Gastric Bypass - Bariatric Surgery Nutritional Assessment

Indications for Testing

Before bariatric surgery, individuals should be assessed for nutritional status. Patients who have undergone surgical modification of the gastrointestinal (GI) tract for management of obesity should also be monitored postsurgery for nutritional deficiencies.

Laboratory Testing

Preoperative Assessment

Individuals being considered for bariatric surgery should have baseline biochemical and nutritional testing performed to identify deficiencies that may predispose them to a higher risk of deficiencies after surgery; deficiencies should be corrected before surgery when possible.    Micronutrient​ testing to consider includes the following:

Vitamins

Vitamin D and calcium levels should be assessed. Vitamin D, 25-hydroxy is a marker of vitamin D stores in the body and is the preferred test. Vitamin D, 1,25 dihydroxy is indicated for renal disease and other special situations and is less commonly used. Calcium may be reduced due to changes in food preferences. Calcium status can be assessed by measuring levels of ionized calcium in serum or whole blood, whereas urinary calcium (24-hour levels) can identify hypocalcuria. 

Vitamin B₁ should be measured to evaluate patients for thiamine deficiency. Whole blood specimens are required for accurate analysis. Patients should also be assessed for vitamin B₁₂ deficiency, which may take longer to present due to the body’s ability to maintain large stores of the vitamin. B₁₂ deficiency may result from intrinsic factor decrease and loss of absorptive surface. Serum methyl malonic acid (MMA) is the recommended test for vitamin B₁₂ deficiency. 

Patients should be evaluated for folate (B₉) deficiency, which is rarely seen due to consumption of fortified foods. Tests for vitamins A, E, and K should be considered as well.

Trace Elements

Recommended mineral testing includes zinc testing and iron studies. Zinc deficiency is common in patients with obesity and may be linked to metabolic syndrome.  Iron studies include laboratory tests for iron, iron binding capacity, and ferritin. Iron deficiency may be present before surgery and may be exacerbated by the loss of absorptive surface and/or changes in food preferences. Ferritin testing can be used to evaluate patients for iron deficiency anemia. Ferritin, an acute phase reactant, is the most sensitive marker for iron deficiency anemia, but may be affected by inflammation. Iron and iron binding capacity testing should be used periodically to verify accuracy of ferritin testing; however, this testing is not as sensitive as ferritin testing.

Copper, magnesium, and phosphorous tests should be considered as well.

Postoperative Monitoring

The postoperative schedule for biochemical and nutritional testing is based on type of surgery: sleeve gastrectomy (SG) or RYGB.