Prominent cardiologist calls for randomized trials to test relationship between diet and coronary disease

On Jan. 7, the U.S. Department of Health and Human Services (HHS) and U.S. Department of Agriculture (USDA) released updated dietary guidelines to improve health and help prevent chronic disease.

However, Steven E. Nissen, MD, of the department of cardiovascular medicine at the Cleveland Clinic, questioned the validity of the guidelines in an editorial published in the Annals of Internal Medicine on Jan. 18.

Nissen noted the preliminary report from February 2015 mentioned that “cholesterol is not a nutrient of concern for overconsumption,” but that statement was not included in the final report, which recommended that “individuals should eat as little dietary cholesterol as possible.”

“Now that the final report is available, it is prudent to examine how, for decades, the American medical establishment erroneously advised the population to severely limit cholesterol intake and to consider whether other conventional dietary advice will eventually prove faulty,” Nissen wrote.

Although the advisory committee for the guidelines was asked to provide science-based advice, Nissen found that the members relied mostly on observational studies and surrogate end points. He said the lack of randomized trials that have evaluated dietary interventions have caused conflicting recommendations. For instance, some people suggest eliminating carbohydrates, while others propose a fat-free diet.

“Typically, dietary studies rely on a similar and flawed method, use of periodic dietary questionnaires to ascertain the eating patterns of participants,” Nissen wrote. “Recall bias and residual confounding plague such methods. There would be less interest in cult diets and poor-quality studies if nutritional research included properly designed and executed [randomized controlled trials], but few exist.”

The guidelines released on Jan. 7 made several suggestions, including that people should consume less than 10 percent of calories per day from added sugars, less than 10 percent of calories per day from saturated fats and less than 2,300 mg of sodium per day.

Nissen pointed out that the increasing popularity of low-fat, low-cholesterol diets have helped reduce dietary fats, but people instead now eat more carbohydrates, which may lead to obesity and type 2 diabetes. He said there isn’t much evidence to know whether low-fat diets and limiting saturated fats are effective coronary disease prevention strategies.

Nissen mentioned that there was a need for federal agencies such as the National Institutes of Health and Centers for Disease Control and Prevention to conduct randomized controlled trials to evaluate dietary interventions.

“Properly performed studies may demonstrate that saturated fat and cholesterol are indeed nutrients of concern, but the opposite conclusion is also possible,” he wrote. “It is time to transition from the current evidence-free zone to an era where dietary recommendations are based on the same quality evidence that we demand in other fields of medicine.”