We now know that the oft-repeated “your brain only runs on glucose!” is wrong. I’ve mentioned it before, and anyone who’s taken the time to get fat-adapted on a low-carb Primal eating plan intuitively knows that your brain doesn’t need piles of glucose to work, because, well, they’re using their brain to read this sentence. Obviously, you eventually adapt and find you have sufficient (if not much improved) cognition without all those carbs. That said, some glucose is required, and that’s where people get tripped up. “Glucose is required” sounds an awful lot like “your brain only uses glucose” which usually leads to “you need lots of carbs to provide that glucose.” And that’s the question today’s edition of “Dear Mark” finds itself attempting to answer: how much glucose is required?
Let’s get to it.
I have a little problem. Even though I’m able to function at work, maintain conversations, and go about my daily life without having segments of my brain suddenly stop working while eating Primal, my friends are worried about my brain. All they know is that the brain needs glucose. What can I tell them? How much glucose does my brain actually require to keep working?
I wouldn’t be too hard on your friends. They mean well and it’s a common misconception. Instead of chiding them, rubbing their faces in the knowledge that you can function quite adequately on a high-fat diet, educate them.
How much glucose the brain requires depends on the context. There’s not one single answer.
If you’re involved in strenuous exercise, your brain will be running primarily on lactate. Yep, lactate – that unwanted metabolic byproduct of muscle metabolism. During exercise, when the muscles are using up most of the available glucose to lift things and move a bunch of intelligent primate flesh through three dimensional space, and where inadequate oxygen (hence breathing hard) leads to incomplete glucose and pyruvate breakdown and increased lactate levels, the brain will draw upon lactate as a direct energy source. Not only that, but lactate appeared to make the brain run more efficiently, more snappily, and when both are available, the brain prefers lactate over glucose. Other research has found that the brain also prefers lactate in the hours and days immediately following a traumatic brain injury. I’m not sure how much glucose the brain requires when it’s accessing lactate, but it’s definitely fewer than 120 grams.
Of course, even when you need some glucose, that glucose needn’t necessarily come from dietary carbohydrate. It can famously come from gluconeogenesis, the process by which the liver converts amino acids into glucose. It can also come from glycerol, a byproduct of fat metabolism. In deep fasting situations, glycerol can contribute up to 21.6% of glucose production, with the rest presumably coming from gluconeogenesis. The glycerol can come from both dietary fat and adipose tissue (the authors of that glycerol fasting study even suggest that fasting burns body fat in order to provide glycerol for glucose production), while the amino acids can come from dietary protein (if you’re eating) or muscle (if you’re starving).
Overall, recent research into the metabolic demands of brain slices (“living” pieces of brains isolated and used for research) shows that incorporating other energy substrates – ketones, lactate, or even pyruvate – into the glucose solution improves oxidative metabolism and neuronal efficiency. Before you say “but this was in vitro, my brain’s not sliced up and submerged in a weird syrupy solution,” know that the whole point of the study was to better replicate the conditions of the kind of real, actual, living, thinking brains we find in human heads. The authors note that the glucose-only solution normally used to fuel brain slices in other studies is limited, because “in the intact brain, complex machinery exists that coordinates energy substrates delivery and adjusts energy substrate pool composition to the needs of neuronal energy metabolism.” In other words, glucose solution is an easy, dependable way to fuel brain slices, but it’s an incomplete representation of how brains work in heads. The authors conclude that “in slices as well as in vivo, the ability of glucose to maintain energy metabolism is limited and neuronal energy supply should be supported by other oxidative substrates.”
So, a healthy, efficient brain is one that draws on several different fuels. A healthy, efficient brain is one that uses ketones (and perhaps lactate and other fuels) to spare some glucose. A complete reliance on glucose indicates an underachieving brain, a brain that could do so much better, a brain that could really use a coconut milk curry and some intense exercise every now and again. As far as we can tell, then, the absolute physiological minimum is 30 grams of glucose. I wish I could provide hard numbers for some of the other contexts beyond near carnivory (like basic 150 grams carbs Primal eating with coconut or maybe figuring out how to rely on lactate fueling), but the numbers don’t really matter in practice. What matters is that our brains don’t need the full 120 grams of glucose, especially if we’re following a Primal Blueprint eating plan.
I hope that helps.
Questions? Comments? Concerns? Leave them here. Thanks for reading!
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.