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.
Your question is a long one. Vegans are vegetarians that do not eat any animal products at all - no eggs and no dairy products. So, they could eat wheat and sugar, bread (as long as it does not contain whey - a milk by-product). However, most vegans are also concerned about health and would not eat a lot of sugar and processed wheat.