Supplements such as beta-carotene selenium and folic acid quaffed at much higher levels than their recommended daily doses are most likely to elevate the risk of developing a host of cancers a study says. It's not that these nutrients are toxic - they're essential and we need them but we need them in a certain balance says Tim Byers study co-author and professor of epidemiology at the University of Colorado School of Public Health US the Journal of the National Cancer Institute reports. We have a window into less than half of the biology of what these nutrients are doing.