New Review Paper on Dietary Fat and Heart Disease Risk

A new review paper on dietary fatty acids and heart disease risk was just published by Dr. Rajiv Chowdhury and colleagues in the Annals of Internal Medicine-- one of the top medical journals (1).  The goal of the paper is to comprehensively review the studies evaluating the effect of dietary fatty acids on heart (coronary) disease.  The review covers observational and intervention studies pertaining to saturated, monounsaturated, trans, omega-6 polyunsaturated, and omega-3 polyunsaturated fats.  The paper is notable for its comprehensiveness (inclusion criteria were very lax).

Here is a summary of the results:

  • In observational studies that measured diet, only trans fat was related to cardiovascular risk.  Saturated, monounsaturated, and polyunsaturated fats were unrelated to risk.
  • In observational studies that measured circulating concentrations of fatty acids, long-chain polyunsaturated fatty acids (DHA, DPA, EPA, AA) were associated with lower risk.  The dairy-fat-derived margaric acid (17:0) was also associated with lower risk.  No other fatty acids were related to risk, including trans fatty acids.
  • In controlled trials, supplementation with omega-3 or omega-6 fatty acids did not alter risk.
The authors conclude:
In conclusion, the pattern of findings from this analysis did not yield clearly supportive evidence for current cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of saturated fats.  Nutritional guidelines on fatty acids and cardiovascular guidelines may require reappraisal to reflect the current evidence.
My view
Key limitations of this meta-analysis are that 1) it lumped together a variety of studies of different design and quality, and 2) it can only be as good as the studies it's based on.  Meta-analyses are considered the highest level of evidence, but I remain somewhat skeptical of them for those reasons.
This paper contains something to upset just about everyone.  Whether you believe saturated, monounsaturated, omega-6, omega-3, or even trans fat influences cardiovascular disease risk, this paper didn't consistently support any of it, instead suggesting that perhaps dietary fat composition isn't where we should be focusing our attention.
Does it challenge my own position?  Perhaps a little bit, but not very much.  After considering new evidence and reviewing old evidence, I've gradually drifted away from the view that omega-6 polyunsaturated fat contributes to cardiovascular disease.  I still think it's probably a bad idea to eat a lot of refined seed oils-- the lipid equivalent of white sugar-- but I don't see much of an argument for avoiding whole nuts and avocados.  Recent controlled trials and meta-analyses have also dampened my enthusiasm for the idea that omega-3 fatty acids have a major impact on cardiovascular disease risk.  Either the trials weren't long enough to see protection, or omega-3 isn't as powerful as we had hoped.
The data on saturated fat are consistent with my long-standing position.  If saturated fat consumption impacts cardiovascular disease risk, the effect must be small because it's difficult to detect.  However, that doesn't mean it's prudent to eat a bacon and butter diet.  These studies reflect typical dietary patterns and have nothing to say about extreme diets.  Personally I'm wary of diets very high in saturated fat because we don't have much good quality data on them, and contrary to what is claimed in some circles, it probably does not resemble what our ancestors ate.
One of the associations that was identified in this meta-analysis is that circulating margaric acid is associated with lower cardiovascular risk.  This is consistent with the findings of my review paper on full-fat dairy, suggesting that the consumption of full-fat dairy is associated with a lower risk of obesity and metabolic dysfunction (2).  We don't know whether those results reflect cause and effect.  However, we do know that there is little evidence to suggest that dairy fat is harmful to health.
Overall, this meta-analysis argues that dietary fat composition may not be a key factor in cardiovascular disease risk.  That leaves us with these important but uncontroversial factors: genetics, overall diet quality, body composition, physical fitness, hypertension, and cigarette smoking.