The Walnut — A Nutritional Nut Case
Today’s Dietitian
By Nancy M. Lewis, PhD, RD; Jaime Ruud, MS, RD; and Kimberly B. Heidal, PhD, MHS, RD
Vol. 6, No. 8, p. 36

Walnuts are a rich source of polyunsaturated fat. The research continues to show that going nuts over walnuts may be a good thing.

Walnuts have been part of the human diet since the beginning of recorded history with references dating back to Persia in 7000 BC.1 Considered food for gods by the Romans, walnuts were named “Juglans Regia” in honor of Jupiter.2 Today, they are known as “English” walnuts, named after English merchant marines whose ships transported walnuts for trade to ports around the world. There are more than 15 varieties of walnuts, but the two most popular are the English and black walnut. The most consumed and commercially important walnut is the English walnut.

California produces the majority of English walnuts in the United States. The San Joaquin and Sacramento Valleys, with their mild climate and rich fertile soil, are the state’s prime growing regions. California walnuts account for 99% of the commercial U.S. supply and two-thirds of the world supply.2 According to the USDA’s National Agricultural Statistics Service, California’s walnut crop reached a record high of 315 tons in 2003, up 12% from 2002 and 35% from 1995.3

Per-capita consumption of walnuts in the United States, based on food disappearance data, has averaged approximately 0.5 pounds per person since 1980 (see Figure 1).4 Walnut consumption has increased since 1999 due to the news about their role in a healthful diet and their potential health benefits.1

Two major pests affect the yield of walnuts: walnut blight, which is a bacterium, and the codling moth, an insect. Walnut blight is perhaps the most damaging disease of walnuts because it attacks the developing nuts on the tree.5 The prevalence of walnut blight varies from year to year depending on weather conditions. In high rainfall years, growers can lose as much as 45% of their yield to walnut blight.

The codling moth is a destructive pest that is difficult to manage, especially if its population swells over a season or two.6 In 1995, walnut growers had yield loss as high as 20% due to the codling moth. Today, through the judicious use of pesticides and integrated pest management tools, growers have been able to effectively control walnut blight and codling moths to produce higher crop yields.

Nutrient Profile
Table 1 provides a nutrient profile of walnuts. Most nuts are high in monounsaturated fatty acids whereas walnuts are a rich source of the polyunsaturated fatty acids linoleic acid and alpha-linolenic acid (ALA). Of these two essential fatty acids, ALA (an omega-3 fatty acid) is present in considerable amounts in walnuts.7 Compared with other nuts, walnuts are one of the highest concentrated plant sources of ALA, providing approximately 2.6 grams per 1-ounce serving (see Table 2). In the seed family, flaxseed is also a good source of ALA, supplying 1.8 grams per 1-ounce serving. The Dietary Reference Intake for ALA is 1.6 grams per day for men and 1.1 grams per day for women.8 In a recent study of 4,440 subjects who participated in the National Heart, Lung, and Blood Institute Family Heart Study, mean daily consumption of ALA was 0.81 grams per day (range of 0.19 to 3.48) for men and 0.69 grams per day (range of 0.13 to 2.45) for women.9

In addition to being a rich source of polyunsaturated fat, walnuts are a source of phytosterols, plant sterols with similar chemical structures to that of cholesterol. Phytosterols are known to inhibit intestinal absorption of cholesterol, thereby lowering total plasma cholesterol and low-density lipoprotein (LDL) cholesterol levels.7
Walnuts also contain high concentrations of antioxidants (ie, > 20 millimoles per 100 grams). Halvorsen et al assessed total antioxidants in a variety of plant foods, including fruits, berries, vegetables, cereals, and nuts.10 Walnuts were second to berries (dog rose) in total antioxidant activity of all the plants analyzed in the study.

In addition, walnuts contain high amounts of gamma tocopherol, the major form of vitamin E found in nuts and seeds, including walnuts, soybeans, and corn. Other components present in notable quantities in walnuts are protein, dietary fiber, folate, selenium, magnesium, potassium, and phosphorus.

Health Benefits
Recent studies support the health benefit of walnuts, making them part of a healthy diet. In March, the FDA affirmed the health claim: “Supportive but not conclusive research shows that eating 1.5 oz of walnuts per day, as part of a low saturated fat and low cholesterol diet, and not resulting in increased caloric intake may reduce the risk of coronary heart disease.”11 A growing body of evidence shows that regular intake of walnuts has a cholesterol-lowering effect in normal and hypercholesterolemic subjects.
Five studies have evaluated the effects of walnuts on serum lipids.12,13,14,15,16 Each of these studies used a randomized crossover design comparing a low-fat, low-cholesterol diet with one rich in walnuts. In general, the experimental diets, which contained between 41 grams (1.45 ounces) and 78 grams (2.75 ounces) per day of walnuts, reduced both total and LDL cholesterol levels.

