The primary culprit I suspected was how close in time my last meal was with respect to the time I was checking my PM numbers.
Including transit times, we know it takes around 2 hours to digest simple carbohydrates from the time they hit your mouth to the time the last bit has entered your blood stream (that’s why they ask diabetics to check 2 hour post prandial blood sugars).
We also know that part of the reasons for the increased satiety we feel on a ketogenic diet are the fact that when we eat fats and proteins, these actually slow down digestion and require more time to process.
How much more time?
This is the question that I haven’t found an answer to. Here is a separate forum thread I found where people discuss this. This corresponds to what I’ve seen other people report that it takes around 4-6 hours to digest fat and proteins. I’d be interested to see some actual sources for this though.
But assuming this is true, assuming you eat a ketogenic meal high in coconut oil and other fats and proteins, your body will be processing these macronutrients from the moment it hits your mouth to at least 4-6 hours out.
I suspect that this is the cause for the discrepancy in ketone readings between PM and AM readings. The PM readings can be affected by the ketones generated by something that was eaten 6 hours ago!
The entire purpose of measuring blood ketones is to gain insight as to what metabolic fuel your body is preferentially using, fats or carbs? A higher ketone number tells you that you’re metabolizing fats as the primary source of fuel. But if this number can be confounded by something that was ingested up to 6 hours ago, it can’t be relied upon to provide an accurate representation as to the state of your metabolism.
This is something that definitely bears more looking into.