The simple science that explains why diets do not work

The simple science that explains why diets do not work

‘Lose Weight Fast’. ‘Lose a stone in two weeks’. ‘Get thin and eat what you want’. The tempting offers of the miracle diets can blind people to the scientific realities behind them.

Obesity, weight loss and dieting are always subjects of interest in the UK. The statistics tell us that as a nation we are getting fatter, and the health professionals tell us that we need to do something about it. As a result there is a thriving industry selling diet books, diet plans and nutritional supplements. It seems that a new miracle diet appears almost every month. Sometimes we are told to reduce carbohydrates, sometimes it is suggested that we eat like a caveman, and now it is even suggested that we hardly eat at all for two days a week.

We all know people who are ‘always on a diet’, but seem oblivious to the fact that they keep regaining the weight. No other product that fails so often seems to inspire such loyalty in its customers. This is excellent news for the diet industry which has plenty of repeat customers, but not good news for the dieter. Repeated weight loss and gain (known as ‘yo-yo dieting’) has not been absolutely proved to damage health, but it is certainly very demoralising.

The science behind weight loss and gain is complex, but not as complex as the diet ‘gurus’ might have us believe. It is not disputed that the human body needs food as fuel for the functions of life. This fuel comes as oxygen from breathing, water, and energy and nutrients from food. Like all forms of energy, food energy is measured in joules or in the older unit of calories. A calorie is the amount of energy needed to raise the temperature of one gram of water by one degree centigrade. This is a very small amount, so the calorie measurement used in nutrition is actually equal to one thousand of these units and is properly known as a kilocalorie.

The average woman needs about 2000 calories a day to stay alive, while for a man the figure is 2200. Related to this is the concept of ‘base metabolic rate’ or BMR. This describes the amount of energy (i.e. calories) that the body requires to carry out the basic life functions of breathing, digestion, temperature maintenance, repair and movement. This rate varies between people and is governed by factors that include physical size, fitness, genetics and medical history. High BMR accounts in part for the people that we all know that can eat huge amounts and never put on weight. However most people who appear to eat lots and still stay slim are very active, or are eating large amounts of the right foods and thus not taking in as many calories as it might appear. Eating large amounts of cake is never going to keep anybody slender!

Some people use their metabolism as an excuse for their weight, with the idea that they have a slow metabolism and so they put on weight very easily. The diet and supplements industry has capitalised on this by advertising products that are supposed to speed up metabolism. The theory is that this will mean that people will burn more calories without effort, and hence promote weight loss.

The bad news is that speeding up metabolic rate is very difficult. Scientific studies of the ‘magic pills’ that purport to change metabolic rate show that any effects are negligible. Measurable changes in metabolic rate generally only occur with medical disorders such as thyroid problems, or with extreme levels of fitness.

The even worse news is that crash diets can slow metabolism. This is because these diets are so low in calories that the body starts to burn the energy stored in muscles. It is the muscles that use most energy, and a body with fewer muscles will require fewer calories. Thus these extreme diets can result in a body that has a lowered BMR, and so when the dieter returns to a more normal pattern of eating they find that they are gaining weight again. As extreme diets are not sustainable for anyone mentally and physically normal, the dieter will definitely start eating more at some stage.

In almost all cases, weight gain is due to eating too many calories. This either means that people are eating more than they realise, or are eating foods which are higher in calories than they realise. Low-fat foods and ‘health’ foods are a common trap for those who want to watch their weight. Low-fat foods usually have a lot of sugar added to make them taste good, and items such as cereal bars are also extremely high in sugar. It is far better to eat the ‘real’ version of the low-fat food but in smaller quantities.

The brutal truth is that if diets worked or there was a miracle fat loss pill, no-one would be fat. The only way to control weight in a sustainable fashion is to balance calories taken in with effort expended. For most of us, it is time to put down the fork and get off the sofa!



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