Exploring the Science: Dairy Fat and Risk of Diabetes

On March 22, 2016 a study was posted online, or “ahead-of-print”, on the website of the scientific journal Circulation. The title of the study is Circulating Biomarkers of Dairy Fat and Risk of Incident Diabetes Mellitus Among US Men and Women in Two Large Prospective Cohorts and it has generated a lot of buzz over the past few weeks. Here are just a few of the headlines I saw:

  • Skim Milk Could Increase Your Risk of Diabetes, Study Suggests
  • The Case Against Low-Fat Milk is Stronger than Ever
  • Are Full-Fat Dairy Foods Better for You After All?

As I mentioned in my introduction post, I plan to use this blog to explore the science behind the headlines so this seemed like a perfect study for this series.

Let’s start with the “publish-ahead-of-print” category. It means that this study has been accepted to be published in the journal Circulation but it hasn’t been published yet. Right now, it is being prepared for publishing which means copyediting, typesetting, proofreading, and author review. Since this process hasn’t been completed yet, the final article that gets published could be different than the article that is currently online in the “publish-ahead-of-print” category. We’ll look forward to seeing the final article once it is published.

Who did the research? It is a group of Boston researchers from well-respected institutions. Most are affiliated with Harvard Medical School, Harvard School of Public Health, and Brigham and Women’s Hospital, and one researcher is affiliated with Tufts University.

Who did they study? Their participants were two large groups of people who have been studied for many years: 1864 women in the Nurses’ Health Study and 1469 men in the Health Professionals Follow-Up Study. These study participants agreed to be studied back in the 1970’s and 80’s. They agreed to answer questions about their health, eating habits, and lifestyle habits over the years so the researchers could learn about how their choices linked with their health.

What did they study? They looked at biomarkers in blood samples that had been taken from the participants in the 1980’s and 1990’s. Biomarker, short for biological marker, is a body factor that can be measured and is linked to a specific body process or disease. In this situation, they looked at biomarkers that would be created in the blood from eating dairy fat. Their research question was: Are people who have higher levels of dairy fat biomarkers less likely to get diabetes?

How did they try to answer their question? They only studied the blood of participants that did not have diabetes when their blood was drawn. Since the participants had agreed to answer questions about their health regularly, they used the participants' reports to see how many of them had diabetes by the end of 2010.

What did they find? They found that participants with higher levels of dairy fat biomarkers in the 1980’s and 1990’s were less likely to get diabetes by the end of 2010. Depending on which one of the three biomarkers they looked at, participants with higher levels had 43-52% lower risk for diabetes than participants with lower levels. Some participants with higher levels of dairy fat biomarkers still got diabetes but it was less likely to happen compared to the participants with lower levels of dairy fat biomarkers.

What does this mean? There is a link between dairy fat biomarkers and the risk of diabetes but this study is showing correlation and not causation. A correlation means that two things are linked but it doesn’t mean that one thing causes the other thing. We can't say that eating dairy fat prevents diabetes because some patients with higher levels of dairy fat still got diabetes. We also can't say that avoiding dairy fat causes diabetes because not all of the patients with lower levels of dairy fat biomarkers got diabetes. We just know that people with higher levels of dairy fat biomarkers have a lower risk of diabetes compared to people with lower levels of dairy fat biomarkers.

For us in the nutrition world, we hope that this study will lead to more research so we can better understand how dairy fat impacts our health. We have been recommending low-fat and fat-free dairy products for many years because they are lower in calories and have less saturated fat than full-fat dairy products. However, some other studies are also suggesting that the saturated fat from dairy products might not be bad for our health. A study like this makes health professionals think critically about what we are recommending and why we recommend it. It's one of the exciting parts of being in the world of nutrition science. We still have so much to learn about nutrition and our bodies and the ever-emerging research keeps us on our toes.