Parkinson's disease (PD) is a type of movement disorder. It happens when nerve cells in the brain don't produce enough of a brain chemical called dopamine. Sometimes it is genetic, but most cases do not seem to run in families. Exposure to chemicals in the environment might play a role.
Why Vegans are Often Deficient in Vitamin B12. The few plant foods that are sources of B12 are actually B12 analogs. An analog is a substance that blocks the uptake of true B12, so your body's need for the nutrient actually increases.
As you can see, true vitamin A foods come from animal sources. A vegan diet simply does not provide the body with adequate vitamin A for optimal health. A vegan diet also reduces thyroid function and bile release, which drastically compromises the already poor carotene-to-cartenoid conversion.