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Bypass has long-term effects on hypertension

Lower blood pressure in the bypass group remained at 10 years

Gastric bypass surgery leads to better long-term resolution of hypertension than purely restrictive methods of surgery, according to a the latest paper from the Swedish Obese Subjects research group, published in Plos One.

The researchers also found a diuretic effect related to gastric bypass independent of weight loss.

Using 10-year data from the Swedish Obese Subjects study, the researchers compared blood pressure in patients who had gastric bypass surgery, to patients who had restrictive surgery (vertical banded gastroplasty and gastric band), and patients in a control group who were treated with non-surgical methods.

The researchers found that while all methods of surgery examined led to lower blood pressure at two years, by ten years, the benefits from the restrictive methods had almost disappeared, while the benefits from bypass remained.

While systolic pressure in the bypass group had decreased by -12.1mmHg and -5.1mmHg, and the diastolic pressure by -7.3mmHg and -5.6mmHg compared to baseline at two years and ten years, the effect was less pronounced in bypass and gastroplasty patients, and by ten years, diastolic pressure was actually lower in the control group.

The finding led researchers to speculate that gastric bypass modulates blood pressure through at least two mechanisms: an initial reduction in blood pressure linked to weight loss, as also experienced in the restrictive operations, and a more long-term mechanism, unrelated to weight loss.

“Based on these observations it is plausible to assume that the exclusion of the gastroduodenum, or the direct loading of undigested food into the jejunum, added or removed a blood pressure regulating factor acting in parallel with the depressor effect of weight loss,” wrote the researchers.

At two years, a significantly lower proportion of both surgical groups were taking antihypertensives than the control group – 27% for the bypass group and 31% for the restrictive group, compared to 43% for the control group. However, at ten years, there were significantly fewer patients – 35% – taking hypertensives in the bypass group, compared to 45% in the restrictive group and 53% in the control group.

Diuretic effect

The researchers also found that, while diurnal urinary outputs were reduced in both surgical groups in absolute terms, the gastric bypass group exhibited higher urinary output compared to body weight. They also found a similar effect at two years in the restrictive group, albeit at less than half the magnitude; this effect also disappeared at the ten-year follow-up.

After adjusting for BMI reduction, patients in the bypass group was 100ml and 170ml higher than the restrictive group at the-year and ten-year mark. However, the proportion of patients using diuretics did not differ significantly between the two groups at either follow-up point.

The researchers wrote that regression analysis demonstrated that changes in urinary output were linearly associated with blood pressure changes only in the bypass cohort, suggesting that blood pressure reduction after gastric bypass can be attributed to a diuretic action.

Despite the drop in blood pressure, the researchers found that gastric bypass patients were eating over one gram of sodium more per day compared to the restrictive cohort, which would ordinarily suggest a rise in blood pressure.

The findings led the researchers to speculate that the bypass operation increases the excretion of sodium in the patient’s urine, and that their findings support the existence of a “sodium sensor” in the upper gut, which inhibits salt appetite and influences natriuresis, but which is eliminated from the intestine by the procedure.

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