Bariatric Surgery - Nutritional Assessment

Content Review: January 2023 Last Update:

Bariatric surgeries are used increasingly to treat overweight and obese patients who have clinically relevant signs and symptoms. Approximately 252,000 bariatric surgeries were performed in the United States in 2018.  These procedures can be successful in reversing many adverse syndromes associated with obesity (including type II diabetes, hypertension, joint pain, obstructive sleep apnea, hyperlipidemia, and coronary artery disease ), but there are risks and complications to consider, such as the exacerbation of preexisting nutritional deficiencies and the development of new ones. Some deficiencies present soon after surgery, but the late presentation of nutritional deficiencies can contribute to poor outcomes or devastating complications. Bariatric surgeries that use a restrictive technique, such as sleeve gastrectomy (SG), may lead to nutritional deficiencies due to low intake, whereas surgeries that use a combination of restrictive and malabsorptive techniques, such as Roux-en-Y gastric bypass (RYGB), may lead to deficiencies due to either low intake or malabsorption.  Therefore, long-term monitoring for nutritional deficiencies is indicated following all types of bariatric surgery.

Quick Answers for Clinicians

Which types of bariatric surgery are performed to induce weight loss?

Bariatric surgeries can be classified as restrictive or malabsorptive. Restrictive procedures reduce the volume or capacity of the stomach. Malabsorptive procedures reduce the amount of calories absorbed by altering the flow of food.  Sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB) are the most common types of bariatric surgery performed. SG is a restrictive procedure; RYGB is a dual restrictive and malabsorptive procedure.  Adjustable gastric binding (AGB) and biliopancreatic diversion with duodenal switch (BPD/DS) are other available options but account for only a small percentage of bariatric surgeries. 

Which tests are recommended during a preoperative gastric bypass assessment?

Individuals being considered for bariatric surgery should have a preoperative nutritional assessment to identify deficiencies that may predispose them to a higher risk of nutritional deficiencies postsurgery.  When possible, these deficiencies should be corrected before surgery.  Assessment typically includes vitamin D, calcium, parathyroid hormone (PTH), ferritin, folate (vitamin B9), and vitamin B12 tests, as well as iron studies.   For more information, refer to the Preoperative Assessment section.

 

Which tests are recommended for postoperative monitoring?

The same tests used for preoperative assessment are also used postoperatively. The biochemical and nutritional testing schedule varies depending on the type of surgery. For more information, refer to the Postoperative Monitoring section.

Indications for Testing

Preoperative nutritional assessment is indicated in all individuals being considered for bariatric surgery. A more extensive evaluation is recommended for those undergoing a malabsorptive procedure.

Following surgery, nutritional monitoring should continue for all patients, at regular intervals or as needed, to monitor 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.   

Key Micronutrient Testing

Current guidelines recommend preprocedural testing for the following micronutrients in all patients  :

  • Calcium (including parathyroid hormone [PTH])
  • Vitamin D
  • Iron (including ferritin)
  • Folate (vitamin B9)
  • Vitamin B12

Calcium status can be assessed by measuring concentrations of ionized calcium in serum or whole blood, whereas measuring urinary calcium (24-hour concentrations) can identify hypocalciuria.  Evaluation of PTH concentrations (alongside 25-hydroxyvitamin D and calcium) is recommended to detect possible hyperparathyroidism. To determine vitamin D status, 25-hydroxyvitamin D (a marker of vitamin D stores in the body) is preferred in the absence of renal disease. 

Iron studies include laboratory tests for iron, iron binding capacity, and ferritin. Ferritin is the most sensitive marker for iron deficiency anemia, which is present in up to 47% of patients before bariatric surgery.  However, ferritin concentrations may be affected by inflammation. Iron and iron binding capacity testing, although less sensitive than ferritin testing, is useful to verify the accuracy of ferritin results.

Folate (vitamin B9) deficiency has been reported in some patients before bariatric surgery,  although the consumption of fortified foods helps reduce its likelihood. A serum or plasma folate test can be used to assess deficiency, followed by a red blood cell (RBC) folate test if initial results are inconclusive. 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. A serum methylmalonic acid (MMA) test is recommended to evaluate vitamin B12 status. 

Additional Micronutrient Testing

More extensive nutritional assessment is recommended in patients undergoing malabsorptive procedures, such as RYGB or biliopancreatic diversion with duodenal switch (BPD/DS), and in those suspected of having a preexisting nutrient deficiency. When more extensive screening is indicated, consider testing for the following micronutrients  :

  • Vitamin B1 (thiamine diphosphate)
  • Vitamin A
  • Vitamin E
  • Copper
  • Zinc
  • Selenium

Postoperative Monitoring

Recommendations for postoperative biochemical and nutritional testing are generally based on the bariatric procedure selected. However, postoperative screening for the following micronutrients should be performed in all patients  :

  • Calcium
  • Vitamin D
  • Iron
  • Folate (vitamin B9)
  • Vitamin B12

Testing for vitamin B1 (thiamine diphosphate) in whole blood may also be considered.

