Wednesday, October 03, 2007

Rebound: A Positive Feedback Loop

A positive feedback loop is a cause and effect response that has no self-correcting mechanism. An example of a positive feedback loop is your bathtub. If you turn the water on and let it run to fill, it will continue running until you manually turn it off, even if that means it floods your bathroom.

Counter intuitively, negative feedback loops are self-correcting. An example of a negative feedback loop is your home thermostat. You set it at the threshold you want, and it continually samples the room temperature and makes corrections to maintain the desired temperature setting (threshold).

In feline diabetes, the way a cat's body uses insulin is both a positive feedback loop and a negative feedback loop. When the body gets too much insulin, the liver releases glycogen (stored glucose) to protect against hypoglycemia. That's the negative feedback loop.

When the body receives too much insulin over time, the liver adjusts its threshold upward. For example, instead of recognizing 50 mg/dl (for felines) as a glucose level that is too low and requires intervention through released glycogen, it starts to think anything lower than 200 mg/dl needs adjustment. So the cat's body stops using the insulin effectively and stays in a continuous hyperglycemic state, also called rebound. The negative feedback loop, which is the natural mechanism, is overwhelmed by too much insulin, which in turns creates the unnatural positive feedback loop.

Since there are no tests for rebound, humans too often continue to increase the insulin dosage in order to alleviate the hyperglycemia, causing the liver to set the 'hypo' threshold even higher. So no matter how much additional insulin the cat receives, the response is in the opposite direction than what is expected.

Unfortunately, a cat in rebound is always subject to hypoglycemia since the liver does not have a endless supply of stored glucose/glycogen. When the glycogen runs out, hypo results.

The only way to break a positive feedback loop is to stop feeding it. In feline diabetes, that means reducing the insulin dosage, hopefully before the cat hypos.

Other examples of positive feedback loops:
  • credit card debt (the more you borrow, the deeper in debt you go and the more you need to borrow to cover the debt)
  • hyperactive behavior in a child; the more they over-react, the more wound up they get
  • alcoholism and drug addiction....any addiction

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