Americans crave information about diets, even as our national weight keeps rising. New studies are highlighting that there is still a lot that we don't know.
A study published Monday in the Annals of Internal Medicine is contributing to water cooler chatter about the benefits of a low-carbohydrate diet compared with one low in fat. The study was a clinical trial in which scientists randomly assigned participants to a low-carbohydrate diet or a low-fat diet and determined the diet's effect on weight and cardiovascular risk factors. They enrolled 148 middle-aged, obese participants.
People assigned to the low-carbohydrate group were told to maintain a diet of total carbohydrate (minus total fiber) of less than 40 grams a day, which is quite restrictive. For comparison, the recommended daily intake of carbohydrates for someone on a 2,000-calorie diet is 300 grams. A single slice of white bread, for example, has about 35 grams of carbohydrates.
The other group was instructed to keep calories from total fat below 30 percent of daily intake. The subjects were also told to keep calories from saturated fat below 7 percent.
The findings produced a win for the low-carbohydrate diet, which resulted in about 7 more pounds of weight loss at one year. Participants in the low-carbohydrate group also had more favorable changes in their blood lipid levels, such as cholesterol.
Meanwhile another study, published in JAMA, the journal of the American Medical Association, surveyed prior studies of popular diets and found that the differences between diets like Atkins, Ornish, Zone and Weight Watchers were quite small and not enough to distinguish one from another.
Perhaps the most amazing thing in all this is that there is such scant scientific information available about the actual effects of diets. That's why the randomized trial involving 148 people became big news this week.
The JAMA survey of studies went back as far as 1966 and found fewer than 50 blinded trials. All told, they included fewer than 8,000 people. The studies were generally small (most had fewer than 100 people), and they typically lasted for only 24 weeks. Finally, these trials weren't able to determine whether a particular diet actually lowered the risk of disease.
Our diets may be exerting powerful effects on our health, but we lack studies that assess health events. We commonly expect this kind of study for drugs and other health care interventions. And yet, dietary recommendations are made all the time, and different diets may have different effects on health risk.
We have mostly built recommendations based on observational studies, the kind that watch what happens to people who eat different diets. But these studies are limited because people who eat different diets often have different lifestyles. Vegans and burger-lovers may vary when it comes to other habits that may also influence health, making it difficult to isolate the effects of the diet.
The diet industry generates billions of dollars annually, but it is built on razor-thin evidence about what is best for any individual. And it is likely that one size doesn't fit all. A diet that is best for one person may not be so for another. Ultimately, our recommendations need to be personalized.
Which diet reduces the risk of heart attacks and cancer? The risk factors alone don't tell you. They provide an indication, but there are many instances where an intervention that decreased cardiovascular risk factors didn't lower the likelihood of heart disease. To determine which diets reduce risk for which people, we need larger studies with longer follow-up.
For many people, it will be enough to know whether a low-carbohydrate diet is better for weight loss. Even then, we need additional confirmatory studies that are larger and more definitive.
But for other people, the issue is health and the desire to lower risk along with weight loss. People will have to wait for the answer to the question about which diet is better for their health: low-carbohydrate or low-fat? Truth is, we need answers about the health effects of all the different types of diets that are being promoted. It's time to do the studies.
Harlan Krumholz is a cardiologist and the Harold H. Hines Jr. Professor of Medicine at Yale University School of Medicine. He directs the Yale-New Haven Hospital Center for Outcomes Research and Evaluation and is a director of the Robert Wood Johnson Clinical Scholars Program at Yale.