Jeff sent me an email with the above title and a link to a press release, “Nut consumption reduces risk of death,” which begins:
According to the largest study of its kind, people who ate a daily handful of nuts were 20 percent less likely to die from any cause over a 30-year period than those who didn’t consume nuts . . . Their report, published in the New England Journal of Medicine, contains further good news: The regular nut-eaters were found to be more slender than those who didn’t eat nuts, a finding that should alleviate fears that eating a lot of nuts will lead to overweight. . . .
For the new research, the scientists were able to tap databases from two well-known, ongoing observational studies that collect data on diet and other lifestyle factors and various health outcomes. The Nurses’ Health Study provided data on 76,464 women between 1980 and 2010, and the Health Professionals’ Follow-Up Study yielded data on 42,498 men from 1986 to 2010. . . .
Sophisticated data analysis methods were used to rule out other factors that might have accounted for the mortality benefits. For example, the researchers found that individuals who ate more nuts were leaner, less likely to smoke, and more likely to exercise, use multivitamin supplements, consume more fruits and vegetables, and drink more alcohol. However, analysis was able to isolate the association between nuts and mortality independently of these other factors. . . .
The authors noted that this large study cannot definitively prove cause and effect; nonetheless, the findings are strongly consistent with “a wealth of existing observational and clinical trial data to support health benefits of nut consumption on many chronic diseases.” . . .
The study was supported by National Institutes of Health and a research grant from the International Tree Nut Council Nutrition Research & Education Foundation.
The press release did not link to the study—what’s with that, anyway, how hard would it be to include a link???—but a quick google led to this article, “Association of nut consumption with total and cause-specific mortality,” by Ying Bao, Jiali Han, Frank Hu, Edward Giovannucci, Meir Stampfer, Walter Willett, and Charles Fuchs.
Here are my quick thoughts (in no particular order):
1. Unlike various other examples we’ve discussed recently, multiple comparisons and statistical significance were not major issues with this study. Sample sizes were huge, and the treatment and outcome variables seemed clear enough.
2. They report that, “as compared with participants who consumed nuts less frequently, those who consumed nuts more frequently were leaner, less likely to smoke, more likely to exercise, and more likely to use multivitamin supplements; they also consumed more fruits and vegetables and drank more alcohol.” The regression analysis controls for age, ethnicity, body-mass index, physical activity, smoking status, alcohol consumption, multivitaminin use, intake of total energy, red/processed meat, fruits, and vegetables, and a bunch of other things. Just to get a sense of things, I’d like to see the raw death rate for each nut-consumption group (adjusting for age but no other predictors) and then their regression estimates. Adjusting using regression makes sense, but I’d like to see what the adjustment is doing. It looks like the regression model is done on the log-hazard, which is standard and again makes sense.
3. The correlation-causation issues are real, and they are clear enough in the press release, which does not seem to say anything not supported by the study.
4. The Bayesian in me wants to do some partial pooling. The point estimate from the study is that a daily serving of nuts reduces the instantaneous mortality rate by 20%. So, if there really is an effect, my best estimate would be lower than 20%. Maybe 10% or 5%. A 20% reduction in risk from one food, that’s a lot! I’m not saying it’s impossible, I’m just saying that a bit of Bayes would pull our estimate toward zero.
As you can see, I can’t bring myself to mock this study, which seems much stronger than the notorious coffee study from a few months ago, and much much stronger than the various n=100 Mechanical-Turk-style psychology studies that we’ve been reporting on with depressing regularity over the past year or so.
Should I start eating nuts? I really don’t know what to believe. Much depends on the prior (that is, on the distribution of true health effects of various diets, as best we can estimate from existing scientific data and models).
One of the pleasures of blogging is that I am completely free to admit uncertainty, unlike in a research article (where there is pressure to demonstrate that some claim has been conclusively proved) or in the journalistic style in where there are clear heroes and villains.