Vitamin B-12 is a large molecule involved in many bodily processes, such as blood cell production and nervous system function. Your kidneys are involved in absorbing B-12 and other vitamins, which prevents them from escaping via urination. Diseased and malfunctioning kidneys absorb poorly and contribute to vitamin deficiencies. Some forms of synthetic B-12 are safer than others for people with chronic kidney problems.
Functions and Recommended Amounts
Vitamin B-12 is needed by your body to synthesize DNA and RNA, produce red blood cells, conduct electrical nerve messages, stimulate metabolism and regulate brain function, especially short-term memory. The recommended daily values of B-12 for adults range from 2.4 to 2.8 micrograms, depending on pregnancy and breastfeeding. Unlike most other B vitamins, which are secreted within days, B-12 normally is stored in the body for many months or years, and not much B-12 is needed on a regular basis. Some conditions, however, such as pernicious anemia, result in poor B-12 absorption by the body.
Kidney Absorption and Storage
Your kidneys are responsible for filtering many nutrients out of the blood and lymph, including vitamin B-12 and other B vitamins, such as folate. In addition to vitamin conservation, tubular uptake by the kidneys is important for vitamin metabolism, storage and homeostasis, according to a study published in a 2006 edition of the medical journal "Renal Physiology." For people with chronic kidney failure or acute kidney injuries, higher levels of vitamins and other nutrients typically are excreted in urine. Elevated levels of homocysteine in the blood indicate tioo little B-12 in your body. Homocysteine damages blood vessels and is linked to cardiovascular disease.
The natural form of B-12 that's in animal-based food is called cobalamin, which is virtually non-toxic and not associated with any serious side effects. However, synthetic versions of B-12 are potentially toxic in large doses and can trigger allergic reactions. The most common B-12 supplement on the market is cyanocobalamin, which forms cyanide -- a neurotoxin -- as a byproduct. Such small amounts of cyanide don't pose much of a threat to people with healthy livers and kidneys, but for those with dysfunctional organs, the risk is amplified. Diseased kidneys cannot detoxify cyanide into thiocyanate, which is harmless and easily excreted.
People with chronic kidney disease have a higher risk of B-12 deficiency, so supplementation may be warranted. However, cyanocobalamin poses a slightly higher risk for kidney patients, so other forms such as methylcobalamin, hydroxocobalamin or adenosylcobalamin may be safer options. Therapeutic doses of B-12 by injection range between 500 and 1,000 micrograms. If you take oral supplements, place the tablet under your tongue for better absorption.