Peanut butter 'good for the heart'

“Peanut butter wards off heart disease,” the Daily Mail has reported. The newspaper said that peanut butter sandwiches could be the secret to beating heart disease after scientists found that snacking on nuts five days a week can halve the risk of a heart attack.

This news comes from a large, in-depth study which followed 6,309 women with type 2 diabetes over an average of 12 years. Researchers collected information on the women’s diet and health status every two to four years. They found that, in women who were free from heart disease at the start of the study, consuming nuts and peanut butter five or more days a week reduced their risk of developing heart disease or strokes during the follow-up period. The study’s strengths are its large size and regular follow-up, but there are design limitations to bear in mind when interpreting its results.

Although nuts are high in monounsaturated ‘good’ fats, they are still very high in overall fat and calories and should not be eaten in excessive daily amounts.

Where did the story come from?

Tricia Li and colleagues of Harvard Medical School and Harvard School of Public Health carried out this research. The study was funded by the National Institute of Health in the US and was published in the peer-reviewed Journal of Nutrition.

What kind of scientific study was this?

This was a cohort study which examined the association between nut intake and coronary heart disease (CHD) events, such as heart attack, in women with type 2 diabetes.

The women who participated in the study were members of the larger Nurses’ Health Study (NHS), which was established in 1976. As part of the NHS study, these women have completed questionnaires every two years to identify new diagnoses of CHD, stroke and other diseases, and have provided information about their lifestyle and possible health risk factors.

This new study specifically looked at women who had been diagnosed with type 2 diabetes using specific diagnostic criteria. The study followed a number of women from 1980, but later added further suitable women who were diagnosed while the study was underway. Participants were followed until the occurrence of a cardiovascular event, death or the end of the follow-up period in June 2002. Women with existing coronary heart disease at the start of the study were excluded. This left the researchers with a total cohort of 6,309 women.

Food frequency questionnaires were completed in 1980 and, afterwards, every two years. Participants reported typical consumption of selected foods and drink over the previous year. Nutritional content of these estimates was analysed.

In the 1980 and 1984 dietary questionnaires, participants were asked how often they had eaten nuts (broken down into servings of 28g/1oz) during the previous year. In later years, they were asked about peanuts and other nuts separately. Peanut butter consumption was also recorded, but a serving was considered to be 16g/1 tablespoon. The researchers added up the total nut and peanut butter amounts consumed. The responses were combined into four exposure categories: almost never, one to three servings a month to one serving a week, two to four servings a week, and at least five servings a week.

In 1989-90, blood samples were taken from 18.5% of the women to check blood cholesterol levels. Cardiovascular events for the study were fatal CHD, nonfatal heart attack, stroke and coronary artery surgery (confirmed by medical records and test results). The researchers did not include angina.

The risk of a cardiovascular event was calculated according to nut consumption, with adjustment for several possible confounders, including body mass index, physical activity, smoking, alcohol consumption, use of aspirin and HRT, and other nutrients and food groups that are believed to have an effect on heart risk.

The researchers calculated ‘person-years’ for individual participants (by multiplying the subjects’ time in the study by the number of participants followed). Time in the study was defined as the date of each woman’s entry into the study until the date of an event.

What were the results of the study?

The study follow-up involved a total of 54,656 person-years from 1980 to 2002, during which time there were 634 cardiovascular events (452 heart attacks and 182 strokes). The researchers found that:

Women who ate more nuts were more physically active and smoked less than those who ate fewer nuts.
Women who ate at least five servings of nuts a week generally had higher intakes of polyunsaturated fat, red meat, fruits and vegetables, and total energy.
Reduced levels of LDL ‘bad’ cholesterol were seen in those women who ate at least five servings of nuts a week. This was only true of those women who had blood samples available, which was around one-fifth of participants. Levels of HDL ‘good’ cholesterol were not increased.

After controlling for other cardiovascular risk factors, higher consumption of nuts and peanut butter (5 or more servings a week) was associated with a 44% decreased risk of a cardiovascular disease event or heart attack (95% confidence interval 3 to 67%) compared to “almost never” consuming them.

However, there was not a general trend towards decreased risk with increased consumption of nuts. Adjustment for other dietary variables made the risk reduction slightly smaller.

What interpretations did the researchers draw from these results?

The researchers concluded that frequent nut and peanut butter consumption was associated with a decreased risk of cardiovascular disease in women with type 2 diabetes.

What does the NHS Knowledge Service make of this study?

This is a large, in-depth study that followed 6,309 women with type 2 diabetes over an average of 12 years, collecting information on their diet and health status every two to four years. The study found that, in women who were free from heart disease at the start, high consumption of nuts and peanut butter reduced the risk of developing heart disease or strokes during the follow-up period.

The study has strengths in its large size and regular follow-up, and there are a few additional points to bear in mind:

The study relied on self-reported consumption estimates in a food frequency questionnaire. Even though this is a validated method, there may be inaccuracies due to the estimations of portion size and variability between individuals in reporting this.
Consumption was unlikely to have remained the same over time, particularly over such an extensive follow-up period as this.
Although the researchers made careful attempts to adjust for other possible confounders that may have affected the risk, it is possible that other factors linked to cardiovascular outcome were not taken into account.
The ideal quantity of nuts needed to achieve the degree of benefit seen cannot be derived from these results. Consumption of over five servings a week was significantly associated with decreased risk, but there was no general trend towards decreased cardiovascular risk with increased consumption of nuts.
The study did not differentiate between different types of nuts or different types of nut butter, so it is not possible to say whether one type of nut could be more beneficial than another.
Blood samples were only taken from the 20% of the women who were willing to provide them, so the association between nut consumption and blood cholesterol levels should be interpreted with caution and may not be applicable to the whole sample.
This was a cohort of women with type 2 diabetes. Results may be different in other population groups.