"In adults with diabetes, two servings of pistachios per day lowered vascular constriction during stress and improved neural control of the heart," said Sheila G West, professor of biobehavioural health and nutritional sciences at Penn State.
"Although nuts are high in fat, they contain good fats, fibre, potassium and antioxidants. Given the high risk of heart disease in people with diabetes, nuts are an important component of a heart healthy diet in this population," West said.
West and her colleagues investigated effects of pistachios on responses to standardised stress tasks in patients with well-controlled Type 2 diabetes who were otherwise healthy.
They used a randomised, crossover study design in which all meals were provided. Each of the diets contained the same number of calories.
After two weeks on the typical American diet - containing 36 per cent fat and 12 per cent saturated fats - participants were randomised to one of two test diets.
Test diets included a standard heart-healthy diet - 27 per cent fat and 7 per cent saturated fat - and a diet containing two servings per day of pistachios - about 3 ounces or 20 per cent of calories from pistachio nuts.
The typical research participant consumed about 150 pistachio nuts per day. The pistachio diet contained 33 per cent fat and 7 per cent saturated fat. Half of the nuts consumed each day were salted and half were unsalted.
At the end of each four-week diet period, the researchers measured blood pressure and total peripheral vascular resistance at rest and during two stress tests - a cold water challenge and a confusing mental arithmetic test.
"After the pistachio diet, blood vessels remained more relaxed and open during the stress tests," West said.
Although laboratory measurements of blood pressure were not affected by pistachios, real-world measures of blood pressure (measured by an automated monitor) were significantly lower after the pistachio diet.
"We found that systolic blood pressure during sleep was particularly affected by pistachios. Average sleep blood pressure was reduced by about 4 points and this would be expected to lower workload on the heart," said Katherine A Sauder, former graduate student in biobehavioural health, who conducted the measurements.
The team found that the pistachio diet lowered vascular constriction during stress. When arteries are dilated, the load on the heart is reduced. The physical challenge involved immersing one hand into icy water for two minutes.
"This cold stressor produces a large vascular constriction response in most people. In comparison with a low fat diet, the pistachio diet blunted that vascular response to stress," West said.
The study was published in the Journal of the American Heart Association.