Your body produces a hormone called insulin in order to keep your blood sugar steady. Insulin decreases the level of glucose in the body. It acts by increasing the rate at which glucose is transported out of the blood and into cells and by stimulating muscle cells to take up sugar from the blood and convert it into glycogen. This glycogen is stores in your muscle tissues and liver for use later. However, your body can only store so much glycogen. If too much sugar comes into your bloodstream at once, it can't all be stored and your body will create a spurt of insulin to help reduce the excess. This leaves too much insulin in your bloodstream, which prevents your body from burning fat and ends up being stored as fat.

Protein doesn't convert to blood sugar at all. So if you eat protein, your body makes another hormone called glucagon, which helps release the stored glycogen back into the bloodstream.

Fat doesn't convert to blood sugar either, but it helps slow down the speed at which carbohydrates turn into blood sugar. It also triggers another hormone which tells your brain when you've had enough to eat.

It has been discovered that of our energy requirement, we obtain about 50% from carbohydrates, 35% from fats and 15% from proteins. Thus, we daily need 400-500gms of carbohydrates, 60-70gms of fats and 65-75gms of proteins. But the Zone diet tries to balance proteins, carbohydrates and fats for the best effect on your blood sugar. The usual ratio is 30% protein, 40% carbohydrate and 30% fat. However, as each individual responds slightly differently to food, you may have to increase the ratio of either carbohydrate or protein to suit your particular body.

The Zone diet uses protein as its core, because it's relatively easy to work out just how much protein you should eat. The lean muscle in your body needs a certain amount of protein to function efficiently, and that amount is determined by how much exercise you do. Competitive sportsmen, for example, need more protein than couch potatoes.

Once you know how much protein you can eat, it's quite easy to add carbohydrates and fat to suit. To make it easy, the Zone diet works in blocks. One block of protein is 7 grams, which you find in about 1 ounce of skinless chicken breast. One block of carbohydrate is 9 grams, which you find in half a slice of bread or 1 cup of broccoli. One block of fat is 1/3 of teaspoon of butter, or one macadamia nut, for example.

Your body can only absorb a certain amount of protein in one meal, so each meal is restricted to a maximum number of five blocks. Using the example foods, a typical three-block meal (the most common size) would be 3 ounces of chicken, 3 cups of broccoli and a teaspoon of butter. In the real world, you may want to spread the broccoli allowance out through a variety of vegetables and grains.

The Zone diet is not really a high protein diet, because it relies on a scientific calculation to work out exactly how many grams of protein your particular body needs. Nor is it a low carbohydrate diet, because to stay in balance, you need to eat more carbohydrate than protein. It is certainly a low calorie diet, with very strict portion control.