If you suffer from daytime sleepiness, forgetfulness, confusion, tiredness or muscle weakness, I recommend that you get a blood test for vitamin B12 deficiency, since nerve damage from lack of B12 is not always reversible. The frequency of B12 deficiency is increasing because:
• Aging increases risk because your stomach loses some of its ability to produce acid that helps you absorb B12, and almost 40 percent of North Americans become deficient in vitamin B12 (Am J Clin Nutr, 2009;89:1981S-1996S).
• B12 is found only in animals, not in plants. Research on meat has encouraged many people to stop eating or cut back on eating animal products, which increases risk for B12 deficiency.

Vitamin B12 deficiency can damage your nerves, blood cells and DNA. It can cause a wide variety of symptoms, including:
• anemia that makes you weak and tired
• blurred or double vision
• numbness and tingling
• depression, anxiety and abnormal behavior
• yellow skin from bilirubin released by broken red blood cells
• restless legs syndrome, fibromyalgia or chronic fatigue syndrome
• heart palpitations
• shortness of breath
• constipation, diarrhea, loss of appetite, excess gas

How Your Body Handles Vitamin B12
All animals contain B12, so you get it in meat, poultry, seafood, dairy products, eggs, and B12-fortified foods. Foods from plants do not contain B12 unless it is added to them, as in fortified breakfast cereals. After you eat a food containing B12, your stomach produces hydrochloric acid which separates vitamin B12 from that food. Then your stomach produces “intrinsic factor” that binds to B12 to help it to be absorbed in your upper intestines (Cell Metab, Nov 4, 2014;20(5):769-778). Older people are susceptible to B12 deficiency because they often have decreased amounts of stomach acid (Curr Opin Clin Nutr Metab Care, 2010 Jan; 13(1): 24-27). Some people develop pernicious anemia because they lack “intrinsic factor,” a protein produced in the stomach to help you absorb vitamin B12. At least two groups of colon bacteria, Pseudomonas and Klebsiella sp., make B12, but this is of little or no benefit since B12 is absorbed only in the upper intestinal tract, not in the lower intestine or colon. (Nature, Feb 21, 1980;283(5749):781-2).

Evaluating People with Low Levels of B12
People who have blood levels of B12 below 300 need to get blood tests for homocysteine and methylmalonic acid. If either of these tests are elevated, that person is likely to be already suffering nerve damage (The American Journal of Clinical Nutrition, July, 2003;78(1):7-21).

People most likely to lack B12 include:
• older adults who do not have enough stomach acid to absorb vitamin B12
• people who lack “intrinsic factor” and suffer from pernicious anemia
• people who have had stomach or intestinal surgery
• those who have had bypass surgery to help them lose weight
• those with intestinal diseases such as celiac disease, Crohn’s or ulcerative colitis
• vegetarians
• pregnant women
• people who take certain medications that can interfere with the absorption of B12, such as chloramphenicol (an antibiotic), proton pump inhibitors or histamine receptor antagonists for excess stomach acidity (cimetidine, famotidine, or ranitidine), or metformin for diabetes.

How Much Vitamin B12 Do You Need?
Most people with vitamin B12 deficiency can be maintained on B12 pills as long as they are able to get their blood levels of B12 above 400 (Blood, 1998;92(4):1191-1198). Healthy adults require 2.4 mcg of B12 per day, and the common B12 pills contain 1000 to 2000 mcg. A 2000 mcg over-the-counter pill contains more than 800 times the recommended daily requirement for B12 in a healthy person. Most people who do not have intrinsic factor still can absorb more than one percent of the B12 they take by mouth (Expert Opin Pharmacother, 2010;11(2):249-256). You will need injections if you can’t get your B12 to normal levels with pills. Realize that lack of B12 can irreversibly damage your brain, so you must do everything possible to get blood levels of B12 above 400.

My Recommendations
Lack of B12 is a serious disease that can cause permanent nerve damage. I believe that everyone should get a blood test for vitamin B12 as part of their annual physical, or sooner if they are experiencing forgetfulness, confusion, or any of the other symptoms listed above. Most people with B12 deficiency can be treated just by taking ordinary over-the-counter B12 pills.