Even the epidemiologists doing the research found the pun irresistible: good health costs peanuts. For what they describe as a relatively affordable benefit, their study has found that a diet high in nuts is linked to lower mortality - associated with death rates cut by as much as a fifth.

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Did you know? Peanuts are not really nuts. They're legumes (like beans), but they are nutritionally similar to true tree nuts.

The results, published among the latest online papers from JAMA Internal Medicine, comes from three large prospective cohort studies.

These involved 71,764 people living in the southern US with mostly low incomes - black and white people, men and women - and 134,265 Chinese people - one a cohort of men, the other of women - living in Shanghai, China.

Consistent across all three separate cohorts, the study showed that nut intake was linked to a lower risk of total mortality (death from any cause), and death from cardiovascular disease.

Numerous previous studies, say the authors - including Dr. Xiao-Ou Shu from the Vanderbilt University School of Medicine, Nashville, TN - have found benefit from eating peanuts, but with the limitation that the people studied represented higher income groups - and the nuts were more expensive, too.

This study therefore looked at the diets of people from low-income groups, and diets that included predominantly peanuts - the least expensive nuts.

"Nuts," that is, in the common sense - since peanuts are in fact legumes, as are peas and beans, but have similar nutrients to true tree nuts, and are counted as nuts in epidemiological studies as well as by most ordinary people.

The combined analysis of the cohort studies cannot conclude a cause-and-effect relationship between eating nuts and a lower death risk because, for example, other factors can be responsible for observations, and in this study, there was no "dose-response" effect - no trend towards additional benefit from eating more and more nuts, just a benefit of eating nuts versus not eating them.

There is nonetheless strength in the prospectively observed link, as Dr. Michael Katz points out in an editor's note to the study:

"The consistency of the results between the cohorts, and with prior studies that have been performed in higher-income populations, increases our confidence that the beneficial effects of nuts are not due to other characteristics of nut eaters."

Key results of the study included:

  • In the US cohort of people, there was a 21% lower risk of death from any cause for individuals who ate the most peanuts
  • In the Chinese groups, high nut intake gave a 17% lower risk of death overall
  • For all the ethnic groups, there was a link to a reduced risk of ischemic heart disease (coronary heart disease, which leads to heart attack and heart failure).

The authors conclude:

"The findings highlight a substantive public health impact of nut/peanut consumption in lowering CVD [cardiovascular disease] mortality given the affordability of peanuts to individuals from all socioeconomic status backgrounds."

They add: "We found consistent evidence that high nut/peanut consumption was associated with a reduced risk of total mortality and CVD mortality.

"This inverse association was observed among both men and women and across each racial/ethnic group, and was independent of metabolic conditions, smoking, alcohol consumption and BMI [body-mass index]."

The studies in both the American and Chinese populations asked detailed questions about participants' diets, including food types and frequencies, and taking a measure of nut, peanut and peanut butter intake.

The American groups were in 12 southern US states, with more than 85,000 participants enrolled between the years 2002 and 2009, aged from 40 to 79 years and mostly from low-income communities.

The Chinese cohorts were between 40 and 74 years of age in 8 communities of urban Shanghai, taking part in the study from 1996 to 2006 and totalling over 136,000 people.

Including the Chinese groups gives "important" new evidence on the benefits of nuts, says Dr. Katz, because there was very little tree nut consumption in the groups and "peanuts are cheap and ubiquitous (and can be ground into delicious peanut butter!)."

For the Americans, peanuts made up about half of the total nut consumption.

"Who cares" if peanuts are not really nuts, asks Dr. Katz - director of health services at the Fielding School of Public Health, University of California, Los Angeles - "if they help us to live longer at an affordable price?"

While "multiple studies" have shown the beneficial effects of eating nuts, media coverage about peanuts is usually concerned with allergy. This area of research took a major new direction last week, though, when study results found that eating peanuts during infancy may protect against allergy.