BioEd Online

The fiery debate over the worth of low-carb regimes is being fuelled by two US researchers. They are using the laws of thermodynamics to argue that calories from protein are better for losing weight than those from carbohydrates.

The idea that eating a diet rich in protein will help you shed more pounds than one stuffed with carbs is driving a vast diet industry. But many nutritionists dispute this view, saying that a calorie is a calorie regardless of where it comes from. Formally, a calorie is the amount of energy needed to heat one kilogram of water by 1ºC.

Now Richard Feinman of the State University of New York and Eugene Fine of Jacobi Medical Center in New York are turning to physics to assert that calories differ. They use the laws of thermodynamics: fundamental rules that describe how heat, work and energy change in a system.

The first law of thermodynamics says that energy is always conserved. In slimming terms, this means that protein, fat and carbohydrate calories are equivalent because none of their energy is magically destroyed when you eat them. Feinman and Fine are not arguing with this idea.

But the second law of thermodynamics says that energy spontaneously disperses if it is not hindered. Feinman and Fine point out that protein and carbohydrate are metabolized in different ways and their energy is therefore dispersed in different forms. When protein is broken down by the body, for example, more energy is released as heat than is converted into chemical energy¹.

The upshot of their argument is that, while a hunk of steak and a slice of bread may carry equal calories, the amount of energy the body actually gleans from them, to fuel movement or store as fat, is different. "To say a calorie is a calorie all the time is false," says Feinman.

Energy burn

The theoretical argument has some experimental support. In 2002, Arne Astrup of Denmark's Royal Veterinary and Agricultural University, Copenhagen, and his team put 12 men in a room and measured precisely how much energy each burned when fed different diets. Those on a regimen rich in pork protein burned 4% more energy than those on a higher carbohydrate diet, the team found, because they lost more energy as heat². So Astrup says he agrees with Feinman "to some extent".

But when it comes to dieting, Astrup and other experts say that Feinman is missing the point. Even if protein and carbohydrate are processed differently in the body, what really matters is whether low-carb diets actually help people lose more weight than other eating plans.

In this regard, studies are scant and conflicting. Some of the best evidence comes from two trials published in the New England Journal of Medicine in May 2003, which showed that those on a low-carb, high-protein diet shed around three times as much weight as those on a low-fat diet after six months. However, the difference was minimal after a year³,⁴.

The main reason that some people shed extra weight on a low-carb diet is because they eat fewer calories overall, experts say, probably because protein makes them feel more full. They may also stick to the diet more rigidly or for a longer time.

Compared with these factors, any differences in the way the different foods are metabolized are negligible, argues George Bray, an authority on obesity at Louisiana State University, Baton Rouge. "The epidemic of obesity isn't due to a small biochemical defect," he says, "it's due to large portions of food eaten by inactive people."

Food science and nutrition researcher Donald Layman of Illinois State University in Urbana-Champaign argues that different diets suit different people. Eventually, decisions about which regime is most appropriate will be based on a range of health factors, such as risk of heart disease or diabetes. "That to me is where nutrition is going," Layman says.