For 20 years a team of researchers watched more than 2,400 Finnish men eat. What they learned gives us a glimpse into some of the long-term health effects of one of today’s most popular dieting fads.

The fascination with high-protein diets isn’t new. In the 1970s, American doctor Robert Atkins popularized the idea with his namesake eating plan, designed to help people lose weight fast. Today, similar regimens are still hitting the market. In April 2018, Pierre Dukan, a French doctor, introduced the Dukan Diet, which also emphasizes limited carbohydrate intake while promoting protein.

Despite the popularity of such diets, the research on how they impact heart health has been relatively scant. A new study, though, published this week by the American Heart Association, shows that eating a lot of protein—derived from both plants and animals—is linked to an elevated risk of cardiovascular failure. Even still, the University of Eastern Finland researchers who undertook the study noted in the paper that many unanswered questions about their findings remain. For example, it’s not yet clear how and why various amino acids abundant in animal protein sources often lead to poor heart function. Also left unanswered: why fermented dairy foods (which also contain protein) such as cheese are worse for heart health than unfermented dairy, such as milk.

This research project began in 1984, when scientists launched the Kuopio Ischaemic Heart Disease Risk Factor Study. As part of the study, scientists recruited 2,441 middle-aged and older men and tracked their daily protein consumption for an average of 22 years.

Each participant was assessed on how much dairy, animal, plant, and overall protein they ate. In each of those four categories, the participants were divided into four quartiles; the researchers then compared the top quartile in each category to the bottom quartile. So, for example, they looked at the risk of heart failure for the 25% who ate the most dairy protein compared to the 25% who ate the least dairy protein.

In general, a higher intake of protein was associated with a greater risk of heart failure. A total of 334 people experienced heart failure over the course of the research period. But those study participants who ate (and drank) the most dairy and animal protein sources had the highest risk for heart failure—and there was no correlation found between heart failure and consumption of fish and egg protein. The results give health experts even more reason to promote diets rich in vegetables, fish, beans, and nuts.

These findings are especially important for establishing policies and health guidance in wealthier countries, where protein consumption is typically higher than in poorer parts of the world. In the US, for instance, the average person eats about 100 grams (a little less than 0.25 lb) (paywall) of protein per day, about double the recommended amount given by the Institute of Medicine. Even vegans, who don’t eat meat, typically—in the US—consume 60 to 80 grams per day from foods such as beans, nuts, and broccoli.

All that protein contributes to broader heart health trends. Over 610,000 people die of heart disease in the US each year, accounting for about 25% of total deaths, according to the US Centers for Disease Control and Prevention. More than 735,000 Americans have a heart attack and 28% of those are a second heart attack annually. The average daily protein intake across the developing world—places that include China and India—hovers around 60 to 80 grams per day, according to the United Nations’ Food and Agriculture Organization. In sub-Saharan Africa, the average is about 55 grams per day.