Exercise-fat link still unclear

Louise asks: Why do some people with big appetites never get fat despite their lack of exercise?

Wynnie says: While most westernised countries are facing an epidemic of obesity, researchers are still trying to understand why some people can eat whatever they like and never put on any weight, while others struggle to shed a single kilo no matter how little they eat.

There's a common misconception that thin people have a high metabolic rate. Individuals with a high metabolism aren't always thin.

Experiments show that, even when metabolism is high, the maximum number of calories the body can burn is about 3,000 each day. It has nothing to do with the number of fat cells, either. Although fat cells multiply in childhood, they stop increasing in number at adulthood; from then on they increase only in size.

Scientists have proposed some theories to solve the mystery.

Set point theory: Every person has a control system dictating how much fat he or she should carry: think of it like a thermostat for body fat. Some people have a high setting, while others have a low one. This theory suggests body fat percentage and body weight are set internally at different points in different people.

When the set point is reduced below the predetermined level, by dieting or during starvation, the body starts a process to force itself to recover its lost weight. No one really knows how to change the set point, although regular exercise seems to be able lower the setting.

Genetics: A gene called FTO is present in humans and animals and may be able explain the differences.

A study of mice found that those that did not have the FTO 'obesity' gene stayed thin even though they ate large amounts of food and never exercised. Mice without the FTO gene were found to burn more calories and at a faster rate than those with the gene. Scientists think this gene can influence appetite, making some people not know when they are full. Those without the gene may find it easier to say no to food.

Metabolism: An enzyme called MGAT2 is present in the gut of humans and animals. This 'fat enzyme' determines if food is broken down to provide energy or stored as fat around the waistline. Scientists at the University of California found that laboratory mice missing the MGAT2 gene could eat a high-fat diet without putting on weight or getting fat. They were also protected against high blood cholesterol, a build-up of fat in the liver cells and glucose intolerance - which can lead to diabetes.

Muscle, not fat: Experiments on volunteers asked to eat at will show that, while excess calories can cause many people to put on body fat, some form muscle instead. Having more muscle boosts the rate at which a person burns calories. Putting on muscle can bolster the metabolic rate by 30 per cent.

To maintain a healthy body weight, it's best to reduce the number of calories you take in from fat, avoid overeating and exercise whenever possible.

Exercise helps to burn calories, can lower 'bad' blood cholesterol levels, increase muscle mass (muscle cells use more calories as fuel than fat cells do) and stimulate the production of 'happy' hormones.

Wynnie Chan is a British-trained nutritionist. If you've got a question for her or would like to be featured in this column, e-mail nutrition@scmp.com