"The answer to living longer is in a nutshell" is the pun in the Metro newspaper, reporting on a study into nut consumption. The study followed 76,464 female and 42,498 male health professionals in the US for up to 30 years.
The participants were asked about their nut consumption at the beginning of the study and then every couple of years. Deaths during the study were also monitored. The researchers found that eating nuts was associated with a reduced risk of death from any cause during the study, and that the more frequently nuts were eaten, the lower the risk of death.
Eating 28g of nuts seven or more times per week was associated with a 20% reduced risk of death. This amount roughly corresponds to the size of a small bag of nuts that you can buy in a pub.
However, on its own the study can't show a cause and effect relationship. As many of the newspapers report, participants who consumed nuts frequently tended to be healthier as they were leaner, less likely to smoke and more likely to exercise and have a healthy diet. Somewhat surprisingly, though, these people were also more likely to drink more alcohol. Although these factors were adjusted for, there could be other differences that were not adjusted for.
This research reinforces the message that nuts can be part of a healthy balanced diet. But we should be wary of added salt – unsalted nuts are always the better option.
Read more about how a healthy diet can help prevent chronic diseases.
Where did the story come from?
The study was carried out by researchers from Brigham and Women's Hospital and Harvard Medical School, Harvard School of Public Health, the Dana-Farber Cancer Institute, Richard M Fairbanks School of Public Health, and Indiana University, US. It was funded by the US National Institutes of Health and the International Tree Nut Council Nutrition Research and Education Foundation.
The International Tree Nut Council Nutrition Research and Education Foundation is a non-profit organisation that represents nine tree nut industries (almonds, Brazils, cashews, hazelnuts, macadamias, pecans, pine nuts, pistachios and walnuts) and supports nutrition research and education.
The researchers state that the funders had no role in the design or conduct of the study, in the collection, management, analysis or interpretation of the data, or in its preparation for publication.
This study was covered accurately by the Metro, the Mail Online and The Daily Telegraph. The Telegraph's coverage was particularly good as it noted some of the study's limitations, such as the fact that it did not prove a causal link between eating nuts and longer life, but pointing out that it backs up other research.
What kind of research was this?
This study investigated whether nut consumption was associated with a reduced risk of death from any cause or death from specific causes. It involved two large independent cohort studies of nurses and other health professionals in the US.
Cohort studies are the ideal study design to address this question, as people need to be followed for a long period of time to see whether nut consumption is associated with a reduced risk of death. The gold standard study design – a randomised control trial – would be impractical because of the length of time involved.
However, cohort studies on their own cannot show causation, as it is difficult to account for all the factors (confounders) that may be responsible for the reduction in risk seen, so a variety of study approaches are needed. Despite this inherent limitation, large studies such as this one are still an important source of evidence.
What did the research involve?
The association between nut consumption and death was examined in 76,464 women participating in the Nurses' Health Study (1980-2010) and 42,498 men participating in the Health Professionals Follow-Up Study (1986-2010). To be included, men and women could not have had cancer, heart disease or a stroke before the start of the study.
Nut consumption was assessed at the start of the study and then every two to four years. Participants were asked how frequently they had consumed a serving of nuts (28g, or just under one ounce) during the previous year. The researchers then calculated average nut consumption during the study or until a diagnosis of stroke, heart disease, angina or cancer.
Deaths were monitored by searching death certificates and relevant documentation, the US National Death Index, and reports from family members and postal authorities.
The researchers then looked at the association between nut consumption and death after adjusting for known or suspected confounding predictors of death risk, including:
- body mass index (BMI)
- level of physical activity
- multivitamin use
- aspirin use
- family and personal history of a variety of conditions
What were the basic results?
During the 30 years of follow-up in the Nurses' Health Study, there were 16,200 deaths. During the 24 years of follow-up in the Health Professionals Follow-Up Study, there were 11,229 deaths.
Nut consumption reduced the risk of death from any cause during the study. The more frequently people ate nuts, the lower their risk:
- participants who ate nuts less than once per week had a 7% reduced risk of death compared with those who ate none (hazard ratio [HR] 0.93%, 95% confidence interval [CI] 0.90 to 0.96)
- participants who ate nuts once per week had a 11% reduced risk of death compared with those who ate none (HR 0.89%, 95% CI 0.86 to 0.93)
- participants who ate nuts two to four times per week had a 13% reduced risk of death compared with those who ate none (HR 0.87%, 95% CI 0.83 to 0.90)
- participants who ate nuts five or six times per week had a 15% reduced risk of death compared with those who ate none (HR 0.85%, 95% CI 0.79 to 0.91)
- participants who ate nuts seven or more times per week had a 20% reduced risk of death compared with those who ate none (HR 0.80%, 95% CI 0.73 to 0.86)
Increasing nut consumption was also associated with reduced levels of risk of death from cancer, heart disease and respiratory disease.
How did the researchers interpret the results?
The researchers conclude that, "In two large independent cohorts of nurses and other health professionals, the frequency of nut consumption was inversely associated with total and cause-specific mortality, independent of other predictors of death."
This study found that eating nuts is associated with a reduced risk of death from any cause, and that the more frequently nuts were eaten, the lower the risk of death. Eating 28g of nuts seven or more times per week was associated with a 20% reduced risk of death.
The researchers say that previous studies found that increased nut intake was associated with a reduced risk of several diseases (including type 2 diabetes mellitus, colon cancer, high blood pressure and diverticulitis), and that nut consumption has been linked to reductions in various risk factors for chronic diseases. Seeing whether nut consumption was associated with reduced risk of death was the next step.
The study has many strengths, but also has several limitations that should be considered when interpreting the results.
It used data from people in two large cohort studies with 24 to 30 years of follow-up. All the participants were US-based health professionals, so it is possible that there may be problems with generalisability to other groups of people. Single cohort studies on their own can't show a cause and effect relationship.
Diet, including nut consumption, was measured at regular intervals, making it more possible that any changes in diet during follow-up were captured. However, nut consumption was self-reported, and data on how the nuts were prepared (salted, spiced, roasted, raw) was not collected.
The researchers also collected extensive data on known or suspected confounding variables and these were adjusted for.
As many of the newspapers report, participants who consumed nuts 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. These factors were adjusted for, but the possibility of residual confounders remains.
The researchers also tried to exclude the possibility that their findings were because of reverse causality: that the association seen was down to the fact that people with chronic conditions and poor health eat less nuts, rather than that people who eat less nuts develop chronic conditions and poor health.
Despite these limitations – many of which are unavoidable because of the study design – this is an impressive piece of research. While it cannot prove that nuts increase life expectancy, this study certainly suggests a potential association between the two.
This research reinforces the message that nuts can form part of a healthy, balanced diet. We should be mindful of salt intake, however – eating more than 6g a day (around one full teaspoon) would be counterproductive, as this could lead to high blood pressure.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 21 November 2013
Mail Online, 21 November 2013
The Daily Telegraph, 20 November 2013
Metro, 21 November 2013
Links to the science
New England Journal of Medicine. Published online November 21 2013