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.
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.
A report last year in the Journal of Nutrition (Vol. 135, pp. 2114-2118) reported that deficiency in zinc in mice was associated with an increase in the risk of cardiovascular disease (CVD) through inflammation and a decreased level of compounds that protect against atherosclerosis.
The new research, published on-line in the journal Free Radical Biology and Medicine (doi: 10.1016/j.freeradbiomed.2006.03.017), reports that supplementation of a high cholesterol diet with zinc reduced the formation of lesions in the arteries of rabbits, but the effects were not linked to changes in cholesterol levels.
Lead researcher Barry Halliwell and colleagues from the National University of Singapore divided 18 white rabbits into three groups. The first was fed a normal diet (control), the second fed a high-cholesterol diet (HCD, one per cent cholesterol), and the third was fed the HCD diet but was supplemented with zinc in the carbonate form (0.1 per cent).
After eight weeks, the researchers measured blood levels of HDL, LDL, and total cholesterol, as well as triglyceride levels. The number of white and red blood cells were also measured.
The researchers found that zinc supplementation did not significantly change total cholesterol, LDL, or triglyceride levels, but a significant decrease in HDL levels was observed - 6.9 millimoles per litre in the HCD group and 2.6 millimoles per litre in the HCD plus zinc group.
This result appears to agree with last year's report that zinc deficiency is associated with increased levels of HDL.
"Since HDL is antiatherosclerotic, it seems unlikely that the protective effects of zinc are mediated by altering plasma lipid levels," wrote lead author Minquin Ren.
When the scientists investigated the formation of lesions in the rabbit aorta. They found that zinc supplementation significantly decreased the area of the lesions by 66 per cent, a result linked to a decrease in iron concentrations in the tissue, which has previously been reported to catalytically promote damaging free-radical reactions and the development of atherosclerosis.
"Other evidence that zinc is influencing iron metabolism is that the red blood cell dropped in the HCD animals from 5.9 to 3.04bn per litre, whereas in the Zn-supplemented HCD animals the drop was smaller, from 5.9 to 4.07bn per litre," said Ren.
"Zinc may thus play a role in inhibiting lesion formation through the indirect prevention of iron-mediated free-radical damage, in that it decreases the iron content of the lesion," concluded the researchers.
Research is reported to be on-going to determine the mechanism behind the reported benefits.