I’ve promised a few people I’d write this up, so here it is.
I keep hearing that peanuts are healthy. When pressed, I’m usually referred to epidemiological evidence, but the evidence I”m referred to always looks at nuts in general, and not peanuts in particular. Considering their mono-unsaturated and poly-unsaturated fat content, it’s not a surprise the nuts are associated with a decreased risk of CHD — it’s been epidemiologically established that increasing the ratio of poly-unsaturated to saturated fat intake decreases heart disease (but increases cancer) mortality. There’s something strange about peanuts, though: peanut oil is atherogenic in rats, rabbits, and primates.
Randomizing peanut oil reduces its atherogenic properties (Tso P., Pinkston G., Klurfeld D.M., Kritchevsky D. (1984) Lipids 19, 11-16), but the randomization process does a lot of things (Kritchevsky D., Tepper S.A., Klurfeld D.M. (1998) Lipids 33, 821-823), so it’s hard to say what the actual cause is; maybe it’s just the lectin content. I’m skeptical of animal studies where a clear mechanism can’t be identified, but animal studies are stronger than nothing, which is all the support there is for the oft-repeated claim that peanuts are healthy. It’s not all that uncommon for results on primate studies to carry over to humans, and I prefer cashews to peanuts, almond butter to peanut butter, and hazelnut oil to peanut oil anyway, so these results were enough to get me to change my diet.
The always interesting Overcoming Bias has a post about the Framingham study, a longitudinal survey which found no correlation between diet and cholesterol. Robin quotes an article from the Framingham newspaper; his take is that Kannel is trying to be clearer than truth here:
Although there is no discernible relationship between reported diet intake and serum cholesterol levels in the Framingham Diet Study group, “it is incorrect to interpret this finding to mean that diet has no connection with blood cholesterol,” Dr. William B. Kannel, director of the Framingham Heart Study has stated.
That made me wonder: isn’t it possible that there’s other evidence that diet can affect cholesterol? After all, the Framingham study merely confirms the null hypothesis, and the director of the study is clarifying the results.
A quick PubMed search confirms that there’s a large body of evidence, including both controlled and epidemiological studies, supporting Kannel’s statement. Moreover, the evidence is consistent with the result of the Framingham study. See, for example, Mattson, Erickson, and Kligman (Am J Clin Nutr 25:589-594, 1972): increasing dietary cholesterol intake from 0 mg/day to 245 mg/day and from 550 mg/day to 730 mg /day raised serum cholesterol from 160 mg/dl to 173 mg/dl and from 184 mg/dl to 201 mg/dl, respectively. But we only see an increase in very low fat and relatively low cholesterol diets; a 250 mg/day increase in cholesterol to an average American diet has no discernible effect on serum cholesterol, i.e, the effect is non-linear, and virtually all Americans are in the inelastic range, which is consistent with the Framingham study. We can see the effects of diet on cholesterol if we look at, for instance, rural China.