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–But critics say that observational studies can’t establish causal relationship.

A prominent group of nutrition researchers have once again linked saturated fats to increased coronary heart disease.

The new paper, published in BMJ, is the third paper in the past year to decry saturated fats. Along with the previous two papers, published in JACC and JAMA Internal Medicine, the BMJ paper uses data from two large observational studies to support the widely held view that high intake of saturated fat should be replaced with unsaturated fats, complex carbohydrates, or plant-based proteins. The senior author of all three papers is Frank Hu, the Harvard University nutrition researcher who has been a leading figure in the prosecution of saturated fats.

But some critics, including a BMJ editorialist, continue to raise concerns that observational data like this is not capable of demonstrating the negative effect of saturated fats. The new paper also raises additional concerns about the role of co-authors who work in the food industry.

The BMJ paper is based on 24-28 years of followup of 73,147 women in the Nurses’ Health Study and 42,635 men in the Health Professionals Follow-up Study. In this report the investigators focused on the role of individual saturated fatty acids (SFAs) and found a strong correlation with all the main SFAs and coronary heart disease. They calculated that replacing 1% daily energy intake of SFAs, —including lauric acid, myristic acid, palmitic acid, and stearic acid— with an equivalent number of calories from polyunsaturated fats, whole grain carbohydrates, or plant proteins would lead to a 6-8% reduction in the risk of coronary heart disease.

“Owing to similar associations and high correlations among individual SFAs, dietary recommendations for the prevention of coronary heart disease should continue to focus on replacing total saturated fat with more healthy sources of energy,” they concluded.

But, in an accompanying editorial, Russell de Souza and Sonia Anand (McMaster University) warn that “the results of cohort studies must always be approached with caution.” In an email interview, de Souza elaborated: “Epidemiologic studies are observational studies. This means that people who tend to eat High (or Low) saturated fats tend to also do other things that may reduce or increase risk of heart disease. For example, in this study, those who ate more saturated fat also ate more protein from animals, ‘refined’ carbohydrate, and trans fats. They also ate less fiber, fruits and vegetables, and whole grain carbohydrates. They were more likely to smoke. Though the authors carefully controlled for many of these confounders in their analyses, the risk of what we call residual confounding is always present.”

Regarding “the breakdown of risks by each particular fatty acids” de Souza said it is “a challenge to translate” these into food guidelines, since it’s “not common knowledge as to which foods contain which fatty acid; and many fatty acids are found in many foods.”

In their editorial de Souza and Anand warn against a close focus on individual nutrients. “Dietary advice about single nutrients that fails to consider dietary context is problematic…. A narrow focus on SFAs might result in diet that meets one target (for example, low in saturated fat) but fails to meet another (owing to a high intake of refined carbohydrates).”

Another potential source of concern regarding the BMJ paper is that three of the co-authors work for Uniliver, the food company that is the largest producer of margarine and other food spreads. I asked Fiona Godlee, editor-in-chief of the BMJ, for a comment about this:

“The BMJ stopped publishing research funded by the tobacco industry a few years ago because of strong evidence that it couldn’t be relied on. We have no such restrictions on research funded by the pharmaceutical or food and drinks industries, but the evidence of systematic bias in industry funded research is something we in the scientific community must keep under review. This study was funded by the NIH and has been subjected to rigorous peer review. The fact that four of its eight authors receive funding or are employed by Unilever, which is a major manufacturer of vegetable oils, is one of the many uncomfortable realities of 21st century medical research.”

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