Are oranges the new forbidden fruit? NO! Go ahead and enjoy them! Oranges and other citrus are packed with Vitamin C and will help to boost your immune system. The Journal of General Dentistry reported a 2008 study that compared the negative effects of lemons, grapefruit and oranges on the enamel surface of teeth. However, the teeth were soaked continuously in the juice from the fruit for four week increments, up to twenty weeks. So use your common sense. Always brush after eating any foods and enjoy those healthy fruits.