Nuts and your health: Cracking old myths

(This article was first printed in the May 2005 issue of the Harvard Men’s Health Watch. For more information or to order, please go to www.health.harvard.edu/mens.)

Few would accuse Harvard's researchers of being health nuts, but they may actually deserve that designation. They have teamed up to show that nuts really are healthy, especially for men at risk for heart disease.

Harvard men — and women

Researchers from Harvard Medical School and the Harvard School of Public Health have examined how eating nuts affects the cardiovascular health of men and women. One study evaluated 21,545 men enrolled in the Physicians’ Health Study. All the participants were between the ages of 40 and 84 when the study began, and none had been diagnosed with heart disease or high blood pressure. Researchers began tracking the subjects in the early 1980s, and they have continued to evaluate many of the men through more than two decades. The nut study, though, is based on a much shorter (12-month) observation period. Over the course of that year, men who ate nuts two or more times a week enjoyed a 47% lower risk of sudden cardiac death and a 30% lower risk of dying from all types of coronary artery disease than men who eschewed nuts.

It’s an important finding, but it has two limitations. First, the observation period was relatively short. Second, the men who ate the most nuts also had slightly better health habits (fewer smokers, more exercisers) and better blood pressures, though they did have more diabetes.

Before you seize on these caveats to dismiss the findings as nutty, consider a previous Harvard study. The subjects were 86,016 women who were free of cardiovascular disease and between 34 and 59 when the research began. Investigators tracked the volunteers for 14 years. During that time, women who ate at least 5 ounces of nuts per week were 35% less likely to suffer heart attacks than women who ate less than 1 ounce a month. Women who ate an intermediate amount of nuts experienced an intermediate degree of protection against fatal and nonfatal heart attacks. Moreover, a careful analysis of other risk factors showed that the protective power of nuts could not be explained away by other dietary patterns (fat, fiber, vegetables, fruits), health habits (smoking, drinking, exercise), or risk factors (blood pressure, cholesterol, diabetes, obesity, family history). And in a separate report, the Nurses’ Health Study linked a high consumption of nuts and peanut butter to a reduced risk of diabetes.

It’s not as nutty as it sounds: Eating nuts promotes cardiovascular health.

Other populations

All the subjects of the Harvard studies were doctors or nurses. Whether they admit it or not, health care providers are as vulnerable to illness as other people. Do nuts reduce cardiac risk in other population groups?

They do.

Scientists in California evaluated diet and heart disease in 31,208 Seventh-Day Adventists. After six years of observation, the researchers found that people who ate nuts at least four times a week had suffered 51% fewer heart attacks than those who ate fewer nuts. But were the nuts themselves responsible, or did the nut eaters benefit from other protective influences? The study evaluated diet, obesity, exercise habits, smoking, blood pressure, cholesterol levels, and age without finding any trends that could subtract from the protective power of nuts. In fact, the only other protective food was whole wheat bread.

The California research included both men and women, but because they were all Adventists, they all had relatively healthful lifestyles. Do the findings also apply to people of other persuasions? A study of 41,837 Iowans tells us they do. As in the other studies, the people who ate the most nuts had the fewest heart attacks. Like the California study, the Iowa research did not detect differences in any of the known cardiac risk factors that could explain away the apparent benefit of nuts. But although it was a careful investigation, the scientists failed to discuss one major risk factor — gender — because they were working with data from the Iowa Women’s Health Study. Still, men would be nuts to overlook results from the opposite sex.

If nuts can reduce a healthy person’s risk of developing heart disease, can they also help patients who have already suffered an attack? To find out, doctors in India randomly divided more than 500 heart attack survivors into two groups. One received standard medical care and a low-fat diet; the other got the same care but ate a diet supplemented by extra portions of nuts, grains, vegetables, and fruits. The special diet resulted in better cholesterol levels, fewer recurrent heart attacks, and a lower death rate. It’s not possible to single out nuts as the protective element, but if nothing else, the study shows that diet can fight heart disease, even in patients recovering from heart attacks.

