A new study has linked excessively high levels of HDL (high-density lipoprotein) -- often dubbed "good cholesterol" -- to premature death.

Researchers at Washington University School of Medicine in the U.S. investigated HDL -- known as the "good cholesterol" -- to establish whether its role was always beneficial. They found that HDL levels that were either too low or too high could have negative effects on health.

Previously, only LDL (low-density lipoprotein) -- known as "bad cholesterol" -- had been an subject of close consideration when interpreting blood test results. Excess LDL favours the build-up of deposits on artery walls, leading to atherosclerosis, or the hardening and narrowing of the arteries. This is the main risk factor for strokes and heart attacks.

Although HDL has a protective role, carrying cholesterol away from the arteries to the liver, this epidemiological study -- which followed 1.7 million male veterans for 10 years -- found that very high levels of HDL could increase the risk of premature death.

The researchers established that patients with kidney disease often had lower levels of HDL cholesterol, which could explain their increased risk of premature death. They also found that levels of HDL cholesterol that were too high or too low were linked to an increased risk of early death, regardless of kidney health.

In other words, the scientists found a U-shaped correlation between levels of HDL and early death, with increased risk at the highest and lowest ends of the scale.

Generally, dietary intake of antioxidants such as vitamin E, vitamin C and beta-carotene, found in fruit and vegetables, and omega-3 fatty acids can help limit the build-up of "bad cholesterol" on artery walls. However, all types of meat -- even lean meats (offal, chicken, etc.) -- are sources of cholesterol, particularly offal.

According to a recent Canadian study, barley and oats could reduce the risk of cardiovascular disease linked to "bad cholesterol" by seven per cent. These two cereals are particularly rich in beta-glucan, a highly viscous soluble fiber that can be found in brans, flours, ground grains or flakes.

The study was published in the Clinical Journal of the American Society of Nephrology.