Why Is B12 Needed After Gastric Bypass surgery?

Gastric bypass, a weight loss surgery procedure, is often successful for people who need to lose a substantial amount of weight. However, the procedure also affects the body's ability to absorb certain nutrients, including vitamin B12. You may need to take supplements for the rest of your life to prevent serious neurological complications after this surgery.

Gastric Bypass

Gastric bypass surgery uses a combination of restricted food intake and malabsorption to speed weight loss. First, the surgeon reduces the stomach to an egg-sized pouch, helping you feel full after eating small amounts of food. Second, she rearranges the digestive system, causing food to bypass part of the small intestine and blocking some absorption of calories and other nutrients, including vitamin B12.

Vitamin B12

The body needs vitamin B12 for proper nerve function, production of red blood cells and other materials involved in you cardiovascular and immune system functions. Most normally healthy individuals get vitamin B12 from animal protein sources, including meat, poultry, fish, dairy products, and eggs. Over-the-counter supplements also provide vitamin B12, as do prescription injections and nasal sprays. Normal absorption of B12 from food sources depends on substances that are often diminished after gastric bypass surgery.

Vitamin B12 Deficiency After Gastric Bypass

After gastric bypass, people cannot eat enough food to meet their needs for vitamin B12, nor do they absorb B12 properly from the little food they consume. Because the liver stores several years' worth of B12, you may not show signs of a deficiency for several years after surgery. However, as many as 70 percent of gastric bypass patients have low blood levels of vitamin B12 after the first year and more than 30 percent have symptoms of deficiency if they rely only on multivitamins for B12 supplementation, according to a 2004 study published in the "Archives of Neurology." Early signs of vitamin B12 deficiency include fatigue, weakness, diarrhea, dizziness, rapid heart beat, poor appetite, anemia, sore or swollen tongue, numbness and tingling sensations. Prolonged B12 deficiency can cause permanent nerve damage, dementia, psychosis, altered mood, spasticity and memory loss.


While routine vitamin B12 supplementation after gastric bypass has no research based evidence to recommend it, most weight loss surgeons advocate supplementation within the first six months after surgery, according to a 2008 study published in the journal "Surgery for Obesity and Related Diseases." Optimal doses may vary among patients but generally an oral dose of at least 350mcg of vitamin B12 is enough to maintain normal blood levels, according to the study 1. Alternative routes include injections of 1,000mcg of vitamin B12 after one to three months or 500mcg by nasal spray every week.