Vitamin D is a fat soluble vitamin found in a small number of foods but we get most of our vitamin D from the action of sunlight on our skin.
Good food sources are oily fish and eggs. Other food sources include fortified foods such as margarine, breakfast cereals and powdered milk.
Some people believe that individuals on the autism spectrum don't have enough vitamin D in their bodies, or that their bodies are poor at making use of the vitamin D available to them.
These people believe that these nutritional deficiencies may be the cause of some of the problems faced by people on the autism spectrum.
They also believe that some of these problems can be overcome by taking supplements of vitamin D.