It's all over the news the last few days: A new study in Archives of Internal Medicine showing that a low carbohydrate diet beats a low fat diet for lowering blood pressure. This was not a big study -- just 146 subjects -- but it did look at the effects of both diets over 48 weeks time, almost a year, which is a reasonably ample time frame. Too, the study involved subjects who were not only obese -- an average BMI of 39 -- but who also had obesity-related health problems such as diabetes, elevated cholesterol and triglycerides, and, yes, hypertension.

For once researchers actually put their "low carb" subjects on a low carb diet; they all started at Atkins Induction levels of 20 grams per day; I could not find any information regarding how many grams per day they worked their way up to. Way too many studies look at "low carb diets" that allow 100 grams per day or more, so this use of a diet similar what is being eaten by the low carb community -- and recommended in low carb books like DANDR and Protein Power -- is welcome. The low fat subjects ate less than 30% of their calories from fat. But that wasn't all; they also took orlistat, aka xenical or Alli, a drug that sequesters fat and causes it to be passed out of the body in feces, thus reducing the number of calories absorbed.

(And thereby causing all sorts of nasty intestinal side effects, ranging from gas and loose stools through bowel incontinence. Indeed, the directions that come with Alli, the over-the-counter version of this drug, warn you to wear dark trousers and take a change of clothes to work, in case you soil yourself. This is not referred to as a "side effect," but rather as a "treatment effect." Um, yeah. I never wanted to lose weight that badly.)

According to the authors of this study, the difference in weight loss between the two groups was not significant. That said, the low carbers did lose more weight, overall, than the low fat group -- 9.5% of their body weight on average as opposed to 8.5%, which actually sounds kinda significant to me. I mean, if you weigh, to pick a nice, round number, 200 pounds, 9.5% is 19 pounds, while 8.5% is 17 pounds. Unless my junior high school math fails me, that's 12% more weight lost for the low carber.

But still, weight loss varies from subject to subject, so across the group differences are less significant. What I do find very interesting, however, is that the low carbers didn't just lose more weight than low fat dieters, they lost more weight than low fat dieters who were taking a widely-touted weight loss drug that further reduces fat absorption.

Both groups had an improvement in their cholesterol and triglycerides, but insulin and glucose markers improved only in the low carb group, and blood pressure dropped dramatically more in the low carb group: systolic pressure (the top number) dropped 5.9 points in the low carbers, and only 1.5 points in the low fat group. Diastolic pressure (the bottom number) dropped an average of 4.5 points in the low carb group, and a big 0.4 points in the low fat group.

47% of the low carb group had their blood pressure medication discontinued in the course of the study, as compared to 21% of the low fat group. Dr. William Yancy, who ran the study, said the difference in the two groups might have been even greater had subjects remained on their blood pressure medication, but they instead took people off medications as their readings normalized.

This is all welcome news, but it comes as no surprise. Why?

You know how people always say, "Oh, you just lose water on that diet."(1) It's not true, as anyone who has given low carbing a serious shot knows. But there is a grain of truth in this: The fast weight loss during the first week of a low carb diet does chiefly come from water loss.

High insulin levels make your kidneys retain sodium, you see. And as anyone who is concerned with blood pressure knows, sodium makes you hold water. Insulin also causes your kidneys to throw off potassium, the mineral that helps your body eliminate water. With too much sodium and not enough potassium, your body retains water, increasing the volume of your blood, causing high blood pressure.

The answer is not to restrict salt and gobble bananas, an approach unlikely to succeed. Rather, by cutting the carbs out of your diet,thereby lowering your insulin levels, you signal your kidneys to throw off the excess sodium and stop wasting potassium. Once your sodium/potassium balance is corrected, your body can eliminate all that excess water, and blood pressure drops. Indeed, blood pressure can drop so quickly on a low carb diet that it's important for hypertension patients to keep a close eye on their pressure and stay in touch with their doctors; they may quickly need their medication adjusted or discontinued.

All of which explains why a low carb diet has once again eaten the low fat diet's lunch (after discarding the bread and chips, of course)for improving health, even when that low fat diet is augmented by a drug that removes even more fat from the digestive tract. Furthermore, a low carb diet caused modestly greater weight loss than the drug-assisted low fat diet, which certainly suggests it would kick the stuffing out of a low fat diet that didn't have a little pharmaceutical help.

Please, the next time someone tells you that your low carb diet may cause weight loss, but it's soooo unhealthy, laugh at them. Just flat-out laugh at them.

Especially if it's your doctor.

(1) Years ago a Lowcarbezine! reader wrote me to say she'd lost well over 100 pounds on Atkins. Her doctor was surprised, but very pleased, by her weight loss -- until informed how the weight had been lost. When told his patient had been on Atkins, the doctor harrumphed, "You only lose water on that diet."