Weight Loss Diet

How Gastric Bypass Surgery Affects B12

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Gastric bypass procedures (GBP) are an increasingly popular method for extreme weight loss. Thousands of people benefit from the surgery every year, and many of them manage to lose impressive amounts of weight. But one of the many side-effects is a lifelong need to take nutritional supplements.

How Gastric Bypass Surgery Works
A surgeon divides the stomach into a small upper stomach ‘pouch’ (around the size of an egg) and a much larger lower pouch. The small intestine is then rearranged so that it connects to both pouches.

Weight Loss DietThis has two key physiological impacts. Firstly, the much smaller stomach can’t take anywhere near as much food as before. This makes the patient feel ‘full’ after consuming far fewer calories. In fact, overeating after GBP can cause nausea, vomiting and other unpleasant effects.

The other effect of GBP is that food will bypass some of the digestive system. This is to encourage the malabsorption of calories, but has the same impact on the absorption of vital nutrients, including vitamin B12.

N.B. ‘Stomach stapling‘ is a similar but outdated procedure that has been more or less entirely replaced by gastric bypass surgery. The terms are now often used interchangeably, even though this is technically incorrect.

Why GBP Causes B12 Deficiency
The whole point of the surgery is that patients can no longer consume anywhere near as much food as they could before. And it prevents some of the calories in this new restricted intake from being absorbed as well.Low Calories Diet

But what was designed to restrict caloric intake also restricts vitamin B12 intake. Post-GBP patients simply cannot get enough B12 in their diets from food alone.

Large stores of B12 are often held in the liver, so a deficiency could take years to present itself. However, the signs normally appear much sooner. A scientific study in 2004 found that 70% of GBP patients had measurably low levels of B12 after just one year. Even among patients taking multivitamins to supplement their intake, 30% still showed signs of B12 deficiency.

Vitamin Supplementation After GBP
Because of the permanent nature of surgery, patients will normally need to take dedicated B12 supplements for the rest of their lives. As noted above, a multivitamin will rarely be sufficient.

Dietary SupplementsMost surgeons recommend starting to take supplements within the first six months of surgery. Research from 2008 found that 350mcg oral supplements were sufficient for most patients, with the alternative being 1000mcg B12 injections after 1-3 months.

However some experts disagree, as B12 administered orally may not be adequately absorbed by the surgically altered digestive system. For this reason, far higher oral doses are sometimes recommended.

Sublingual tablets are often suggested as a better option. There is no evidence that sublingual absorption is affected, and that wouldn’t make sense anyway – this route involves the vitamin being absorbed directly into the bloodstream.

We always recommend making sure that any B12 supplement you take contains methylcobalamin rather than synthetic cyanocobalamin.

Other Affected NutrientsVitamin Pills
GBP has a wide range of physical and physiological effects. Beyond the initial risks of surgery and the complications that may occur, the permanent lifestyle changes are not insubstantial.

Besides the risk of B12 deficiency, GBP patients are also regularly found to have low levels of calcium, zinc, iron, folate (aka folic acid or vitamin B9), vitamin B1 (aka thiamine), protein and vitamin A. Many or all of these nutrients will need to be obtained through dietary supplements for the rest of the patient’s life.

Other Side Effects
There are plenty of other side effects to consider as well, but that’s a separate article in itself. If you’re interested, this site would be a good place to start.


From top to bottom, images are courtesy of Mister GC, nixxphotography, YaiSirichai, and Danilo Rizzuti, all at FreeDigitalPhotos.net.

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2 thoughts on “How Gastric Bypass Surgery Affects B12”

  1. I really like the Gastric Bypass Surgery section as it is very informative. Gastric bypass is really becoming popular because it is an efficient way of losing weight.

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