Q & A: What is the basic mechanism that controls my body weight?

What is the basic mechanism that controls my body weight?

Your weight - particularly the amount of fat your body stores - is a reflection of the balance between your energy intake and energy expenditure. Energy intake is everything you eat or drink which contains calories (kilocalories) - units of energy, and energy expenditure is the amount of calories that your body burns up for fuel in living and working.

Energy expenditure can be divided into three categories. The first is your basal metabolic rate (BMR), which is the amount of calories you burn up in a given time just existing - i.e., lying down doing absolutely nothing. This accounts for approximately 60% of energy used in an average person. The second category is the amount of energy you use in activity - moving, talking, and so on, which accounts for an average of 30% of calories used. Lastly there is dietary thermogenesis, described as meal-induced heat production, which is the calories used up in eating, digesting, absorbing and using food. This accounts, surprisingly enough, for about 10% of our use of calories.

All these factors are variable, as, of course, is the amount we eat (i.e., the amount of energy we provide the body with). If energy input and energy output remain in balance, our body weight remains stable. If energy input is higher then energy output, the surplus energy eaten as food and not required by the body is stored as body fat. If energy output is higher than energy input, the body fat stores are mobilised and converted back into energy and if this happens over a
period of time, we lose body fat and therefore weight.

The amount of your energy output over a period of time is known as your metabolic rate. The rate at which the body burns up calories both at rest, during physical activity and via dietary thermogenesis can all vary considerably with factors such as your sex, age and weight, your fitness, your body composition, the duration and intensity at which you carry out work, and the type of food you eat. There are also other factors at work - genetic, hormonal, and so on. The World Health Organisation estimates that basal metabolic rate (BMR) alone can vary by as much as 25% up or down between individuals of similar weight. Add on to that all the other variables and you can see why some people have to eat a little, and some people a great deal, to maintain their body weight in status quo.

Is it true that fat people have a slow metabolic rate?

Emphatically no. Quite the reverse. All other factors being equal, the heavier you are, the higher your metabolic rate will be. This is because a heavy body uses more energy both at BMR levels (see previous question) and to move itself around (imagine the surplus fat as a block of concrete on your back and you will see why); because fat-free mass (metabolically active muscle and other tissue) also generally increases with weight, and because dietary thermogenesis increases.

This is why when fat people lose lots of weight their new metabolic rate will be lower than it was when they were fat.

Could my weight be due to faulty genes?

A wide-ranging report on obesity published in 2000 by the World Health Organisation concluded that 'while it is possible that single or multiple gene effects may cause overweight and obesity directly, and indeed do so in some individuals, this does not appear to be the case in the majority of people".

However, the consensus of expert opinion is that your genes can have at least some influence on your susceptibility to be overweight. An overview of all the studies on the genetic influence suggests that between 25 and 40% of cases of obesity have a hereditary factor and that intra-abdominal fat appears to have a genetic link of up to 60%. One study of twins concluded that 60% of body fat is determined by genetics.

If one of your parents is overweight you may have inherited that parent's 'fat genes'. If both your parents are overweight it may be that you are very unlucky and they both have faulty fat genes - but it is probably more likely that, as a family, you eat too many calories and don't take enough exercise to maintain energy balance. As one US researcher remarked, the human gene pool cannot possibly have altered so much in the last 30 years to account for the vast rise in the
incidence of obesity. And as another researcher remarks, "You don't get fat in a famine." Whatever the reasons, one estimate is that a child with two fat parents has a 70% chance of growing up obese.

Even if you have inherited a tendency to put on weight, it doesn't mean that you cannot lose weight or maintain a reasonable weight but that, for example, you may be more likely than someone without 'fat genes' to put on weight give the same amount of food.

A few specific examples of how your genes may influence your weight:

• By giving you a BMR lower than average.
  
  


• By giving you a high percentage of body fat and a low percentage of lean tissue (muscle), which would predispose you to needing less calories than average for your height and weight.
  
  


• By giving you a larger than average appetite.
  
  


• By giving you a blunted thermic response to food, meaning that your dietary-induced thermogenesis will be lower than average, meaning that more calories will be stored rather than burnt.

Research is being carried out in all these areas and in future it may be possible to reprogramme gene defects so that such people don't get fat, but the message meanwhile is that by maintaining a suitable energy balance (calories in versus calories out) the majority of people can maintain a reasonable body weight.

Is there a foolproof formula for speeding up my metabolic rate?

Yes - increase the amount of regular exercise that you do. Recent US research clearly demonstrated that high-intensity (i.e., hard!) exercise raised the heart rate, metabolic rate and energy expenditure for several hours. However, ideal exercise should include not just aerobic work such as brisk walking or cycling, but also strength training to increase the amount of lean tissue (muscle) in your body. For every extra pound of muscle that you manage to add to your body, you use an extra 50 or so calories a day. A recent study found that regular weight training - the best way to increase muscle bulk - boosts your basal metabolic rate by about 15%.

Does how or when I eat, rather than what I eat, affect my metabolism?

There is support for the theory that consumption of small, regular meals will keeps your metabolism ticking over well and is a much better way to burn off the calories than, for example, one meal a day. Levels of the thyroid hormones that control your metabolic rate begin to drop within hours of your last meal - probably in response to the prospect of starvation (which was a reality when humans evolved). Also the thermogenic effect of dividing your calorie intake up over several meals might be slightly higher than if you have them all at one sitting.

One recent study done for the International Obesity Task Force found that obese people had a better chance of losing weight by eating little and often than by eating larger, less frequent meals. Another study concluded that an infrequent meal pattern is associated with a tendency towards obesity. Despite the fact that yet another study found that there was no difference in the metabolic rates of people who ate in a variety of different patterns, for most people I believe it is worth trying the 'little and often' method as other benefits are to be gained - hunger is better controlled and there may be less inclination to binge if meals are regular.

Eating in the hour or two after vigorous exercise may encourage the metabolism of food rather than its laying down as fat as the body's metabolic rate is generally speeded up during this time and similarly, moderate exercise (e.g. a walk) after eating may increase the body's natural thermogenic response to food and burn off more calories.

It seems that there is good reason to eat slowly, though - one 2000 study inAmerica found that obese people eat faster than slim people and have a greater tendency to store fat around their stomachs.

There is no convincing evidence that eating food at any particular time of day (e.g. early in the morning) rather than at another time of day (eg in the evening) speeds up your metabolism and neither is there for the idea that you should eat nothing except fruit before noon every day which is a creed of Fit for Life and food combining.