Tuesday January 31 2012
This study can't prove that milk improves cognitive function
“Drinking just one glass of milk a day could boost your brain power,” the Daily Mail has reported today. Milk is being hailed as a memory aid, the newspaper says, with a study showing that dairy products could “help stave off mental decline”.
The study on which the story is based found that adults with higher intakes of milk and other dairy products did better in memory and other brain function tests than those who drank little or no milk.
However, the Mail’s excitement is misplaced – the study did not show that milk was responsible for better mental performance. The type of study reported cannot show cause and effect. All it showed was that, at one point in time, people who drank more milk performed better in mental tests than those who drank less. It is possible that many other things influenced people’s performance in mental function tests, including occupation, stress levels, even how well they were feeling at the time they took the tests.
Milk may be good for your bones but so far there is no good evidence that it improves mental performance.
Where did the story come from?
The study was carried out by researchers from the University of Maine in the US and the University of South Australia. It was published in the peer-reviewed International Dairy Journal. It was partly funded by the Maurice de Rohan International Scholarship, the University of South Australia and the National Heart, Lung and Blood Institute, the National Institute on Aging and the National Institutes of Health, all in the US.
The Mail reported the study uncritically. Its suggestion that milk could help stave off mental decline is not supported by this research. It’s worth noting that the study was released to the press by a US PR company on behalf of the National Fluid Milk Processor Promotion Board, which is an industry-funded organisation set up by the US government to promote milk. This may explain how it found its way into the Daily Mail.
What kind of research was this?
This was a cross-sectional analysis of nearly 1,000 participants that aimed to investigate whether dairy food intake was associated with mental functioning. This type of study can provide a “snapshot” of various lifestyle factors and people’s health at one point in time, but it cannot establish cause and effect. A cohort study that recorded people’s dairy consumption over time and then tested their mental function more than once would provide more reliable results although even this type of study cannot establish cause and effect.
The researchers say that as the whole population ages, cognitive decline and dementia place a severe strain on both families and healthcare systems. Change in diet may have a role in preventing cognitive decline, but they say little attention has so far been paid to the relationship between dairy foods and mental performance.
The researchers say there is growing evidence that dairy products may be of benefit to cardiovascular health. Many experts would dispute this. Some dairy foods are high in saturated fat, which is associated with obesity and heart disease. Most dietitians advise a restricted intake of dairy products or consumption of low-fat varieties.
What did the research involve?
Researchers recruited 1,049 adults of all ages who were taking part in research looking at cardiovascular health and mental functioning. They collected health and lifestyle data from the participants by various methods including self-reports, medical examination, diagnostic interviews, health records and neuropsychological testing.
After excluding those who did not fulfil eligibility criteria (for example, because dietary or cognitive data were missing or because they had suffered a stroke), they were left with 972 participants.
To measure mental functioning of the participants, the researchers used a validated series of tests measuring memory, verbal recall, visual–spatial perception, organisational and verbal skills, and abstract reasoning ability. For dietary intake, they used a recognised questionnaire that included questions about nutrition and lifestyle.
The dietary component of this questionnaire included questions about dairy products. Milk was considered separately from total dairy foods. Total dairy foods were grouped as followed:
- yoghurt and dairy desserts
- cream and ice-cream
Participants were asked how frequently they consumed such foods, with six possible responses:
- once a week
- 2-3 times a week
- 5-6 times a week
- once or more a day
Participants were also asked which type of milk they consumed – full fat, reduced fat or skimmed.
The researchers used validated statistical methods to analyse the relationship between mental performance scores and dairy intake. They adjusted their results for other factors that might affect the results, including age, education, smoking and alcohol.
What were the basic results?
The researchers report that participants who consumed dairy products at least once a day had “significantly higher scores on multiple domains of cognitive function” compared with those who never or rarely consumed dairy foods. In addition, those who reported eating dairy foods between two and four times a week performed significantly better on some of the tests than those who ate dairy foods once a week. The association between greater dairy food consumption and mental performance remained significant after adjusting for a number of risk factors. There was no significant association, however, between intake of specific categories of dairy foods (such as milk, cheese or yoghurt) and results of the tests.
How did the researchers interpret the results?
The researchers say their results support an association between high dairy food consumption and cognitive function. Although little is known about how dairy foods might influence mental functioning, they say that one possibility is that dairy food consumption may be beneficial for mental functioning through its “favourable effect” on cardiovascular risk factors such as obesity.
Contrary to the headlines, this study does not show that dairy food consumption has benefits for mental functioning. All it can do is provide a “snapshot” of a group of people’s dairy consumption and their mental functioning at one point in time. Some limitations are that:
- It relied on people self-reporting their dairy intake, which introduces the possibility of error.
- It is possible that many other factors (known as confounders) might have affected the results, including exercise habits, alcohol and stress levels, although researchers tried to adjust their findings for some of these.
- As the authors acknowledge, the dietary questionnaire did not specify size of portions or servings, which undermines the accuracy of estimated intakes.
Dairy products contain many nutrients that are needed for good health, in particular for the development of healthy bones and teeth. However, they are also high in saturated fat, which is associated with heart disease and obesity. At present there is no good evidence that dairy foods are especially beneficial for brain functioning.
Find out how dairy fits into a healthy diet using the Eatwell Plate.