Two studies comparing low-carbohydrate and low-fat diets were published earlier this month. One, released to great acclaim (and an adulatory New York Times editorial) was a randomized trial comparing a low-carb and a (supposed) low-fat diet, completed in New Orleans by Tulane University. The other was an analysis of 48 randomized trials comparing named diet programs (e.g., Atkins, Jenny Craig, South Beach, Ornish and others) versus other types of diets, done by a group of researchers in Canada. This study also assessed low-carb and low-fat diets, as well as diets that they characterized as “moderate,” which were in between the low-carb and low-fat diets. This study didn’t get as much notice in the press, but it has a number of important things to say about diet and weight loss. Both studies raise questions about the role of different types of diets in weight loss – and ultimately, in the long-term health of cancer patients.
We’ll look at the Tulane study first. This study randomized 148 residents of New Orleans to either a low-fat or a low-carbohydrate diet. Half of the participants in the study were African-Americans, a group that is usually neglected in research studies, and for this, the researchers are absolutely to be applauded. The study also included intensive counseling throughout the year-long intervention, and featured several assessments of cardiac risks. About 80% of the patients in both groups continued all the way through the 12 months of the study. This is really an excellent design and implementation for a research study. The low-carb diet patients were told to eat a diet that contained less than 40 grams of digestible carbohydrates per day (that’s total carbs minus total fiber). The low-fat group was told to eat a diet of less than 30% total fat, with about 55% total carbohydrates. They were not given any specific caloric requirements, and were told to not change their exercise habits in the study. This was a study, then, that focused closely on the results of changing fat and carbohydrate percentages.
The most telling differences between the groups emerged at the end of the 12-month study. By this time, the low-carb group was eating a diet with a caloric composition that was 34% of calories from carbs, 24% from protein and 41% from fat. The low-fat group was eating a diet that was 54% of calories from carbs, 19% from protein and 30% from fat. They were, in other words, pretty much on target with what they were asked to do. The study doesn’t provide the specific weight lost at the end of 12 months for the two groups. However, the researchers reveal that the low-carb group lost 7.7 pounds more than the low-fat group; gained about 3.7 pounds more in lean body mass, and decreased fat mass by 8 pounds more. They also increased their HDL (“good”) cholesterol levels, and improved their total to HDL ratio, which is an important measure of cardiac risk, and reduced their triglycerides and C-reactive protein levels more. This is clearly a better result than the low-fat group had, although, in fact, the low-fat group did improve on all these variables, and both groups lowered their overall caloric and fat intakes.
But there are some concerning aspects to this trial. For one thing, both groups lowered their fiber intake, from about 16 or 17 grams per day down to 15 grams per day, despite having been given a target of 25 grams per day of fiber. One could expect some lowering of fiber with a low-carb diet, since whole grains are a major source of dietary fiber, and are generally significantly reduced in a low-carb diet. But this is quite worrisome for the low-fat group. It suggests that, rather than filling up on whole grains and fiber-rich vegetables, this group may have increased refined carbohydrates to make up the increase in calories from carbs that researchers observed. This is, unfortunately, what happens with many people who adopt low-fat diets; that is, they replace the fat in their diet with refined grains and simple sugars. This is also what happens with the companies that produce low-fat, “healthy” products, or “healthier,” low-fat versions of existing products. Most often, they replace the fat with sugar. This is in contrast to what often happens with most low-carb diets. Their proponents do seem to be much more effective in enforcing a ban on sugar and refined flour than most low-fat diet proponents, and that certainly is a plus for them – and consistent with one of the major tenets of the Life Over Cancer diet. Some other things to note are that, while the low-fat group increased their beta-carotene consumption substantially, the low-carb group had a drop in beta-carotene consumption, suggesting that their consumption of yellow-orange vegetables and fruits was lower than we would like to see it. Vitamin C, likewise, increased substantially in the low-fat group while dropping in the low-carb group. And folate consumption decreased in both groups, but dropped twice as much in the low-carb group as in the low-fat group. This suggests that green vegetable consumption as well as whole grains may have been less than optimal in both groups. Even refined grains are supplemented with folic acid, and the drop in grains that took place in the low-carb group may be partly due to the overall exclusion of grains on a low-carb diet. No details on vegetable and fruit consumption, or whole versus refined grains, are available in the publication, so it’s hard to tell for sure about the nutritional quality of the diets in these two groups. Because of the uncertainty about the nutritional quality of these diets – along with concerns about the long-term effects of these low carb diets – jumping to the conclusion that we should all be on low-carb diets would certainly be unwarranted!
We’ll take a closer look at these concerns in the third and final installment in this series. Our next article will take a look at the meta-analysis out of Canada that examined 48 randomized trials of well-known, “name brand” diet programs.