In a new study published in the American Journal of Clinical Nutrition, Dr Mario Kratz, an associate professor in Fred Hutch’s Public Health Sciences Division, led a team that looked at dairy’s impact on regulating blood sugar levels in people with metabolic syndrome.
The project was motivated by previous observational studies suggesting people who ate the most yogurt or full-fat dairy foods tended to have a lower risk of Type 2 diabetes.
Findings from the new Fred Hutch-led trial showed the body’s ability to regulate blood sugar levels was not directly affected by whether participants consumed dairy foods. However, consumption of either low-fat or full-fat milk, yogurt, and cheese reduced insulin sensitivity.
Metabolic syndrome is a group of risk factors that raises risk of heart disease, diabetes, stroke and other health problems. Insulin sensitivity refers to how the body’s cells respond to insulin. High insulin sensitivity allows the cells of the body to use blood sugar more effectively, reducing blood sugar.
The study involved 72 volunteer men and women who had the metabolic syndrome. Using a parallel-design, randomized, controlled trial, the research team randomized the volunteers into three groups over 12 weeks: A limited dairy diet, consisting of no dairy foods other than — at most — three servings of skim milk per week; a low-fat dairy diet, consisting of more than three servings of skim milk, nonfat yogurt and low-fat cheese per day; and a full-fat dairy diet, consisting of more than three servings of whole milk, full-fat yogurt and full-fat cheese per day.
After the 12 weeks, Kratz and his team measured a variety of biomarkers, including blood sugar throughout a glucose tolerance test, systemic inflammation and liver-fat content. They found blood sugar regulation was not directly affected by whether participants consumed dairy foods.
However, participants on the full-fat dairy diet gained a modest amount of weight, and participants on both low-fat and full-fat dairy diets saw a decrease in insulin sensitivity.
A reduction in insulin sensitivity could lead to an increased risk of Type 2 diabetes in the long term; however, because blood sugar levels were not affected by dairy foods, the long-term impact of reduced insulin sensitivity in people eating a diet rich in dairy on Type 2 diabetes risk is unclear.
“Unlike previous observational studies which suggested a beneficial relationship between fermented dairy foods such as yogurt, as well as high-fat dairy foods, and better metabolic health, our rigorous randomized, controlled trial could not confirm that eating more dairy foods lowers people’s blood sugar levels,” Kratz said.
“While more work needs to be done examining the impact of diets rich in dairy in healthy populations, the finding of reduced insulin sensitivity that resulted from higher dairy intake may be concerning for people with metabolic syndrome and similar conditions such as prediabetes or Type 2 diabetes.”
Kratz said a single study should always be interpreted with caution. First, dairy-rich diets had not reduced insulin sensitivity in all previous trials. He added it is important to consider that even though the dairy-rich diets reduced insulin sensitivity, this did not lead to higher blood sugar levels in these participants.
Because blood sugar is the clinical endpoint the research team cared more about (and it was the primary endpoint in this trial), he said interpretation is not straightforward.
Also, when evaluating the overall health effects of a food group such as dairy, the food’s impact on the regulation of blood sugar levels is just one of many considerations.
Kratz is a member of the American Journal of Clinical Nutrition editorial board. He received funding for this study from several dairy organizations, including the United States National Dairy Council and the Dairy Farmers of Canada.
The funding dairy-related organizations suggested changes to details of the study design prior to the conduct of the study, some of which were implemented. Otherwise, the funding organizations had no impact on the design or conduct of the trial, or on the analysis and interpretation of study data.
The research team also received multiple grants from the National Institutes of Health and the Department of Veteran Affairs that indirectly supported this project.