As well as lowering cholesterol, it appears that consuming whole walnuts and walnut oil also reduces the risk of cardiovascular disease in other ways, according to a new study funded by the California Walnut Board.
The investigating team, comprising researchers from Penn State, Tufts University and University of Pennsylvania, writes about its findings in a paper due to appear in the 1 June print issue of the Journal of Nutrition, a version of which is already available online.
Senior author Penny Kris-Etherton, Distinguished Professor of Nutrition at Penn State, says in a statement:
“We already know that eating walnuts in a heart-healthy diet can lower blood cholesterol levels.”
“But, until now, we did not know what component of the walnut was providing this benefit. Now we understand additional ways in which whole walnuts and their oil components can improve heart health,” she adds.
For the study, Kris-Etherton and colleagues randomly assigned 15 volunteers to one of four treatments, each comprising an “acute” or one-time comsumption of whole walnuts (85 gms), their skin (6 gms), defatted nutmeat (34 gms), or oil (51 gms).
The volunteers were healthy overweight and obese adults with moderate hypercholesterolemia (high blood cholesterol).
They underwent a number of biochemical and physiological tests, both before ingestion and at various times after (30 minutes, one hour, two hours, four hours and six hours after).
The study was a cross-over study, so each volunteer eventually underwent all four treatments, and the associated tests.
The test results showed that a one-time consumption of walnut oil boosted blood vessel functioning. Also, consumption of whole walnuts helped “good cholesterol” (HDL) transport and remove excess cholesterol from the body more effectively.
First author Claire Berryman, a graduate student in nutritional sciences at Penn State, says the results suggest after a meal containing walnut oil, blood vessels perform better, “which is very important given that blood vessel integrity is often compromised in individuals with cardiovascular disease”.
She explains that the walnut oil was particularly good at preserving the function of the cells that line the walls of blood vessels, the endothelial cells, which play an important role in cardiovascular health.
Speculating on which compounds may be involved, the researchers point to the alpha-linolenic acid, gamma-tocopherol and phytosterols in walnuts.
Alpha-linolenic acid is a type of omega-3 fatty acid found in plants. Gamma-tocopherol is a major form of vitamin E found in many plant seeds, and phytosterols are cholesterol-like molecules found in plants that can lower cholesterol levels.
Berryman says although further studies now need to confirm these findings, especially as the “science around HDL functionality is very new”, they imply we need to improve dietary advice for avoiding heart disease.
Kris-Etherton agrees, and says more studies are also needed to find out exactly what the underlying mechanisms might be that tie walnut consumption to lowering of cardiovascular risk. But in the meantime:
“Our study indicates that simple dietary changes, such as incorporating walnuts and/or their oil in a heart healthy diet, may reduce the risk of heart disease.”
In 2010, researchers also found that a diet rich in walnuts and walnut oil may prepare the body to deal better with stress.