Vitamin C (also known as ascorbic acid) is a water-soluble vitamin. Vitamin C is found in a wide variety of fruit and vegetables including peppers, broccoli, Brussels sprouts etc. It can also be taken as a dietary or nutritional supplement.
Some people believe that individuals on the autism spectrum don't have enough vitamin C in their bodies, or that their bodies are poor at making use of the vitamin C available to them.
Other people believe that individuals on the autism spectrum do not have the correct amount of dopamine in their bodies or that it is not working effectively. They believe that vitamin C can be used to change the amount and the action of the dopamine that is there.
Both groups of people believe that taking supplements of vitamin C (often alongside other vitamins and minerals) can help to change the brain chemistry of individuals on the autism, spectrum resulting in improvements in behaviour.