Michael Cala interviews nutrition expert and cancer-prevention specialist Moshe Shike, M.D. about whether low carb diets really work.

Discovery Health: People trying to lose weight often say they've had limited or no success following the 20-20-60 diet. Consumer magazines praise the "high-protein low-carbohydrate" diet as a panacea for the American weight problem. Your views?

Dr. Shike: These "high-protein, low-carbohydrate" diets have not been proven to be safe or effective in the long run. We know, for example, that high-protein diets may be harmful to the kidneys, and are associated with calcium loss, which can result in bone problems.

Also, an extreme diet of any kind — say, one high in meats and whole-milk dairy products — may be harmful because high-fat diets have been associated with a number of chronic diseases, including cancer and heart disease. Also, when a whole class of foods is restricted, critical nutrients may be lacking from the diet.

In a fat-rich, low-carbohydrate diet, the body switches to a form of metabolism that produces ketones. The presence of ketones in the blood system causes the blood to become acidic. Persistent acidity can lead to nausea, muscle breakdown, headaches, irritability, kidney problems and weak bones. Another problem with the low-carbohydrate diets is that they may be deficient in essential nutrients such as calcium, potassium and various vitamins.

Discovery Health: A number of books on the market virtually guarantee weight loss simply by adjusting the percentages. "Raise the protein, lower the carbs," as it were.

Dr. Shike: It's true that consuming high-protein or high-fat diets may initially induce weight loss in some people. But what has to be understood is that weight loss is based on calorie restriction — not on what is being consumed!

Fad diets prohibit a lot of foods — in the case of high-protein diets, carbohydrate intake is severely restricted. And guess what? People lose weight not because of the altered food balance, but simply because they are restricting calories. Of course they will lose weight!

Now, if you are going to restrict calories — which is vital to losing weight — isn't it better to restrict them in a way that is consistent with a healthy diet? Nobody needs excess protein and fat in their diets for the many health-related reasons we have already discussed.

Although the potential negative effects that I mentioned earlier may take some time to become apparent, it is important to understand that the long-term safety of these fad diets has not been established.

Discovery Health: But what about the fact that high-protein diets appear to reduce insulin requirements in diabetics? Isn't this a good thing?

Dr. Shike: In the overweight person, increased insulin requirements are related primarily to excess body weight. People who are obese tend to develop an insensitivity to the natural insulin produced by the body — insulin resistance, as it's often called.

When insulin resistance becomes severe enough, these individuals display high blood sugar levels and may require medications or supplementary insulin injections as a way to counter insulin resistance. They have developed non-insulin-dependent diabetes (NIDDM), or Type 2 diabetes.

Once you reduce body weight to normal levels, insulin insensitivity decreases. The formerly obese person now becomes more responsive to naturally produced insulin. When that happens, he or she requires less supplemental insulin and may not even require insulin injections as long as normal body weight is maintained.

Such individuals should still have their blood sugar monitored closely.

Discovery Health: Is this also true for people who don't require insulin but might have "borderline" blood sugar irregularities?

Dr. Shike: Losing excess weight can help prevent or delay the onset of diabetes. In people who routinely test high for blood sugars, losing weight frequently lowers it, and may even return blood glucose levels to the normal range. Overweight, non-insulin-dependent diabetics will also find the disease under much better control when they lose weight.

Discovery Health: So, what you're saying is that high-fat and protein diets do work, but for reasons other than those that are claimed for them?

Dr. Shike: Diets work to induce weight loss when they restrict calories. What I have just said about the health and weight-loss benefits of calorie restriction, and the direct relationship between obesity, insulin insensitivity and poor sugar metabolism is nothing new. It's not something that has been "discovered" in the past few years — it has been understood for decades.