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).