High blood concentrations of beta-carotene and other carotenoids, plant pigments found in carrots, sweet potatoes, spinach, kale, collard greens, papaya, bell peppers, tomatoes, have been linked to a lower risk of cancer, especially lung, mouth, throat, and cervical cancers.
A blood beta-carotene concentration less than 0.28 micromol/L has been linked with a higher risk of several cancers, while a concentration of more than 0.28 to 0.37 micromol/L have been associated with a reduced risk of several cancers in prospective blood concentration studies. However, three other large randomized studies reported no cancer benefit from beta-carotene supplements given at 20, 30, or 50 mg/d for 4 to 12 years.