A study “Association of Dietary, Circulating, and Supplement Fatty Acids With Coronary Risk; A Systematic Review and Meta-analysis” published March 18 in the Annals of Internal Medicine should be the “nail in the coffin” of the lipid hypothesis (linking saturated fat consumption to coronary heart risk). I want to help out, to hammer one tiny nail in the coffin of this zombie idea. Herewith, my hammer swings.
The study is a systematic review of all available evidence on the lipid hypothesis, including observational studies, prospective cohort studies and RCTs. Taken together, the evidence does not support any link between consuming saturated fat and coronary heart risk. Its “surprising” results have come up in several conversations this week; one friend (you know who you are) speculated that the research may have been funded by a nefarious, self-interested funder (the beef industry perhaps?). This is not the case – as you can see if you follow the link above.
My friends, and many others, are suspicious because the finding conflicts with so much existing evidence. Except, they do not; rather, the finding confirms the balance of existing evidence. The findings are at odds with current dietary guidelines and conventional wisdom. This is a very different issue altogether.
Since this issue has come up in several conversations, I want to lay out what I discovered when I examined the evolution of the evidence for this hypothesis, as well as the evolution of dietary guidelines.
The origin of the lipid hypothesis lay in poor handling of then-available observational data. To wit, Ancel Keys’ Seven Countries Study (1980), which examined observational data on changes in fat consumption and heart disease levels of different countries. It was named for the seven countries that saw an increase in heart disease cases correspond with increased fat consumption; the study ignored considerable additional observational data that was available at the time – which, taken together, supported the linkage – but weakly. Nevertheless, Time magazine covers, and sadly, national dietary guidelines based on the findings followed.There have been many more observational studies since then. Taken together, their findings do not support the lipid hypothesis. Check out this excellent overview of the evidence.
The mechanism? The concern over fat gathered steam when studies showed that saturated fat increases LDL cholesterol — the bad cholesterol — the artery-clogging stuff. They assumed this increased the risk of heart disease. When further studies did not confirm saturated fat elevated coronary heart risk, researchers started to dig more deeply into the mechanism. They found the more important predictor of risk is the ratio a person has of LDL to HDL, the good cholesterol. Note, compared with carbohydrates, saturated fat can increase HDL and lower fat deposits in the blood called triglycerides, which, is protective against heart disease. Heck, even the American Heart Assn admits this. In fact, more recent studies, such as those examining the health effects of consuming full-fat dairy – see here and here, suggest there are health benefits from eating higher saturated fat diets.
Nor do subsequent prospective, cohort studies (e.g. Framingham) support the lipid hypothesis. See this systematic review Siri-Tarino, P. W., Sun, Q., Hu, F. B., & Krauss, R. M. (2010). Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease. The American journal of clinical nutrition, 91(3), 535-546. They found “no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD”.
Many RCTs to measure the effects (in terms of fatal or non-fatal heart attacks) of saturated fat have been either inconclusive, poorly designed, or completely unsupportive of the hypothesis. A few such studies are (I could not find a systematic review of only RCTs):
- Research committee. Low-fat diet in myocardial infarction. A controlled trial. The Lancet 1965;2:501-4.
- Rose GA, Thomson WB, Williams RT. Corn oil in treatment of ischaemic heart disease. British Medical Journal 1965;i:1531-3.
- Research committee to the medical research council. Controlled trial of soya-bean oil in myocardial infarction. The Lancet 1968;ii:693-700.
- Dayton S, and others. A controlled clinical trial of a diet high in unsaturated fat in preventing complications of atherosclerosis. Circulation 1969;40(suppl 2):1-63.
- Leren P. The effect of plasma cholesterol lowering diet in male survivors of myocardial infarction. A controlled clinical trial. Acta Medica Scandinavica 1966;suppl 466:1-92.
- Woodhill JM, and others. Low fat, low cholesterol diet in secondary prevention of coronary heart disease. Adv Exp Med Biol 1978;109:317-30.
- Burr ML, and others. Effects of changes in fat, fish, and fibre intakes on death and myocardial reinfarction: diet and reinfarction trial (DART). The Lancet 1989;2:757-61.
- Frantz ID, and others. Test of effect of lipid lowering by diet on cardiovascular risk. The Minnesota Coronary Survey. Arteriosclerosis 1989;9:129-35.
This brings me back to the just-published systematic review of the available evidence from all three methods (observational, prospective cohorts; and, RCTs) Chowdhury, R., S. Warnakula, et al. (2014). “Association of Dietary, Circulating, and Supplement Fatty Acids With Coronary Risk: A Systematic Review and Meta-analysis.” Ann Intern Med 160(6): 398-406.
Which, unsurprisingly, found that “current evidence does not clearly support cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats”.
Let us hope government guidelines will finally be changed to reflect the evidence. We can’t take such a change for granted though. The folks involved with developing dietary guidelines have been ignoring the evidence they are wrong for quite awhile (see here and here).
I am not giving dietary advice. I am encouraging my many econometrically literate friends to take a look at the evidence themselves. Like me, I think you will be surprised what you find.