A 柚子 a day keeps the doctor away? Appearing in the New England Journal this week is a juicy study that suggests that consuming fresh fruit once daily can substantially lower your risk of cardiovascular disease. In fact, the study suggests that 16% of cardiovascular death can be attributed to low fruit consumption. For those of you keeping score, that's pretty similar to the 17% of cardiovascular deaths that could be prevented if older people stopped smoking.
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What we're dealing with here is a prospective, observational cohort of over 500,000 Chinese adults without a history of cardiovascular disease. At baseline, they were asked how often they consumed a variety of foods, and gave a qualitative answer. Most of the analyses compare people eating fruit "daily" to those who ate fruit "rarely or never".
Those fruit-eaters were substantially different from the non-fruit eaters, but not, perhaps, in the way you might expect. For example, waist circumference and BMI were higher in the fruit-eaters and fruit-eaters were much more likely to live in urban rather than rural areas. Fruit-eaters also ate more meat, all suggesting that, in China at least, eating more fruit might be a marker of better nutrition overall. Reporting the cardiovascular effects of more frequent eating of other foods would reveal whether this is the case, but that data was not shown.
More in line with our Western expectations, fruit-eaters had a substantially higher income, more education, and were less likely to smoke or drink alcohol.
After more than 3 million person-years of follow-up, there were 5,173 cardiovascular deaths. If you followed a group of 1000 fruit-eaters for a year, you'd expect less than 1 cardiovascular death. Following a similar-sized group of never-fruit eaters, you'd expect 3.7 deaths.
These observations withstood adjustment for socioeconomic factors, smoking, physical activity, BMI and consumption of other types of food, though unmeasured confounding always plays a role in dietary studies.
Why does it work? We don't know. Though the frequent fruit-eaters had lower blood pressure and lower blood sugar, these factors did not explain the protective effects of the fruit.
Indeed, maybe it's not something in fruit that is beneficial at all, but something that isn't. Like sodium. Fresh fruit isn't salty and salt-intake was not captured in this study. Missing data like that makes it hard to trust that the observed relationship is truly causal.
Still, there isn't much harm in advising patients to eat fresh fruit more regularly, which is I suppose, what makes studies like these so appealing.