A healthy vegan diet can be designed to meet the majority of your nutrition needs. However, there are a few nutrients that benefit from extra attention in the form of a vitamin supplement.
Vitamin B12 is the most important supplement for people on a vegan diet to take. Produced by single-celled organisms that live in soil, B12 is not found abundantly in whole plant foods.
The reason that vitamin D is a recommended supplement on a vegan diet is that the best food sources of this nutrient are fatty fish, seafood, and eggs.
Iron is needed for making healthy red blood cells and moving oxygen throughout the body. Plant foods contain non-heme iron, while animal foods contain heme iron.
The best place to get iodine is from food sources, such as iodized salt. Seaweed is also rich in iodine, but can be an inconsistent source, and even contribute too much if you eat it often.