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, reviewed by
Kimberly Langdon M.D.
Research shows that there is a high prevalence -- approximately 40% -- of B12 deficiency in hypothyroid and Hashimoto’s patients. This prevalence may be due to the high association between anemia and hypothyroidism. Anemia is when your body lack’s enough healthy red blood cells to carry oxygen to your body’s tissue, causing tiredness and weakness. Pernicious anemia can impact the absorption (or lack thereof) of vitamin B12. Supplementing with B12 may lead to an improvement in hypothyroid symptoms.
A B12 deficiency may cause a buildup of homocysteine (an amino acid) in the blood and might decrease levels of substances need to metabolize neurotransmitters. Elevated homocysteine levels are associated with cognitive impairment, and studies show that low vitamin B12 status is associated with more rapid cognitive decline. However, supplementing with B12 may only useful for cognition if there is a nutritional deficiency of B vitamins. A study in the American Journal of Clinical Nutrition evaluated the effect of supplementing with folic acid, vitamin B12, and vitamin B6 on cognitive function in women at high risk of cardiovascular disease. Compared to the placebo group, supplementation did not affect the average cognitive change from the baseline. However, in a small group of women with low baseline dietary intake of B vitamins, supplementation significantly slowed the rate of cognitive decline.
Due to its role in energy metabolism, B12 is sometimes considered to enhance energy, athletic performance, or endurance. This is partially true, considering that correcting anemia caused by vitamin B12 deficiency may improve the associated symptoms of weakness and fatigue. B12 will likely not improve symptoms in those with already sufficient levels of this vitamin. Since vitamin B12 is a water-soluble vitamin, your body cannot store extra amounts. Instead, any excess passes through your body. So, if you are deficient in vitamin B12, you may see an improvement in your energy levels.
Vitamin B12 does not have a Tolerable Upper Intake Level (UL)—defined as the highest daily intake level that is likely to pose no adverse health effects—because of its low potential for toxicity. Vitamin B12 does have the potential to interact with certain medications like chloramphenicol (Chloromycetin®), proton pump inhibitors, histamin H2 recepto antagonists (like cimetidine (Tagamet®), famotidine (Pepcid®), and ranitidine (Zantac®)), and metformin. People who take these medications regularly should talk to their doctor about vitamin B12 supplementation.
The Recommended Dietary Allowance (RDA) of vitamin B12—or, the average daily level of intake sufficient to meet the nutrient requirements of nearly all healthy individuals—is 2.4 mcg for adult men and women. Women who are pregnant or breastfeeding should take 2.6 mcg daily and 2.8 mcg daily, respectively.
Vitamin B12 plays a vital role in cell production. So adequate levels of this vitamin are required to promote healthy hair, skin, and nails, which have a fast cell turnover rate. Research shows that supplementing with B12 can improve dermatologic symptoms in people deficient in vitamin B12.
Vitamin B12 deficiency may prevent red blood cells from developing properly. Typically, red blood cells are small and round. However, in a vitamin B12 deficiency, they become larger and misshaped, meaning that the red blood cells cannot move from the bone marrow into the bloodstream at their regular rate. A small percentage of people with a vitamin B12 deficiency may develop anemia, which is when the body doesn't have enough red blood cells to transport oxygen to vital organs. So, supplementing with vitamin B12 can help your body produce red blood cells.
Vitamin B12 is in animal products like fish, meat, poultry, eggs, milk, and milk products. Vitamin B12 is generally not present in plant foods, but fortified breakfast cereals contain vitamin B12 with high bioavailability.