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Blog: What's in a number?
How our lives are dominated by numbers yet we somehow get it wrong

7. After the holidays, have you put yourself on a high fad diet?

No, the title is not a misprint. For "fad" read "fat", "carbohydrate" or "protein" all of which have become the latest get slim quick method at some time or another. In fact this article was inspired, if that is the right word, by a recent article in The Times where you can replace one of the d's in my title with a t! The Times should know better than to put forward this unscientific claptrap, but in all fairness to that quality newspaper, how can all the newspapers produce so much apparently original material for a relatively modest price? The answer is that the article comes from some general source, accessed probably by a lot of other newspapers as well, and not written by a specialist Times journalist. So standards drop a bit.

Just to illustrate that we can have our cake and eat it (sic!), in the next few pages of the Weekend section, the Times told us presumably with a straight face how we can burn off all our excess fat by a mere 12 min per day of calisthenics. I do not deny the advantage of calisthenics, but one thing it doesn't do is burn much fat. Have you noticed, though, how to burn fat properly, the ladies have to be dressed in skin tight outfits emphasising the shapes of their breasts, legs and buttocks? For it to work for the gentlemen, they have to be dressed in a skin tight T-shirt, revealing their abs but they have to wear baggy trousers.

Of course many people are in denial mode and love this sort of stuff and the majority when they try it out, whatever the new diet is, will find that it doesn't work. Doesn't any of these people think of the numbers? Essentially if you eat more food energy than you use up in exercise and normal activity, then the difference is deposited as fat. Poisoning your system by having an excess of one form of energy over another achieves very little except overworking your liver.

We should be measuring our food energy in kilojoules (kJ) but the Calorie persists. This is actually a kilocalorie and is an old fashioned heat unit, invented before different types of energy were properly understood. One kilocalorie is the heat required to raise the temperature of one kilogramme of water by 1 C. Unfortunately that isn't quite the whole story as the calorie is an ill-defined unit, as explained in my book "Measuring the World". For some reason or other, people in the USA like to use medieval units in general, such as feet and inches, but they switch to an old-fashioned metric unit for food intake. This makes it very difficult for example to work out the effect of exercise on body weight.

In Britain, food packages are usually marked in kilojoules, but because they are also marked in calories, kilojoules are almost entirely ignored. what makes it worse is that the calories are often given first. So what follows will be a little confusing to most people, as I'll be using proper energy units, which as noted have the benefit of converting mechanical energy, such as exercise, directly to food equivalents.

The average intake of a sedentary person should be about 8,000 kJ per day for a female of about 1.7 m tall and 10,000 kJ per day for an adult male of about 1.8 m. The actual values vary quite a bit depending on physical activity. Now suppose the adult walks about 2 km per day, then the additional energy needed is 500 kJ approximately, depending on the weight of the person. The exercise indicated would take, say, 25 min., maybe less. Therefore you would expect the calisthenics supposedly there to burn fat would in 12 min. consume probably only about 250 kJ.

In fact the real way of burning fat is to exercise steadily (at below 80% of maximum heart rate) for as long as possible. This obviously depends on fitness. Suppose you are fit enough to cover 8 km per day, which might take an hour and a half or less, then you would be consuming an extra 2000 kJ per day. The fat that this burns off can be readily calculated, as the energy of fat is 38 kJ/g. so you would lose 2000/38 = 50 g (approx.) of body fat per day. Assuming that the fat is also bound up with water, you would likely lose about 100 g per day by eating normally (i.e. the sedentary person amount) but exercising for an hour and a half per day.

This sounds terribly slow, but after 10 days you lose a kilogramme and after a month 3 kg. That would be starting to sound significant and don't forget it is permanent and you would not be subject to the yo-yo dieting effect that is so profitable for the diet industry and is so expensive for you if you pick up on the latest fad. Calisthenics of 12 min. per day sounds much easier of course. It will tone up your muscles and make you look a bit thinner, if you're not in very good shape to start with, but it won't burn much fat as these energy estimates show.

Incidentally, if you can lose more than 100 g per day then you are either severely starving yourself, or losing water or muscle tone. None of these options are good as they are all at best temporary. Most people won't have the patience for this regime, which is why they will be in exactly the same position next year as they are in now, whereas with a balanced approach you could be tens of kilos lighter, or at your ideal weight, by the same time.

Before you reject these ideas out of hand just ask yourself how long it took you to put the excess weight on in the first place. To know the answer to this you would have had to have weighed yourself accurately. Most domestic weighing machines are not reliable enough to measure relatively small changes in short periods of time. The remarkable thing about our bodies, though, is that even though our daily food intake might vary considerably (by tens of percent) over several decades, without special effort we usually maintain average food intake to within 1% or better. For example, a 1% excess of food would be about 80 kJ per day which would increase body weight (if left unchecked) by about 4 g per day (including 2 g of water). So after 10 years of this, body weight would increase by 10 x 365 x 4 = 15 kg approx. I would like to know why this figure is so small!

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