The Zone Diet works on the principle that 100, 000 years ago, we were meat eaters, and our metabolism is designed to handle the demands of a meat-based diet.

Madonna, Demi Moore, and Jennifer Aniston swear by the results of the Zone Diet, which was created by Barry Sears, PhD. The Zone Diet contains 40% carbohydrate, 30% protein, and 30% fat and is known as the 40-30-30 plan.

As the food industry evolved, more carbohydrates have been introduced into our daily diet, thus causing an imbalance in our metabolism to burn fat. The reason for our extra weight can also be attributed to the many grains and starches in our diet (pasta, rice, breads, and potatoes). The Zone Diet's approach calls for a return to the diets of our ancestors where meat, fruits, and vegetables are the main dietary foods.

The Zone Diet suggests that you need the right ratio of carbohydrates to proteins and fats in order to control the insulin in the bloodstream. Too much of the hormone (insulin) can increase fat storage and inflammation in the body (conditions that are associated with obesity, type 2 diabetes and heart disease). Sears asserts that by using the Zone Diet, you are actually optimizing the body's metabolic function. Through the regulation of blood sugar, you allow your body to burn excess body fat.

Although you are not prohibited from any particular food group, it is best to avoid food with high fat and carbohydrates such as grains, starches, and pastas. The ideal sources of carbs are fruits and vegetables and for monounsaturated fats olive oil, almonds, and avocados are recommended. The Zone Diet claims to use food as a drug for overall good health, weight loss and prevention or management of heart disease and diabetes.

Sears says that you can test to see whether you are 'hormonally' correct by eating following the Zone diet and see how you feel four hours later. To simplify the Zone Diet, fill one-third of a plate with low-fat protein, and then two-thirds with fruits and vegetables.

Celebrities and some health experts say that the Zone's recommendations do not stray far from the USDA's (United States Dietary Association) dietary guidelines and therefore are advocates of the Zone Diet. Others argue that the Zone Diet has flawed ratios but Sears argues that the Zone diet is a low-glycemic-load diet that has adequate protein.

A critic of the Zone diet such as the AHA (American Heart Association) classifies the Zone Diet as high protein and does not recommend the Zone Diet for weight loss. They contend that the Zone Diet has not been proven effective in the long term for weight loss. The AHA issued an official recommendation warning against diets like the Zone Diet. They believe that the Zone Diet is hazardous as it restricts the intake of essential vitamins and minerals present in certain foods. The AHA also contends that the protein ratio in the Zone diet is too high even if the minimal fat ratio is good. Robert H. Ecker M. D of the A. H. A. finds the Zone Diet's theory on insulin flawed and argues that there is no scientific proof that the hormone insulin plays a big role in weight regulation.