In our previous article, we took a look at the recent Tulane low-carb versus low-fat diet study. We can get some perspective on this diet by looking at the analysis from Canada that was published in the same week. This study examined 48 randomized trials of the “name-brand” diet programs. Interestingly, 20 of these trials involved the Atkins diet compared with other diets. This kind of study is called a meta-analysis. A meta-analysis takes results of several studies, accumulates all the data from the studies into a single database, and analyzes all the results together. This is a more powerful type of analysis than simply looking at one study, since it has a much higher number of study participants. This study had a different definition of “low-carb” and “low-fat” diets than the Tulane study. While the Tulane study considered low-carb diets to have less than 40 grams of carbs per day, the Canada analysis classified low-carb diets as those having 40% of calories from carbs, with 30-55% coming from protein. Low-fat diets were defined as those having 20% fat or less, with 60% of calories coming from carbs. Between these two diet types, they defined a third type of diet, which they called “moderate macronutrients” diets, which contained 21-30% fat and 55-60% carbs (macronutrients are the “big three” nutrient classes of protein, fat and carbohydrate).
In this analysis, the Atkins, South Beach and Zone diets fit into the low-carb categories. The low-fat diet group included only the Ornish diet, and an English diet program developed by Rosemary Conley, as well as a variant of the LEARN (“Lifestyle, Attitudes, Relationships and Nutrition”) diet that prescribed 20% fat. The moderate macronutrients group included Weight Watchers, Biggest Loser, Jenny Craig, Nutrisystem and Volumetrics. These diets are often thought of as being low-fat, and they are, in fact, lower than the “low-fat” cutoff of 35% drawn by the Institute of Medicine. However, many nutritionists feel that diets calling for 20% fat or less are the true low-fat diets. The 35% fat cutoff is far too close to the standard American diet, and diets with this much fat have often been found to not being successful in modifying cardiac risk factors. The Life Over Cancer diet, as used for prevention and remission maintenance at the Block Center, fits into the category of the true low-fat diet (some versions of the diet prescribed for patients going through chemo or suffering from cancer-related weight loss call for more than 20% fat, but these are not meant for weight loss).
The Canadian group recorded amounts of weight lost in each study at 6 and 12 months, and published average weight losses at 12 months for the low-carb, low-fat and moderate macronutrients diet. Here are the averages:
Diet Pounds lost after 12 months
Low-carb 17
Low-fat 15
Moderate Macronutrients 12
Obviously, we cannot deny that the low-carb diet can result in weight loss. But in this study, it appears that both the low-carb and the low-fat diets did better than the moderate macronutrients data. The amounts lost by the low-carb and the low-fat diets did not differ significantly, but both diets resulted in more weight loss, from a statistical viewpoint, than the moderate macronutrients. Other factors noted in these trials were that groups that were given exercise regimens had better weight loss than those that did not (4.7 pounds more after 12 months) and that groups that received behavioral counseling, especially during the first 3 months of the diet, had better weight loss than those that did not (2.4 pounds more after 12 months). Interestingly, a similar analysis of 23 randomized trials of low-carb (4% to 45% carbs) versus low-fat (10-30% fat) diets, published in 2012 by some of the same investigators as the Tulane study, also found that weight loss did not differ in low-carb (13 pounds) versus low-fat (11 pounds) diets.
How does this study compare with the Tulane study? The first thing to note is that the comparison diet of the Tulane study was not a true low-fat diet. Tulane’s “low-fat” diet aimed at a fat intake of 30% of calories. That puts it squarely in the moderate macronutrients category. How does the weight loss of the Tulane groups stack up against the diet classes of the Canada analysis? After 12 months, the low-fat group in the Tulane study lost 4 pounds, whereas the low-carb group lost 11.7 pounds (this doesn’t mean that the Tulane diets were defective relative to the named diets, but likely reflects that different populations were recruited for the studies, and that they faced different challenges in losing weight). The difference in the weight lost in the Tulane study between the low-carb and the low-fat group was 7.7 pounds. The difference in weight loss between the moderate macronutrients groups in the Canadian analysis and the low-fat and low-carb diets was 3-5 pounds.
What we don’t know is what would have happened in the Tulane group if a true low-fat diet of less than 20% of calories from fat had been implemented. The results are arguably about what could have been expected from comparing a low-carb diet with a moderate macronutrients diet. Would this population have lost about the same amount of weight as the low-carb group if they were placed on a true low-fat diet? That could very well have been the outcome if a 20% fat diet had been successfully implemented in Tulane’s study. We understand that adhering to a 20% fat diet does require a real commitment to a plant-based diet, and to cutting back significantly on the meat, cheese, whole-milk dairy, sugar and refined flour that saturate the American diet. Not everyone is willing or able to do this – but cancer patients – many of whom are fighting for their survival, are often among those who are!
In our next and final installment in this series, we’ll take a look at the long-term health effects of low carb and low fat diets.