On average, a normal human burns about 100 calories for every mile run, so you can imagine that The Flash is easily burning through a few thousand calories before you can say, “Thank you, ma’am.” A normal man about Barry Allen’s size would need no more than 3,000 calories a day, even on an intense exercise schedule, which means The Flash is burning off breakfast, lunch, and dinner before you can blink. In fact, according to estimates from a sports physician who spoke with MTV, running for five minutes at the speed of sound, Barry burns 8,889 calories, but if he runs constantly at that speed for 20 minutes, he burns a whopping 70,000 kcals.
The solution is simple: Barry eats constantly to make up for his supercharged metabolism. In the DCEU films, he refers to himself as a “snack hole,” and has no problem eating an entire pizza by himself during a car ride. Eating a whole pizza might not sound like a superheroic feat, but have you tried running a mile afterward?
However, we run (metaphorically) into another problem here. How does Barry’s body fit all that food? To power even a few minutes of running, he’d need to consume more than a human stomach could possibly contain. “He’d have to eat 200 million cheeseburgers to run at superspeed once,” said a doctor interviewed by Business Insider. And not every part of the food we eat can be turned into energy, so how often does he use the bathroom? Can a toilet even survive the damage Barry must be unloading?
Unfortunately, this is where suspension of disbelief factors into the equation. There’s no totally scientific way to explain Barry’s metabolic and digestive capabilities. Call it Speed Force-gestion, if you like.