Question: On a ketogenic diet, how rapidly does the
brain flip between glucose and ketones for fuel? Can it use both fuel
sources simultaneously?
So I think that answers at least part ofAnswer: The above question sort of encompasses a few
your question: when first starting a low-carbohydrate diet, it takes the
brain about 3 weeks to adapt to using ketones for fuel; even then it
only gets about 75% of its total fuel from them. At the same time, after more extended periods on the diet (perhaps
6-8 weeks), switching back and forth from a carb-based to a ketone-based
brain metabolism seems to cause most people no problems. They can sort
of drop in and out of ketosis (even throughout the day under certain
conditions) and not really notice anything one way or the other.
different potential things and I’m not 100% sure which you’re asking so
I’ll just cover them all. First realize that one fuel that the brain
cannot use is fatty acids, at least not directly. This has led to the
oft-stated belief that the brain can only use glucose. But this is
incorrect as the brain has an alternative fat derived fuel which are
ketones (or ketone bodies, the two major of which are
beta-hydroxybutyrate and acetyl-acetate).
Ketones are produced primarily in the liver (from the breakdown of
fatty acids) and exist predominantly as an alternative fuel source for
the brain (they can also be used by skeletal muscle) during periods of
low-carbohydrate availability. This probably was originally important
during periods of complete starvation; now very low-carbohydrate diets
(defined here as any diet containing less than 100 grams per day of
carbohydrates) effectively ‘exploit’ this mechanism.
Now, on a carbohydrate based diet, the brain runs essentially on 100%
glucose since ketones are generally not produced in significant amounts
under those conditions (there are a couple of odd exceptions, one is
following very long duration endurance exercise where a post-exercise
ketosis can occur due to changes in fuel metabolism). So what happens
when you remove most or all carbohydrates from the diet? Does the brain
magically switch to using ketones? For the most part, no. Studies
done way back when show that there is an adaptation phase that may last
about 3 weeks while the brain ramps up its ability to use ketones for
fuel.
Even there, after that roughly 3 week period, the brain still only
derives about 75% of its total fuel requirements (about 400 calories per
day or thereabouts) from ketones; the other 25% come from glucose
(which the body can produce through a variety of pathways that I won’t
detail here; all of this is explained in excruciating detail in my first
book The Ketogenic Diet).
Mind you, this is only relevant on a very low-carbohydrate diet. Even
if the brain could still use ketones on a carb-based diet they wouldn’t
be produced in large enough amounts for it to be relevant.
So I think that answers at least part of
your question: when first starting a low-carbohydrate diet, it takes the
brain about 3 weeks to adapt to using ketones for fuel; even then it
only gets about 75% of its total fuel from them. This scans pretty well
with what many experience on the diet, they don’t feel fantastic for
the first 2-3 weeks of the diet (while they are adapting). Some of that,
mind you, is related more to mineral intake than anything else (early
studies found that sufficient intake of sodium, potassium and magnesium
eliminated all of the fatigue and lethargy that occurred on very low
carbohydrate diets).
But there is a related question that often comes up which has to do
with switching back and forth between fuels (this is especially relevant
for some cyclical ketogenic diets such as what’s described in The Ketogenic Diet or in my Ultimate Diet 2.0).
Here I am unaware of any research on the topic and most of what I have
to say is just based on empirical evidence, what people have reported
over the 15+ years they’ve been giving me feedback.
Certainly early in the diet there is often a period where the
alternation of high and low carbs often causes some people distress,
they get the same headaches and issues going from high-carbs back to
low-carbs for a couple of weeks. Probably just a function of
‘interrupting’ the adaptation to ketone metabolism in the brain and
there might be some rationale to doing 2-3 straight weeks of a ketogenic
diet prior to inserting refeeds or carb-loads.
At the same time, after more extended periods on the diet (perhaps
6-8 weeks), switching back and forth from a carb-based to a ketone-based
brain metabolism seems to cause most people no problems. They can sort
of drop in and out of ketosis (even throughout the day under certain
conditions) and not really notice anything one way or the other.
Interestingly, even after extended periods off of a low-carbohydrate
diet, most people don’t report the same early adaptation phase that they
went through the first time on the diet; they go back onto a ketogenic
diet and don’t notice anything.
This suggests to me that there is some type of long-term and/or
almost permanent change in the brain in terms of its ability to use
ketones for fuel with long-term exposure to them. Again, I have exactly
zero research to back this up; it’s just an observation. But even
there you’d still expect to see the same basic 75/25 split, just with an
easier switching back to ketone metabolism after that initial
adaptation phase.
Hope that answers your question.
