Eating leafy greens may help prevent memory loss

"A salad a day keeps brains 11 years younger," the Mail Online reports.

This oddly specific headline was prompted by new research into whether eating a diet high in leafy green vegetables protects against age-related memory loss and decline in thinking skills (cognitive abilities).

This study found eating approximately 1 serving a day of leafy green vegetables and foods rich in certain vitamins, such as vitamin K, may have some protective effect.

But it's too early to say such a diet could prevent dementia. Some participants were only followed-up for 2 years, with an average follow-up time of 4.7 years.

This is problematic given it can take much longer for people to develop memory loss and dementia. A longer follow-up period would have provided more reliable results.

Also, this was a comparatively small sample of older people, 95% of whom were of white ethnicity and from only 1 city in the US.

Where did the story come from?
The study was carried out by researchers from Rush University and Tufts University, both in the US.

It was funded by the USDA Agricultural Research Service and published in the peer-reviewed journal Neurology.

The study prompted a range of different headlines in the UK media.

While the Mail Online reported "eating a salad every day may keep your brain a decade younger", The Independent said "eating salad and leafy greens could prevent dementia", while The Times added "one portion of spinach a day can fend off dementia".

This study certainly doesn't hold spinach wholly responsible for protecting against cognitive decline.

It's also incorrect of the Mail Online to talk about "a salad a day" given the contents of a salad can vary widely and wasn't a measure used in this study (the study looked at the nutrients found in each vegetable).

And it's premature to claim such a diet could prevent dementia based on the evidence provided in this research.

What kind of research was this?
This was a prospective cohort study of 960 people from the Memory and Aging Project (MAP).

This is a study of volunteers from more than 40 retirement communities, senior public housing units, churches and senior centres in the Chicago area.

Prospective cohort studies are the best kind of studies to examine specific outcomes – in this case, dementia over time.

What did the research involve?
The researchers began collecting data in 1997. Participants were first assessed using standard methods to make sure they didn't have dementia before they were enrolled in the study.

These initial tests were followed by annual assessments for dementia, as well as 2 additional assessments that specifically looked at memory.

Food frequency questionnaires were added to the study in February 2004. At this point the cohort had 1,306 people eligible for analysis.

Of these, only 960 had done both the memory assessments and the food frequency questionnaire.

Researchers broke the intake of leafy green vegetables down into 5 categories (quintiles) ranging from 0.07 portions a day (lowest) to 1.14 portions a day (highest).

They also looked at the following nutrients separately to determine if any specific foods could be targeted for preventing memory decline:

phylloquinone – also known as vitamin K, found both in food and as a dietary supplement
folate – also known as folic acid or vitamin B9, found in dark leafy green vegetables and liver
lutein-zeaxanthin – a vitamin found in leafy vegetables, green or yellow vegetables such as cooked kale and cooked spinach, and egg yolks
beta-carotene – the red-orange pigment found in carrots, sweet potatoes, mangoes and pumpkin among others
alpha tocopherol – or vitamin E, found in turnip greens, broccoli and asparagus
nitrate – found in spinach, rocket and beetroot juice
kaempferol – found in foods like apples, grapes, tomatoes, green tea, potatoes and many others
The researchers adjusted for a number of factors that might have influenced these results, known as confounders, which included age, education, physical activity, obesity and smoking history.

What were the basic results?
The average age of the participants was 81 years and 74% were women.

They had 15 years of education on average, which would seem to show that most of them went to college or university for 15 years, and were mostly of white ethnicity. They were followed for 4.7 years on average.

Leafy green vegetable intake varied from an average of less than 1 serving a day (0.09) to 1.3 servings a day.

Compared with those with the lowest intake of leafy greens, those with the highest intake were more likely to be higher educated, male, take part in more cognitive and physical activities, and have fewer cardiovascular and depressive symptoms, which could in theory have additional protective effects on memory.

The researchers found eating approximately 1 serving a day of leafy green vegetables was linked to slower loss of memory with ageing.

In age-adjusted models, people in the highest quintile of leafy green vegetable intake (median 1.3 servings a day) had a slower rate of cognitive decline.

Using the results from the memory testing, researchers estimated a "memory age" for each participant.

Participants who ate the most leafy greens were estimated to have a memory age around 11 years younger compared with those eating the least.

How did the researchers interpret the results?
The researchers stated eating approximately 1 serving of leafy green vegetables may help slow the decline of cognitive abilities in older age, perhaps because of the protective effects lutein, folate, beta-carotene and phylloquinone have on the brain.

The addition of a daily serving of leafy green vegetables to one's diet may be a simple way to contribute to brain health.

Conclusion
This study adds to the current body of research that a healthy, balanced diet could potentially slow memory loss. But the study didn't measure dementia rates.

The study has some strengths, such as using a standardised assessment of memory at regular intervals and using a standardised questionnaire.

But it also has some limitations that mean we can't say with any certainty that leafy green vegetables could prevent memory loss, let alone dementia:

The follow-up was short at 4.7 years on average, and some people were followed for as little as 2 years.
Dementia itself wasn't measured.
How quickly memory declines depends on the cause. It can take several years before the diagnosis is made, and people can live with a diagnosis of dementia for between 8 and 10 years.
This study was conducted in only 1 city in the US, involving just 960 people who were of retirement age, limiting generisability to other populations.
The participants were 95% white, so the findings may not apply to other ethnicities.
Other things might have influenced the results. For example, older people in retirement homes are likely to be wealthier, meaning less well-off older people weren't included in the study. Dietary choices are known to vary according to wealth.
Diets in retirement homes are also more likely to be controlled by the nursing staff, and this study only measured the older people's diets once they entered the retirement homes. This can't tell us anything about dietary habits before entering the home and how these might have affected memory.
The food frequency questionnaire is reliant on people's recall, and given the participants are older and candidates for memory decline, exact reports of what foods they ate could be over- or underestimated.
Despite the limitations, this study provides a weak link between eating leafy green vegetables and reducing cognitive decline and memory loss.