If you enjoy swimming, and enjoy a post-swim nibble, the title of this article may cause you to worry. After all, if you want to lose weight, the Government’s advice is simple – “eat less and do more”. For most people, this makes absolute sense. After all, we have had the message drummed into us for the last 30 years. In addition, everyone knows that body weight is simply a matter of balance of calories in versus calories out. Eat too many calories, and you will put on weight. Do a few more lengths in the pool instead of having lunch, and you will lose weight. For some people, this can work – but most people find that it doesn’t. So what is going on?

Most dieting books will do the sums for you. On the assumption that you need to eat 3,500 calories to gain one pound of fat, it follows that to lose this pound requires you to under eat by the same amount. An average, fairly active male swimmer (if there is such a thing?), generally needs about 2,500 calories each day. Cut this back by a mere 500 calories each day, and by the end of the week you will have a deficit of 3,500 calories, and will have lost one pound of fat. Women need about 2000 calories a day, so an intake of 1500 calories will have the same effect. This may sound reasonable but, if it were true, you could effectively lose a pound of fat each week … and in time, disappear to weighing just a few pounds of skin, muscle and bone. In practise this does not happen.

Most people experience a fairly rapid, initial, weight loss (due largely to the loss of water and glycogen). In spite of counting calories and sticking to the diet of 500 calories less than you need each day, weight loss tends to slow down, and plateau. Some people may even start to put weight ON again, despite the required calorie deficit. This is because the body has a very efficient system which can regulate metabolism. This, over the centuries of evolution, has allowed us to survive plentiful eating after a good harvest, followed by a period of famine. These days, we go through the same cycles of eating and starvation – although the modern term is “dieting!” A reduced food intake can create all sorts of changes in the body, including psychological changes. This is when dieters become grumpy, disheartened, and lose the motivation to continue. For those who feel that eating less isn’t doing the trick, they try to tip the balance and start to do more.

Surely another swimming session each week, or a few more lengths in the pool will keep those pounds melting away. This is the classic “eat less, do more” approach to weight loss which the Government would like more of us to do. Now, I don’t know about you, but when I “do more” I tend to get hungry … and want to eat more. Even for those not trying to lose weight, it is nice to come out of the pool and reward yourself with a favourite snack. Perhaps you grab an energy drink from a vending machine at the pool, or leisure centre. The energy in these drinks comes from the sugar syrup – high fructose corn syrup (HFCS). This is a very cheap ingredient and a very un-natural one. Fructose sugar is found in small quantities in fruit and also honey. At this concentration, and in this form, it is fine. However, manufactured HFCS is a highly processed and concentrated source of fructose – which the body is un-used to handling. If you gulp down one of these energy drinks, a surge of fructose sugar is blasted into your bloodstream. The effect on your metabolism is rather like a right hook from Mike Tyson. In an effort to deal with the blow of sugar, the fructose is taken to the liver, where it is re-packaged – as fat. Ironically, it is the composition of these energy drinks and many other snacks containing HFCS (many of them packaged as "low fat" and "diet") that can make you fat.

Find out more about calories, weight loss and the psychology of dieting in my latest book Forget the Fear of Food.