Refer to the following tables for postoperative recommendations specific to SG, RYGB, and BPD/DS procedures.

Notably, recommendations vary for individuals who become pregnant following bariatric surgery; applicable micronutrient testing should be performed during each trimester.  

Sleeve Gastrectomy
Micronutrient(s) Recommendation(s) Testing

Vitamin A

Assess as needed in patients who report steatorrhea or symptoms of vitamin A deficiency 

In the presence of vitamin A deficiency, consider testing for deficiencies in other fat-soluble vitamins, iron, and copper 

Vitamin A, serum or plasma

Vitamin B1 (thiamine)

In the presence of signs, symptoms, or risk factors for thiamine deficiency, assess within 6 mos of surgery and then every 3-6 mos until symptoms subside 

Vitamin B1 (thiamine diphosphate), whole blood

Folate (vitamin B9)

Assess at 3 mos, 6 mos, and 12 mos after surgery and annually thereafter 

Folate, serum

Vitamin B12

Establish a postoperative baseline and assess annually thereafter 

In patients using medications that increase the risk of B12 deficiency, assess every 3 mos within the first 12 mos of surgery and annually thereafter or as clinically indicated 

In patients taking higher doses of folate, measure B12 to identify possible deficiency 

MMA, serum

Total homocysteine (optional)

Vitamin D and calcium

Assess at 3 mos, 6 mos, and 12 mos after surgery and annually thereafter 

In the presence of vitamin D deficiency, consider testing for deficiencies in other fat-soluble vitamins 

25-hydroxyvitamin D, serum

Calcium, serum

Calcium, 24-hour urinary (as needed; consider if history of renal stones)

PTH, intact with calcium (as needed)

Bone formation/resorption markers (as needed; refer to the ARUP Consult Osteoporosis topic for more information)

Vitamin E

Assess as needed in symptomatic patients 

In the presence of vitamin E deficiency, consider testing for deficiencies in other fat-soluble vitamins 

Vitamin E, serum

Vitamin K

Assess as needed in symptomatic patients 

In the presence of vitamin K deficiency, consider testing for deficiencies in other fat-soluble vitamins 

Vitamin K1, serum

Copper

Assess annually 

Following treatment for copper deficiency, assess every 3 mos after concentrations have returned to normal 

Consider measuring copper in patients supplementing zinc  or in patients with a known vitamin A deficiency 

Copper, serum

Ceruloplasmin, serum

Iron

Assess within the first 3 mos of surgery and then every 3-6 mos until 12 mos postsurgery; assess annually thereafter 

Iron panel

CBC

Total iron binding capacity

Ferritin, serum

Soluble transferrin receptor (if available)

Zinc

Assess annually 

Consider measuring zinc in patients supplementing copper 

Zinc, serum or plasma

Sources: Mechanick, 2020 ; O’Kane, 2020 

Roux-en-Y Gastric Bypass
Micronutrient(s) Recommendation(s) Testing

Vitamin A

Assess within the first 12 mos of surgery and annually thereafter, or as needed in patients with symptoms of vitamin A deficiency or protein-calorie malnutrition 

In the presence of vitamin A deficiency, consider testing for deficiencies in other fat-soluble vitamins, iron, and copper 

Vitamin A, serum or plasma

Vitamin B1 (thiamine)

In the presence of signs, symptoms, or risk factors for thiamine deficiency, assess within 6 mos of surgery and then every 3-6 mos until symptoms subside 

Vitamin B1 (thiamine diphosphate), whole blood

Folate (vitamin B9)

Assess at 3 mos, 6 mos, and 12 mos after surgery and annually thereafter 

Folate, serum

Vitamin B12

Establish a postoperative baseline and assess annually thereafter 

In patients using medications that increase the risk of B12 deficiency, assess every 3 mos within the first 12 mos of surgery and annually thereafter or as clinically indicated 

In patients taking higher doses of folate, measure B12 to identify possible deficiency 

MMA, serum

Total homocysteine (optional)

Vitamin D and calcium

Assess at 3 mos, 6 mos, and 12 mos after surgery and annually thereafter 

In the presence of vitamin D deficiency, consider testing for deficiencies in other fat-soluble vitamins 

25-hydroxyvitamin D, serum

Calcium, serum

Calcium, 24-hour urinary (as needed; consider if history of renal stones)

PTH, intact with calcium (as needed)

Bone formation/resorption markers (as needed; refer to the ARUP Consult Osteoporosis topic for more information)

