The pistachio study included 28 adults whose LDL cholesterol level was higher than the optimal range. Their average LDL level was in "borderline high" range when the study started.
Apart from their cholesterol levels, participants were healthy. None was taking cholesterol-lowering drugs.
First, participants spent two weeks on a standard American diet rich in full-fat cheese, oil, and butter and lacking pistachios.
Next came a month on a low-fat diet without pistachios, another month on a healthy diet that included one daily serving of pistachios, and a third month eating a similar diet with two daily servings of pistachios, with two-week breaks between each type of diet.
Participants got all their food, packaged into appropriate serving sizes, from the researchers. And they stuck to their assigned diets pretty well, the study shows.
Average LDL levels fell when participants ate pistachios -- not enough to get their LDL levels into the optimal range, but enough to get it out of the "borderline high" category.
LDL cholesterol level dropped by 9% during the month that participants ate a daily serving of pistachios and by 12% when they had two daily servings of pistachios.
As for the low-fat diet, it didn't trim LDL cholesterol. That surprised the researchers. What happened? The researchers aren't sure, but they note that the low-fat diet was lower in polyunsaturated fats (which include heart-healthy omega-3 fatty acids) and higher in carbohydrates than the pistachio diets.
Pistachios didn't affect levels of HDL cholesterol, which is often called "good" cholesterol.
The study was funded in part by the California Pistachio Commission.