Increasing the ratio of plant-based protein in your diet may reduce your risk of cardiovascular disease and coronary heart disease, finds a new study led by researchers at Harvard T.H. Chan School of Public Health.

According to the researchers, these risk reductions are likely driven by the replacement of red and processed meats. The researchers also observed that a combination of consuming more plant protein and higher protein intake overall provided the most heart health benefits.

While global dietary guidelines recommend higher intake of plant protein, the ideal ratio of plant to animal protein has remained unknown. The study is the first to investigate this ratio and how it impacts health, specifically heart health.

Risk reductions are likely driven by the replacement of red and processed meat with several plant protein sources, particularly nuts and legumes.

“The average American eats a 1:3 plant to animal protein ratio. Our findings suggest a ratio of at least 1:2 is much more effective in preventing cardiovascular disease. For coronary heart disease prevention, a ratio of 1:1.3 or higher should come from plants,” said lead author Andrea Glenn, visiting scientist in the Department of Nutrition. Glenn worked on the study as a postdoctoral fellow at Harvard Chan School and is now an assistant professor in the Department of Nutrition and Food Studies at NYU Steinhardt.

The study was published Dec. 2 in the American Journal of Clinical Nutrition.

The researchers used 30 years of data on diet, lifestyle, and heart health among nearly 203,000 men and women enrolled in the Nurses’ Health Studies I and II and the Health Professionals’ Follow-up Study. Participants reported their dietary intake every four years. The researchers calculated each participant’s total protein intake, measured in grams per day, as well as their specific intakes of animal and plant proteins. Over the course of the study period, 16,118 cardiovascular disease cases, including over 10,000 coronary heart disease cases and over 6,000 stroke cases, were documented.

After adjusting for participants’ health history and sociodemographic and lifestyle factors, the study found that eating a higher ratio of plant to animal protein was associated with lower risks of cardiovascular disease and coronary heart disease. Compared to participants who consumed the lowest plant to animal protein ratio (~1:4.2), participants who consumed the highest (~1:1.3) had a 19 percent lower risk of cardiovascular disease and a 27 percent lower risk of coronary heart disease. These risk reductions were even higher among participants who ate more protein overall. Those who consumed the most protein (21 percent of energy coming from protein) and adhered to a higher plant to animal protein ratio saw a 28 percent lower risk of cardiovascular disease and a 36 percent lower risk of coronary heart disease, compared to those who consumed the least protein (16 percent of energy). No significant associations were found for stroke risk and the ratio; however, replacing red and processed meat in the diet with several plant sources, such as nuts, showed a lower risk of stroke.

The researchers also examined if there’s a point at which eating more plant protein stops having added benefits or could even have negative implications. They found that risk reduction for cardiovascular disease begins to plateau around a 1:2 ratio, but that coronary heart disease risk continues to decrease at higher ratios of plant to animal protein.

According to the researchers, replaving red and processed meat with plant protein sources, particularly nuts and legumes, have been found to improve cardiometabolic risk factors, including blood lipids and blood pressure as well as inflammatory biomarkers. This is partly because plant proteins are often accompanied by high amounts of fiber, antioxidant vitamins, minerals, and healthy fats.

“Most of us need to begin shifting our diets toward plant-based proteins,” said senior author Frank Hu, Fredrick J. Stare Professor of Nutrition and Epidemiology at Harvard Chan School. “We can do so by cutting down on meat, especially red and processed meats, and eating more legumes and nuts. Such a dietary pattern is beneficial not just for human health but also the health of our planet.”

The researchers pointed out that the ratios they identified are estimates, and that further studies are needed to determine the optimal balance between plant and animal protein. Additionally, further research is needed to determine how stroke risk may be impacted by protein intake.

Other Harvard Chan authors included Fenglei Wang, Anne-Julie Tessier, JoAnn Manson, Eric Rimm, Ken Mukamal, Qi Sun, and Walter Willett.

The Nurses’ Health Studies and Health Professional Follow-up Studies are supported by National Institutes of Health grants UM1 CA186107, R01 CA49449, R01 HL034594, U01 HL145386, R01 HL088521, U01 CA176726, R01 CA49449, U01 CA167552, R01 HL60712, and R01 HL35464.