Explaining overweight through the effect of insulin and glucose metabolism on fat tissue can so far only be termed a hypothesis. It is a hypothesis build primarily on physiological knowledge. The next logical step is to check if the hypothesis fits the observations that have been done and if it can adequately explain these observations. If not, we need a new hypothesis. If it indeed can explain observational data then this serves to strengthen the hypothesis and to increase the likelihood of it being correct.
So does it all fit the facts?
One way to find out would be to ask overweight people in a controlled environment to consume as much energy as they want and to expend as much energy as they feel, as long as no carbohydrates are consumed. If our hypothesis is correct, we would expect these people to lose weight while on this diet. To further increase the quality of our data, we could include a control group whose energy intake is equal, but with no restriction in carbohydrate intake.
The good thing is that we don’t have to do these studies. They have already been done. In randomized controlled trials (several groups of people who are randomized to different interventions) carbohydrate restriction does usually cause a larger weight loss than fat or calorie restriction. This observation holds true for both sexes and in a wide age range. When total weight loss has been found to be similar in such trials, it is usually because carbohydrates have been reintroduced into the diet of the low carbohydrate group thus reducing the amount of weight lost.
But, remember that this is about fat tissue reduction, not weight reduction. The important question is of course whether carbohydrate restriction is more effective at reducing fat tissue mass than calorie reduced diets of similar energy intake. And indeed they are. I’ve checked. In most randomized controlled trials that has measured the different body tissues, low carbohydrate diets cause larger reductions in fat mass and thus body fat percentage. I’m not the only one who’s checked by the way. In 2006, Krieger et al did a meta-regression of diet trials and concluded that:
“Low-carbohydrate, high-protein diets favorably affect body mass and composition independent of energy intake, which in part supports the proposed metabolic advantage of these diets. “
Weight loss is almost never a loss of just fat mass. Other tissues are reduced as well and mostly skeletal muscles. Even though many trials of low carbohydrate diets have shown a relatively large reduction in fat free mass, there are other findings still, that support the notion that carbohydrate restriction favorably affects non fat tissues. In one trial, Volek et al (2002) demonstrated that carbohydrate restriction for six weeks caused not only a significant loss of fat mass, but also an increase in muscle mass. This same finding has also been reported by Steven M. Willi and colleagues who did a trial on six overweight adolescents. In addition, other trials have shown a larger retention of muscle mass with weight loss with low carbohydrate diets compared to low fat diets. But it seems that a requisite for proper muscle mass retention is an adequate protein consumption.
Carbohydrate restriction do usually results in larger loss of fat mass and body weight compared to diets higher in carbohydrate. So far our observational data support our hypothesis. There has even been reporting of correlations between insulin levels and the amount of fat lost. It all seems to fit the facts.
A funny “byproduct” of many of the dietary trials that have been made in recent years, is the discussion of thermodynamics in weight loss diets. What sparked the discussion was the finding that carbohydrate restriction has caused up to twice the weight loss as a low fat diet containing just as many calories. Several studies reported a larger weight loss with carbohydrate restriction even though the control group consumed just as many calories but with a higher carbohydrate content. These observations have sparked a debate in the scientific community as to whether any thermodynamic laws have been broken in the process. The common dogma of nutritional science is that a calorie is a calorie, meaning that you should lose just as much weight cutting fat from your diet than from cutting carbohydrates. The thing is that from a physiological point of view this doesn’t make any sense. From a physiological point of view, carbohydrate restriction could easily cause a greater weight reduction than fat restriction despite equal energy intake. Two new expressions are being used to describe this advantage of low carbohydrate diets; decreased caloric efficiency or a metabolic advantage. As it happens none of the laws of thermodynamics have yet been broken, but as Richard Feinman puts it: saying that a calorie is a calorie is a violation of the second law of thermodynamics.
Physiological data tells us that dietary carbohydrates are the main regulator of whether the fat tissue gives up energy or stores energy. If less is given up than what is stored we gradually will increase the size of our fat tissue. In addition to these physiological data, diet trials show us that we’ve most likely got our physiology right. Carbohydrate restriction does indeed cause large reductions in fat mass and is the most effective way to reduce fat tissue size. If we add to this by addressing epidemiological data showing increased intake of refined carbohydrates correlating with increased rates of overweight and obesity, we have ourselves a pretty strong case.
So does it all fit the facts? Well yes, it certainly appears so. In fact it fits so well that any dietary overweight treatment that is not based on a reduction in the intake of carbohydrates could easily be called malpractice. Especially when we know that the most common long term effect of a reduction in fat intake is an increase in body weight (even George Bray with coauthors admitted this much in their Handbook of obesity). If a doctor recommends energy and fat restriction, than this consequently is also a recommendation of a weight loss followed by an increase in weight to a level surpassing the starting weight.