Recent research has confirmed that eating pistachios may trim down the body's response to the stress of everyday life.

According to a Penn State (US) study, elevated reactions to stressors can be reduced by including pistachios in the diet.

"A 10-year follow-up study of young men showed that those who had larger cardiovascular responses to stress in the lab were more likely to contract hypertension later in life," says Dr Sheila G West, associate professor of bio-behavioural health.

"Elevated reactions to stressors are partly genetic, but can be changed by diet and exercise. Lifestyle changes can make the biological reactions to stress smaller."

West and her colleagues inspected the effects of pistachios on uniform stressors on participants who had high cholesterol but normal blood pressure. They used a randomised, crossover, controlled feeding study plan. All three diets included an equal number of calories.

After a two-week run-in diet containing 35 per cent fat and 11 per cent saturated fats, each test diet lasted for four weeks. During this time, participants consumed only foods supplied by the study.

The researchers reported the results of this study at Experimental Biology 2007 on April 30 in Washington, DC.

The diets given to the participants included a Step I Diet, a standard heart healthy diet with 25 per cent fat and 8 per cent saturated fat.

The Step I Diet incorporated a diet containing 1.5 ounces (42.52 grams) of pistachios with 30 per cent total fat and 8 per cent saturated fat and a diet containing 3 ounces (85 grams) of pistachios containing 34 per cent fat and 8 per cent saturated fat.

At the end of each four-week diet regime, the researchers measured blood pressure and total peripheral vascular resistance at rest and during two stress tests.

The two tests consisted of a physical test and a psychological test.

The physical test consisted of putting one foot in a bucket of ice water for 2.5 minutes.

The psychological test asked participants to listen to two numbers, add them in their head and say the answer. Then they were asked to pay attention to another number and add it to the second number they heard, not the sum they spoke.

"The ice water is a stimulus for the sympathetic nervous system, but it is very different from the stressors we encounter every day," says West. "We also wanted to see if the reaction occurred when the stress was nonphysical, so we used the math test."

The researchers found that both pistachio containing diets abridged the stress effects on blood pressure, but that the 1.5 ounce pistachio diet reduced systolic blood pressure by 4.8 millimetres of mercury while the 3-ounce pistachio diet only reduced systolic blood pressure by 2.4 millimetres of mercury.

The diets had no effect on normal, resting blood pressure.

"When we only look at blood pressure, these results are confusing," says West. "If it is the pistachios, why is it not dose related?"

When the researchers examined total peripheral vascular resistance, it was clear that the 3-ounce diet led to greater relaxation of arteries. Because the body strongly controls blood pressure, rather than allowing blood pressure to drop further, the heart compensated by pumping more forcefully.

"The relaxation of blood vessels after the 3-ounce pistachio diet likely reduced the workload on the heart," says West. "This pattern of change would be beneficial if it is maintained long term. It is possible that other foods that are high in unsaturated fat and antioxidants would have a similar effect."