In contrast to other water-soluble vitamins it is not excreted quickly in the urine, but rather accumulates and is stored in the liver, kidney and other body tissues.
As a result, a vitamin B12 deficiency may not manifest itself until after 5 or 6 years of a diet supplying inadequate amounts.
Vitamin B12 functions as a methyl donor and works with folic acid in the synthesis of DNA and red blood cells and is vitally important in maintaining the health of the insulation sheath (myelin sheath) that surrounds nerve cells.
The classical vitamin B12 deficiency disease is pernicious anaemia, a serious disease characterized by large, immature red blood cells.
It is now clear though, that a vitamin B12 deficiency can have serious consequences long before anaemia is evident.
The normal blood level of vitamin B12 ranges between 200 and 600 picogram/milliliter (148-443 picomol/liter).
Although deficiency is far more common than excess when it comes to vitamin B12 status cases have been reported where blood levels exceeded 3000 picograms/milliliter. Such high levels may be caused by bacterial overgrowth as outlined in the article Vitamin B-12 Overload.
Vitamin B12 exists in several forms and contains the mineral cobalt, so compounds with vitamin B12 activity are collectively called "cobalamins". Methylcobalamin and 5-deoxyadenosylcobalamin are the forms of vitamin B12 that are active in human metabolism .
The amount of vitamin B12 actually needed by the body is very small, probably only about 2 micrograms or 2 millionth of a gram/day.
Unfortunately, vitamin B12 is not absorbed very well so much larger amounts need to be supplied through the diet or supplementation.
The richest dietary sources of vitamin B12 are liver, especially lamb's liver, and kidneys.
Eggs, cheese and some species of fish also supply small amounts, but vegetables and fruits are very poor sources.
Several surveys have shown that most strict, long-term vegetarians are vitamin B12 deficient.
Many elderly people are also deficient because their production of the intrinsic factor needed to absorb the vitamin from the small intestine decline rapidly with age.
Many experts previously believed that strict vegetarians were the primary group that could develop a vitamin B12 deficiency. If you had adequate stores of this important nutrient in your body, the experts believed you were “safe” for many years from developing a B12 deficiency.
Now we know better.
Vitamin B12 depletion and deficiency are much more common than previously thought, especially in the over-60 population. In fact, it’s believed that almost one in four people over 60 have deficient levels of this vital vitamin.
Equally disturbing are emerging signs that other age groups harbor suboptimal blood levels of B12 as well.
Why is vitamin B12 deficiency such a big deal?
Your body depends on vitamin B12 for a host of functions, including…
- Helping to maintain normal energy levels*
- Promoting healthy neurological activity, including mental alertness*
- Supporting normal homocysteine levels for healthy cardiac function*
- Helping to ease occasional stress and sleeplessness*
- Maintaining healthy cell growth and repair*
- Promoting normal immune function*
- Supporting normal metabolism of carbohydrates and fats*
- When your blood levels of vitamin B12 are low, one or more of these functions may be disrupted.*
Without adequate blood levels of B12, you can experience symptoms related to low energy, mental fatigue, mood changes, sleep difficulties, and even occasional indigestion.*
Your body relies on the efficient conversion of carbohydrates to glucose – your body’s source of fuel – just like your car needs to be able to use gas to run smoothly. Vitamin B12 plays a major role in that conversion in your body.* Likewise, B12 enables your body to convert fatty acids into energy as well.*
Contrary to what you might have heard, there’s really no solid evidence that supplemental vitamin B12 helps you lose weight.
Overall, vitamin B12 is a nutrient your body cannot do without for efficient, healthy metabolism of fats and carbohydrates.*
Sources of Vitamin B12
Food
Vitamin B12 is naturally found in animal products, including fish, meat, poultry, eggs, milk, and milk products. Vitamin B12 is generally not present in plant foods, but fortified breakfast cereals are a readily available source of vitamin B12 with high bioavailability for vegetarians . Some nutritional yeast products also contain vitamin B12. Fortified foods vary in formulation, so it is important to read product labels to determine which added nutrients they contain.
Table 2: Selected Food Sources of Vitamin B12 [13]
| Food | Micrograms (mcg) per serving | Percent DV* |
|---|---|---|
| Clams, cooked, 3 ounces | 84.1 | 1,402 |
| Liver, beef, cooked, 3 ounces | 70.7 | 1,178 |
| Breakfast cereals, fortified with 100% of the DV for vitamin B12, 1 serving | 6.0 | 100 |
| Trout, rainbow, wild, cooked, 3 ounces | 5.4 | 90 |
| Salmon, sockeye, cooked, 3 ounces | 4.8 | 80 |
| Trout, rainbow, farmed, cooked, 3 ounces | 3.5 | 58 |
| Tuna fish, light, canned in water, 3 ounces | 2.5 | 42 |
| Cheeseburger, double patty and bun, 1 sandwich | 2.1 | 35 |
| Haddock, cooked, 3 ounces | 1.8 | 30 |
| Breakfast cereals, fortified with 25% of the DV for vitamin B12, 1 serving | 1.5 | 25 |
| Beef, top sirloin, broiled, 3 ounces | 1.4 | 23 |
| Milk, low-fat, 1 cup | 1.2 | 18 |
| Yogurt, fruit, low-fat, 8 ounces | 1.1 | 18 |
| Cheese, Swiss, 1 ounce | 0.9 | 15 |
| Beef taco, 1 soft taco | 0.9 | 15 |
| Ham, cured, roasted, 3 ounces | 0.6 | 10 |
| Egg, whole, hard boiled, 1 large | 0.6 | 10 |
| Chicken, breast meat, roasted, 3 ounces | 0.3 | 5 |
*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers determine the level of various nutrients in a standard serving of food in relation to their approximate requirement for it. The DV for vitamin B12 is 6.0 mcg. However, the FDA does not require food labels to list vitamin B12 content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet. The U.S. Department of Agriculture's Nutrient Database Web site [13]) lists the nutrient content of many foods and provides a comprehensive list of foods containing vitamin B12.
