I have some data on this question which are old but should still be valid. The information comes from a health-screening study in which people were given appointments throughout the day (about 9 am to about 4pm) and took a glucose load soon after arrival. Plasma glucose was measured on fasting blood and again 2 hours after the glucose load.

For fasting plasma glucose, time of day made no difference but for the 2-hour glucose the average value was higher early (9-10 am) or late (after 3 pm) than it was between 10 am and 2 pm. The difference was about 1.2 mmol/l (just over 20 mg/100 ml) between mid-morning and late afternoon.

So the answer as far as fasting glucose is concerned seems to be No, even though there is diurnal variation in glucose tolerance (which has been known for a long time).

John Whitfield
Brisbane, Australia

YES

Yes! It always takes time to keep our body in discrete metabolic balance.

I think circadian rhythm impacts FBG testing since cortisol hormone which is an important hormone that has an influence on glucose metabolism shows a a diurnal variation reaching in the evening half the value of the morning.

Cortisol's actions are to increase the expression of gluconeogenic enzymes and produce peripheral insulin resistance to raise blood glucose concentrations.

We posted this question because sometimes clinicians request that the lab measure a fasting plasma glucose in the afternoon or perform an OGTT in the afternoon. Certainly we can do this but such testing is less sensitive than AM testing. If you're old enough (like me), you might remember when researchers would do a cortisol-primed OGTT to try to elicit hyperglycemia.