Vitamin E

Assess as needed in symptomatic patients 

In the presence of vitamin E deficiency, consider testing for deficiencies in other fat-soluble vitamins 

Vitamin E, serum

Vitamin K

Assess as needed in symptomatic patients 

In the presence of vitamin K deficiency, consider testing for deficiencies in other fat-soluble vitamins 

Vitamin K1, serum

Copper

Assess annually  

Following treatment for copper deficiency, assess every 3 mos after concentrations have returned to normal 

Consider measuring copper in patients supplementing zinc  or in patients with a known vitamin A deficiency 

Copper, serum

Ceruloplasmin, serum

Iron

Assess within the first 3 mos of surgery and then every 3-6 mos until 12 mos postsurgery; assess annually thereafter 

Consider testing for other causes of nutritional anemia, including deficiencies in vitamin B12, folate, copper, selenium, or zinc, when iron deficiency has been ruled out 

Iron panel

CBC

Total iron binding capacity

Ferritin, serum

Soluble transferrin receptor (if available)

Selenium

Assess as needed in patients presenting with unexplained anemia or symptoms consistent with selenium deficiency 

Selenium, serum

Zinc

Assess annually  

Consider measuring zinc in patients supplementing copper 

Zinc, serum or plasma

Sources: Mechanick, 2020 ; O’Kane, 2020 

Biliopancreatic Diversion With Duodenal Switch
Micronutrient(s) Recommendation(s) Testing

Vitamin A

Assess within the first 12 mos of surgery and annually thereafter, or as needed in patients with symptoms of vitamin A deficiency or protein-calorie malnutrition 

In the presence of vitamin A deficiency, consider testing for deficiencies in other fat-soluble vitamins, iron, and copper 

Vitamin A, serum or plasma

Vitamin B1 (thiamine)

In the presence of signs, symptoms, or risk factors for thiamine deficiency, assess within 6 mos of surgery and then every 3-6 mos until symptoms subside 

Vitamin B1 (thiamine diphosphate), whole blood

Folate (vitamin B9)

Assess at 3 mos, 6 mos, and 12 mos after surgery and annually thereafter 

Folate, serum

Vitamin B12

Establish a postoperative baseline and assess annually thereafter 

In patients using medications that increase the risk of B12 deficiency, assess every 3 mos within the first 12 mos of surgery and annually thereafter or as clinically indicated 

In patients taking higher doses of folate, test B12 to identify possible deficiency 

MMA, serum

Total homocysteine (optional)

Vitamin D and calcium

Assess at 3 mos, 6 mos, and 12 months after surgery and annually thereafter 

In the presence of vitamin D deficiency, consider testing for deficiencies in other fat-soluble vitamins 

25-hydroxyvitamin D, serum

Calcium, serum

Calcium, 24-hour urinary (as needed; consider if history of renal stones)

PTH, intact with calcium (as needed)

Bone formation/resorption markers (as needed; refer to the ARUP Consult Osteoporosis topic for more information)

Vitamin E

Assess as needed in symptomatic patients 

In the presence of vitamin E deficiency, consider testing for deficiencies in other fat-soluble vitamins 

Vitamin E, serum

Vitamin K

Assess as needed in symptomatic patients 

In the presence of vitamin K deficiency, consider testing for deficiencies in other fat-soluble vitamins 

Vitamin K1, serum

Copper

Assess annually  

Following treatment for copper deficiency, assess every 3 mos after concentrations have returned to normal 

Consider measuring copper in patients supplementing zinc  or in patients with known vitamin A deficiency 

Copper, serum

Ceruloplasmin, serum

Iron

Assess within the first 3 mos of surgery and then every 3-6 mos until 12 mos postsurgery; assess annually thereafter 

Consider testing for other causes of nutritional anemia, including deficiencies in vitamin B12, folate, copper, selenium, or zinc, when iron deficiency has been ruled out 

Iron panel

CBC

Total iron binding capacity

Ferritin, serum

Soluble transferrin receptor (if available)

Selenium

Assess as needed in patients presenting with unexplained anemia or symptoms consistent with selenium deficiency 

Selenium, serum

Zinc

Assess annually  

Consider measuring zinc in patients supplementing copper 

Zinc, serum or plasma

Sources: Mechanick, 2020 ; O’Kane, 2020 

ARUP Laboratory Tests

Nutritional assessment and/or postoperative monitoring of patients undergoing bariatric surgery includes the following recommended tests. Refer to the ARUP Laboratory Test Directory for additional nutrient testing options.

References

Medical Experts

Contributor

Frank

Elizabeth L. Frank, PhD, DABCC
Professor of Pathology (Clinical), University of Utah
Medical Director, Analytic Biochemistry, Calculi and Manual Chemistry; Co-Medical Director, Mass Spectrometry, ARUP Laboratories