Dietary supplements
In dietary supplements, vitamin B12 is usually present as cyanocobalamin , a form that the body readily converts to the active forms methylcobalamin and 5-deoxyadenosylcobalamin. Dietary supplements can also contain methylcobalamin and other forms of vitamin B12.
Existing evidence does not suggest any differences among forms with respect to absorption or bioavailability. However the body’s ability to absorb vitamin B12 from dietary supplements is largely limited by the capacity of intrinsic factor. For example, only about 10 mcg of a 500 mcg oral supplement is actually absorbed in healthy people .
In addition to oral dietary supplements, vitamin B12 is available in sublingual preparations as tablets or lozenges. These preparations are frequently marketed as having superior bioavailability, although evidence suggests no difference in efficacy between oral and sublingual forms .
Prescription medications
Vitamin B12, in the form of cyanocobalamin and occasionally hydroxocobalamin, can be administered parenterally as a prescription medication, usually by intramuscular injection . Parenteral administration is typically used to treat vitamin B12 deficiency caused by pernicious anemia and other conditions that result in vitamin B12 malabsorption and severe vitamin B12 deficiency .
Vitamin B12 is also available as a prescription medication in a gel formulation applied intranasally, a product marketed as an alternative to vitamin B12 injections that some patients might prefer . This formulation appears to be effective in raising vitamin B12 blood levels , although it has not been thoroughly studied in clinical settings.
Vitamin B12 Deficiency
Vitamin B12 deficiency is characterized by megaloblastic anemia, fatigue, weakness, constipation, loss of appetite, and weight loss.
Neurological changes, such as numbness and tingling in the hands and feet, can also occur .
Additional symptoms of vitamin B12 deficiency include difficulty maintaining balance, depression, confusion, dementia, poor memory, and soreness of the mouth or tongue .
The neurological symptoms of vitamin B12 deficiency can occur without anemia, so early diagnosis and intervention is important to avoid irreversible damage .
During infancy, signs of a vitamin B12 deficiency include failure to thrive, movement disorders, developmental delays, and megaloblastic anemia .
Many of these symptoms are general and can result from a variety of medical conditions other than vitamin B12 deficiency.
Folic acid and vitamin B12
Large amounts of folic acid can mask the damaging effects of vitamin B12 deficiency by correcting the megaloblastic anemia caused by vitamin B12 deficiency without correcting the neurological damage that also occurs .
Moreover, preliminary evidence suggests that high serum folate levels might not only mask vitamin B12 deficiency, but could also exacerbate the anemia and worsen the cognitive symptoms associated with vitamin B12 deficiency .
Permanent nerve damage can occur if vitamin B12 deficiency is not treated.
For these reasons, folic acid intake from fortified food and supplements should not exceed 1,000 mcg daily in healthy adults .
Groups at Risk of Vitamin B12 Deficiency
The main causes of vitamin B12 deficiency include vitamin B12 malabsorption from food, pernicious anemia, postsurgical malabsorption, and dietary deficiency . However, in many cases, the cause of vitamin B12 deficiency is unknown.
The following groups are among those most likely to be vitamin B12 deficient.
- Older Adults
- Individuals with pernicious anemia
- Individuals with gastrointestinal disorders
- Individuals who have had gastrointestinal surgery
- Vegetarians
- Pregnant and lactating women who follow strict vegetarian diets and their infants
Vitamin B12, folate, and vitamin B6 are involved in homocysteine metabolism.
Interactions with Medications
Vitamin B12 has the potential to interact with certain medications. In addition, several types of medications might adversely affect vitamin B12 levels. A few examples are provided below. Individuals taking these and other medications on a regular basis should discuss their vitamin B12 status with their healthcare providers.
- Chloramphenicol
- Proton pump inhibitors
- H2 receptor antagonists
- Metformin
The Dietary Guidelines for Americans describe a healthy diet as one that:
Emphasizes a variety of fruits, vegetables, whole grains, and fat-free or low-fat milk and milk products:
Milk and milk products are good sources of vitamin B12.
Many ready-to-eat breakfast cereals are fortified with vitamin B12.
Includes lean meats, poultry, fish, beans, eggs, and nuts.
Fish and red meat are excellent sources of vitamin B12. Poultry and eggs also contain vitamin B12.
Is low in saturated fats, trans fats, cholesterol, salt (sodium), and added sugars.
Stays within your daily calorie needs.
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source of reference..
http://ods.od.nih.gov/factsheets/vitaminb12/
http://www.yourhealthbase.com/vitamin_B12.html
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