Small Intestinal Bacterial Overgrowth (SIBO) is a digestive condition where excessive bacteria colonize the small intestine, causing malabsorption and digestive symptoms. It is caused by an abnormal increase in bacterial populations in the small intestine, particularly species normally found in the colon, which compete for nutrients and damage the intestinal lining. The Methylmalonic Acid test is the most important blood test for SIBO because it detects vitamin B12 deficiency resulting from bacterial-induced malabsorption.
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SIBO is caused by an overgrowth of bacteria in the small intestine, where bacteria normally exist in much lower numbers than in the colon. This overgrowth typically occurs when the normal mechanisms that control bacterial populations fail, such as decreased stomach acid production, impaired intestinal motility, structural abnormalities like intestinal strictures or diverticula, or immune system dysfunction. Risk factors include conditions like diabetes, scleroderma, Crohn disease, previous bowel surgery, and the use of medications that reduce stomach acid or slow intestinal movement.
The Methylmalonic Acid test is the most important blood test for SIBO because it detects vitamin B12 deficiency, one of the most significant nutritional complications of the condition. When excessive bacteria in the small intestine compete for vitamin B12 or damage the intestinal lining where B12 is absorbed, methylmalonic acid accumulates in the blood because B12 is required to metabolize it. Elevated methylmalonic acid levels indicate that SIBO is causing functional B12 deficiency even before symptoms become severe. While breath tests using glucose or lactulose are the primary diagnostic tools for SIBO itself, blood testing with methylmalonic acid helps assess the metabolic impact and severity of malabsorption.
You should get tested if you experience chronic bloating, abdominal pain, diarrhea, or unexplained weight loss that does not respond to typical treatments. Testing is especially important if you have conditions that increase SIBO risk like diabetes, inflammatory bowel disease, or a history of abdominal surgery. You should also consider testing if you develop signs of vitamin deficiency such as fatigue, numbness or tingling in your hands and feet, or cognitive changes, as these may indicate that SIBO is causing significant malabsorption requiring treatment.
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What this means
Your methylmalonic acid level is elevated, which indicates your body is not getting enough functional vitamin B12 at the cellular level. This can explain symptoms like fatigue, weakness, or neurological issues such as numbness and tingling. Even if a standard B12 blood test showed borderline results, this elevated methylmalonic acid confirms true B12 deficiency that needs attention.
Recommended actions
Start vitamin B12 supplementation (sublingual or oral B12, typically 1000-2000 mcg daily)
Include more B12-rich foods like meat, fish, eggs, and dairy in your diet
Discuss absorption issues with your doctor if you have digestive problems or take certain medications
Retest in 8-12 weeks after starting supplementation to monitor improvement
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