Vitamin B12 and folate are distinct nutrients that differ in
their synthesis and uptake from the diet. However, they have roles in
the body that overlap in one crucial respect- both are cofactors of the enzyme methionine synthase, a key enzyme in the synthesis of the amino acid methionine and the multifunctional cofactor S-adenosyl methionine (SAM). As a result, a deficiency in one of these nutrients can mimic a deficiency in the
other. These two "vitamins" are therefore often considered together.
Humans obtain both vitamin B12 and folate from their diets, and thus the USDA has set minimum daily requirements for each of these essential nutrients. Sources of these vital nutrients are summarized in the table below.
|Meat, fish, poultry, fortified cereals
|Leafy green vegetables, whole-grain breads, enriched breads and cereals
|Source: USDA Food and Nutrition Information Center (url: http://www.nal.usda.gov/fnic/index.html), viewed June 27, 2006.
Biosynthesis will not be considered in detail for either nutrient. However, it should be noted that biosynthesis of folate in bacteria is a target for antibiotic action.
Effects of Vitamin Deficiencies
In all people, vitamin B12 deficiency leads to impaired red blood cell production and a condition called megoblastic anemia. In addition, approximately 30% of patients with vitamin B12 deficiency have neurological disorders. In infants and small children, vitamin B12 deficiency can also cause developmental defects. There are no natural plant sources of vitamin B12, therefore a strict vegetarian diet, devoid of vitamin-fortified breads and cereals, can lead deficiencies. Vitamin B12 deficiency can also occur in people who eat a healthy diet. For example, pernicious anemia is an autoimmune disorder that affects dietary uptake of vitamin B12, resulting in vitamin B12 deficiency.
Folate deficiency also leads to megoblastic anemia. In addition, folate is vital during fetal development, thus folate deficiency in pregnant women can lead to neural tube defects in their offspring. In addition to lack of folate in the diet, folate deficiency is also caused by malabsorption of this essential nutrient.
The student is referred to an excellent module on vitamin
B12 and its role in metabolism prepared by Dr. Louise Canfield.