The Effects of Nuts and Extra Virgin Olive Oil on Brain Health

There are few foods, concludes a review in a top-rated nutrition journal, that have experienced such “vindication” as nuts. According to the Harvard Nurses’ Health and Health Professionals Follow-Up studies, which followed nearly 100,000 men and women for 20 years, those eating walnuts tended to live longer than those who didn’t. Compared to non-consumers, the potentially prolonged life expectancy at age 60 for those eating nuts more than five times a week was about two years.

What about nuts and cognition? There are favorable effects of nut extracts on nerve-like cells in petri dishes and the human equivalent of an ounce (28 g) a day of walnuts or five or six almonds or hazelnuts a day, on the memory function of rats, but let’s skip right to the human data. Most observational studies of walnuts or other nuts found a positive association between nut intake and cognitive status. For example, in the Harvard Nurses’ Health Study, the intake of nuts five or more times a week appeared to delay brain aging by two years compared to those eating nuts less than once a month.

So, frequent nut eaters tend to live longer and think better, but that doesn’t mean the nuts have anything to do with it. Elderly walnut eaters, for example, eat less animal protein, less saturated fat, and less sodium. They are more likely to be non-smokers, engage in vigorous physical activity, and not be obese. Most studies control for these lifestyle factors, but walnut eaters also tend to have greater education and white-collar jobs. In other words, walnut consumption may just be a “marker of healthy lifestyle choices supported by advantageous socioeconomic position.” Thankfully, we now have a growing number of interventional studies that have put nuts to the test.

In my video on putting purported brain foods to the test, I detailed the first randomized, controlled trial, which was on the effects of walnut consumption on cognitive performance in college students, finding a slight advantage in certain critical thinking skills in those randomized to banana bread with walnuts versus without.

There have been two three-month-long controlled studies on almond-enriched diets that failed to have any cognitive consequences, but a three-month study of peanuts did. Middle-aged men and women randomized to add two or three ounces (56 or 84 g) of peanuts to their daily diets experienced improvements in short-term memory, verbal fluency, and processing speed compared to their usual diets without peanuts. However, there was no attempt at a placebo, and the peanuts were a special “Hi-oleic” cultivar with elevated monounsaturated fats (38 percent versus 26 percent in regular peanuts), and so, it’s not clear if any effects would translate to normal nuts.

Is it possible the almond studies just didn’t last long enough to have an effect? A six-month study randomizing middle-aged to older adults to 1.5 ounces (42 g) of almonds a day, 3 ounces (84 g) almonds a day, or a control snack mix approximating the same calories and macronutrients resulted in no differences in any cognitive measure between the three groups. Almonds just don’t seem to affect cognition. What about walnuts?

More than 700 healthy elders, aged 63 to 79, were randomized to eat about one to two ounces (28 to 56 g) of walnuts a day or to abstain from walnuts for two years in the first published quote-unquote “long-term” randomized trial testing a single whole food for cognitive outcomes in older adults. (In dietary intervention studies, two years really is actually exceptional). In the walnut group, not a single participant developed clinically significant cognitive impairment. But the exact same thing happened in the walnut abstention group. In other words, either walnuts have no cognitive effects in older adults, or the study population wasn’t sick enough or followed for long enough to show a difference.

There was one interventional nut study involving older people already suffering from cognitive impairment. And what appeared to improve their brain function? A single Brazil nut a day. Older men and women with mild cognitive impairment were randomized to six months of adding a Brazil nut to their daily diet or not, and although there was no meaningful change in overall cognition, there were significant improvements in certain measures, such as a boost in verbal fluency. Unfortunately, there was no placebo in play. The researchers suggested they could have tried another type of nut as a control, but since they were just using a single nut, they could have powdered it and stuffed it into a few capsules to pit it against a perfectly-matched placebo.

The longest running study of nuts and cognition was the PREDIMED trial, which I’ve detailed previously. Though it wasn’t the researchers’ original intention, it turned out thousands of people were effectively randomized for about four years to either 1) switch from consuming about three tablespoons (44.40 ml) a day of half-virgin olive oil to four tablespoons (59.15 ml) of all-virgin olive oil, or 2) go from eating about a half an ounce (14 g) of nuts a day to a whole ounce (28 g), or 3) pretty much continue their regular diet. It was primarily a cardiovascular prevention trial; so, the cognition of most participants was not measured at the beginning, but assuming because of the randomization the three groups all started out approximately equal, researchers pulled out a few hundred individuals from each group to see if their cognition differed in the end. Two separate batches of people were analyzed. The first found that both those who switched to extra-virgin olive oil and got the added nuts ended up with better overall cognition compared to those from the do-nothing group, but in the second batch only those who switched to extra-virgin olive oil appeared to have lower cognitive impairment rates.

The problem with both of these sub studies is that cognition was only measured at one point in time. But there was a third study in which a few hundred participants had their cognition tested at the beginning and again at the end to assess changes over time. Compared to the control group, the added nut group significantly improved their memory, and the extra-virgin swap group increased their general and “frontal [lobe] cognition” (which refers to attention and executive function). So, if you’re eating half a handful of nuts a day, it may be worth to going up to a full palmful, and if you’re using regular olive oil, it may be worth the switch to extra-virgin, in which more of the olive fruit polyphenols are preserved.