Urinary sodium excretion after gastric bypass surgery.

Background: Gut-kidney signaling is implicated in sodium homeostasis and thus blood pressure regulation. Roux-en-Y gastric bypass (RYGB) surgery for morbid obesity confers a pronounced and long-lasting blood pressure lowering effect in addition to significant weight loss.

Objectives: We set out to establish whether RYGB is associated with an intrinsic change in urinary sodium excretion that may contribute to the reported blood pressure lowering effects of the procedure.

Setting: University hospital METHODS: Five female patients (age range: 28-50 yr) without metabolic or hypertensive co-morbidities were included in a study involving four 24-hour residential visits: once before surgery and 10 days, 3 months, and 20 months after surgery. Creatinine and sodium were measured in fasting plasma samples and 24-hour urine samples and creatinine clearance, estimated glomerular filtration rate, and indices of urinary sodium excretion were calculated. Fasting and 60-minute postprandial blood samples from each study day were assayed for pro-B-type natriuretic peptide (NT-proBNP).

Results: Increases in weight-normalized urinary sodium excretion of up to 2.3-fold in magnitude occurred at 20 months after surgery. Median fractional excretion of sodium at 20 months was double that seen before surgery. Fasting NT-proBNP levels were stable or increased (1.5- to 5-fold). Moreover, a small postprandial increase in NT-proBNP was observed after surgery.

Conclusions: Renal fractional excretion of sodium is increased after RYGB. A shift toward increased postoperative basal and meal associated levels of NT-proBNP coincides with increased urinary sodium excretion. The data support a working hypothesis that an enhanced natriuretic gut-kidney signal after RYGB may be of mechanistic importance in the blood pressure lowering effects of this procedure.