Monday November 9 2009
“Children who drink full-fat milk are less likely to become overweight than those who drink the skimmed version,” reported the Daily Mirror. It said that a study of eight-year-old children found that those who drank the most full-fat milk had a lower body mass index (BMI).
The finding that full-fat milk was associated with lower BMI is unexpected and the researcher herself was surprised by this. Two theories are put forward: either the children who drink full-fat milk have fewer snacks and sugary drinks, or their diets are generally healthier with fewer total calories.
Both theories are plausible, but these results need to be repeated in a larger group of children to see if the same links are found. People should not give their children full-fat milk to reduce their BMI on the basis of this research.
Where did the story come from?
The research was from a PhD thesis by Susanne Eriksson from the University of Gothenburg in Sweden. Sources of funding were not reported. The study has not yet been published in a peer-reviewed journal. Peer review aims to ensure that the methods and conclusions of research stand up to independent scrutiny. As such, this will be an important part of any further research or publication in the future.
The study investigated nutritional intake, bone mineralisation (such as bone density) and metabolic markers (such as vitamin D levels in the blood) in a group of healthy eight-year-olds and related these to body composition, growth, socio-economic factors, physical activity and health.
The media reported this research appropriately, emphasising the preliminary nature of the conclusion and offering possible alternative explanations for the results. The Daily Telegraph’s headline, that ‘children who drink full-fat milk weigh less than those who do not’, accurately describes the research without suggesting that drinking full-fat milk causes children to weigh less.
What kind of research was this?
This cross sectional study looked at the relationship between diet, bone density and other health factors (such as being overweight) in eight-year-olds. It is a PhD thesis and includes four separate studies and several separate analyses. Just one part of this thesis, which found an association between milk consumption and BMI, is reported in the news.
A cross sectional study cannot prove causation (that one thing causes another) and must be viewed in the context of other evidence.
What did the research involve?
The study’s main area of investigation was the possible association between various measures of nutrition (such as diet), metabolic markers (such as vitamin D blood levels) and bone mineralisation (such as bone density) in healthy eight-year-olds. The researcher also measured other factors that might affect the strength of any associations, such as body composition, BMI, weight, growth, socio-economic factors, physical activity or general health.
The main purpose of the study was to investigate how bone health was affected by milk intake. The association between milk intake and BMI was a secondary finding.
The study involved 92 children, who had been part of a previous study about diet when they were four years old, and 28 newly recruited children. The children answered a questionnaire about everything they had eaten in the past 24 hours. This is quite a short recall period and may have led to inaccurate recordings as day-to-day variation in diet was not recorded.
The children’s height and weight were measured to calculate their BMI, and blood samples were taken. Bone mineralisation was assessed using a process known as dual energy X-ray absorptiometry (DEXA scan).
A variety of mathematical techniques were used to statistically test for associations. A type of modelling called multivariate linear regression was used to test the extent to which milk intake was associated with weight.
What were the basic results?
The researcher says that the population was representative of Sweden, except that a higher than expected number of the children’s parents had a university degree.
In total, 17% of the children were overweight. The researcher found that children who had been in the earlier study at four years of age had similar food choices at eight years, suggesting that food habits are established at an early age. The researcher reported that:
- Intake of beverages, chocolate and sweets were influenced by parental education.
- The ethnicity of the mother influenced the type of milk that was drunk.
- Children who consumed full-fat milk regularly had a lower BMI than those who seldom or never drank milk.
- Socio-economic status influenced the children’s nutritional intake of milk and soft drinks, but not other items in their diet.
How did the researchers interpret the results?
The researcher says, “BMI correlated strongly to fat mass and … saturated fat and intake of full fat milk were inversely associated with BMI.”
The finding that full-fat milk is associated with lower children’s BMI is unexpected, and the researcher herself was surprised by the results. Two theories are put forward: either the children who drink full-fat milk have fewer snacks and sugary drinks, or their diets are generally healthier with fewer total calories. Both are plausible, but the study was in a relatively small group of children, and there are several possible confounding factors, including socio-economic factors, which could have caused the results.
Overall, this study raises more questions than it answers and, in the tradition of a PhD thesis, will no doubt lead to larger studies.