Whether manganese is useful in the treatment of diabetes by helping glucose metabolism or in people with osteoarthritis by stimulating mucopolysaccharide production to heal joints is still undemonstrated and questionable. It is more likely that a manganese deficiency reduces our ability to handle glucose and may thus worsen a diabetic condition. Manganese has also been tried in treatment for multiple sclerosis and myasthenia gravis. When given with B vitamins, manganese may alleviate fatigue or weakness by enhancing nerve impulses. Research has found most tumors and cancer cells to be very low in this mineral, which suggests a possibility that manganese may have a role in preventing cancer cell production and protecting against cancer growth.

Deficiency and toxicity: From a nutritional point of view, manganese may be one of the least toxic minerals. There is no known natural toxicity from manganese in food or from taking reasonable amounts in supplements. Lung problems can be caused, however, by breathing in the dust when mining the inorganic mineral.

In Chile, where much manganese is mined, workers sometimes develop a strange syndrome they call locura manganica, or "manganese madness." The first symptoms may be anorexia, weakness, and apathy. However, there may be an initial manic phase, characterized by inappropriate laughter, increased sexuality, insomnia, and even delusions or hallucinations. Violence and other mental change may occur. The earlier mania may shift to depression, impotence, and excessive sleeping. Parkinsonian symptoms such as tremors and muscle rigidity may also appear in the later stages. These symptoms may appear, as in Parkinson's disease, from a loss of dopamine in the brain cells. L-dopa, which converts to dopamine in the brain, is used in the treatment of manganese toxicity to reduce symptoms. Avoiding further manganese inhalation is obviously also suggested.

Manganese deficiency in animals has been studied extensively. In rats, manganese deficiency can lead to sterility or, if occurring during pregnancy, to poor growth in the offspring and decreased lactation in the mother. There is decreased bone growth, especially in length. Poor brain function may occur from decreases in several manganese activities. A decreased threshold for seizures has also been measured. Poor bone and cartilage health and spinal disc degeneration are possible with low manganese. The relevancy of these findings to humans is currently only theoretical and needs further documentation.

In children, severe manganese deficiency may lead to convulsions, paralysis, or blindness. In adults, dizziness, weakness, and problems with the hearing, such as strange ear noises, are associated with manganese deficiency. Decreased strength and ataxia (unstable gait) have also been related; in addition, weight loss, irregular heartbeat, and skin problems have been described by some authors.

Manganese deficiency may cause decreased glucose tolerance, or ability to remove excess sugar from the blood, as occurs also with chromium or zinc deficiency. Low manganese levels may even cause decreased function of the pancreatic cells, and this problem might be helped by manganese supplementation. Research on this relationship between manganese and glucose tolerance and other suggested effects of manganese deficiency is at a preliminary stage, and, to my knowledge, none of these effects has yet been proved. Whether the decreased manganese levels found in cancer are indicative of a causal relationship (which implicates a role of SOD in cancer) or a result of increased nutrient use is of great interest as well.