Wednesday, November 5, 2008

How to Give a Rat Metabolic Syndrome

I was doing my usual journal rounds today when I came across an article in the American Journal of Hypertension that caught my eye. It's called "Metabolic Syndrome: Comparison of the Two Commonly Used Animal Models." Metabolic syndrome is a cluster of symptoms including large waist circumference, elevated triglycerides, elevated blood pressure, and insulin resistance. It's the quintissential modern metabolic disorder, and it affects 24% of Americans (NHANES III). So what are the two most commonly used animal models of metabolic syndrome?
  • A strain called the spontaneously hypertensive rat (SHR), fed a high-sucrose (table sugar, 50% fructose) diet.
  • Sprague-Dawley (generic lab strain) rats fed a high-fructose diet.
When fed sugar, these rats develop insulin resistance, impaired glucose tolerance, elevated triglycerides and hypertension. Fructose causes leptin resistance in rats. Leptin resistance causes metabolic syndrome in rats. These studies trace a line directly from sugar to the metabolic syndrome.

On to humans. Total sugar and fructose consumption have been increasing in the U.S. in recent decades, along with metabolic syndrome. I think the average numbers may hide some important information, because there is a fraction of the population that consumes far more than the average amount of sugar through soda. Leptin resistance seems to be central to the metabolic syndrome, and typically precedes the other symptoms. The evidence suggests that the rat research on metabolic syndrome is applicable to humans.

I don't think sugar acts alone in causing the metabolic syndrome in humans. I believe the liver is a central player in the disorder, as many of the markers used to diagnose it are measures of processes that occur in the liver (triglyceride synthesis, glucose and insulin disposal). Insulin resistance in the liver is sufficient to cause many of the hallmarks of the metabolic syndrome in mice. The fructose portion of sugar and high-linoleic (omega-6) vegetable oils act synergistically to cause liver dysfunction in rats and probably humans.

I also believe wheat contributes to the process, perhaps through its ability to cause hyperphagia (overeating) or intestinal damage. So we're back to the three big killers in the modern diet:
  • Refined vegetable oils
  • Sugar
  • Wheat

1 comment:

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