Prepare yourself for the post yuletide barrage of diets and detox propaganda that we will be subjected to in the coming days. Also prepare yourself for the fact that, as in previous years, they won't work.To understand why they don't work you have to get down to real issues involving evolution, nutrition, physiology, thermodynamics and social pressure. No wonder we keep looking for the quick fix. In this article I have simplified the reason why we put on weight and how best to deal with it.Man has been around for about 50,000 years. About 10,000 years ago agriculture began; until then man was a hunter-gatherer. The diet was a mixture of feast and famine, when there was game and berries available, our predecessors would gorge themselves and when they were hungry they would go out and look for more food.The body evolved a very efficient storage system that enabled man to survive long periods of starvation.Even when the agricultural age started, things did not improve that much. At harvest time there was an abundance of food and although the principle of storage existed there were still long periods of hunger. So for the majority of time man has existed, he has evolved to deal with deprivation. In my lifetime this has changed in western societies to a time of abundance. So the first problem is we are walking around in a land of milk and honey in a body designed to cope with famine. The food that we eat is broken down into its component parts via enzyme activity in the digestive process. Everything we eat converts to simple sugars, amino acids or fatty acids, which pass into the blood stream. Here the simple sugars and fatty acids can be burned off as energy through muscle activity while any surplus will be converted to body fat. Balancing the calories in to the calories out will mean no laying down of new body fat. If you want to burn off stored fat, however, you have to create a calorie deficit.Body fat is stored as globules in adiposities. These are holding cells where the fat is waiting to be called on to revert to fatty acids to supply energy to working muscles. However if in the mean time you have another meal then the body fat just piles up. It gets worse, new research indicates that the fat cells are able to grow new fat cells known as dysfunctional fat cells.In terms of scale, what are we talking about? It is not unreasonable to increase body fat by 500gms from one meal taking about an hour. This is enough fat to supply 4500cals. To shift it could take a week of increased activity for two reasons. Reversing the process of converting stored fat is far more complex and takes more time than laying it down. Also fat is used in conjunction with carbohydrates for energy, so the total energy needed to shift the fat would be closer to 10,000cals when accounting for the carbs.Applying the principle of prevention being better than cure the best thing to do is to watch what we eat. The problem here is if we go onto a regime that is too onerous then we will not stick to it. There is a social side to eating and drinking; we don't just do it to survive. Also we like eating fatty foods, they taste the best so we have to make a compromise. The first step is to try to stop or reduce the laying down of new fat. If you have a meal break of an hour, use half of it to eat and the other half to walk. This will burn the fatty acids on their way to the store. Don't eat large meals late at night but instead make your main meal your breakfast, so you are applying the principle of fuelling prior to activity.These simple changes will reduce the storage of new fat but, to attack existing fat stores, you need to cut down on the calories you take in and increase the calories you burn through exercise.