Let’s push aside the marketeers, and the health food cultists (who may have ethical reasons to steer you away from that big juicy steak), and focus on the scientific research.
Previous studies of nut consumption have taken a narrow view – for example, it’s known that eating nuts helps prevent heart disease, type 2 diabetes, colon cancer, and other serious conditions. However, there was always the question: perhaps eating nuts reduced your risk of heart disease, but increased your risk of dying from other causes. In other words, let’s get to the real proof of the pudding: does eating nuts reduce your rate of dying from all causes?
A group of seven researchers at Harvard and Indiana University, headed by Ying Bao (M.D., Sc.D.) published an unusually convincing study in the November 21 New England Journal of Medicine. The results were picked up and reported by Reuters and Live Science. The researchers concluded that eating just one ounce of nuts every day reduces the risk of dying by 20% over a 30-year time period. The results apply whether you are male or female, and peanuts (which are beans, not nuts – don’t ask me why!) work just as well as “tree nuts” (walnuts, hazelnuts, almonds, etc.).
Why should we believe this study compared with the clatter of noise from the people who want to improve our health with açaí, gingko, garcinia, and green coffee?
First, let’s put this study into perspective among the different ways you can perform a health study on people. There are various “levels of evidence” that describe how convincing a study may be and there’s a great summary from the University of Minnesota:
– The lowest level of evidence is a case study, in which you describe individual patients, with or without controls. This is like, my grandmother had a shot of whiskey every day and lived past 100, so whiskey must help you live a long time.
– More convincing is a “cohort study” or “longitudinal study”, in which you follow a large number of people for a long time and compare many different factors. This is the type of study Dr. Bao and her colleagues carried out – see what people eat, and then see what happens to them.
– Still more convincing would be a randomized controlled study in which you control what the subjects are allowed to eat or not eat. This doesn’t work very well with human beings, especially when the effects are so small that they are only evident after many years of data collection.
– Most persuasive are high-level reviews, in which many studies are collected and analyzed by people who are expert in the field.
Obviously, the researchers did not lock up the test subjects and force them to eat nuts, or not eat nuts, so the most convincing level of study they could conduct is the “cohort” type above. So why do I say it’s convincing?
– The data came from two huge long-term surveys: the Nurses’ Health Study of 121,700 female nurses from 11 states, and the Health Professionals Follow-up Study of 51,529 male health professionals from all 50 states. After eliminating people who did not fill out the diet questionnaires and people with a history of cancer, heart disease or stroke, there were still 118,962 people in the study, which is a heck of a lot of people.
– Nurses and doctors are well-trained professionals who chose to work in health services, so not surprisingly they were cooperative and conscientious in filling out the diet surveys.
– Nurses and doctors come from a homogeneous socioeconomic group; this reduces confusion that might result from many different personal life styles.
– The data were controlled for other factors that could have also contributed to death rate: smoking, alcohol consumption, obesity, exercise habits and other diet habits (such as eating more fruits and vegetables).
– The results were “dose-dependent” – that is, the more nuts people ate, the better their mortality (up to the seven-times-a-week level). It’s easier to find an explanation for this kind of effect than a “threshold” effect, where things suddenly change when you exceed a particular level.
Science Speculation: As regular readers know, I advise reading research results with a critical eye.
The present study was partly paid for by the International Tree Nut Council, but I don’t dock the researchers for that: they disclosed the relationship, they were subject to university policies preventing the funding source from influencing the research, and best of all, the results are consistent with (and more thorough) than previous studies.
But here’s another angle: what if nuts are not the cause of longevity, but just correlated with longevity? In other words, what if some other factor that was not controlled for is actually the cause? For example, suppose that drag racing promotes long life (this is only a supposition) and drag racers like to eat nuts. How do you account for things like this?
I was prompted to ask this question because of some fascinating books that divide North America into nine or eleven “nations” based on their persistent values and cultures, which are rooted in the occupations and way of life of the people who settled those areas. Notable books are those by Joel Garreau and Colin Woodard. Now it’s true that these books are more focused on people’s political attitudes – see a fascinating summary in the Tufts alumni magazine about gun control politics. And it’s also true that the authors cannot avoid political bias (“Yankeedom Contempt For the South And The Rest of The Country Show Through” says one Amazon reviewer). Nonetheless, it’s clear that there are distinct regional cultures in the US which encompass not only politics but also religion, values, favorite foods, sports, leisure activities, and a thousand other details. Without more information, you can’t be sure whether geography has just as big an effect as eating more nuts.
So naturally I sent Dr. Bao an e-mail asking whether she had controlled for place of residence. She sent the following answer:
Thank you very much for your interest in our work. We did not control state or region.
However, to answer your question, I just ran the models again further controlling for states in women (nurses are from 11 states) and regions in men (men are from 50 states and I classified them into 5 regions: northeast, south, west, midwest, and other). The results remained largely unchanged.
Compared to the never group, hazard ratio of 7 or more times per week is 0.78 (95% CI: 0.68-0.90) in women and 0.80 (95% CI: 0.73-0.89) in men.
In other words, the head of the research team not only has a thorough grasp of her large data base, she is gracious enough to crank the analysis once more in reply to an inquiry from someone she’s not acquainted with. Thank you, Dr. Bao!
My conclusion from all this? That this is one of the very few pieces of diet advice that you can take to the bank: if you want to live longer, eat nuts daily. Just eating nuts won’t keep you alive, of course, but eating nuts will help you regardless of what else you do to stay healthy.
Are nuts believable as a healthy addition to your diet?
Drawing Credit: frankes, on openclipart.org