There are a lot of different transporters for glucose that allow tissue to use it. I posted about GLUT4 recently.
Today here is a little bit of information about the glucose transporter GLUT3, and why it is used by the brain.
GLUT3 is most abundant in brain, kidney, and placenta. Let’s discuss the brain a little bit. Why does the brain use GLUT3 to get its glucose supply? What is it about GLUT3 that suits the needs of the brain and its high glucose demand?
(Believe it or not, your brain uses a lot of glucose. When at rest, about 60% of the glucose used by your entire body is used by your brain! And the brain does not need insulin to get its glucose!)
GLUT3 is a high-affinity glucose transporter. That means only a small amount of substrate is needed to saturate it. It has a high affinity for glucose.
In very simple and unscientific terms, this means GLUT3 is able to pull in glucose even when it is not available in high quantities.
(The affinity is usually expressed as the Km (Michaelis constant) of the enzyme. It is an inverse measure of affinity, low Km means high affinity.)
Hang with me, there is something important coming up.
The GLUT3 Km for glucose is 1.6 mM. That is much lower than the normal BG, meaning the GLUT3 enzyme is usually saturated with glucose under all but the most extreme low conditions. The high-affinity for glucose of GLUT3 means the brain is usually supplied with a constant glucose source.
According to Biochemistry, 5th Edition (Section 30.2):
Glycolysis slows down when the glucose level approaches the KM value of hexokinase (~50 μM), the enzyme that traps glucose in the cell (Section 16.1.1). This danger point is reached when the plasma-glucose level drops below about 2.2 mM (39.6 mg/dl) and thus approaches the KM value of GLUT3.
And here is a picture of the relevant page in my copy of the book:
Yes, low BG impacts you in many ways. And there is no reason to hang with a BG under 60 for days and days.
But I think the general recommendations and fear of brain damage are somewhat overstated for particular BG values, unless you drop below 40. Hopefully this can reduce some of the worry associated with it.
Next time, ask your endo about the Km for GLUT3, and what BG levels still allow it to supply glucose to the brain. Blow his brain a little bit. I would love to hear discussion about it.