A Less Fattening Fat

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The key to long-term weight control seems to be a diet that allows people to feel satisfied while consuming fewer calories. Exercise or at least an increase in daily activity will also help most people lose weight without being hungry. Increasing the satiety per calorie consumed has largely been ignored as a strategy to promote weight loss without hunger despite growing evidence that this may be the key to long-term weight control.

Despite the current popularity of low-carbohydrate diets there is no credible scientific evidence that restricting dietary carbohydrates promotes weight loss. On the other hand, there is some evidence that diets higher in fat do tend to promote weight gain. In general research has shown that diets higher in fat, rather than carbohydrate, tend to be fattening. The tendency of dietary fat to promote weight gain can be largely explained by the high calorie density of fat.

Research has shown that over a period of several weeks, increasing the percentage of fat in the diet while holding calorie density constant has little impact on calorie intake or body weight.1 Calorie density seems to be a far better predictor of ad libitum (at will) calorie intake than the percentage of dietary fat and carbohydrate. In general, a diet with a lower calorie density makes people feel satiated on fewer calories.

The challenge of reducing calorie density in the long run is that most people prefer more calorie-dense foods to those with a low calorie density. Dietary fat carries flavors and increases the pleasure of eating, but all dietary fats are very calorie dense. It is difficult to eat a diet with more fat and still be able to lose weight without hunger.

Exceptions to the Calorie Density Rule

Like most nutrition principles, the impact of calorie density on ad libitum calorie intake has been shown to have some exceptions. Here are three examples:

? Soft drinks and fruit juices have a low calorie density and yet they have a low satiety value compared to eating hard candy (which has a high calorie density) or whole fruit.

? The calorie density of whole fruits is similar to the juice but liquid calories low in fiber seem far less satiating than do those of solid foods.

? Alcohol is a liquid and is calorie dense. But research has shown that it is far less fattening than one would expect, perhaps because it is rapidly oxidized by the liver and not converted to fat or carbohydrate in the body.

When I was a graduate student in nutrition at Rutgers University in the 1970s, I studied the impact of different dietary fats and cholesterol on blood lipids in chickens. When I fed chickens a diet in which most of the fat came from medium chain triglycerides (MCT) I noted that the MCT-fed birds put on far less weight and fat than those fed other kinds of fats and oils. Back then no one was sure why MCT was less fattening. However, calorie density is not a likely explanation because the calorie density of MCT is only slightly less than other fats and oils.

While much additional research in animals has shown MCT is less fattening than other fats and oils, there is less convincing evidence it may aid weight loss in humans. A review article did find that MCT causes a modest increase in metabolic rate compared to longer chain fats and may increase satiety and facilitate weight control.2 It appears MCT is a considerably less fattening fat in humans as well. Perhaps it will prove to be a way to improve the flavor and texture of food without also promoting excessive calorie intake. However, more long-term data is needed about its safety and effectiveness before it should be promoted as a functional food for weight loss.

Bottom line:

MCT oil is not generally sold to the public but used commercially in foods for premature infants and adults with fat malabsorption problems. Using it for weight control is premature. In large amounts it causes GI upset in many people. It may eventually prove to be useful as a less fattening fat (perhaps in combination with other fats) in a salad dressing.

By James J. Kenney, PhD, RD, LD, FACN.

References:

1. Critical Rev Food Sci & Nutr 2000;40:481-515

2. J Nutr 2002;132:329-32

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