Over the past three years (apart from nine months during pregnancy) my weight has been at its lowest since teenagerdom. The weird thing is that, while I was trying to diet and exercise I languished at a weight two stone heavier than at present.

Since I stopped dieting the weight has dropped off. Now I only eat food I enjoy, including butter, red meat, chocolate, potatoes etc, I am satisfied quicker, so eat less. And I have not stopped exercising altogether, I do one session a week called Body Pump in which I build up my muscle mass.

So how is it that I am slimmer than I have ever been in my adult life? One reason is that my basal metabolic rate or BMR has increased. Dr Nicola Lowe, senior lecturer in nutrition at the University of Central Lancashire, explains: "The amount of calories you burn depends on your muscle mass. Fat isn't metabolically active, it just sits there. Muscle mass uses energy to keep it going. So your BMR will be higher if you have a more muscle or lean tissue in your body. Increasing lean tissue mass is important for increasing BMR which will help us burn more calories when we're at rest."

At Body Pump I use large weights, which build up my muscle mass enough to burn more calories while I'm resting.

But you do not have to pump iron. BMR can be raised simply by being fidgety. "Involuntary exercise, like fidgeting and leg jiggling burn energy," says Dr Lowe. "There are other factors that can affect your BMR too. Research shows that caffeine taken before exercise can enhance your ability to burn calories when you're exercising."

The second way to burn calories is in dietary-induced thermogenesis or DIT and it is the energy expended as a result of eating food. "It's the energy we need to digest, absorb, metabolise and store food and it represents 10 per cent of our total energy expenditure," says Dr Lowe.

Catherine Spencer-Smith, a sports physician at Pure Sports Medicine, adds that the less we eat, the less efficient our metabolism becomes: "Back in the '80s people used to go on these really extreme diets and worked out frantically. But it has since been proven that if you eat less than 1,400 calories a day, then you are at risk of winding your metabolism in the wrong direction. It is better to eat well. The things that help make you feel full for longer are slow burning carbohydrates which use up energy to break down, and protein."

As I enjoy good, wholesome food mostly prepared at home, this helps keep the engine revving too (which is why I'm probably able to enjoy a chocolate or two for pudding without suffering the consequences too badly).

The third way to burn calories is of course the dreaded exercise which accounts for 30 to 80 per cent of our energy expenditure. But you do not need to burn as many calories as you think during your workout, as you will continue burning once you stop. "When you're running you will be burning more energy because of that activity but your BMR will stay high as a result of the exercise for a short amount of time after," says Dr Lowe.

So my class will help to burn off extra calories eaten that day. "If you exercise early in the morning there's a nice after-effect," says Dr Spencer-Smith. "It winds the machine up and keeps it burning through the day by raising the BMR so you're actually burning more calories." But she warns, that shouldn't be an excuse to reward yourself with extra food. Just one eclair might be enough to ruin all the good work.