It's the season of eating, and when the calendar turns, many people will start thinking about dieting. But one biochemist points out that not all calories are created equal. This ScienCentral News video has more.
Metabolism Physics
In one 2003 poll, 32 million Americans said that they were on low carb diets. Now one scientist says a law of physics might explain the science behind them.
"The first law of thermodynamics is the one that's easy to understand, it's the conservation of energy," says Feinman. "There's a fixed amount of energy in the world, and in the context of nutrition it means that any energy that you take in in the form of food must either show up as work that you do, or heat that you generate, or the chemical transformation that you carry on in your body, making new protein, and so on; and the rest will be leftover as fat. So that's always true. The second law of thermodynamics, however, is a much more difficult law to understand, and it's a dissipation law. It's a law of efficiency. It says that not all processes are equally efficient."
Feinman, who published his research in Nutrition Journal, says getting the body's fuel— glucose— from protein is less efficient than getting it from carbs, which means low carb diets make the body use more energy. "Your brain and some cells in your body have to have glucose, and there are several ways they can get that glucose," he says. "You take in sugar or starches, that's a direct supply of glucose. You can make that glucose from protein. If you do that, that's a very inefficient process, you're going to use energy to turn protein into glucose, and in the end you're going to have to get that energy from burning something, usually fat."
image: ABC News
In other words, says Feinman, the more inefficient the diet is at turning calories into glucose, the more effective it should be for weight loss. He also says that not all calories are created equal— it depends on the source. "For many years nutritionists have been saying, 'a calorie is a calorie,'" says Feinman."That is, weight gained or lost only depends on the calories in the diet, regardless of the macronutrient composition, that is, protein, carbohydrate, fat. We knew this was not true, so we set out to show that this was not true. I think the bottom line is once you have the idea that all calories are the same, you're not going to try to find the best diet. And I think it's very important to try to find out what's going to be most effective. We don't know that yet, but unless we work at it, we won't find it."
The American Dietetic Association (ADA) says there is no one diet for everyone, and recommends balance and moderation. "Whatever studies have been done in terms of thermodynamics, I think that when you take in an excess of what your body burns off, no matter how quickly something may burn, the fact is that it still will be hard for you to lose weight," says Bonnie Taub-Dix, spokesperson for the ADA and a registered dietician for 28 years. "There's no one diet that's the answer, and it's not about what's 'low' and what's 'high.'"
Taub-Dix also warns against haphazardly removing carbs from the diet. "When someone's just cutting out carbohydrates, they're cutting out a lot of B vitamins, they're cutting out fiber."
Feinman himself is on a low carb diet, and recently held a conference on different aspects of low carb diets, While he provides a biochemical explanation for them, he agrees that they don't work for everyone. "We teach medical students that the best diet is the diet your patient can stay on."
Feinman's research appears in the July 28, 2004 issue of Nutrition Journal, and the January, 2005 issue of Discover. He received no outside funding for the study.
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