A recently published study found low-carb ketogenic diet resulted in lower blood pressure than a weight loss regime using orlistat plus a low-fat diet (less than 30% of calories from fat). Orlistat is marketed as Xenical or Alli. It prevents the body from absorbing fat that was eaten. The study subjects were overweight (mean Body Mass Index of 39.3) with a mean age of 52. Although both groups lost about 10% in a year (20-25 lb.), the low-carb group showed better blood pressure results. The scientists did not have a ready explanation for this result, but postulated that the low-carb diet could have a diuretic effect, or the lower insulin levels that occur on a low-carb diet were involved.

Usually, losing weight causes blood pressure to drop:

It is well documented that weight loss due to diet alone is associated with a reduction in blood pressure. The Cochrane Collaboration recently completed a review indicating that a weight loss of 4-8% was associated with a decrease in blood pressure of about 3 mm Hg.

In other studies, orlistat produced greater weight loss and a bigger drop in blood pressure than a control group. However, one of the researchers in this study said other studies usually involve people who were overweight, but not “unhealthy”, whereas the recent study involved “real” people with problems typical of overweight people such as diabetes and high blood pressure.

There are some studies involving hypertensive patients - people with high blood pressure - and orlistat. The Cochrane Collaboration
reviewed eight studies of at least 24 weeks duration, and found blood pressure did drop in those on orlistat (and went up for those on another weight loss drug, sibutramine, even though that group lost the same amount of weight). The Cochrane authors also said long term studies of these drugs are needed on mortality and morbidity.

Bottom line:

Studies on healthy subjects are not necessarily able to predict what happens when a person with diabetes takes that medication. Low-carb diet not only promotes weight loss, but lowers blood pressure.

Sources:
http://www.theheart.org/article/1042243.do
http://www.nationalpost.com/life/health/story.html?id=5c9bdf08-b9b5-4a1a-8aed-a7835c8e90cd
http://latimesblogs.latimes.com/booster_shots/2010/01/lowcarb-diets-may-trump-lowfat-in-lowering-blood-pressure.html