The 40-30-30 diet has recently become a popular nutritional regimen in the sports and fitness world. Individuals on this diet plan obtain 40 percent of their calories from carbohydrates, 30 percent from fat, and 30 percent from protein, in contrast to the 60-25-15 ratio that has traditionally been recommended by nutritionists. Supporters of the 40-30-30 plan say that decreasing carbohydrate intake and boosting protein consumption will enable dieters to burn more fat. For athletes, this translates into a glycogen-sparing effect, which can extend endurance.

The theory behind the 40-30-30 diet is that a lower carbohydrate intake, coupled with an increased percentage of protein, will help to keep insulin levels low in the blood. From what you've already learned about insulin's importance in glycogen manufacture and storage, you might be wondering what benefits low insulin levels would offer. While insulin is essential for processing carbohydrate, it has also been shown to inhibit fat metabolism.

According to 40-30-30 proponents, lowering blood insulin levels will allow the body to burn fat more efficiently. Furthermore, whereas a high-carbohydrate, low-fat meal triggers the release of insulin, a higher percentage of protein will trigger the release of glucagon. This hormone has the opposite effect of insulin: it enables the body to burn fat. Therefore, in theory, an athlete who is looking to use more fat for energy during exercise would want to eat less carbohydrate and more protein and fat.

Many researchers in sports nutrition, however, have stated otherwise. The fact remains that the consumption of any moderate meal consisting of 60 percent of calories from carbohydrate, 25 percent from fat, and 15 percent from protein, will produce a moderate amount of insulin. The primary role of insulin is to metabolize carbohydrate -not to store fat. Since insulin is busy doing its job on a carbohydrate-rich meal, the concentration of insulin soon falls.

The claim that the 40-30-30 diet helps athletes to lose body fat also remains unproven. This is not because 40-30-30 keeps insulin low in the blood, however. Instead, those who lose body fat are more likely to have done so because they've benefited from calorie counting to stay within the recommended 2,000-calorie limit. An individual following the 40-30-30 plan ends up eating less simply because he or she is taking greater care to count calories and to regulate food consumption.

In the end, most nutrition researchers, sports dietitians, and exercise physiologists remain firm in their conviction that the optimal athletic diet consists of 60 percent of calories from carbohydrate, 25 percent from fat, and 15 percent from protein. This recommendation is based on a veritable mountain of validated and convincing research.

The experts also advocate supplementing with carbohydrate before and during exercise to improve endurance performance, and consuming carbohydrate after exercise to replenish glycogen stores.