Friday, May 30, 2008

Insulin--Your Friend and Your Enemy, Part II

In Part I we saw that insulin is a food storage hormone, somewhat like the grocery stocker who puts shipments of food onto the correct shelves in the grocery store. We can't survive without insulin, but sometimes insulin hurts us as well as helps us.

As people get older, they can develop a condition called insulin resistance. In early insulin resistance, insulin is still secreted by the pancreas, but gradually the insulin does a poorer and poorer job as a food stocker. This happens at different rates in different tissues.

Remember gluconeogenesis--the process our liver uses to make extra glucose out of the food we eat? Normally when insulin is released, the liver responds by shutting off its production of extra glucose. That makes sense. If we're getting glucose in our food, there is no reason for the liver to make more of it. But when insulin stops doing its job properly, the liver doesn't get a clear message to stop making glucose, and blood glucose levels rise higher than they should. Think of bags of sugar starting to litter the aisles in the grocery store.

Elsewhere in the body, our muscles normally respond to insulin by taking up glucose from the blood and storing it as muscle glycogen. But if insulin starts doing a poor job of stocking muscle cells with glucose, the glucose stays in the blood and muscle cell "shelves" start to become empty. In this case, not only are the bags of sugar littering the aisles, but they are missing from the shelves where they are supposed to be.

So insulin resistance produces a condition in which there is excess glucose in the blood from gluconeogenesis and also from the fact that muscle cells are taking up less glucose to store as glycogen. The pancreas responds by producing more insulin (more grocery stockers), and eventually the liver and the muscles get the message and respond appropriately.

However, fat is the last tissue to become insulin resistant. The original insulin/grocery stocker continues to allow triglycerides to be stocked into fat cells. When extra glucose is present in the blood, the insulin/grocery stocker allows some of the extra glucose to be stored as fat in the fat cells, too. So far, so good. But when the pancreas decides that our bodies need more insulin to function properly, the insulin/grocery stockers won't let customers take very much back out of the fat cells. Fat inventory increases, but is unavailable to be used by customers that might need it. At this point, insulin is starting to become your enemy.

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