Insulin is the hormone that allows blood sugar to be used by the cells. A side effect of insulin is that it causes fat to be deposited, and it stimulates the brain to produce hunger signals. 

If carbohydrates are severely restricted, the body goes into a state of ketosis, burning fat with the subsequent production of ketone bodies in the bloodstream.  As a result the blood sugar level stabilize; insulin level drops; and because body is burning fat.  

Followers of these eating plans believe that a decrease in carbs results in lower insulin levels, which causes the body to burn stored fat for energy.

Initially, when you follow a low-carb diet you may lose more weight than if you followed a low-fat, low-calorie diet. However, this advantage slowly decreases after the first six months.

Well-known low-carb diets include the Atkins diet, Zone diet and Protein Power.

Results

Initially, when you follow a low-carb diet you may lose more weight than if you followed a low-fat, low-calorie diet. However, this advantage slowly decreases after the first six months. After one year, the amount of weight loss is about the same between the two diets.

Four factors contribute to weight loss with low-carb diets:

• Loss of water weight. When you initially decrease your carbohydrate intake, your body burns glycogen. Glycogen contains large amounts of water, so burning glycogen leads to the release of water and increased urination, causing weight loss.
• Decreased appetite. Studies suggest that a low-carb diet decreases appetite, but why this occurs isn't clear.
• Increased feeling of fullness. Low-carb diets are higher in fat and protein. Fat and protein take longer to digest, which makes you feel fuller longer.
• Reduced calories. Most low-carb diets reduce your overall calorie intake because they strictly limit the variety of foods you can eat. Carbohydrates — including bread, pasta, rice, cereals, milk, most fruit and any sweets — usually provide over half of people's daily calories. On a low-carb diet, however, carbohydrates are limited or avoided, thus leading to a significant reduction in calorie intake.

Theoretically, in order to maintain weight loss if you do lose weight, you need to continue the program. But a low-carb diet doesn't appear to be easier to maintain than any other diet. Studies comparing low-carb diets and low-fat diets found that after a year, people dropped out of both diets at similar rates. This suggests that the low-carb diet, like so many diets, is no easier to stick to long term.

Risks

No one knows the long-term health effects of low-carb diets. Though some studies have looked at the benefits and risks, none has been conducted over a long enough period to show whether these diets increase the risk of health conditions that develop over many years, such as heart disease, cancer, and kidney or bone problems.

Low-carb diets limit carbohydrates — for example, bread, grains, rice, starchy vegetables and fruit — and emphasize sources of protein and fat. Many types of low-carb diets exist, each with varying restrictions on the types and amounts of carbohydrates. Examples of low-carb diets include Atkins diet, Zone diet and Protein Power.

Purpose

Low-carb diets are generally used to lose weight. You might choose this type of diet because you enjoy the types and amounts of food featured in the diet. Or, you may believe that a low-carb diet will help you lose weight quickly and easily.

Diet details

Premise
Carbohydrates raise blood sugar levels, which then kicks in insulin. The theory behind low-carb diets is that insulin drives blood sugar into the cells and prevents fat breakdown in the body. This means you won't burn excess fat and lose weight.
Proponents of low-carb diets take this one step further. They say that if carbohydrates raise blood sugar and insulin levels and cause weight gain, a decrease in carbs will result in lower blood sugar and insulin levels, leading to weight loss. And because you're not eating the carbs, your body breaks down fat to provide needed energy. Some people do lose weight on low-carb diets, but the weight loss probably isn't related to blood sugar and insulin levels.
Typical menu
In general, low-carb diets focus on meat, poultry, fish, dairy products, eggs and some nonstarchy vegetables. The diet excludes or limits most grains, beans, fruits, breads, sweets, pastas and starchy vegetables. Some low-carb diet plans allow fruits, vegetables and whole-grains.