The long-standing concern that high consumption of saturated fats increases the risk of cardiovascular disease was weakened by several meta-analyses that, with the exception of trans-fats, found no clear relationship between total fat, and a weak relationship or no relationship between saturated fatty acid intake and cardiovascular events or deaths.
A 2014 meta-analysis, published in the Annals of Internal Medicine by Chowdhury et al., found no association between most dietary and circulating fatty acid types and coronary artery disease events. The study found that saturated fats were not associated with increased CVD risk, and polyunsaturated and monounsaturated fats were not cardio-protective. The Chowdhury study, with findings at odds with years of advice linking dietary fat and health, generated a great deal of publicity, controversy, and confusion. The Chowdhury study conclusion that available evidence did not support limiting saturated fat was repeated in a New York Times commentary proclaiming, “Butter is Back,” and a Time magazine cover displaying an artistic butter swirl and the bold headline,
“Eat Butter” even though butterfat is very high in saturated fatty acids (70%). The following year, a Gallup poll registered a sharp decline in the number of U.S. adults limiting fat in their diets.
The “butter is back” episode seems to be another unfortunate example of the media unduly publicizing a flawed study that purports
to demonstrate that “everything we thought was true is wrong.” It is also a lesson on the need for caution in interpreting meta-analysis studies. The University of California, Berkeley Wellness Letter’s lead story summarizing the Chowdhury study was titled “The end of the debate? Fat chance.” According to the Wellness Letter, the studies reviewed usually did not explicitly specify the nutrients that replaced saturated fatty acids when their intake was lowered, and the replacement foods may have been high in the added sugars and refined carbohydrates that are as unhealthy as the fats they replaced. An editorial in Lancet made the same point, that to understand the effects of changed levels of saturated fat, requires taking into account the nature of the replacement energy source. Another criticism of the Chowdhury study is that the authors did not compare the effects of saturated fat with those of polyunsaturated fat.
Doubt about the validity of the Chowdhury study also comes from a JAMA article by Barnard, Willett, and Ding that suggests caution about the use of meta-analysis, especially when studying links between diet and various disease conditions. They point out most people’s diets are quite variable, recall is difficult, and that nutrition intervention studies vary in many methodological details, weakening the argument for combining their results. They noted that the Chowdhury meta-analysis gave weight to both a Malmö Diet and Cancer cohort study where barely 1% of participants got less than 10% of energy from saturated fat intake, and to the Oxford Vegetarian Study that included participants with reported saturated fat intake ranging from 6% to 7% of energy among vegans to approximately twice that amount in the other diet groups.
In contrast to the Chowdhury meta-analysis, the Oxford study found that study participants in the group with the highest one-third of saturated fat intake had nearly triple the risk of fatal ischemic heart disease compared with the lowest one-third. Although the Malmö study found no significant association between saturated fat intake levels and risk of cardiovascular events, there were no participants with low saturated fat intake. The Malmö study authors stated that saturated fatty acids-cardiovascular disease
hypothesis “is thus not fully testable in this population.” Barnard, Willett, and Ding noted that, nevertheless, the Malmö study was given substantial weight in the Chowdhury meta-analysis, which concluded that available evidence did not support limiting saturated fat.
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