Manganese In Human Nutrition
From "Mad Cow Disease" by Mark Purdey

Manganese in small amounts plays an important role in human nutrition. It forms an essential cofactor of numerous enzymes and is necessary for the utilization and balanced metabolism of many other nutrients. Manganese is a catalyst in the synthesis of fatty acids and cholesterol and is essential for the production of sex and thyroid hormones. Because manganese plays a role in the mitochondrial "power stations" of the nerve neurones, deficiency can result in symptoms that are similar to those of overload — lack of coordination, irritability, psychological difficulties and even paralysis, convulsions, blindness and hearing loss.

Whole grains, egg yolks, nuts, seeds and green vegetables provide manganese if it is present in the soil. Manganese is poorly absorbed from food but readily absorbed into the brain via the inhalatory-nasal-olfactory route. Thus toxicity is much more likely from environmental atmospheric sources than from food, although diets high in manganese-containing foods, such as soy and tea, can exacerbate the condition of manganese overload, especially when fed in large amounts to infants or growing animals.