Washington:Too much of a good thing may not always be better for your health. Researchers have shown that high levels of high-density lipoprotein (HDL) cholesterol - commonly touted as "good cholesterol" for helping to reduce risk of stroke and heart attack - may increase a person's risk of premature death as much as its low levels. The research suggests that intermediate levels of HDL cholesterol may increase longevity. "The findings surprised us," said the study's senior author Ziyad Al-Aly, Assistant Professor at Washington University School of Medicine in St. Louis, Missouri. "Previously it was thought that raised levels of the good cholesterol were beneficial. The relationship between increased levels of HDL cholesterol and early death is unexpected," Al-Aly said. Cholesterol is a fatty substance found in blood that can narrow and block heart vessels, causing cardiovascular disease and stroke. For years, HDL cholesterol has been credited with helping to remove plaque-building "bad cholesterol" from arteries. For this study, researchers studied kidney function and HDL cholesterol levels in more than 1.7 million male veterans from October 2003 through September 2004. Researchers then followed participants until September 2013. In the study, published in the Clinical Journal of the American Society of Nephrology, the researchers showed that both high and low HDL cholesterol levels were associated with an increased risk of dying among study participants with all levels of kidney function. "Too low and too high are both associated with higher risk of death," Al-Aly said. Whether maintaining intermediate HDL cholesterol levels may increase longevity will need to be explored in future studies, Al-Aly said.