In a recent article in the Annals of Internal Medicine, investigators from the Temple University School of Medicine report on a study they conducted to find out why people lose weight on low carbohydrate diets. Prior studies have documented weight loss on these diets, along with favorable metabolic changes, but speculation as to why people lose weight on diets high in fat and protein has continued to rage. The most common explanation is that it's mainly water weight that is lost, or that restricting carbs mysteriously causes a change in metabolism that burns more calories.
The Temple investigators placed their research subjects in an inpatient "metabolic unit" where they could be closely observed, and where each and every calorie they consumed would be measured and tabulated. (This in striking contrast to prior studies on low-carb diets, where compliance to the diets was assessed by the investigators asking subjects whether they had strictly adhered to the assigned diet, and receiving the response, "Uh, sure.")
In this new study, three obese men and seven obese women with type 2 diabetes were admitted to the metabolic unit, and for a week they were allowed to eat whatever they wanted, including food from outside sources. The ten subjects packed it away, averaging 3000 calories, and 300 grams of carbohydrates, per day. Then, for the next two weeks they were all placed on a diet restricting their carbohydrates to 21 grams per day. During this time they were allowed to eat as much and as often as they wanted, as long as they chose it from a low-carb, high-fat and high-protein menu (which included plenty of low-carb snack foods.)
What the investigators found was that the weight loss experienced by these subjects during their two-week low-carb diet - weight loss that averaged about 2 1/2 pounds - was due to a voluntary restriction in calories. On the low-carb diet, subjects simply ate fewer calories - on average, about 1000 fewer calories each day - despite the fact that they had free access to food, and had displayed a certain enthusiasm for excess calories during the "control" period. Furthermore, subjects said they found the low-carb diet to be just as satisfying as the non-carb-restricted diet. In addition to taking in fewer calories and losing weight, these diabetic subjects had an improvement in triglyceride and cholesterol levels, and their sensitivity to insulin improved, allowing a reduction in insulin doses.
Based on these results, the investigators surmised that carbohydrates stimulate hunger. Thus a carbohydrate-restricted diet tends to curb hunger, and fewer calories are ingested. This, they say, is why people lose weight on low-carb diets - it's tissue weight, and not water weight, that is lost.
In an accompanying editorial, a metabolism expert congratulates the investigators on a well-conducted study, but speculates that the reduction in calories seen in these subjects may not, after all, be due to carbohydrate restriction. It may be a less specific response, he suggests, to any diet that limits food choices. If you can eat only certain foods, you quickly become sated with those foods and eat less - or break the diet.
DrRich comments:
DrRich is gratified that this new study reconfirms the law of conservation of mass. DrRich is okay with occasionally violating the laws of congress or the laws of medicine, but becomes dyspneic when someone suggests violating the laws of physics.
To lose weight you must burn more calories than you take in; it's as simple as that. If low-carb diets cause people to lose weight, then one way or another - despite the fact that fat is higher in calories, gram for gram, than carbohydrates - low-carb diets must result in fewer calories ingested. This study, at last, defines how this reduction in caloric intake with a low-carbohydrate diet occurs.
