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To what extent does eating more nuts help to slow cognitive decline?

A higher intake of nuts in older adults at risk of cognitive decline could help to slow the decrease in cognitive performance, although the evidence to date is far from clear. Clinical writer Rod Tucker takes a closer look.

The term ‘dementia’ embraces a number of diseases that affect memory, thinking and the ability to perform daily activities. In short, dementia represents the loss of cognitive health, which is key feature of healthy aging.

Unfortunately, the level of dementia appears to be on the rise, becoming a growing and global problem. According to the World Health Organization, there are currently an estimated 55 million people living with dementia worldwide, and this figure is expected to rise in the coming years. Moreover, dementia the seventh leading cause of death and one of the major causes of disability and dependency among older people globally.

There are no known cures for dementia, but efforts have been directed at a range of potential modifiable risk factors, outlined in a 2020 Lancet review. One such factor is diet – particularly the Mediterranean diet, which consists of a high intake of vegetables, legumes, fruits, nuts, cereals and olive oil. But is there evidence to support a higher intake of the individual components in the Mediterranean diet as a means of reducing the risk of cognitive decline?

One constituent that has gained interest in recent years as a strategy to modify cognitive decline, is a higher intake of nuts, but the strength of the evidence supporting this proposition requires more analysis.

Nut intake and cognitive decline

Tree nuts and peanuts (henceforth, nuts) are nutrient-dense foods with a myriad of biologically active ingredients such as unsaturated fatty acids and high-quality vegetable protein, combined with an array of vitamins and beneficial minerals, dietary fibre, phenolic compounds and phytosterols. Peanuts, though botanically classified as legumes rather than nuts, do contain a similar nutrient profile as tree nuts and to all intents and purposes can be considered a nut.

To date, there is compelling evidence for the cardiovascular protective effect of a higher nut intake, largely through an improvement in the lipid profile. For example, it is recognised that a daily intake of small amounts of walnuts lowers LDL cholesterol.

Despite the cardiovascular benefits, much less is known about whether the intake of nuts impacts on brain health, and in particular, if this reduces the risk of cognitive decline. Nevertheless, nuts are likely to have an important effect on the brain as recognised by a recent systematic review on depression.

The review, which included 10 studies with over 60,000 participants, concluded that a higher nut consumption could be associated with a lower risk of depression, fewer depressive symptoms and better mood state in the general population.

In relation to cognitive health, another recent review suggested that while the available evidence was limited and inconclusive, there was likely to be a possible role for nuts for the maintenance of cognitive health and the prevention of cognitive decline, particularly in older adults and those at higher risk.

Randomised trial evidence

With supportive data from observational studies, the real test of efficacy can only really be determined from randomised, interventional, controlled trials. So, how does this evidence stack up?

Unfortunately, findings have been somewhat mixed. For instance, the PREDIMED-NAVARRA randomised trial explored the impact of a Mediterranean diet on cognitive function. Some 522 participants at high vascular risk were assessed and compared a Mediterranean diet supplemented with either extra-virgin olive oil or mixed nuts versus a low-fat control diet. Interestingly, the study found that both Mediterranean diets appeared to improve cognition compared with a low-fat diet.

In contrast, the MedLey Study, which used a Mediterranean diet in older adults compared to a control diet, failed to detect any effect of a Mediterranean diet on cognitive function among healthy older adults. Similarly, a trial in which healthy older adults were randomised to receive an additional intake of almonds found no improvement in cognitive function. A second interventional trial, this time using walnuts, while showing no effect on cognition in healthy elders, did observe that that walnuts might delay cognitive decline in subgroups at higher risk.

The most recent study to examine the effect of consuming nuts on cognitive function, published in June 2023, looked at the two-year impact on cognitive performance in 6,630 adults with a mean age of 65 years who were deemed at risk of cognitive decline.

Nut consumption was categorised as less than one serving per week, one to three servings, between three and seven servings or more than seven serving per week, with a serving defined as 30g. Although the study was prospective in nature, and therefore subject to several recognised limitations, the researchers did see a benefit from increased nut consumption.

Using those who consumed less than one serving of nuts per week as a comparator, the results showed that eating between three and seven servings per week or more than seven portions both demonstrated more favourable changes in general cognitive performance. Although not the main focus of the study, the researchers also found a potential synergistic interaction between nut consumption and depression. In other words, participants with depressive symptoms at baseline tended to benefit more from the consumption of nuts than those without depression.

There seems to be plenty of observational evidence that nut consumption may help to delay the onset of cognitive decline, as randomised interventional studies have not provided consistent findings. In addition, no studies have specifically addressed the effect of eating nuts on hard clinical outcomes such as the development of dementia or Alzheimer’s disease.

While the latest prospective study does suggest a possible cognitive benefit, there is an urgent need for more randomised, interventional trials before recommending increased consumption of nuts to either delay cognitive decline or prevent the onset or progression of cognitive impairment and dementia.

But given a possible link between the cardiovascular and dementia biochemical pathways, it remains plausible that inclusion of more nuts into an individual’s diet might not only improve their cardiovascular health but could help to ameliorate cognitive decline.