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12 May 2017 - The Conversation - Dairy got the all-clear this week

Eirini Trichia, PhD student, MRC Epidemiology Unit, University of Cambridge; Nita Forouhi, Programme Leader, MRC Epidemiology Unit, University of Cambridge
A new study, published recently in the European Journal of Epidemiology, appeared to give dairy products a clean bill of health. The researchers found no evidence for an increased risk of cardiovascular disease or death from consuming dairy products – even full-fat dairy products.

These findings were widely covered in the media – headlines mostly singled out cheese, saying it does not increase the risk of heart attack or stroke. Some of the reports claimed that saturated fats do not increase the risk of cardiovascular disease, but the study didn’t examine saturated fat consumption itself.

It’s worth taking a closer look at the science behind the headlines.

Key finding

This new study is a type of study known as a meta-analysis because it pools data from earlier studies and analyses the combined data. In this instance, there were 29 studies that the researchers felt were of sufficiently high quality to include in their analysis. There were 783,989 participants in all, with an average age of 57 years. Each of the studies had asked healthy people about their usual diet and then followed them up over five to 25 years. Over the course of the follow up, 93,158 participants died (from any cause), 28,419 developed coronary heart disease and 25,416 developed cardiovascular disease (heart attack and stroke).

The international team of researchers who conducted the meta-analysis found no statistically significant link between any of total dairy consumption (high- and low-fat combined), high-fat dairy consumption, low-fat dairy consumption, and risk of death, risk of coronary heart disease or risk of cardiovascular disease.

Among individual dairy products, milk consumption was not associated with cardiovascular disease or death, and neither was yoghurt consumption. For cheese and fermented dairy products (including sour milk products, cheese or yoghurt), there was a minor reduction in the risk of cardiovascular disease, but it seemed to be because of one large study. When this study – which showed an extreme result – was removed from the analysis, the link disappeared.

Strengths and weaknesses

Although not new, this research included the largest sample of participants to date on this topic and investigated both total and individual types of dairy products. The researchers applied sound meta-analysis methods and were careful to use extra analyses (called sensitivity analyses) to check their initial findings. However, there are some limitations of the individual studies, and meta-analysis, which are worth mentioning.

Although up to 29 studies were identified for the meta-analysis, the actual number available for each analysis was considerably less than 29. For example, for high-fat there were five studies for looking at death, nine for heart disease and seven for cardiovascular disease. And for yoghurt consumption, there were only three studies. This can compromise the confidence we can place in some of the results. For example, we can’t be certain that the association of yoghurt intake with cardiovascular disease is indeed null, as fermented dairy products – especially yoghurt – have been associated with reductions in cardiovascular risk, weight gain and type 2 diabetes in other research.

Another weakness is that the meta-analysis combined data from observational studies. Observational studies are not as robust as clinical trials. But this is a common limitation of studies of diet as it’s often not possible to keep people under close scrutiny – as required in clinical trials – for many years while comparing one type of diet with another.

With observational studies, there is always a risk that some other factor – one not being investigated – is the actual reason for the observed associations. To get around this, researchers take “confounding factors” into account when analysing the data. In this instance, the studies took into account such factors to very different degrees – some only including a very limited number of factors such as age, sex, smoking, or social status (such as education), while there are also other important factors such as obesity, physical activity, alcohol intake, ethnicity and other foods consumed that were missed in some studies. It is possible such health and lifestyle factors not taken into account by the researchers may have influenced the results.

Inconsistency in the definitions of groups of dairy products might also obscure interpretation. For example, some studies might include full-fat yoghurt in high-fat dairy products, whereas other studies might include both types of yoghurt in low-fat dairy products. Some dairy products such as cream and butter were not individually assessed in the meta-analysis.

Wider context

There is emerging evidence from other research, that not all foods that are rich in saturated fats have the same impact on health. This is probably because foods rich in saturated fat also contain many other ingredients. For instance processed red meat is associated with an increased risk of type 2 diabetes, whereas
dairy products, particularly yoghurt, are associated with a decrease in risk of type 2 diabetes, as we have also shown in two studies (EPIC InterAct and EPIC-Norfolk).

Most dietary research relies on people reporting their dietary habits, and this can be prone to error and inaccuracies, which can mask the detection of links between diet and health. Our own research has shown that measuring biomarkers of food intake in blood or urine can open up new possibilities for understanding links with disease and bypass errors due to self-reporting in questionnaires.

New evidence suggests that saturated fat in dairy may not be as bad for us as we once thought, but there isn’t enough evidence, just yet, to change dietary guidelines which, in the UK, recommend that saturated fats should make up fewer than 11% of all calories consumed from food.