Because they are soluble in water, excess B vitamins are generally readily excreted, although individual absorption, use and metabolism may vary…"  The elderly and athletes may need to supplement their intake of B 12 and other B vitamins due to problems in absorption and increased needs for energy production. [ medical citation needed ] In cases of severe deficiency, B vitamins, especially B 12 , may also be delivered by injection to reverse deficiencies.  [ unreliable medical source? ] Both type 1 and type 2 diabetics may also be advised to supplement thiamine based on high prevalence of low plasma thiamine concentration and increased thiamine clearance associated with diabetes.  Also, Vitamin B 9 (folic acid) deficiency in early embryo development has been linked to neural tube defects . Thus, women planning to become pregnant are usually encouraged to increase daily dietary folic acid intake and/or take a supplement. 
All living things need vitamins for growth and health and each vitamin has specific roles to play. The body either cannot manufacture them at all or cannot normally manufacture them in sufficient amounts, and so must absorb them from food. They are essential for a healthy life and contribute to good health by regulating metabolism and assisting the biochemical processes that release energy from digested foods. Many reactions in the body require several vitamins, and the lack or excess of any one can interfere with the function of another.