How do nuts help?

Doctors don’t know for sure, but they have several theories.

The best bet is that nuts help reduce blood cholesterol levels either by displacing other harmful foods or by lowering cholesterol on their own. A series of studies dating back to 1993 supports this possibility. All the experiments were intense, short-term trials, in which volunteers were given strictly controlled diets for periods of up to 6 weeks.

Although the details vary, all the studies shared a basic design, testing diets enriched with nuts against diets that lacked nuts but were otherwise identical in their fat and caloric contents. For example, a 1993 trial evaluated 18 healthy male volunteers who were divided into two groups, then placed on diets providing 30% of calories from fat and conforming to the Step 1 diet of the National Cholesterol Education Program. Half the men got their dietary fat from the usual foods; the others got two-thirds of their fat from walnuts. At the end of a month, the walnut eaters had lower cholesterol levels, enjoying a 12% drop, which could translate into a 20%–30% decline in the risk of heart disease if it was maintained.

Walnuts were also used in a 2000 study that reported lower cholesterol levels; almonds and macadamia nuts have produced similar benefits in other trials, most of which were funded by the nut industry. By themselves, nuts seem to produce a modest fall in cholesterol, but when they are combined with other healthful foods, the results can be spectacular. Researchers in Canada proved the point by comparing three low-fat vegetarian diets. All three produced lower cholesterol levels, but the diet that contained about 2.2 ounces of nuts a day reduced LDL (“bad”) cholesterol levels by about 30% — which would be a good day’s work for a powerful cholesterol-lowering drug.

Although these results are impressive, trials don’t necessarily translate into real life, particularly when they involve portions that most folks would call nutty. Still, much smaller “doses” of nuts — as little as two portions a week in the Physicians’ Health Study — appear to reduce risk in large groups of people picking their own foods and leading ordinary daily lives.

To account for the apparent benefits of even modest quantities of nuts, the Harvard scientists speculate that protection involves more than improved cholesterol levels. Nuts are high in fat, but they contain “good” fats, which may reduce the risk of abnormal heart pumping rhythm, perhaps including ventricular fibrillation, the cause of sudden cardiac death. It’s why fish oils are considered protective, but walnuts are the only nuts that contain appreciable amounts of omega-3 fatty acids, the chief beneficial elements in fish. Nuts are also rich in arginine, a tiny molecule that increases production of nitric oxide (NO). NO improves endothelial function. A 2004 Spanish study demonstrated this beneficial effect for walnuts, and it is probably true for other nuts as well.

Under the shell

What’s in a nut? Plenty; nuts pack many nutrients in a small package.

The major nutrients are fats, but they are special fats. Unlike animal products, nuts have no cholesterol and contain only tiny amounts of saturated fat. Instead, nuts have lots of mono- and polyunsaturated fats that resemble the fats in olives and other vegetables that may help protect the heart. Nuts may also help by providing vitamin E, an antioxidant. They are also rich in protein, B vitamins, and minerals; Brazil nuts are particularly high in selenium, the mineral that’s been linked to a reduced risk of prostate cancer. And nuts are an excellent source of fiber, which reduces the risk of heart disease.

Nuts to you?

Before you go nuts, consider two provisos. First, because they are high in fat, nuts are high in calories. All fats, whether they are harmful saturated fats or desirable unsaturated fats, have 9 calories per gram, making them the most calorie-dense nutrient. So unless you want to gain weight, don’t add nuts to your diet without subtracting a similar number of calories; for example, substitute nuts for chips or cookies. Second, remember that many processed nuts that are so handy for snacks are fried in oil and/or laced with salt, which can raise your blood pressure.

You’ll have to choose your nuts carefully to make them work for your health. Even then, they are not the answer to heart disease, but they can be a part of it. By incorporating nuts into a balanced, healthful diet you can take a big step away from heart disease.

(This article was first printed in the May 2005 issue of the Harvard Men’s Health Watch. For more information or to order, please go to www.health.harvard.edu/mens.)

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