Map of life expectancy at birth from Global Education Project.

Friday, April 15, 2016

The Lipid Hypothesis

You may have heard that the long-standing conventional wisdom about the ill effects of saturated fats has come under sharp questioning.  A team of investigators led by NIH employee Christopher Ramsden has resurrected and reanalyzed data from the Minnesota Coronary Experiment, a nutritional trial done back in 1968-73, which was a foundation of the conventional wisdom that substituting vegetable fat for animal fat in the diet is protective against heart disease. They have also done a meta-analysis of published studies and had earlier re-analyzed data from the Sydney Diet Heart Study of the same era.

To make a very long story short, what they found is that diets high in linoleic acid -- a component found particularly in certain vegetable oils including safflower and corn oil -- and low in saturated fat do reduce LDL cholesterol. But . . . 

According to data from these studies which were never published, that did not correspond to a reduction in the risk of heart disease, coronary events, or death. I would say this conclusion is not quite definitive. For one thing, the Minnesota study had a non-representative population of residents of mental hospitals and a nursing home. They had a lot of comorbidity. Also the high linoleic acid diets they were fed are considerably more extreme than the diets people would have in real life. For one thing, olive and canola oil are not so high in in linoleic acid; and most people would continue to consume substantial animal at even if they did try to substitute some amount of vegetable oil. Other problems are that the available data from the study don't allow direct testing of the association between LDL levels and outcomes; and that autopsy data to assess atherosclerosis is available for only a small number of participants.

Still, this work has reopened what the conventional wisdom had long considered a closed can of worms. It's been extremely controversial and of course people who have been entrenched in the conventional wisdom their whole careers aren't about to suddenly "Oops, you got us." This is not my expertise and I'm certainly not going to pronounce a verdict. It is still pretty solid that trans-fats are bad for you, statins do protect against heart disease (although maybe not so much because of their cholesterol lowering effect after all -- something long suspected), and excess sugar is also bad.

But we don't really understand what's going on with lipids in the diet or the blood plasma. It's certainly not as simple as people have long believed. The "low fat" fad of a few years back was definitely a mistake.

Most important of all, science is not like religion. It's open to correction and change. However dug in some people are about the lipid hypothesis, it's not like the Bible. It can be rewritten, and it is being rewritten. Don't get cynical and start down that well-worn path of saying, they're always changing the advice about diet, so why pay any attention? Some advice has been changing lately, but some of it has gotten more solid than ever. It's always getting better, so I say you should keep paying attention.Keep eating your veggies and whole grains, and steer clear of the sugar bombs. But I would say you can relax about dairy if you're okay with it on ethical grounds.

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