If this article portrays things accurately, the nutrition literature is in even worse shape than I thought

Forget Pizzagate. This is the stuff we really care about.

John Ioannidis writes:

Assuming the meta-analyzed evidence from cohort studies represents life span–long causal associations, for a baseline life expectancy of 80 years, eating 12 hazelnuts daily (1 oz) would prolong life by 12 years (ie, 1 year per hazelnut) [1], drinking 3 cups of coffee daily would achieve a similar gain of 12 extra years [2], and eating a single mandarin orange daily (80 g) would add 5 years of life [1]. Conversely, consuming 1 egg daily would reduce life expectancy by 6 years, and eating 2 slices of bacon (30 g) daily would shorten life by a decade, an effect worse than smoking [1].

Could these results possibly be true? Authors often use causal language when reporting the findings from these studies (eg, “optimal consumption of risk-decreasing foods results in a 56% reduction of all-cause mortality”).[1] Burden-of-disease studies and guidelines endorse these estimates. Even when authors add caveats, results are still often presented by the media as causal.

This is indeed ridiculous; it’s the piranha problem.

Indeed, it’s just as stupid as pizzagate and beauty-and-sex-ratio and ovulation-and-voting and himmicanes and all the other noise-mining social science hype we’ve been screaming about all these years—but worse, because it’s life and death we’re talking about here. More’s at stake than academic careers and who gets their research featured on NPR.

I just want to make sure of one thing, though, and that is that Ioannidis characterized this work correctly.

The above quote references two papers:

[1] Schwingshackl L, Schwedhelm C, Hoffmann G, et al. Food groups and risk of all-cause mortality: a systematic review and meta-analysis of prospective studies. Am J Clin Nutr. 2017;105(6): 1462-1473.

[2] Poole R, Kennedy OJ, Roderick P, et al. Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes. BMJ. 2017;359:j5024.

So let’s see. I’m kinda busy so I’ll just look at article [1], which is here, and I’ll focus on the hazelnuts. I don’t see anything in that paper about hazelnuts specifically, but there is this in the abstract:

With increasing intake (for each daily serving) of whole grains (RR: 0.92; 95% CI: 0.89, 0.95), vegetables (RR: 0.96; 95% CI: 0.95, 0.98), fruits (RR: 0.94; 95% CI: 0.92, 0.97), nuts (RR: 0.76; 95% CI: 0.69, 0.84), and fish (RR: 0.93; 95% CI: 0.88, 0.98), the risk of all-cause mortality decreased . . .

Later on in the paper a serving is defined as “28 g nuts/d,” and they provide some references to the original studies; I assume that hazelnuts count as nuts for this serving size.

The next question is how to map an estimated risk ratio of 0.76 to increased life expectancy. There’s gotta be some standard formula for this: take the instantaneous probability of death and integrate it over the distribution of ages and you get life expectancy; multiply these probabilities by 0.76 and you’ll get a different life expectancy; take the difference and, according to Ioannidis’s calculations, you get 12 years. Here’s an overview from David Spiegelhalter, where he goes through the calculations and finds that a 13% increase of risk, from age 40 onward, corresponds to a 1-year decrease in life expectancy. This would imply that the risk ratio of 0.76, from age 40 on, corresponds to roughly a 2-year increase in life expectancy. I guess that Ioannidis’s calculation assumes the risk ratio starting at birth, not age 40, so that will bump up the effect on life expectancy. It doesn’t seem to me like this would give you another factor of 6, but maybe I’m missing something.