I have written an article in Medium that I have completely forgotten about mentioning here. Bad girl… well, it is better later than never. Shortly after I published that article and it went viral, it was republished by Marika Sboros on her website with a beautiful header image that I love to look at. Whichever you prefer to read, just read it.
What’s All The Fuss About?
We often hear that glucose is the human primary fuel and ketones are the secondary (backup) fuel for those times when glucose is not available. But is that really true?
Thinking that glucose is primary is nothing short of suggesting a dentist is primary because that is the mandatory “first thing” (primary) you need to do if you have a tooth ache. It is not primary; it is very urgent. If you had a choice, and had no tooth ache, visiting the dentist would be the last thing on your list.
And so it is with glucose versus ketones and also versus alcohol, for that mater, as one of the readers noted in a comment. Indeed, alcohol is also a fuel, and it yields 7 Calories per gram! Recall that glucose provides 4 kcal and fat 9 kcal, so what is the order of fuel-burning preference? 4, 9, 7 or 4, 7, 9, or 7, 9, 4, or 9, 7, 4, or 9, 4, 7, or 7, 4, 9? What exactly is the process based on which the body decides which fuel it uses first? Is it by how much energy it can get from it or by some other magic?
The Fuel Order Question
This is not at all intuitive. How does our body choose which fuel it should use first? From the perspective of energy output, our body should definitely not choose glucose first since per gram the output is only 4 kcal, the smallest usable energy possible to be gained from any of the fuels. From the perspective of energy output, the first choice would have to be fat.
However, just like visiting the dentist provides a huge payoff when there is urgency, but not at any other time, perhaps our body is smart and chooses its fuel based on the urgency by which it must be removed from the body with minimal negative consequences. And this, indeed, is what is happening.
Urgency & Priority vs Primary
And here is the important part. The human body is a smart one. It had millions of years in which to figure out the order via evolution. It picks based on an order of urgency, and priority will be given to the most dangerous of fuels, which have negative consequences if they remain in the body for long. The body will use the dangerous fuel first over fuel that can be stored without negative consequences.
So what will the order of preference be in burning the 3 fuels?
Alcohol, glucose, and then fat (aka ketones).
And this is in order of urgency and not based on which is primary. In the article I only discussed 2 fuels and I added now alcohol as the 3rd. Here is a quote from the original article that explains the two fuels:
Few professionals, even within the field of nutrition, realize that ketones are not our backup fuel. Ketones are not any more backup than glucose is the primary fuel. The human body has no primary or backup fuel: it has two fuels. I presume that the urgency for glucose use — meaning the urgent removal of glucose from the blood — is misleading and makes people think that if the body switches to using glucose the moment it is provided, it must be its primary fuel. But this is a mistaken argument, which assumes that the first task that has to be done is the preferred task. It is true that when we add glucose to our blood by, for example, eating carbohydrates, the body must immediately switch to remove that glucose from the blood.
The maximum comfort level for glucose in the blood is 99 mg/dL (5.5 mmol/L) and if we eat carbohydrates such that this is exceeded, it is literally an emergency and the body must remove it from there. Too much glucose in the blood for extended time is toxic. However, having to remove glucose immediately does not make it into the primary fuel, it just makes it into an urgent task.
Had I thought of adding alcohol into the original article, alcohol would be the first fuel used because it is the most toxic, glucose as second because of its toxicity, and fat as third because it is not toxic at all.
Children Are in Ketosis
In the original article, you will find discussion on Cahill’s landmark article (1), which you may or may not be familiar with. His work is focused mostly on ketosis during starvation. By starvation, understand “fasting” in modern terms. Babies, for example are in ketosis all through their early teenage years–as per Cahill–even 20 minutes between feeding initiates starvation responses in babies. But there are articles showing that the fetus is also in ketosis in the womb, where there is no starvation, and babies remain in ketosis even after eating–clearly no starvation there. So there is more to ketosis than meets the careless eye.
In my Facebook migraine groups (here and here) the migraineurs measure their blood glucose and blood ketones over a 5-hour postprandial period, and post their data for my analysis. I usually graph this and discuss the course of action to take in nutrition. Many moms and dads are in the groups for their children, and the children also submit their 5-hour postprandial tests (also fasting and pre-meal).
It is fascinating to see that every single child , so far through age 16, is in ketosis even after a breakfast containing fruits and milk.
I suppose few people have the same opportunity as I have in being able to measure the blood ketones of various ages of children for five hours postprandial plus fasting and pre-meal measures. And so ignorance is bliss…
Thus, while today in most countries around the world any type of food is just a short walk/drive away 24/7 and we need not experience hunger and starvation, our children are still in ketosis 24/7. Shouldn’t that tell us something about the importance of ketosis?
With all due respect, I disagree with glucose as the primary human fuel. Glucose is not primary or secondary or any-ary. It is a fuel that is urgent to be used up as fast as possible and so it gets used first (after alcohol) but not because it is primary but because we need to get rid of it faster than fat.
1 Cahill, G. F. Starvation in man. N Engl J Med 282, doi:10.1056/nejm197003052821026 (1970).
Comments are welcome, as always, and are moderated for appropriateness