The term 'metabolic rate' refers to the energy (calories) you expend over a day just keeping your body functioning - your heart beating and your lungs breathing, for example. This is often called the basal or resting metabolic rate.

Scientists have measured the exact amount of calories overweight and healthy weight people burn while sitting or lying quietly. This was done by measuring the amount of oxygen breathed in and the amount of carbon dioxide breathed out.

Results from these studies have consistently shown that overweight people use more energy to keep their bodies working. This is because they have larger bodies with bigger muscles and internal organs.

However, after taking into account differences in body size, lean and obese people have been shown to have similar metabolic rates.

Basal metabolic rate can be influenced by body composition. Muscle requires more energy to function than fat. As we get older, we tend to gain fat and lose muscle. This explains why basal metabolic rate tends to decrease with age.

Similarly, two people of the same age and weight may have different metabolic rates if one is fitter (and has more muscle) than the other.

Of course, we also use energy to move around during the day. Basal metabolic rate and the energy required for physical activity make up our total energy expenditure, or total energy needs. Your total energy needs depend very much on how active you are.
But I eat very little

Often, overweight people believe they eat very little and still put on weight, hence the 'slow metabolism' theory, but research has shown people tend to eat more than they think.

When asked to write down everything they've consumed in a day, people tend to report eating far less than they actually do. This may be to impress the researcher or because they genuinely forget to include some items.

On the day they recorded their intake, they may even have chosen lower calorie foods than they'd normally eat.

The bottom line is you'll gain weight if you consume more calories than your body needs. This can be a difficult fact to face, but recognising the need for change is vital for successful weight loss.

The good news is people not only manage to lose weight but are able to keep control of it in the long term.
Can I speed up my metabolism?

There are many pills, supplements and foods that claim to speed up the metabolism and burn fat. Most of these claims are unproven.

Some chemicals, such as nicotine and caffeine, do have a small effect. However, any increase in the metabolism may be accompanied by an increase in the heart rate and other side-effects, and so aren't recommended as a treatment for obesity.

The fact is no single ingredient will melt body fat away without some effort on your part to reduce the amount of calories you eat.
How can I burn calories?

The best way to burn calories is to increase your levels of physical activity. The amount you'll burn depends on your body size, age and gender, but as a rough guide:

60 minutes of gardening burns 300 kcals
60 minutes of walking burns 200 kcals
90 minutes of football burns 600 kcals
45 minutes of dancing burns 450 kcals
In the long run, improving your body composition (increasing muscle and decreasing fat) by being more physically active will lead to small but important increases in your basal metabolic rate.