A deficiency of certain B vitamins can cause anemia, fatigue, poor appetite, abdominal pain, depression, numbness and tingling in the arms and legs, muscle cramps, respiratory infections, hair loss, eczema, poor growth in children, and birth defects. Some alternative medical practitioners believe vitamin B deficiencies weaken the immune system and increase vulnerability to cancer, but there is no scientific evidence to support the claim, according to the American Cancer Society.
The B vitamin group includes B1 (thiamin), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folic acid, a manufactured supplement or folate, which is found in certain foods) and B12 (cobalamin), explains the American Cancer Society. These vitamins are essential for growth and development, especially in the developing fetus. Evidence indicates that taking folic acid during pregnancy reduces the risk of birth defects, states MedlinePlus, and folic acid has been added to grain products since 1998.
Randomized, controlled studies suggest that man-made folic acid supplements increase the risk of colorectal and breast cancer, according to the American Cancer Society, which does not generally advocate supplements for people other than pregnant women and those with dietary deficiencies. Some practitioners believe a diet rich in vegetables and enriched grain products can help protect against cancer.