Vitamin D and vitamin D3 are both forms of vitamin D, but there are some differences between the two. Structure: Vitamin D is a group of fat-soluble secosteroids that includes vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is synthesized by plants, while vitamin D3 is synthesized by the body when the skin is exposed to sunlight. Function: Both vitamin D2 and vitamin D3 are important for maintaining strong bones, supporting immune function, and regulating the absorption of calcium and phosphorus. Bioavailability: Vitamin D3 is more bioavailable than vitamin D2, meaning that it is more easily absorbed and used by the body. Some research suggests that vitamin D3 may be more effective at raising and maintaining vitamin D levels in the body compared to vitamin D2. Sources: Vitamin D2 can be found in some fortified foods, such as some types of bread and cereals, and in some supplements. Vitamin D3 can be found in fatty fish, egg yolks, and in supplements, and it can also be produced by the body when the skin is exposed to sunlight. Overall, both vitamin D2 and vitamin D3 are important for maintaining good health, but vitamin D3 may be more effective at raising and maintaining vitamin D levels in the body. It's always a good idea to speak with a healthcare provider about the appropriate dosage of vitamin D for your specific needs.