Kerry L. Ivey is a nutritional epidemiologist, clinical dietitian, and bioinformatician. She is a Harvard Medical School assistant professor of medicine at Brigham and Women’s Hospital and the corresponding author of a review of yogurt and human health in the Journal of Nutrition, Health and Aging. 

People have been making health claims about yogurt for hundreds of years. The Mongol emperor Genghis Khan, in the late 12th and early 13th centuries, reportedly believed yogurt would give his army strength and bravery; it’s only recently that the science is catching up. Bravery: probably not. But a lot of my research looks at how we can stay vital and healthy as we age, and if we can increase our protein intake while reducing chronic disease risk, which yogurt can help us do, then maybe Genghis Khan was onto something. 

Now, there’s an interesting paradox here. Yogurt, particularly full-fat yogurt, can contain saturated fat, which has traditionally raised concerns it increases LDL cholesterol and cardiovascular disease risk. Meta-analysis of randomized controlled trials has found that probiotic yogurt can actually reduce total LDL cholesterol in people with mild to moderate elevated cholesterol, while large observational studies suggest that people who regularly eat yogurt tend to have better heart health.

In large U.S. studies, eating yogurt at least five times per week was associated with a 16 percent lower risk of high blood pressure, and among people with high blood pressure, eating yogurt at least twice a week was associated with a 17 percent lower risk of cardiovascular disease in women and 21 percent lower risk in men. And particularly interesting given concerns about saturated fat, a large Danish study found that replacing low-fat and high-fat milk with whole-fat yogurt was associated with an 11-13 percent lower risk of heart disease. These findings illustrate what we call the “food-matrix effect,” where the health effects of a whole food cannot necessarily be predicted from one nutrient, such as saturated fat, considered in isolation.

The problem with these observational studies is that they can’t show causality. Results are susceptible to confounding by other variables like overall diet, lifestyle, and socioeconomic factors. Exactly how and why yogurt is beneficial, and for whom, at what stage of life, is not clear. 

All of this paints a complicated picture for the consumer standing in the grocery store, looking at so many different products. Some have high protein, some have more sugar, some have more fat. There’s no straightforward answer, no magic number of servings of yogurt that everyone should eat. I think of it more like a nutrient-dense component of an overall healthy dietary pattern. That might be having yogurt with fruit and whole grains at breakfast or having some yogurt as a snack instead of something more highly processed or higher calorie, or even using plain yogurt in savory dips or dressings. 

But we still have a lot to learn — both about the mechanisms by which yogurt is beneficial, and about the strains of bacteria, which are actually very, very different from one another all the way up to their taxonomic class. My hope is that by continuing to study this food, we can get closer to personalized nutrition — the right type of yogurt with the right type of bacteria, for the right person, at the right time. 

— As told to Sy Boles, Harvard Staff Writer