why A1c doesn't correlate to bg readings

I'm reading Taubes and there's a few good bits of info in here.

Glycation of proteins happens in the presence of any monosaccharide, and it turns out that fructose is more chemically active than glucose in glycating proteins. So A1c, which measures glycation of a specific protein, will be correlated to the sum of both blood glucose and blood fructose levels, and is actually more highly correlated to blood fructose.

We only measure blood glucose though, not fructose. I suspect this is why people find the equations converting average bg readings to A1c don't work very well.

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http://www.ornery-geeks.org/consulting/
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