Sunday, February 6, 2011

Unintended Consequences: Salt Intake, Obesity and Insulin Resistance

When William Banting wrote his Letter on Corpulence in 1863, the medical community attacked him like piranhas in a lake filled with hemophiliac swimmers. Gary Taubes describes this best in his book Good Calories, Bad Calories:
"The medical community of Banting's day didn't quite know what to make of him or his diet. Correspondents to the British Medical Journal seemed occasionally open-minded, albeit suitably skeptical; a formal paper was presented on the efficacy and safety of Banting's diet at the 1864 meeting of the British Medical Association. Others did what members of established societies often do when confronted with a radical new concept: they attacked both the message and the messenger. The editors of The Lancet, which is to the BMJ what Newsweek is to Time, were particularly ruthless. First, they insisted that Banting's diet was old news, which it was, although Banting never claimed otherwise. The medical literature, wrote The Lancet, "is tolerably complete, and supplies abundant evidence that all which Mr. Banting advises has been written over and over again." Banting responded that this might well have been so, but it was news to him and other corpulent individuals." [emphasis mine]

It was news to me as well. No one told me that I should eat that way. But what else was news to me is this, apparently restricting sodium intake can cause and/or aggravate insulin resistance. In a country with what some call a "diabetes epidemic" why are nutritionists still advocating restricting sodium intake? Especially when lower salt intake does little for lowering blood pressure, which is why it's pushed on the general public in the first place. One review found that a reduction in dietary sodium would amount to systolic blood pressure being lowered by a whopping mean of 1.1 mmHg, and diastolic by an even more underwhelming mean of 0.6 mmHg.

I suppose part of it may be because it seems a lot of the studies done were published in a journal called American Journal of Hypertension over the last 20(!) years, and I guess the "experts" and the "nutritionists" don't read it. I mean, don't read period. Because if they did, they'd have read Taubes and the plethora of other studies that would tell them that what they're telling people is flat out wrong and is killing them.

Not only can salt restriction apparently be bad for insulin resistance, but one study showed giving people more salt helps alleviate it and makes them more sensitive to the insulin.
"For the group as a whole, urinary sodium excretion increased on sodium chloride to 267 ± 118 mEq/day versus control (placebo) phase of 135 ± 53 mEq/day, P < .001. Total glycemic response in the oral GTT (area under the glucose curve) was 8.0% lower during sodium supplementation, P < .001. Secondary analysis revealed that the effect of sodium was noteworthy in 1) type 2 diabetic subjects (n = 8), 2) sodium -sensitive subjects (n = 10), and 3) nondiabetic subjects receiving antihypertensive drug treatment (n = 6). The total insulinemic response to oral GTT was also lowered by sodium loading among diabetic subjects. Thus, an abundant sodium intake may improve glucose tolerance and insulin resistance, especially in diabetic, salt-sensitive, and or medicated essential hypertensive subjects." [emphasis mine]

(The effect of sodium supplementation on glucose tolerance and insulin concentrations in patients with hypertension and diabetes mellitus, from the American Journal of Hypertension, Volume 14, Issue 7, July 2001, Pages 653-659)

One of the most recent and largest studies from the journal Metabolism, took data from 152 healthy people who were prescribed a high or low salt diet. It too found that a low salt diet was associated with increased insulin resistance.

People are expensive to do studies on. You have to pay them to participate. Lab rats on the other hand are cheap and you don't have to pay them anything and you get to dissect them when the study is over so you can obtain even more information on what was going on physiologically. One rat study found:
"In the present study, higher body weight, blood glucose, plasma insulin, triacylglycerols (triglycerides) and cholesterol, hematocrit and heart rate were observed in animals on low salt intake compared to the normal-salt group. Blood pressure and glucose uptake were lower on low-than on normal-salt diet." [emphasis mine]

(Insulin resistance due to chronic salt restriction is corrected by α and β blockade and by l-arginine, from Physiology & Behavior, Volume 88, Issues 4-5, 30 July 2006, Pages 364-370)

If you've been paying attention, insulin resistance (and by extension diabetes) is associated with obesity. Apparently one study showed that restricting salt intake in female Wistar rats predisposed their female offspring to obesity.

Most of the studies say that more and larger studies are needed. As mentioned above, studies are expensive. There were some studies in humans that found the opposite or no correlation between salt intake and insulin resistance, but of these I saw, they used a very small number of people, in one only eight and were very short term. That's not enough subjects or time to get meaningful data. We need large studies with lots of people, that way individual variation is controlled for. All of the rat studies I found showed a correlation with low salt intake and insulin resistance, and since rats are cheap, there were more rats involved than people. It may be that some people are not as effected by low sodium intake as others.

So here we have unintended consequences, in that nutritionists are advocating you messing with your diet for what might amount to a slightly lower blood pressure, at the expense of something else completely unforeseen. How do you like being an unwitting participant in one of the biggest health studies ever made? Because that's what you are if you live in any country that follows the dietary guidelines set forth by the USDA. You may think you have no problem eating as you are, but the food manufacturers have made it a point to remove salt and fat from the food they sell you. So unless you are making a concerted effort to add it back in, you may well be deficient and not even know it.

Common sense is indeed not very common. If excess salt intake had anything to do with ill health, the human race in western Europe and elsewhere would not have survived into the modern era. Last I checked, prior to the 20th century, people did not have refrigeration and used, get this, salt and LOTS of it to cure meat. I doubt there's any way to find out exactly how much they consumed, but it was way more than we do now. And since we don't have reports that medieval people were dropping dead from stroke or heart disease (plague is another matter), I dare say the salt wasn't a primary cause of mortality. Of course, absense of evidence is not evidence of absence, and maybe they just didn't live long enough for the salt to kill them.

No comments:

Post a Comment