Monday December 5 2016
Nuts are rich in vitamins and minerals
"People consuming at least 20 grams of nuts daily less likely to develop potentially fatal conditions such as heart disease and cancer," The Independent reports. That was the main finding of a review looking at 20 previous studies on the benefits of nuts.
Researchers found consistent evidence that a 28 gram daily serving of nuts – which is literally a handful (for most nuts) – was linked with around 20% reduced risk of heart disease, cancer and death from any cause.
However, as is so often the case with studies into diet and health, the researchers cannot prove nuts are the sole cause of these outcomes.
It's hard to discount the possibility that nuts could be just one component of a healthier lifestyle pattern, including balanced diet and regular physical activity. It could be this overall picture that is reducing risk, not just nuts.
The researchers tried to account for these types of variables, but such accounting is always going to be an exercise in educated guesswork.
Also, many non-lifestyle factors may be involved in any individual's risk of disease. For example, if you are a male with a family history of heart disease, a healthy diet including nuts can help, but still may not be able to eliminate the risk entirely.
The link between nuts and improved health is nevertheless plausible. As we pointed out during a discussion of a similar study in 2015: "Nuts are a good source of healthy unsaturated fats, protein, and a range of vitamins and minerals … Unsalted nuts are the healthiest option."
Where did the story come from?
The study was carried out by researchers from Norwegian University of Science and Technology, Trondheim, Norway, Imperial College London, and other institutions in the US.
It was funded by Olav og Gerd Meidel Raagholt's Stiftelse for Medisinsk forskning (a Norwegian charitable foundation), the Liaison Committee between the Central Norway Regional Health Authority and the Norwegian University of Science and Technology (NTNU), and Imperial College National Institute of Health Research (NIHR) Biomedical Research Centre (BRC).
The study was published in the peer reviewed medical journal BMC Medicine on an open-access basis, so it is free to read online.
The UK media presents the results reliably but without discussing the inherent potential limitations of the type of observational evidence examined by the researchers.
What kind of research was this?
This was a systematic review that aimed to examine the link between nut consumption and risk of cardiovascular disease, cancer and death.
Previous studies have suggested an intake of nuts is beneficial, and some have found it could be linked with reduced risk of cardiovascular disease and cancer. Other studies though have found no link. The researchers consider the possibility that there is a weak link and that's what they aimed to look at.
A systematic review is the best way of compiling all literature on a topic available to date. However, systematic reviews are only as good as the underlying evidence. Studies looking at dietary factors are often observational and it is difficult to rule out the possibility of confounding variables from other health and lifestyle factors.
What did the researchers do?
The researchers searched two literature databases to identify any randomised controlled trials (RCTs) or prospective cohort studies that had looked at how nut intake in adults was linked with cardiovascular disease, cancer and death from any cause.
Studies had to report information on nut intake specifically (ideally by dose and frequency). Researchers assessed the quality of studies for inclusion.
Twenty prospective cohort studies met the inclusion criteria. Nine studies came from the US, six from Europe, four from Asia, and one came from Australia. All studies included adult populations; five were in women only, three in men only, and 12 in a mixed population.
The researchers did not find any suitable RCTs to include in their analysis. This is not especially surprising as RCTs involving diet are notoriously difficult to carry out. You could never be sure that everyone who was randomised into the "eat no nuts" group would stick to the plan, or vice versa.
Also they'd need large samples and long follow-up times to capture disease outcomes, so are not usually feasible.
What did they find?
Twelve studies (376,228 adults) found nut consumption reduced the risk of cardiovascular disease. Each 28 gram/day serving was linked with a 21% reduced risk of cardiovascular disease (relative risk [RR] 0.79, 95% confidence interval [CI] 0.70 to 0.88).
This was for any nut intake, but risk reductions were also found when analysing peanuts or tree nuts separately. Increasing intake was associated with reduced risk up to 15grams/day, above which there was no further risk reduction.
Looking at specific outcomes, 12 studies also found a 29% reduced risk of heart disease specifically (RR 0.71, 95% CI 0.63 to 0.80).
However, 11 studies didn't find a significant link with the outcome of stroke specifically (RR 0.93, 95% CI 0.83 to 1.05).
Nine cohorts (304,285 adults) found that one serving of nuts per day reduced risk of any cancer by 15% (RR 0.85, 95% CI 0.76 to 0.94). By separate analysis, the risk reduction was slightly higher for tree nuts (20%) than peanuts (7%).
Fifteen cohorts (819,448 people) recorded 85,870 deaths. One serving of nuts a day was linked with a 22% reduced risk of death during study follow-up (RR 0.78, 95% CI 0.72 to 0.84).
Looking at specific causes of death, each serving of nuts a day was linked with reduced risk of respiratory deaths (0.48 (0.26–0.89); three studies) and diabetes deaths (RR 0.61, 0.43 to 0.88; four studies).
There was no link with deaths from neurodegenerative diseases, and inconsistent links with deaths from kidney disease and infectious diseases. No other disease-related causes were reported.
Overall, the researchers estimate that 4.4 million premature deaths in 2013 across America, Europe, Southeast Asia and Western Pacific could be attributable to nut intakes below 20 grams/day.
What did the researchers conclude?
The researchers conclude: "Higher nut intake is associated with reduced risk of cardiovascular disease, total cancer and all-cause mortality, and mortality from respiratory disease, diabetes, and infections."
This systematic review finds evidence that nut intake may be linked with reduced risk of cardiovascular disease, cancer and death.
The systematic review has several strengths. It identified a large number of studies with a large total sample size. It also included only prospective cohorts assessing nut consumption and then followed up later disease outcomes.
It excluded cross sectional studies, which assess diet and disease at the same time, and so can't show the direction of effect. It also excluded cohorts that have retrospectively questioned diet when the person already has the disease, which could be subject to recall bias.
However, there are still a number of inherent limitations which mean these studies cannot easily prove that nuts are the magic dietary ingredient that are solely and directly responsible for these outcomes.
There were no randomised controlled trials of nut consumption. All studies were observational where people were choosing their own diet.
The researchers took care to include studies that only looked at nut consumption as an independent factor and looked at results that had adjusted for any confounders. However, the factors that the studies adjusted for, and how well they were assessed, will have varied across studies.
As such it's very difficult to prove that nuts alone are the causative factor and they are not just one component of a generally healthier lifestyle pattern, including balanced diet, regular physical activity, not smoking, and moderating alcohol.
When it comes to frequency or quantity of intake, it is likely there is an element of inaccuracy when people report how much they eat. For example, most people wouldn't weigh out how many nuts they're eating each day.
The review also provides limited information about specific types of nuts. Considering peanuts in particular, the studies included in the review didn't specify whether these are plain nuts, or whether they could have added salt and oils.
It is also likely that cardiovascular and cancer outcomes were not assessed the same way in all studies, for example whether by participant self-report or by checking medical records.
Overall there does seem to be a link between nut consumption and health, but nuts alone won't reduce your risk of cardiovascular disease or cancers, if your lifestyle is still generally unhealthy.
If you want to live a long and healthy life then you should exercise regularly and eat a balanced diet high in fruit and vegetables and low in salt, sugar and saturated fats, while avoiding smoking and moderating your consumption of alcohol.
Nuts are high in "good fats" and can be eaten in moderation as part of a healthy diet. Unsalted nuts are best as excessive amounts of salt can raise your blood pressure.