An article in today's New York Times reports on new research around the issue of what type of diet is best for fat loss. The Journal of the American Medical Association published the results of a clinical trial by Dr. David Ludwig of Boston Children’s Hospital and his collaborators that showed a low-carb diet is better for those at risk of gaining fat - and it did so through a novel approach. But first some history.
For more than 50 years, various medical groups, as well as the government, have been pushing a low-fat diet for heart health and weight loss, a diet which ends up including more carbohydrates. The theory has always been that a calorie is calorie, or all calories are equal.
But this is a recent idea that began in the 1950s and 1960s - the article quotes a women's magazine from that era about how “Every woman knows that carbohydrate is fattening,” as two British dietitians began a 1963 British Journal of Nutrition article.
Dr. Atkins first began his research and publishing on the benefits of a low-carbohydrate diet in the early 1970s. His work is based on the knowledge that carbohydrates generate more insulin than either fat or protein, and insulin's function is to shuttle energy (glucose) - often in the form of triglycerides (an ester derived from glycerol and three fatty acid molecules) when the diet is high is sugars and starches - into muscle cells (following exercise or exertion) or into fat cells in the absence of glycogen depletion through physical exertion.
The Atkins diet, based on studies of type-II diabetics, proposed that constant exposure to high insulin levels makes the body less sensitive to insulin, which requires more insulin, thus further decreasing insulin sensitivity. A diet based on fats and proteins supplies adequate energy (the liver can convert protein and fats into ketones for energy) and generates very little insulin secretion. Atkins found that doing a two-week period of almost zero carbohydrates can allow the body to become more sensitive to insulin. At this point, the diet reintroduces low-glycemic (producing low levels of blood sugar) carbohydrates (fibrous vegetables, some fruit, and beans) as the new normal diet pattern. It worked, but most doctors rejected his work until the late 1990s when other research (particularly that of Dr. Loren Cordain, author of The Paleo Diet) began to appear that supported his work.
This is context in which this new study appears.
Here is the main portion of the NYT article: What Really Makes Us Fat, By Gary Taubes
What was done by Dr. Ludwig’s team has never been done before. First they took obese subjects and effectively semi-starved them until they’d lost 10 to 15 percent of their weight. Such weight-reduced subjects are particularly susceptible to gaining the weight back. Their energy expenditure drops precipitously and they burn fewer calories than people who naturally weigh the same. This means they have to continually fight their hunger just to maintain their weight loss. The belief is that weight loss causes “metabolic adaptations,” which make it almost inevitable that the weight will return. Dr. Ludwig’s team then measured how many calories these weight-reduced subjects expended daily, and that’s how many they fed them. But now the subjects were rotated through three very different diets, one month for each. They ate the same amount of calories on all three, equal to what they were expending after their weight loss, but the nutrient composition of the diets was very different.
One diet was low-fat and thus high in carbohydrates. This was the diet we’re all advised to eat: whole grains, fruits, vegetables, lean sources of protein. One diet had a low glycemic index: fewer carbohydrates in total, and those that were included were slow to be digested — from beans, non-starchy vegetables and other minimally processed sources. The third diet was Atkins, which is very low in carbohydrates and high in fat and protein.
The results were remarkable. Put most simply, the fewer carbohydrates consumed, the more energy these weight-reduced people expended. On the very low-carbohydrate Atkins diet, there was virtually no metabolic adaptation to the weight loss. These subjects expended, on average, only 100 fewer calories a day than they did at their full weights. Eight of the 21 subjects expended more than they did at their full weights — the opposite of the predicted metabolic compensation.
On the very low-carbohydrate diet, Dr. Ludwig’s subjects expended 300 more calories a day than they did on the low-fat diet and 150 calories more than on the low-glycemic-index diet. As Dr. Ludwig explained, when the subjects were eating low-fat diets, they’d have to add an hour of moderate-intensity physical activity each day to expend as much energy as they would effortlessly on the very-low-carb diet. And this while consuming the same amount of calories. If the physical activity made them hungrier — a likely assumption — maintaining weight on the low-fat, high-carb diet would be even harder. Why does this speak to the very cause of obesity? One way to think about this is to consider weight-reduced subjects as “pre-obese.” They’re almost assuredly going to get fatter, and so they can be research stand-ins — perhaps the best we have — for those of us who are merely predisposed to get fat but haven’t done so yet and might take a few years or decades longer to do it.
If we think of Dr. Ludwig’s subjects as pre-obese, then the study tells us that the nutrient composition of the diet can trigger the predisposition to get fat, independent of the calories consumed. The fewer carbohydrates we eat, the more easily we remain lean. The more carbohydrates, the more difficult. In other words, carbohydrates are fattening, and obesity is a fat-storage defect. What matters, then, is the quantity and quality of carbohydrates we consume and their effect on insulin.
From this perspective, the trial suggests that among the bad decisions we can make to maintain our weight is exactly what the government and medical organizations like the American Heart Association have been telling us to do: eat low-fat, carbohydrate-rich diets, even if those diets include whole grains and fruits and vegetables.