An Evidence-based Approach to Phytochemicals and Other Dietary Factors

4 Nuts

In the not too distant past, nuts were considered unhealthy because of their relatively high fat content. In contrast, recent research suggests that regular nut consumption is an important part of a healthful diet.1 Although the fat content of nuts is relatively high (14–19 g/oz or 49–67%), most of the fats in nuts are the healthier, monounsaturated and polyunsaturated fats (see Table 4.1)². The term “nuts” includes almonds, Brazil nuts, cashews, hazelnuts, macadamia nuts, pecans, pistachios, walnuts, and peanuts. Despite their name, peanuts are actually legumes like peas and beans. However, they are nutritionally similar to tree nuts and have some of the same beneficial properties.

Disease Prevention

Cardiovascular Disease

Coronary Heart Disease

In large prospective cohort studies, regular nut consumption has been consistently associated with significant reductions in the risk of coronary heart disease (CHD).3 One of the first studies to observe a protective effect of nut consumption was the Adventist Health Study, which followed more than 30 000 Seventh Day Adventists over 12 years.⁴ In general, the dietary and lifestyle habits of Seventh Day Adventists are closer to those recommended for cardiovascular disease prevention than those of average Americans. Few of those who participated in the Adventist Health Study smoked, and most consumed a diet lower in saturated fat than the average American. In this healthy group, those who consumed nuts at least five times weekly had a 48% lower risk of death from CHD and a 51% lower risk of a nonfatal myocardial infarction (MI) compared with those who consumed nuts less than once weekly.⁴ In Seventh Day Adventists who were older than 83 years of age, those who ate nuts at least five times weekly had a risk of death from CHD that was 39% lower than those who consumed nuts less than once weekly.⁵ A smaller prospective study of more than 3 000 black men and women reported similar results.⁶ Those who consumed nuts at least five times weekly had a risk of death from CHD that was 44% lower than those who consumed nuts less than once weekly.⁶

The cardioprotective effects of nuts are not limited to Seventh Day Adventists. In a 14-year study of more than 86 000 women participating in the Nurses’ Health Study (NHS), those who consumed more than 5 oz of nuts weekly had a risk of CHD that was 35% lower than those who ate less than 1 oz of nuts monthly.⁷ Similar decreases were observed for the risk of nonfatal MI and death from CHD. More recently, a 17-year study of more than 21 000 male physicians found that those who consumed nuts at least twice weekly had a risk of sudden cardiac death that was 53% lower than those who rarely or never consumed nuts, although there was no significant decrease in the risk of nonfatal MI or nonsudden CHD death.⁸ A follow-up analysis in this cohort of male physicians found that nut consumption was not associated with incident heart failure.⁹ The Iowa Women's Health Study, which followed more than 30 000 postmenopausal women for 12 years, is the only published prospective study that did not observe a significant inverse association between nut consumption and CHD mortality, although a slight but significant decrease in all-cause mortality was observed in those who consumed nuts twice weekly.¹⁰ Overall, the results of most prospective cohort studies suggest that regular nut consumption is associated with a substantial decrease in the risk of death related to CHD. In fact, a recent pooled analysis of four of the US epidemiological studies mentioned above found those with the highest intake of nuts (around five times per week) had a 35% lower risk of CHD.¹¹

Results of controlled clinical trials indicate that at least part of the cardioprotective effect of nuts is derived from beneficial effects on serum total and low-density lipoprotein (LDL) cholesterol concentrations.3 At least 18 controlled clinical trials have found that adding nuts to a diet that is low in saturated fat results in significantly reductions in serum total cholesterol and LDL cholesterol concentrations in people with normal or elevated serum cholesterol. These effects have been observed for almonds,^12–15 hazelnuts,¹⁶ macadamia nuts,^17–19 peanuts,^20,21 pecans,²² pistachio nuts,^23,24 and walnuts.^25–30 More recently, a cross-sectional study found that frequent nut and seed consumption was associated with lower serum levels of inflammatory biomarkers in a multi-ethnic population.³¹ Although the evidence is circumstantial, these findings suggest that compounds in nuts may lower the risk of cardiovascular disease by decreasing inflammation.