In a study involving 10 hypercholesterolemic men, Munoz et al reported that compared with the control diet, the walnut diet reduced serum total and LDL cholesterol by 4.2% and 6.0%, respectively.14 Zambon et al compared the effects of a walnut-rich diet with those of a cholesterol-lowering Mediterranean diet on serum lipid levels of hypercholesterolemic men and women.13

Mean total cholesterol level decreased by 9% during the walnut diet and 5% during the control diet. In a study of healthy Japanese men and women, Iwamoto et al showed that replacing 55% of the energy from fat in a reference diet with walnuts lowered the serum cholesterol concentrations by 5% for women and 4% for men.15
In another study of hypercholesterolemic men and women, Ros and colleagues found that substituting 32% of the energy from monounsaturated fat with walnuts not only decreased total cholesterol and LDL cholesterol but increased endothelium-dependent vasodilation by 64%, suggesting that a diet rich in walnuts has a beneficial effect on vascular health as well.17

Besides improving serum lipid profiles, walnuts may offer additional protection. Reports from prospective studies indicate that high intakes of ALA are associated with reduced risk of fatal ischemic heart disease (IHD).18,19 ALA appears to protect against IHD through its antiarrhythmic effect.19,20 ALAs may also reduce plasma triacylglycerol, an additional independent determinant of atherosclerosis.21 Djousse et al, using data from a large multicenter, population-based study designed to evaluate genetic and nongenetic factors of cardiovascular disease, concluded that high consumption of dietary ALA was inversely related to triacylglycerol concentrations in both men and women.9

There are other components in walnuts that may potentiate cholesterol-lowering effects and independently affect risk factors for heart disease, as well as other chronic diseases. These components include dietary fiber, folic acid, magnesium, vitamin E, and other phytochemicals. However, the data on these components are limited and additional research is needed to confirm their role in reducing heart disease risk.

Walnuts have many benefits. They are one of the highest concentrated plant sources of ALA, an omega-3 fatty acid. There is evidence that ALA may play an important role in heart disease prevention, although more research is needed to confirm this observation. Walnuts also contain high levels of antioxidants compared with other plant foods. In addition, walnuts are an important source of protein, fiber, vitamins, and minerals. Dietitians can help educate their clients about the health benefits of walnuts and ways to incorporate them into the diet. Their convenience and versatility make it easy to add the recommended amount of 1.5 ounces per day.

— Nancy M. Lewis, PhD, RD, is an associate professor of nutrition in the department of education and health sciences at the University of Nebraska.

— Jaime Ruud, MS, RD, is the co-owner of Nutrition Link consulting, a private practice in Lincoln, Neb.

—Kimberly B. Heidal, PhD, MHS, RD, is with the University of Nebraska in Lincoln.

This research was supported in part by funds provided through the Hatch Act and by USDA Cooperative Agreement No. 58-0101-2-143.

The authors would like to gratefully acknowledge Carol Berg Sloan, RD, nutrition consultant to the California Walnut Commission, for her help in preparing this article.

References for this article are available upon request by e-mailing TDEditor@gvpub.com.

Sidebar 1:
Heart Healthy and Much More
A new study shows that unique composition of walnuts improves endothelial function.1

Walnuts may offer cardioprotective effects beyond cholesterol lowering. Researchers at the Hospital Clinic of Barcelona conducted a randomized crossover trial of 21 men and women (aged 25 to 75) with high cholesterol. Subjects followed a cholesterol-lowering Mediterranean diet and a diet of similar energy and fat content in which 1.4 to 2.3 ounces of walnuts daily replaced 32% of the energy from monounsaturated fat. Participants followed the diet for four weeks. Results of the study showed that the walnut diet restored the artery’s elasticity, allowing increased blood flow on demand.

According to Emilio Ros, MD, primary investigator of the study, “This is the first time a whole food, not its isolated components, has shown this beneficial effect on vascular health. In addition, the walnut diet decreased total cholesterol and LDL [low-density lipoprotein] cholesterol levels.

“People with risk factors for heart disease, such as smoking, high blood cholesterol, diabetes, hypertension, or obesity, may have arteries that do not dilate properly. The results of this study provide physicians, dietitians, and clients with a powerful yet simple dietary tool in the fight against heart disease,” says Ros.

Sidebar 2:
It’s Easy to Add a Handful a Day
• Sprinkle walnuts on cold cereal, oatmeal, or yogurt.
• Toss some walnuts into stir-fries or pasta.
• Sprinkle walnuts onto salads.
• Top off your favorite soup or casserole with chopped walnuts.
• Toasted walnuts are superb with sautéed vegetables.
• Chopped walnuts add crunch and texture to fish and chicken.
• Walnuts are great in baked goods such as breads, muffins, and pancakes.
• For a sweet treat, heat 1 tablespoon of sugar and toss in 1/4 cup of sliced walnuts.
— Source: www.walnuts.org

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