Nuts tracked 26% lower risk of high blood pressure across 8 studies and 20,665 cases

Twenty-six percent sounds like a lot to hang on a handful of almonds.

Writing in the British Journal of Nutrition, Metoudi and colleagues arrive at it after pooling eight prospective cohort studies covering 20,665 new cases of high blood pressure. They report a 26% reduction in risk for people eating 30 grams a day against people eating none. Thirty grams is a small handful, and the comparison group eats no nuts at all.

A duller number travels further and rests on firmer ground. Pooling all eight cohorts, the team found further evidence for a reduced risk of hypertension with higher intake of nuts, and scaled to a serving of nuts per day the estimate came out about a fifth lower.

The grade that makes this different

Most single-food findings arrive without any formal assessment of how much weight to put on them, which is why the nutrition aisle of the news is so exhausting. This paper carries one. Using World Cancer Research Fund criteria to assess the strength of evidence, the authors graded the likelihood of causality as probable.

That word is doing specific work. The World Cancer Research Fund scheme has five rungs, and probable sits second from the top, above limited-suggestive and below convincing. Reaching the top rung generally takes randomized trials, and nobody has assigned several thousand people to eat nuts every day for a decade and waited to see whose readings climbed. For a food, probable is close to the ceiling.

The grade also carries a caveat of its own: the same team that ran the analysis assigned it. Read it as a considered opinion about the weight of evidence, with the interests that implies.

Why blood pressure is the outcome worth having

Blood pressure is the force of your blood pushing against the walls of your arteries, and the reason clinicians chase it is that nothing about it announces itself. High blood pressure usually has no symptoms, so the only way to find out if you have it is to get regular checks. Left alone it does slow structural damage: when blood pressure stays high over time, it causes the heart to pump harder and work overtime, possibly leading to heart attack, stroke, heart failure, and kidney failure.

So a dietary change that shifts the odds of ever developing it is worth more than one that nudges a reading. This study counted new diagnoses over years of follow-up, which is harder to do than measuring anyone before and after a snack, and more useful to know.

What eight cohorts can and cannot show

Everyone in these studies chose what to eat, which is the whole design and the limitation you cannot analyze away. People who eat nuts most days differ from people who eat none, and they differ in ways that are hard to measure and that also protect the heart. They tend to have more money, exercise more, smoke less, and eat better across the board. The authors pooled adjusted estimates, which is the standard correction, and standard corrections handle the confounders somebody thought to record.

Two other things temper the result. The studies disagreed with each other more than chance would explain, especially in the dose-response analysis, which is what usually happens when different cohorts measure a food in different ways. The searches covered PubMed and Embase databases and nothing else, so the pool is what those two hold in English.

What the paper does have is an unusually clean bill on publication bias. There was no indication of publication bias with Begg’s test or Egger’s test or by visual inspection of the funnel plot. In a field where the small flattering results are the ones that get written up, that absence counts for something.

The shape of the dose curve

There was some indication of nonlinearity, meaning the benefit did not rise in a straight line with intake. The gap between eating no nuts and eating some looks larger than the gap between some and more.

Nutrition research turns up this shape constantly, usually because the zero group is unusual. People who eat no nuts at all sit outside the spectrum rather than at the bottom of it, and they tend to eat differently in other ways too. Either reading points the same practical direction. Getting from none to a handful is where the evidence concentrates, and the case for going further is thinner than the case for starting.

What to do with a bag of almonds

Nuts are one of the few foods where the advice has been stable for twenty years and this analysis does not disturb it. A healthy eating plan includes a variety of foods, including vegetables, fruits, and whole-grain products, and good nutrition can help people live longer and lower their risk of health problems like heart disease and type 2 diabetes.

Metoudi and colleagues end on a recommendation rather than a hedge, writing that “these findings support dietary guideline recommendations to include a serving of nuts per day”.

Nuts are also one of four components of the Portfolio diet, the cholesterol-lowering pattern that recently turned up a 10% lower all-cause mortality signal across four cohorts.

Two honest qualifications belong with that. Nuts carry a lot of energy for their size, so a handful added to a day is a different proposition from a handful that replaces something else. And this is evidence about not developing high blood pressure in the first place. If you already have it, the treatment is the treatment, and almonds are not part of it.