Why are some carotenoids considered phytochemicals rather than provitamins?

Answer 1

There are two types of carotenoids—those that can be converted to vitamin A and those that cannot. Those carotenoids that cannot be converted to vitamin A are call nonprovitamin A carotenoids and are not technically vitamins, but instead are considered phytochemicals or zoonutrients depending on whether they come from plant- or animal-based foods, respectively. Several nonprovitamin A carotenoids are themselves associated with decreased risk for heart disease and cancer, and others appear beneficial in decreasing the risk for age-related eye disease. (Phytochemicals and zoonutrients are thought to have health-promoting properties and the foods that contain them are sometimes called "functional foods.".)