My answer: Nothing. As you may recall, I've adopted an intermittent fasting protocol, and I'm exercising on an empty stomach.
Some people think I'm insane for diving into CrossFit workouts ten or twelve hours into a fast. How do you have the energy? they ask. Don't you bonk halfway through your workout? Are you not aware that breakfast is the most important meal of the day?
Certainly, fasted training isn't a good idea unless your cortisol levels are already in check. But if you're (one of the few of us) getting good sleep, keeping your stress levels low, and aren't pushing your body beyond capacity when exercising, working out without food in your belly is something to consider.
Fasted training can help increase endurance and boost the body's ability to store muscle glycogen efficiently. And as Chris Highcock pointed out, it can help you recover faster from endurance exercise. Plus, from an evolutionary perspective, it's been pointed out that our cavemen ancestors likely exerted themselves most strenuously while hunting for food -- not after they'd filled their stomachs with a meal.
And by the way, breakfast is by no means "the most important meal of the day." That's just a marketing slogan that breakfast cereal makers dreamed up decades ago (and still reverberating in the echo chamber of lazy science, agribusiness marketing, and the informational cascade). Don't swallow the hype: Skipping a meal doesn't "slow your metabolism" and make you fat.
