Article: Brenda Entwisle from FAIRLADY

Obese people have faster metabolisms than slimmer people do, and burn more energy to keep their bodies ticking over.

Carrying extra weight has a similar effect on the body as weight training, building muscles and digestive tissue, which burn kilojoules.

Metabolism researcher Andrew Prentice says often it is not the metabolism of an obese person that's to blame for weight gain, but his or her memory.

Do you engage in unconscious eating, where picking at your kid's leftovers or the muffin grabbed on the run doesn't make it into your diet diary? In one study, obese people who claimed to be "diet-resistant" (unable to lose weight despite cutting kilojoules) underreported their eating by 50% and over-represented their energy expenditure by 70%.

So, although metabolism is purely physiological, our relationship with food and exercise is influenced by unconscious psychological elements. Once we come to a greater understanding of these, we can begin to address the elements of metabolism over which we have some control: our food intake, physical activity level and muscle mass.

Dieting and metabolism

"The more diets I've go on, the less weight I'm able to lose" is a common complaint. Can sustained yo-yo dieting slow down your metabolism irrevocably?

You'll be pleased to know that despite the abuse some of us heap on our metabolism over the years, it is a remarkably buoyant mechanism.

"Sustained very-low-kilojoule dieting with inadequate protein consumption can cause deterioration in thyroid function and a drop in resting metabolic rate by up to 17%," says registered dietician Anne Till.

"It is, however, very rare for permanent thyroid damage to occur, and most people's metabolism will recover to previous values within at most six months."

That gets rid of the disempowering notion that we can mess things up beyond recovery. Still, it is a far better tactic to bring our understanding of metabolism into a weight-loss programme than to fight against it.

The trick is to "embark on a diet that creates only a small energy deficit rather than a large one," advises Till. If you drastically reduce your kilojoule intake, your body adopts a starvation response and slows down your metabolism.

By reducing your kilojoule intake in smaller steps, you still encourage your body to dip into its fat stores, but without lowering your metabolic rate.

Be sure to include enough protein in your diet so your body isn't forced to deplete the protein stores in your muscles. And exercise to maintain the muscle mass that keeps your metabolic rate revved.

Keytel cites a study in which subjects on a diet-only programme lost more weight than subjects on a diet-and-exercise programme, but 2kg of this was muscle mass.

The latter lost slightly less weight overall, but increased their muscle mass by 1kg. Focusing on diet only will result in weight loss, but a balanced focus on diet and exercise will create a more sustainable weight loss.