Question 1: Once and for all, are carbohydrates fattening?
Answer: No. Carbohydrates are not fattening. Neither is protein, nor fat, for that matter. Eating more total Calories than you burn is fattening. It doesn't matter if those excess Calories come from carbohydrate, protein or fat. Although, if you look at the physiology behind metabolism, one could argue that excess carbohydrate Calories are less fattening than excess fat Calories because it requires more energy to convert carbohydrates into stored body fat. Excess fat Calories can be stored as body fat rather easily.
For example, it only requires 3 Calories to convert 100 Calories of dietary fat into stored body fat, while it takes 23 Calories to convert 100 Calories of carbohydrate into stored body fat.
Question 2: So if carbohydrates aren't fattening, why do high-protein diets like Atkins and South Beach seem to work so well?
Answer: The exact mechanism of weight loss on high-protein diets is not known and scientists are looking at this question. Current research is inconclusive. What is known is that you lose a lot of water weight by cutting carbohydrates as on the Atkins diet or phase one of the South Beach diet. Stored carbohydrate (glycogen) also holds water in the muscles. For each ounce of stored glycogen there are three ounces of stored water. So as you deplete glycogen stores you also release water. The result of carb cutting is a significant initial weight loss -- mostly from water rather than fat.
Fat loss does occur on these diets. No doubt about that. The mechanism for this appears to be due to the total calories consumed. There still is no clear evidence that the weight loss is related to the high protein nature of the diet, but the relatively low Calorie counts of these diets. While the South Beach diet isn't considered a low-carb diet (it allows complex carbs), it is relatively high protein (and fat).
A high-protein / high fat diet can help decrease your appetite because you feel full longer. Because protein takes longer to digest than carbohydrate, it tends to be more filling and satiates more than carbohydrates. Protein and fat stay in the stomach longer and may curb your appetite and reduce hunger, another way that these diets can help you lose body fat.
While these diets are ideal for some people, and have lots of evidence to support that they work (at least for initial weight loss), they aren't ideal for athletes who need carbohydrate to fuel exercise. Another down side to these, and any diet, is that people typically can not stay on them for a lifetime. While any diet can provide a quick fix or a temporay weight loss, many people regain their weight when the diet is over. Just look at the statistics -- after years of fad diets, the American population is fatter than ever, and Europe is gaining at an incredible rate as well.
The key to losing weight is still to learn how to eat in a balanced, flexible way that you can enjoy for the rest of your life, not just something you tolerate for a few weeks or months. One tip to recognizing a healthy, realistic 'diet' is to ask yourself if this is an eating plan you can maintain for the rest of your life. Is it restrictive? Are there 'bad' or 'off-limits' foods? Do you really want to go without bagels, pasta, or bread for the rest of your life?
I didn't think so. For more discussion of high protein, low carb diets and athletes, check out Eating for Exercise and Are South Beach and Atkins Diets Right for Athletes?.
