Like folate, and vitamin B6, vitamin B12 is a coenzyme that plays an important role in cell division and is vital to growth. It also helps your body burn fat and carbohydrate.
Among the best sources of vitamin B12 are liver, kidneys, meat, poultry, fish, oysters, milk, eggs, and fortified cereals.
A deficiency of vitamin B12 causes pernicious anemia, characterized by pale skin; numbness or tingling in the fingers and toes; and (sometimes) vertigo, pain or weakness in the arms and legs, and inability to sense vibration. Those at risk include vegetarians (plants don't contain the vitamin), people with a genetic inability to absorb the vitamin, and the elderly.*
Because some of its symptoms are similar to those produced by a folate deficiency, pernicious anemia is sometimes mistakenly treated with folate. This may clear up the pale skin and shrink the red blood cells to normal size, but it won't prevent the nerve damage that causes the numbness or tingling. If your doctor ever prescribes folate for anemia, ask him or her: "Are you sure I don't need vitamin B12 shots instead?"
High doses of vitamin B12, whether from food or from supplements, are not known to be harmful. Because evidence on this point is limited, however, the Food and Nutrition Board (FNB) recommends caution.
Because 10 to 30 percent of older people may not be able to absorb enough vitamin B12 from their food, the Food and Nutrition Board advises people over 50 to meet their daily allowance "mainly by consuming foods fortified with vitamin B12 or a supplement containing vitamin B12."
Vitamin B12 and Heart Disease
Vitamin B12 appears to help vitamin B6 and folate (another B vitamin) curb levels of homocysteine in the blood, thereby protecting against heart disease. (See Folate.) Of the three vitamins, B12's contribution seems to be the second strongest after folate's. All three are under intense study and should make headlines over the next few years. (Diet Power's News/Update page will carry the latest developments.)
Your Daily Allowance
Diet Power sets your Personal Daily Allowance (PDA) of vitamin B12 at the FNB's Recommended Dietary Allowance (RDA), always specified in micrograms: 2.4 for everyone 14** or older except pregnant women, who need 2.6 micrograms per day, and nursing mothers, who need 2.8 micrograms per day.
** Please remember that Diet Power is not designed for people under 15.
The FNB has not established a Tolerable Upper Intake Level (UL) for vitamin B12.
Revising Your Allowance
If your doctor recommends a different PDA, you can change it with the Personal Daily Allowance Editor.
Color Coding of This Nutrient
The vitamin-B12 bar in your personal Nutrient History is:
· blue for "good" if you've logged 100 to 200 percent of your PDA
· red for "bad" if you've logged less than 100 percent of your PDA
· yellow for "caution" if you've logged more than 200 percent of your PDA
· missing if you've logged no vitamin B12.
In the nutrient profile of a food or recipe, the vitamin-B12 bar is:
· green for "good" if getting your entire PDA of calories from this item would give you more than 150 percent of your PDA of vitamin B12
· magenta for "bad" if getting all your calories from the item would give you less than 50 percent of your PDA of vitamin B12
· blue for "neutral" otherwise
· missing if the amount of vitamin B12 is either zero or (when the term Vitamin B12 is grayed out) unknown.
How Complete Are Diet Power's Vitamin-B12 Readings?
For the 8500 generic items in the Food Dictionary: fairly complete. Only 11 percent list their vitamin-B12 content as "unknown."
For the 2500 chain-restaurant items: totally incomplete. All list vitamin B12 as "unknown."
For all 11,000 items combined: not terribly complete. About 29 percent list vitamin B12 as "unknown."
These figures mean that if you frequently log chain-restaurant foods (or user-added foods with missing vitamin-B12 readings), your Nutrient History may underreport your intake of vitamin B12 by a few points.
To see whether a particular food has a vitamin-B12 reading, open the Food Dictionary and check the food's nutrient profile. If you find a question mark beside "Vitamin B12," it means the amount is unknown. (To see all foods with unknown vitamin-B12 readings, click the dictionary's PowerFoods tab and sort the foods by vitamin-B12 power; then scroll toward the bottom of the list until you see foods with question marks in the "Power Rating" column.)
Vitamin B12 on Food Labels
Food labels are not required to report vitamin B12 content, but some do. They may cite the amount in micrograms, percent of Daily Value, or both.
The Daily Value for vitamin B12 is 6.0 micrograms. (Why so much higher than the RDAs? Because all Daily Values are based on the RDAs of 1968, which for some nutrients were different from today's.) The Daily Value is not necessarily right for you, however¾it's a rough estimate meant to cover most of the U.S.population.
For more on label regulations, see Labels, Food.