In the last year, I have seen two experienced cyclists who suffered recurrent numbness and tingling of their hands when they rode long distances. Both had low blood levels of vitamin B12 and one was cured by taking 1000 mcg per day of vitamin B12 pills, the other by eating fish. If you suffer these or any other symptoms of nerve damage, you should get a blood test for vitamin B12 and if it is below 300 pg/mL, you probably need further tests and may be cured just by taking vitamin B12 pills. Vitamin B12 deficiency must be treated as soon as possible, since the nerve damage it causes can become permanent if you wait too long to start treatment.

Signs of B12 Deficiency
Vitamin B12 deficiency in North America occurs in six percent of people under 60 and 20 percent of those over 60 (BMJ, Sept 4, 2014;349:g5226). Vitamin B12 deficiency can cause signs of nerve damage anywhere in your body:
• forgetfulness or decline in mental ability
• daytime sleepiness, tiredness or chronic fatigue
• muscle weakness
• blurred or double vision
• numbness or tingling
• irritability, depression or anxiety (Psychopharmacology, Jul 2010;211(1):55–68)
• abnormal behavior (Psychiatry, Jan 21, 2013)
• heart palpitations
• shortness of breath
• constipation or diarrhea
• loss of appetite
• unexplained weight loss
• excessive intestinal gas
• dizziness or feeling faint
• headaches
• abnormal gait
• tremor
• restless legs syndrome
• paranoia
• psychosis
Vitamin B12 deficiency can also cause a yellow skin tone from bilirubin released by broken red blood cells, pale skin, a red smooth tongue or a sore tongue, and mouth ulcers.

How Your Body Handles Vitamin B12
All animals contain B12, so you get it when you eat meat, poultry, seafood, dairy products or eggs. Plants do not contain B12. Some breakfast cereals and other foods made from plants are fortified with vitamin B12.

After you eat a food containing B12, your stomach produces hydrochloric acid which separates vitamin B12 from that food. Then your stomach produces a protein called “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 unusually susceptible to B12 deficiency (Annu Rev Nutr, 1999;19:357-77) because they often have decreased amounts of stomach acid (Curr Opin Clin Nutr Metab Care, 2010 Jan; 13(1): 24–27). Some people develop B12 deficiency (pernicious anemia) because they lack intrinsic factor.

Human feces may contain large amounts of vitamin B12 because some intestinal and colon bacteria can make B12. At least two groups of intestinal bacteria, Pseudomonas and Klebsiella sp., make B12, but this is of little or no benefit for you 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).

Risk Factors for Vitamin B12 Deficiency
• Aging. The frequency of B12 deficiency increases as you age because your stomach loses some of its ability to produce acids that helps you absorb B12 (Am J Clin Nutr, 2009;89:1981S–1996S). Almost 40 percent of older North Americans become deficient in vitamin B12
• A vegetarian diet: B12 is found only in animals; it is not found 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.
• Mis-diagnoses: conditions such as restless legs syndrome, fibromyalgia and chronic fatigue syndrome may be caused by B12 deficiency.
• Medications that decrease 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. Almost 10 percent of diabetics taking metformin have low levels of B12 (Endocr Connect, Oct 2019;8(10):1324–1329).
• Stomach or intestinal surgery: People who have had bypass surgery or other stomach and intestinal surgery may no longer produce enough intrinsic factor.
• Intestinal diseases, such as celiac disease, Crohn’s or ulcerative colitis.
• Pregnancy: Pregnant women should be checked for B12 deficiency.

Evaluating People Who Have Abnormal Levels of B12
Normal B12 levels are between 200 and 900 pg/mL. A B12 level below 300 pg/mL requires further tests for homocysteine and methylmalonic acid. If either level is high, you are likely to be B12 deficient and may already be suffering from nerve damage (The American Journal of Clinical Nutrition, July 2003;78(1):7–21) and should be treated immediately, since some of the nerve damage may not be reversible. If your B12 level is higher than 900 pg/mL, you need to be checked for liver disease, diabetes, or leukemia.

Many doctors start treatment for B12 deficiency with injections of B12. However, most patients being treated for B12 deficiency can be maintained on B12 pills as long as they can get their blood levels of B12 above 400 pg/mL (Blood, 1998;92(4):1191–1198). Healthy adults require 2.4 mcg of B12 per day, and the average B12 pill contains 1000 to 2000 mcg. A 2000 mcg over-the-counter B12 pill contains 833 times (2000/2.4) the recommended daily requirement for B12 for healthy people. Even people who do not have intrinsic factor usually can absorb more than one percent of the B12 they take by mouth (Expert Opin Pharmacother, 2010;11(2):249–256). You need injections if you can’t get your B12 to normal levels with pills. Realize that lack of B12 is a serious disease that can damage your brain, so if you are deficient, you should do everything possible to get blood levels of B12 above 400 pg/mL.

My Recommendations
Lack of B12 is serious because it 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. Many people with B12 deficiency can be treated just by eating more animal products, and most can be treated by taking B12 pills. Some people may need B12 injections.