Back in mid summer 2007 there was this thread on the Bernstein forum. Mark, posting as iwilsmar, asked about his gradual yet progressively rising fasting blood glucose (FBG) level over a 10 year period of paleolithic LC eating. Always eating less than 30g carbohydrate per day. Initially on LC his blood glucose was 83mg/dl but it has crept up, year by year, until now his FBG is up to 115mg/dl. Post prandial values are normal.
He wanted to know if he was developing diabetes.
I've been thinking about this for some time as my own FBG is usually five point something mmol/l whole blood. Converting my whole blood values to Mark's USA plasma values, this works out at about 100-120mg/dl. Normal to prediabetic in modern parlance. However my HbA1c is only 4.4%, well toward the lower end of normality and healthy. That's always assuming that I don't have some horrible problem resulting in very rapid red blood cell turnover. I don't think so...
I spend rather a lot of my life in mild ketosis, despite the 50g of carbs I eat per day. So I can run a moderate ketonuric urine sample with a random post-chocolate blood glucose value of 6.5mmol/l.
What is happening? Well, the first thing is that LC eating rapidly induces insulin resistance. This is a completely and utterly normal physiological response to carbohydrate restriction. Carbohydrate restriction drops insulin levels. Low insulin levels activate hormone sensitive lipase. Fatty tissue breaks down and releases non esterified fatty acids. These are mostly taken up by muscle cells as fuel and automatically induce insulin resistance in those muscles. There are a couple of nice summaries by Brand Miller (from back in the days when she used her brain for thinking) here and here and Wolever has some grasp of the problem too.
This is patently logical as muscle runs well on lipids and so glucose can be left for tissues such as brain, which really need it. Neuronal tissue varies in its use of insulin to uptake glucose but doesn't accumulate lipid in the way muscle does, so physiological insulin resistance is not an issue for brain cells.
However, while muscles are in "refusal mode" for glucose the least input, from food or gluconeogenesis, will rapidly spike blood glucose out of all proportion. This is fine if you stick to LC in your eating. It also means that if you take an oral glucose tolerance test you will fail and be labelled diabetic. In fact, even a single high fat meal can do this, extending insulin resistance in to the next day. Here's a reference for this.
The general opinion in LC circles is that you need 150g of carbohydrate per day for three days before an oral glucose tolerance test.
I did this carb loading thing, then performed my own OGTT. It came out very normal except for mild reactive hypoglycaemia.
So, I often walk around with a fasting blood glucose of 5.9mmol/l and in mild ketosis, yet have normal pancreatic and muscle function, provided I carb load before the test. BTW my FBG dropped to 4.3mmol/l after three days of carb loading.
That then raises the question as to whether Mark "iwilsmar" and myself are typical of LC eating people, or an oddity or two.
This brought to mind the self selected macronutrient study performed on mice by Ortman, Prinzler and Klause. They allowed mice to select their own diet and, lo and behold, the mice chose (by calories, not weight!) 82% fat and 5.6% carbohydrate. Sensible mice.
NB These German mice should each be given Professorships of Nutrition at medical schools in the most obese nations of the world. Quite what we should do with the current professors I'm not sure, but I bet the mice could think of something.
Anyway, these mice are cool. The only thing that bugged me when I first read the paper was that they had a higher fasting blood glucose than those poor mice fed the normal junk which passes for laboratory mouse "chow".
This now fits in to an overall pattern. Elevated non esterified fatty acids induce physiological insulin resistance and a higher than expected FBG level. A simple switch to higher carbohydrate eating (in myself) allows the normal underlying pancreatic and muscle function to show. It also fits in with the FBG of 3.5mmol/l found in the carbohydrate fuelled natives in the Kitava studies.
So do I worry about a FBG of over 5.5mmol/l?
Not while my HbA1c is 4.4%.