From Obesity to Obstetrics: Practical Magnesium Guidance during Pregnancy Post-Bariatric 

From Obesity to Obstetrics: Practical Magnesium Guidance during Pregnancy Post-Bariatric 

Bariatric surgeryWritten by Ingrid Kruizinga

Medical Affairs Specialist

Magnesium status after bariatric surgery 

Contrary to what many people think, magnesium levels usually don’t drop after gastric bypass surgery. In fact, studies show that magnesium in the blood often stays the same or even increases after the operation.

This improvement in magnesium levels is mostly linked to better blood sugar control and less fat tissue in the abdomen and suggests that magnesium absorption is not impaired after surgery [1-3].

Obesity and baseline risk of magnesium deficiency

However, low levels of magnesium are already prevalent in people with obesity before surgery, especially in those with type 2 diabetes. Clinical studies show lower serum magnesium in obesity and an association between low magnesium and diabetes and poorer metabolic control. [4, 5]

Pregnancy: supplementation and testing

In pregnancy after bariatric surgery, several guidelines recommend magnesium supplementation of 200-1000 mg [6, 7]. At the same time, meta-analyses evaluating outcomes such as preeclampsia or preterm birth show mixed results, so population-wide clinical benefits remain inconclusive [8-10]. The most defensible position today is to prioritize dietary sufficiency and consider supplements for individuals at risk (e.g., pregnancy, low dietary intake, gastrointestinal losses, diabetes, or high baseline risk), especially given magnesium’s central role in maternal–fetal physiology [11].

Assessing magnesium status is challenging. Serum magnesium is widely used because it’s feasible, but it poorly reflects total-body magnesium. So better biomarkers or composite assessments are needed [8, 12]. In most obesity/bariatric clinics, magnesium is not part of standard blood checks.

Why magnesium is (often) excluded from compact multis: formulation constraints

When it comes to supplement design, getting a useful amount of magnesium into a pill isn’t easy. Most forms of magnesium either don’t absorb very well or only contain a small amount of actual (“elemental”) magnesium. For example, magnesium oxide is mostly magnesium by weight (about 61%), but the body doesn’t absorb it well. On the other hand, magnesium citrate absorbs better, but it only contains about 16% magnesium [13]. That means if you want a meaningful dose—say 200 mg of magnesium—the pill has to be fairly large. This is why many compact multivitamins either leave magnesium out and instead suggest taking a separate magnesium supplement if needed.

Practical implications

Advise magnesium supplementation when intake is insufficient or risk is elevated (e.g., pregnancy, hyperemesis, diarrhoea, diuretics, diabetes, multiple gestation). A pragmatic add-on is 200 mg elemental magnesium/day (e.g., citrate), Routine magnesium testing is not universal and serum Mg has limited sensitivity; interpret results in clinical context (albumin, symptoms, concurrent electrolytes).

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Source:

  1. Haenni, A., I. Nilsen, and H.E. Johansson, Increased circulating magnesium concentrations after Roux-en-Y gastric bypass surgery in patients with type 2 diabetes. Surg Obes Relat Dis, 2018. 14(5): p. 576-582.

  2. Johansson, H.E., et al., Serum magnesium status after gastric bypass surgery in obesity. Obes Surg, 2009. 19(9): p. 1250-5.

  3. Mikalsen, S.M., et al., Improved Magnesium Levels in Morbidly Obese Diabetic and Non-diabetic Patients After Modest Weight Loss. Biol Trace Elem Res, 2019. 188(1): p. 45-51.

  4. Lecube, A., et al., Diabetes is the main factor accounting for hypomagnesemia in obese subjects. PLoS One, 2012. 7(1): p. e30599.

  5. Hyassat, D., et al., Prevalence of Hypomagnesaemia among Obese Type 2 Diabetic Patients Attending the National Center for Diabetes, Endocrinology and Genetics (NCDEG). Int J Endocrinol Metab, 2014. 12(3): p. e17796.

  6. Kaska, L., et al., Nutrition and pregnancy after bariatric surgery. ISRN Obes, 2013. 2013: p. 492060.

  7. Harreiter, J., et al., Management of Pregnant Women after Bariatric Surgery. J Obes, 2018. 2018: p. 4587064.

  8. Dalton, L.M., et al., Magnesium in pregnancy. Nutr Rev, 2016. 74(9): p. 549-57.

  9. Yuan, J., et al., Oral Magnesium Supplementation for the Prevention of Preeclampsia: a Meta-analysis or Randomized Controlled Trials. Biol Trace Elem Res, 2022. 200(8): p. 3572-3581.

  10. de Araújo, C.A.L., et al., Magnesium supplementation and preeclampsia in low-income pregnant women – a randomized double-blind clinical trial. BMC Pregnancy Childbirth, 2020. 20(1): p. 208.

  11. Zarcone, R., G. Cardone, and P. Bellini, Role of magnesium in pregnancy. Panminerva Med, 1994. 36(4): p. 168-70.

  12. Arnaud, M.J., Update on the assessment of magnesium status. Br J Nutr, 2008. 99 Suppl 3: p. S24-36.

  13. Werner, T., et al., Assessment of bioavailability of Mg from Mg citrate and Mg oxide by measuring urinary excretion in Mg-saturated subjects. Magnes Res, 2019. 32(3): p. 63-71.

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