I had a theory for most of my adult life: hard days at work burn a lot of calories. The kind of days where you're deep in complex problems for eight hours straight, making decisions, debugging, writing. By evening, I'd feel utterly drained. Surely all that thinking was metabolically expensive. Surely it justified skipping the interval session I'd planned.
Then I looked at the numbers. And then I looked at what that cognitive exhaustion was actually doing to my workouts. The first part was humbling. The second part was alarming.
Your brain is expensive, but thinking is cheap
Your brain consumes roughly 20 to 25% of your body's total energy at rest. For an organ that accounts for about 2% of your body weight, that's extraordinarily costly. It runs almost exclusively on glucose, consuming about 120 grams per day, and it can't store meaningful reserves. It needs a constant supply delivered through the bloodstream.
Here's the part that surprised me: almost all of that energy goes to baseline maintenance. Keeping 86 billion neurons electrically charged. Maintaining synaptic connections. Running the background processes that keep you conscious and breathing and aware of your surroundings. The "idle cost" of having a brain is enormous.
The incremental cost of actually thinking hard? Almost nothing. Studies measuring brain glucose consumption during demanding cognitive tasks find increases of only about 5% above baseline. Over the course of a full day of intense mental work, that translates to maybe 100 to 200 extra calories. That's a banana and a half.
Even chess grandmasters, who sit for hours in states of intense concentration, burn only about 1.67 calories per minute while playing, compared to 1.53 calories per minute at rest. A 10% premium for one of the most cognitively demanding activities humans engage in.
The reason is structural. Your brain's energy budget is dominated by housekeeping. Maintaining the electrochemical gradients across neuronal membranes, the resting activity of vast neural networks, the constant baseline hum of consciousness. Conscious thought, the stuff that feels effortful, is a thin layer of activity on top of this expensive foundation. It's like turning on a desk lamp in a building where the HVAC system already runs 24 hours a day. The lamp is real. It's just not where the bill comes from.
So no, your day of deep work didn't burn enough extra calories to justify the extra slice of pizza. I found this mildly deflating. But what I found next was far more consequential.
The 15% problem
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