Why do fasting levels not predict hyperinsulinaemia?
Many people seem to think that they can predict whether someone is hyperinsulinaemic or not from fasting insulin. “My fasting insulin values is less than 5 µU/mL, this means I’m not hyperinsulinaemic, Right?”
Umm… well.. maybe?
After three years of close examination of the Kraft database, I still can’t answer this question for one simple reason: With the exception of Kraft pattern IV, fasting insulin within each Kraft pattern is so widely distributed that all the patterns considerably overlap. Unless your fasting insulin is greater than 30 µU/mL we have no way of telling if you are hyperinsulinaemic or not.
But why do people think we can?
I think there are some misunderstandings about statistics and how to apply them in real life. The challenge lies in understanding the wide-spread of results that occurred in the dataset – the standard deviation.
We tend to focus on the mean (sometimes called the arithmetic mean) of a variable, or the sum of the sampled values divided by the number of items in the sample. But this arithmetic mean is not necessarily a robust statistic as it is highly influenced by outlying variables. The mean is not necessarily the most likely value, or even the most central value of your sample. So you need to understand the standard deviation of your key variable, and this statistic is not reported in Dr Kraft’s book, “Diabetes Epidemic & You”.
If you look at a classic standard deviation curve, you can see that about 68% of the results lie within one standard deviation (SD) from the mean, 95% within 2 SD and 99% within 3 SD.
When you look at the means and standard deviations of fasting insulin taken from ~4400 people with normal glucose tolerance (all results in µU/mL*), you can see that the standard deviation is at least 50% of the mean.
When graphed, you can see that all of the patterns (excluding pattern IV) almost all overlap right up to 30 µU/mL.
My statistical knowledge isn’t good enough to work out the probability of someone being in a particular pattern if they have a fasting insulin < 5 µU/mL. Chances are, it is pretty good, but we also can’t exclude them being a pattern III. From the same token, we also can’t say that you are NOT a pattern I if you have a fasting insulin of 16 µU/mL. 13% of all people with pattern I will have a fasting insulin between 12-17 µU/mL.
So how should you tell if your are hyperinsulinaemic? Try and get an insulin sample taken 2 hours after your normal breakfast. That will tell you far more than fasting levels.
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