Cardioprotective Compounds in Nuts

Substituting dietary saturated fats with polyunsaturated and monounsaturated fats like those found in nuts can decrease serum total and LDL cholesterol concentrations.3 However, in some clinical trials, the cholesterol-lowering effect of nut consumption was greater than would be predicted from the polyunsaturated and monounsaturated fat content of the nuts, suggesting there may be other protective factors in nuts.³² Other bioactive compounds in nuts that may contribute to their cholesterol-lowering effects include fiber (Chapter 16) and phytosterols (Chapter 18).³³ See Table 4.1 for the unsaturated fat, fiber, and phytosterol content of selected nuts. Walnuts are especially rich in α-linolenic acid, an omega-3 fatty acid with several cardioprotective effects, including the prevention of cardiac arrhythmias that may lead to sudden cardiac death (Chapter 20). Other nutrients that may contribute to the cardioprotective effects of nuts include folate, vitamin E, and potassium.^3,33–35 The US Food and Drug Administration has acknowledged the emerging evidence for a relationship between nut consumption and cardiovascular disease risk, by approving the following qualified health claim for nuts³⁶: “Scientific evidence suggests but does not prove that eating 1.5 oz/day of most nuts as part of a diet low in saturated fat and cholesterol may reduce the risk of heart disease.”

Type 2 Diabetes Mellitus

Recent results from the NHS suggest that nut and peanut butter consumption may be inversely associated with the risk of type 2 diabetes mellitus (DM) in women.37 In this cohort of more than 86 000 women followed over 16 years, those who consumed an ounce of nuts at least five times weekly had a risk of developing type 2 DM that was 27% lower than in those who rarely or never consumed nuts. Similarly those who consumed peanut butter at least five times weekly had a risk of developing type 2 DM that was 21% lower than in those who rarely or never consumed peanut butter. While these findings require confirmation in other studies, they provide additional evidence that nuts can be a component of a healthful diet. Compounds in nuts that could contribute to the observed decrease in type 2 DM include unsaturated fats, fiber, and magnesium.

Body Weight

A major concern is that increased consumption of nuts may cause weight gain and obesity. However, several cross-sectional analyses of large cohort studies, including the Adventist Health Study 5 and the NHS,⁷ have shown that individuals who consume nuts regularly tend to weigh less than those who rarely consume them. Recently, a 28-month prospective study conducted in Spain found that participants who consumed higher amount of nuts had lower risk of weight gain than those who rarely ate nuts.³⁸ A similar association was observed in the NHS II.³⁹ These epidemiologic data indicate that in free-living subjects, higher nut consumption does not cause greater weight gain; rather, incorporating nuts into diets may be beneficial for weight control. It is possible that higher amounts of protein and fiber in nuts enhance satiety and suppress hunger.

Safety

Nut Allergies

Allergies to peanuts and tree nuts (almonds, cashews, hazelnuts, pecans, pistachios, and walnuts) are among the most common food allergies, affecting at least 1% of the US population.40 Although all food allergies have the potential to induce severe reactions, peanuts and tree nuts are among the foods most commonly associated with anaphylaxis, a life-threatening allergic reaction.⁴¹ People with severe peanut or tree nut allergies need to take special precautions to avoid inadvertently consuming peanuts or tree nuts, by checking labels and avoiding unlabeled snacks, candies, and desserts.

Adverse Effects

Brazil nuts grown in areas of Brazil with selenium-rich soil may provide more than 100 µg of selenium in one nut, while those grown in selenium-poor soil may provide 10 times less.42

Intake Recommendations

Regular nut consumption, equivalent to an ounce of nuts five times weekly, has been consistently associated with significant reductions in CHD risk in epidemiological studies. Consuming 1–2 oz of nuts daily as part of a diet that is low in saturated fat has been found to lower serum total and LDL cholesterol in several controlled clinical trials. Since an ounce of most nuts provides at least 669 kJ (160 kcal), simply adding an ounce of nuts daily to one's habitual diet without eliminating other foods may result in weight gain. Substituting unsalted nuts for other less healthful snacks, or for meat in main dishes, are two ways to make nuts part of a healthful diet.

Summary

• Nuts are good sources of fiber, phytosterols, and unsaturated fat.

• The results of most prospective cohort studies suggest that regular nut consumption (equivalent to 1 oz at least five times weekly) is associated with a significantly lower risk of cardiovascular disease.

• One prospective cohort study has found that regular nut consumption is associated with significantly lower risk of developing type 2 diabetes.

• Most prospective studies have shown that people who consume nuts regularly weigh less than those who rarely consume nuts. Nonetheless, since an ounce of most nuts provides approximately 669 kJ (160 kcal), substituting nuts for other less healthful snacks is a good strategy for avoiding weight gain when increasing nut intake.