It may be all the exercise involved in shelling them, or it may be the presence of a family of anti-oxidant compounds. Whatever it is, research at Penn State University suggests that pistachio nuts, eaten as part of a healthy diet, can help decrease unhealthy, oxidized LDL cholesterol.
"Our previous study showed the benefits of pistachios in lowering lipids and lipoproteins, which are a risk factor for heart disease," said Penny Kris-Etherton, professor of nutrition, Penn State. "This new study shows an additional effect of pistachios so now there are multiple health benefits of eating pistachios."
The researchers note in the May 20 issue of the Journal of Nutrition that "pistachios are high in lutein, beta-carotene and gamma-tocopherol relative to other nuts; however, studies of the effects of pistachios on oxidative status are lacking."
Beta-carotene is the precursor to vitamin A and gamma-tocopherol is a common form of vitamin E. Lutein is found in dark green leafy vegetables and is important in vision and healthy skin. All three compounds are oil soluble vitamins.
Antioxidants are of interest because oxidized low-density lipoproteins (LDL) are implicated in inflammation and plaque buildup inside blood vessels, which are major culprits in heart disease. Antioxidants should prevent LDLs from oxidizing, migrating into the blood vessel walls and causing inflammation.
But researchers note that anti-oxidant studies have offered mixed reviews.
"Currently, studies on antioxidants do not show major benefits," says Kris-Etherton. "Maybe we are not studying people long enough. Maybe there is something in the food that travels with the antioxidants. The antioxidant story is very disappointing to the scientific community."
The reason for the disappointment is that studies on specific antioxidants currently do not show health benefits, but epidemiological studies seem to indicate benefits. Many people feel that we have not figured out antioxidants yet, said Kris-Etherton.
If antioxidants are important, then pistachios fit the bill as antioxidant-laden food.
The researchers conducted a randomized, crossover design, controlled feeding experiment to test the effects of pistachios on antioxidant levels when added to a heart healthy moderate-fat diet.
The participants began the study by eating a typical American diet. They then tested three diets for four weeks each with about a two-week break between each diet. All three diets were variations on the Step I Diet, a cholesterol-lowering diet in general use. The diets included, as a control, a Step I Diet with no pistachios. The pistachio-enhanced diets were Step I Diets with 10 and 20 percent of the energy supplied by pistachio nuts, respectively.
Both pistachio diets produced higher blood serum levels of beta-carotene, lutein and gamma-tocopherol than the typical American diet. Compared to the pistachio-free diet, the pistachio-enhanced diets produced greater blood plasma levels of lutein and gamma-tocopherol. After eating both pistachio-enriched diets, the participants had lower oxidized-LDL concentrations in their blood than after the pistachio-free diet.
When the researchers controlled for the change in LDL-cholesterol produced by the pistachio-enhanced diets, increases in beta-carotene and gamma-tocopherol were still associated with decreased oxidized-LDL for the 3-ounce pistachio-enhanced diet.
"Our results suggest that a heart-healthy diet including pistachios contributes to a decrease in serum oxidized-LDL levels, in part through cholesterol lowering, and also due to an added benefit of the antioxidants in the pistachios," said Kris-Etherton.
