Men who follow a healthful diet could be protecting their brains, according to a new study that tracked a large group of men for more than two decades.

Researchers from the Harvard T.H. Chan School of Public Health in Boston, MA, analyzed data from a study that had followed 27,842 men for 26 years.

The men had all filled in detailed surveys about their food and drink intake at the start of the study in 1986 — when they were aged 51 years, on average — and then every 4 years until 2002.

The follow-up lasted until 2012, by which time their average age was in the mid- to late-70s.

During the last few years of the follow-up, they had also completed short tests to find out whether they had noticed any decline in their own ability to think and remember things.

The analysis showed that consuming higher amounts of certain foods and drinks was tied to lower risk of decline in memory and thinking skills.

The foods that most strongly showed this effect was leafy greens, red and dark orange vegetables, berry fruits, and orange juice.

The journal Neurology recently published a paper about the study and its findings.

"Our studies," says first author Dr Changzheng Yuan, who works in the school's departments of nutrition and epidemiology, "provide further evidence [that] dietary choices can be important to maintain your brain health."

Subjective cognitive function test

The purpose of the subjective cognitive function (SCF) tests that the men completed were to discern changes in memory and thinking abilities that they had noticed themselves.

The SCF test contains six items, and the study authors note that its "validity was supported by strong associations" with a gene that is linked to Alzheimer's disease.

The subjective test can uncover decline in memory and thinking skills before they begin to show up in objective tests.

The men completed the SCF test twice: once in 2008 and again at the end of the follow-up in 2012. Typical questions included:

"Do you have more trouble than usual remembering a short list of items, such as a shopping list?"

"Do you have more trouble than usual following a group conversation or a plot in a TV program due to your memory?"

The authors note that they "categorized the average of the [two] scores as good, moderate, and poor SCF."

Early warning of mild cognitive impairment

Any memory decline revealed in the SCF results could herald the start of mild cognitive impairment (MCI).

MCI is a condition that often precedes Alzheimer's disease, the most common form of dementia. However, not everyone with MCI will develop Alzheimer's.

In the United States, around 80 per cent of those "who fit the definition of amnestic MCI" develop Alzheimer's disease within 7 years, according to the National Institute on Aging.

Amnestic MCI is the form of MCI that is most often tied to memory loss.

In the recent study, 55 per cent of the men scored "good" on the SCF test, 38 per cent scored "moderate," and 7 per cent scored "poor."

Consumption of fruits and vegetables

The team split the men into five groups according to their fruit and vegetable intake. The results showed that the group that ate the most vegetables consumed about 6 servings per day, and that the group that ate the least consumed 2.

The daily consumption of fruit ranged from 3 servings for the group that ate the most to half a serving for the group that ate the least.

A comparison of the vegetable consumption against the SCF scores revealed that:

The men who ate the most vegetables were 34 per cent less likely to report having experienced a reduction in memory function.

Of the men who ate the most vegetables, 6.6 per cent scored poor on the SCF, compared with 7.9 per cent of those who ate the least.

The results also showed a 47 per cent less chance of having a poor SCF score among the men who drank orange juice every day compared with those who only drank it once per month. The link was most relevant for older men who drank orange juice every day.

In addition, men who ate the most fruit each day were the least likely to have a poor SCF score, but this link lost its strength after the team considered the effect of other foods.

The team also found that high levels of fruit and vegetable consumption near the start of the study period was linked to a lower chance of having a poor SCF score some 20 years later.

Whether or not the men kept eating lots of fruits and vegetables — up to 6 years before taking the SCF test — had no effect on the link.

The link does not prove cause and effect

Due to its design, the study cannot conclude that consuming lots of fruits, vegetables, and orange juice actually reduces the chances of developing memory loss.

The results do, however — by revealing the links between the two, particularly over a long period of time — support the idea that eating lots of fruit and vegetables helps avert memory decline.

The authors argue that the fact that the men did not undergo tests of memory and thinking ability at the start of the study in order to assess decline over the whole period is not necessarily a big weakness.

All the men were or had been in professions that involved years of training requiring a high level of cognitive competence, such as dentistry, optometry, and veterinary.

They also note that because the study was confined to men in these groups, the findings do not necessarily apply to all men or women.

"One of the most important factors in this study is that we were able to research and track such a large group of men over a 20-year period of time, allowing for very telling results.