A new study has found that antioxidants like vitamins C and E activate a mechanism that stimulates the growth of new blood vessels in cancer tumors, helping them to grow and spread. The researchers say their findings highlight the potential risk of taking antioxidant supplements when they’re not needed.

To grow and metastasize, cancer tumors need a constant supply of oxygen- and nutrient-rich blood. And that requires the formation of new blood vessels from existing ones, a process called angiogenesis. When tissues are starved of oxygen (hypoxia), cancer cells in the affected area send chemical signals to get the endothelial cells that line the blood vessels to form new vessels.

A new study by researchers at Sweden’s Karolinska Institutet has examined mechanisms of angiogenesis and found that antioxidants play an unexpected role in the growth and spread of tumors.

“We’ve found that antioxidants activate a mechanism that causes cancer tumors to form new blood vessels,