Atherosclerosis is the process whereby fatty substances such as cholesterol and calcium form plaque on the inner lining of an artery, causing them to harden. If enough builds up the plaque can reduce blood flow through the artery, and if it ruptures blood clots can form, which can block the flow of blood to the heart and cause a heart attack, or stroke.
Atherosclerosis occurs naturally in humans as part of the aging process, but certain factors including high blood cholesterol, smoking, high blood pressure, obesity and diabetes increase the risk.
The new study, published in the July issue of the Journal of Nutrition (Vol. 136, pp. 1886-1889), reports the effects of eating a diet rich in green and yellow vegetables on the extent of atherosclerosis in mice.
The researchers, from the University of California in Los Angeles, fed 53 mice a vegetable free control diet, and 54 mice a diet with 30 per cent replaced with freeze-dried peas, green beans, broccoli, corn, and carrots for sixteen weeks.
The extent of atherosclerosis, as measured by the content of choesteryl ester (the form of cholesterol that accumulates on the inner lining of the artery), was found to be 38 per cent lower in the vegetable-diet group, than in the control group.
The vegetable diet also resulted in lower levels of total cholesterol (12 per cent lower), VLDL and ILDL cholesterol (32 per cent lower), and serum amyloid A (37 per cent lower).
Serum amyloid A is a protein that was used by the researchers to assess the degree of inflammation in the mice.
"These results further support the idea that increased vegetable consumption inhibits atherosclerosis progression through antioxidant and anti-inflammatory pathways," wrote lead author Michael Adams.
The researchers could not identify a mechanism, or indeed the active substances that confer these beneficial effects, but noted that the vegetables contained a variety of micronutrients, such as carotenoids, vitamin C, vitamin E, and selenium that are potent antioxidants.
They also noted that broccoli is a good source of sulforaphane, a compound that has been implicated in anti-inflammation.
"Although the pathway(s) involved remain unclear, we conclude that the consumption of a diet containing 30 per cent green and yellow vegetables results in a substantial inhibition of atherosclerosis progression in a mouse model of atherosclerosis," said Adams.
This study is in agreement with a meta-analysis published in January (The Lancet, Vol. 367, pp 320-326) that reported that eating five or more portions of fruit and vegetables per day could cut the risk of stroke by 26 per cent.
The "Five-a-day" message is well known, but applying this does not seem to be filtering down into everyday life. Recent studies have shown that the average American consumes about three portions a day.
A report from the European Union showed that global fruit and vegetable production was over 1 230 million tonnes in 2001-2002, worth over $50 billion (€ 41 000 million). Asia produced 61 per cent, while Europe and North/Central America both producing nine per cent.