On the face of it, the high calorie and fat content of nuts may lead you to think that you should eat them sparingly. But for decades, nutrition experts have encouraged nut eating because of the bevy of nutrients in just a small serving. A number of studies have suggested that nuts can help cut the risk of heart disease and type 2 diabetes, and aid weight loss, too. 

Now, in one of the largest studies to date, researchers from Harvard University have found that just a few servings of nuts per week may help keep your heart healthy. And more specifically, they found that walnuts and peanuts—but not peanut butter—may give your heart a boost.

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The researchers, whose findings were published today in the Journal of the American College of Cardiology, analyzed data from more than 210,000 health professionals (a majority of whom were female nurses) over as many as 32 years from the Nurses’ Health Study and the Health Professionals Follow-Up Study.

Compared with those who never or almost never ate nuts, those who ate an ounce of nuts five or more times per week had a 14 percent lower risk of cardiovascular disease (defined as a heart attack or stroke, or death from cardiovascular disease) and a 20 percent lower risk of coronary heart disease (defined as a fatal or nonfatal heart attack or stroke) during the study period.

“National recommendations are to eat between three to seven 1-ounce servings of nuts per week,” says the study’s lead author, Marta Guasch-Ferré, Ph.D., a research fellow in the nutrition department at the Harvard T.H. Chan School of Public Health. “Our results fit nicely within that range, showing a lot of benefits associated with eating five servings or more.” 

The researchers also looked at the benefits associated with eating walnuts, peanuts, and peanut butter, and found that those who ate one or more servings of walnuts per week had a 19 percent lower risk of cardiovascular disease, a 21 percent lower risk of coronary heart disease, and a 17 percent lower risk of stroke. Those who ate two or more servings of peanuts per week had more modest reductions in risk of cardiovascular disease and coronary heart disease—13 and 10 percent, respectively.

“The data suggest that even alone, peanuts and walnuts are associated with a reduction in cardiovascular disease,” says Richard D. Wainford, Ph.D., an associate professor of pharmacology and medicine at the Whitaker Cardiovascular Institute at the Boston University School of Medicine. He was not involved with the study.

(Some of the study's authors reported having received funding from the Peanut Institute and the California Walnut Commission, but said it wasn't used for this particular study.)

But Guasch-Ferré says researchers found no heart benefits associated with consuming peanut butter. “This doesn’t mean that it’s bad for your health,” she says, “we just didn’t observe an effect.”

This may be because people tend to pair peanut butter with unhealthy foods, such as white bread and sodas. In an editorial accompanying the study, Emilio Ros, M.D., Ph.D., director of the Lipid Clinic and senior consultant at the Endocrinology and Nutrition Service at Hospital Clínic in Barcelona, Spain, also notes that peanut butter is typically mixed with salt and sweeteners such as honey or sugars, which may cancel out any positive health benefits of the peanuts themselves. 

Wainford notes that the new findings mesh nicely with previous studies. However, the study didn't prove that nuts were directly responsible for boosting participants’ health because the analysis required people to remember what they ate, which could introduce error. It also may be that nut eaters are simply healthier than those who don’t eat nuts to begin with.

Additionally, because all of the study participants were white, the findings can't be extended to everyone, Guasch-Ferré says, but there’s nothing to suggest the results would be different. 

What’s So Special About Nuts, Anyway?

As with whole grains and legumes, even small servings of nuts are packed with nutrients.

They’re rich in unsaturated fats (the good kind of fats thought to have heart-protective properties), such as oleic acid and polyunsaturated fatty acids (found in walnuts and sunflower seeds).

They’re also chock-full of complex carbs, fiber, protein, minerals such as calcium and magnesium, tocopherols (a form of vitamin E), polyphenols (nutrients known for their antioxidant properties), and phytosterols—plant compounds similar in structure to cholesterol, which may be responsible for the cholesterol-lowering effect of nuts.

Ros says in his editorial that nuts are highly satisfying, which may partly explain why past studies have found that despite being high in fat, they might help you lose weight.

He also notes that walnuts may be particularly heart-healthy because they’re a rich source of alpha-linoleic acids, a type of omega-3 fatty acid thought to protect against heart disease and high cholesterol and blood pressure. People may also get more of an antioxidant boost from walnuts because they’re commonly eaten raw rather than roasted, which keeps their papery antioxidant-rich skin intact. Peanuts, which are technically legumes, are similar nutritionally to nuts in terms of the types of fatty acids, proteins, carbohydrates, and antioxidants they contain, Guasch-Ferré says.

In fact, she suggests that it’s a good idea to incorporate a variety of nuts into your diet in general, because each type supplies a different mix of nutrients. “For example," Guasch-Ferré says, "almonds and hazelnuts are higher in monounsaturated fatty acids, and walnuts are higher in polyunsaturated fatty acids (omega-3s)."