increased activity levels can increase metabolic rate because you breathe harder and your heart rate increases, which causes a demand for more energy," says Dr Steer. "Any exercise, however, has to be really vigorous to make a significant difference and your metabolic rate soon reverts to normal as your heart rate recovers," she adds.
The good news is that regular exercise - especially weight-bearing activities such as working out with free weights, weights machines at the gym or classes such as Body Pump - builds muscle and the greater your muscle mass, the higher your BMR.
Find out more about strengthening your muscles at home
Can supplements affect your metabolism?
Supplements containing adrenalin which is used to treat allergies, available in the US (where it's called ephedrine) and other some countries, can speed up metabolism and decrease appetite. A side effect of speeding up metabolism, however, is an increased heart rate and blood pressure, which could be dangerous.
Caffeine and red chillies can raise BMR temporarily, but the effects are small and, like adrenalin, caffeine also raises heart rate and blood pressure - not advisable on a regular basis. If you want to raise your BMR you are better off keeping your cash and eating slightly less or going for a brisk walk instead.
Calorie count and metabolism
Three things determine how many calories your body needs every day.
1. Your Resting Metabolic Rate (RMR), accounting for just over 70 per cent of the calories you burn, is the number of calories you burn just to stay alive - so that you keep breathing, your heart beats and your blood pumps around your body.
2. Around 12 per cent of your daily calories are used for what's known as thermogenesis - needed to keep you warm, digest food and fuel your response to stress.
3. The rest are burned during activity - whether it's just doing your daily tasks or more strenuous exercise.