Question: I’ve been doing the CKD effectively. However, I have a major exam on Friday. Is there any effect on limited carbs on cognitive processes? Does limiting carbs ( 20g / day) have a negative effect or could it retard my performance on a major exam, i.e. MCATs, Series7, etc? Is there any study or suggestion you could give based on your research?
Answer: First a quick definition for anyone who isn’t familiar with the abbreviation: as discussed in the Comparing the Diets Series , a CKD refers to a cyclical ketogenic diet. This is simply a diet that alternates between periods of very low-carbohydrate eating (typically 4-6 days) and very high-carbohydrate eating (1-3 days). Dan Duchaine’s Bodyopus, Mauro DiPasquale’s Anabolic Diet and my own Ultimate Diet 2.0 are all examples of CKD’s. My first book The Ketogenic Diet discusses CKD’s generally in mind-numbing detail.
Now back to the question: does ketosis negatively impact on cognitive function? And the answer is one huge it depends. Certainly early studies found that, in the short-term (first 1-3 weeks), low-carbohydrate diets tend to cause some problems. For this reason short-term studies (usually a week long) tend to report decrements in a lot of things including cognitive performance.
Empirically, as well, many report fatigue, lethargy and a sort of mental ‘fog’ until they adapt to the diet (the brain adjusts to using ketones for fuel over those first 3 weeks). I’d note that supplementing with sodium, potassium and magnesium seems to go a long way towards limiting or eliminating that feeling of fatigue.
So, for most I certainly wouldn’t recommend starting a very low-carbohydrate/ketogenic diet right before some major test or cognitive challenge. Odds are it’s going to cause problems.
But what about someone who has adapted to being in ketosis. There there tends to be huge variance. Some people are sort of neutral to it but I know of many who report far better brain functioning when they are in ketosis. I couldn’t tell you the mechanism, this is just one of those self-reported things. But it tends to be highly variable (and I can’t think of any studies that have examined cognitive performance after long-term adaptation to low-carbohydrate diets).
CKD’s add another complication, outside of some exercise research on Cyclical Ketogenic Diets and Endurance Performance that looked only at performance, I’m unaware of any work on CKD’s and cognitive performance. I bring this up as some people do report changes switching back and forth between very low and very high carbohydrate intakes.
Quite in fact, many who find that they feel ‘great’ in ketosis feel a bit dopey or sleepy when they switch back to high carb intakes. This is probably related to either blood glucose swings or a big increase in brain serotonin (which tends to cause lethargy and fatigue) but it does occur.
Similarly, some seem to go through at least a brief re-adaptation (in terms of fatigue, etc.) going back from high carbs to low-carbs. Again, this is pretty variable, many people can switch back and forth from one extreme to the other and don’t seem to notice anything. I have no idea why, just reports I’ve seen.
So back to the question, should you switch out of ketosis for your test? It’s a hard question to answer and you’d have to think back to your previous switches from low- to high-carbs during the CKD. If you find that you’re fully adapted to ketosis and function fine mentally, and that you get dopey going back to high-carbs, I’d probably suggest you stay on low-carbs through the test.
If you’re one of those people who don’t seem to have ever fully adapted to being in ketosis (they do exist), you might want to move back to at least moderate carbs a day or two before your test. Unfortunately, there’s just too much variability for me to give you any advice beyond that.
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