Postoperative follow-up for the malabsorption deficiencies related to bariatric procedures should be performed every six months, including obtaining levels of alkaline phosphatase and others previously discussed. In addition, the Endocrine Society guideline3 recommends measuring levels of vitamin B12, albumin, pre-albumin, iron, and ferritin, and obtaining a CBC, a liver profile, glucose reading, creatinine measurement, and a metabolic profile at one month and two months after surgery, then every six months until two years after surgery, then annually if findings are stable.
Furthermore, the Endocrine Society3 recommends obtaining zinc levels every six months for the first year, then annually. An annual vitamin A level is optional.9 Yearly bone density testing is recommended until the patient’s BMD is deemed stable.3
Additionally, Koch and Finelli10 recommend performing the following labs postoperatively: hemoglobin A1C every three months; copper, magnesium, whole blood thiamine, vitamin B12, and a 24-hour urinary calcium every six months for the first three years, then once a year if findings remain stable.
Use of alcohol should be discouraged among patients who have undergone bariatric surgery, as its use alters micronutrient requirements and metabolism. Alcohol consumption may also contribute to dumping syndrome (ie, rapid gastric emptying).11
Any patient with a history of malabsorptive bypass surgery who complains of neurologic, visual, or skin disorders, anemia, or edema may require a further workup to rule out other absorptive deficiencies. These include vitamins A, E, and B12, zinc, folate, thiamine, niacin, selenium, and ferritin.10
Osteomalacia
Metabolic bone diseases can result from genetics, dietary factors, medication use, surgery, or hormonal irregularities. They alter the normal biochemical reactions in bone structure.
The three most common forms of metabolic bone disease are osteoporosis, osteopenia, and osteomalacia. The WHO diagnostic classifications and associated T-scores for bone mineral density1,2 indicate a T-score above –1.0 as normal. A score between –1.0 and –2.5 is indicative of osteopenia, and a score below –2.5 indicates osteoporosis. A T-score below –2.5 in the patient with a history of fragility fracture indicates severe osteoporosis.1,2
In osteomalacia, bone volume remains unchanged, but mineralization of osteoid in the mature compact and spongy bone is either delayed or inadequate. The remolding cycle continues unchanged in the formation of osteoid, but mineral calcification and deposition do not occur.3-5
Osteomalacia is normally considered a rare disorder, but it may become more common as increasing numbers of patients undergo gastric bypass operations.12,13 Primary care practitioners should monitor for this condition in such patients before serious bone loss or other problems develop.9,13,14
Vitamin D deficiency (see “Vitamin D Metabolism,”4,15-19 below), whether or not the result of gastric bypass surgery, is a major risk factor for osteomalacia. Disorders of the small bowel, the hepatobiliary system, and the pancreas are all common causes of vitamin D deficiency. Liver disease interferes with the metabolism of vitamin D. Diseases of the pancreas may cause a deficiency of bile salts, which are vital for the intestinal absorption of vitamin D.17
Restriction and Malabsorption
The case patient had undergone a gastric bypass (duodenal switch), in which a large portion of the stomach is removed and a large part of the small bowel rerouted—with both parts of the procedure causing malabsorption.11 It is in the small bowel that absorption of vitamin D and calcium takes place.
The duodenal switch gastric bypass surgery causes both restriction and malabsorption. Though similar to a biliopancreatic diversion, the duodenal switch preserves the distal stomach and the pylorus20 by way of a sleeve gastrectomy that is performed to reduce the gastric reservoir; the common channel length after revision is 100 cm, not 50 cm (as in conventional biliopancreatic diversion).13 The sleeve gastrectomy involves removal of parietal cells, reducing production of hydrochloric acid (which is necessary to break down food), and hindering the absorption of certain nutrients, including the fat-soluble vitamins, vitamin B12, and iron.12 Patients who take H2-blockers or proton pump inhibitors experience an additional decrease in the production and availability of HCl and may have an increased risk for fracture.14,20,21
In addition to its biliopancreatic diversion component, the duodenal switch diverts a large portion of the small bowel, with food restricted from moving through it. Vitamin D and protein are normally absorbed at the jejunum and ileum, but only when bile salts are present; after a duodenal switch, bile and pancreatic enzymes are not introduced into the small intestines until 75 to 100 cm before they reach the large intestine. Thus, absorption of vitamin D, protein, calcium, and other nutrients is impaired.20,22
Since phosphorus and magnesium are also absorbed at the sites of the duodenum and jejunum, malabsorption of these nutrients may occur in a patient who has undergone a duodenal switch. Although vitamin B12 is absorbed at the site of the distal ileum, it also requires gastric acid to free it from the food. Zinc absorption, which normally occurs at the site of the jejunum, may be impaired after duodenal switch surgery, and calcium supplementation, though essential, may further reduce zinc absorption.9 Iron absorption requires HCl, facilitated by the presence of vitamin C. Use of H2-blockers and proton pump inhibitors may impair iron metabolism, resulting in anemia.20