I’m currently on a multi-day fast. Less than 24 hours into it, I measured my ketone levels and I was at .5 mmol, which did not surprise me. This morning, I measured my blood after being up for about an hour and my levels were shockingly low – .3 mmol. This evening, after fasting just under 48 hours, they were up to 1.0 mmol.

So I set out to figure out why my ketone levels were low in the morning, and it turns out that I am not alone. I found a lot of discussion groups that discussed this topic, but the best explanation I found was from Thomas DeLauer, in this YouTube video. He explained that when your body wakes up, you have a rise in cortisol. This triggers the liver to go through glycolysis. Glycolysis is the process of your body converting any stored carbohydrates into glucose. This causes the pancreas to secrete insulin. When there’s insulin in the blood, ketones are lower. As the day goes on, and activity increases, so do the number of ketones in your blood.

So I got that question answered. But I then looked back to prior multi-day fasts from last year and realized that my ketone levels are rarely very high. I think the highest I registered was 2.4 mmol after 3 days of fasting. That led me to question why this was.  Mark Sisson came to the rescue in this article that offers several explanations on why my ketone levels stay relatively low.

The theory I choose to believe is this: Being keto-adapted for over 3 years now, I have built so much fat-burning metabolic energy in my muscles that I can burn free fatty acids directly and don’t require additional ketones. Who knows if that’s right, but it’s certainly the theory I like best! At any rate, I’m going to take the advice of others and worry more about how my body feels and my energy level rather than ketone levels.

There is a lot of other great information for athletes in the Mark Sisson article, so definitely take a moment to read it.