My life is crazy, and most days I’m running like my hair’s on fire. (You know the feeling, right?) But no matter how busy things get, I take time to work out every day. What’s more, I’m always looking for ways to improve the quality of my exercise routine. And lately, I’ve found a way to get far more mileage from each workout: by working out during a fast. If you haven’t tried it yourself, I highly recommend it.

Why workout during a fast?

Working out during a fast burns more fat.

About six hours after a meal, your body enters a fasting state. When this happens, it burns off its stored sugar and then starts breaking down fat and converting it into ketone bodies for fuel. In other words, you start burning fat instead of sugar for energy.

When you exercise in this fasted state, it blasts the fat off you. In fact, research shows that if you work out before eating breakfast in the morning rather than afterward, you can burn nearly 20 percent more fat. Fasted exercise is also a great way to undo the damage if you’ve over-indulged in pizza or scarfed down too much Chunky Monkey.

Research shows that even when you’ve sinned, it can keep those extra pounds from piling on. In one six-week study, researchers asked male volunteers to stuff themselves with junk food every day. Some participants didn’t exercise at all while gorging themselves, while others either fasted before exercising in the morning or ate a big breakfast before working out. What happened?

The men who didn’t exercise got way chubbier (no surprise). The men who ate breakfast before exercising also gained weight, although only about half as much as the controls. Those who workout during a fast, however, gained virtually no weight, even though they ate the same bad diet.

Working out during a fast lowers your insulin levels.

Excess insulin packs fat on your body and puts you at risk for diabetes, so one of the biggest favors you can do for yourself is to drive that insulin down. Either fasting or exercise will help you do this—and when you pair them up, the result is metabolic magic. In the study I cited above, for instance, the controls and the non-fasting exercisers exhibited insulin resistance (which leads to higher insulin levels) after their junk food overload.

The fasted exercisers, however, showed no signs of insulin resistance, in spite of their terrible diet. In another study, participants exercised in a fasted state at least three times weekly for 12 weeks. By the end of the study, they lost an average of one-quarter of their baseline fat mass, and their fasting insulin levels fell by 25%. That’s HUGE.

Working out during a fast improves your athletic performance.

Right now, you’re carrying around a big supply of energy in the form of fat. The problem is that you can’t access that energy easily, because your body is trained to burn sugar—not fat. That’s why you’ll “hit the wall” if you run out of blood sugar in the middle of a race or a game. When you work out during a fast, however, you teach your body how to tap readily into your fat stores for energy. When you do this, you become metabolically flexible.

On the day of a big race or game, you’ll still want to eat beforehand (that’s not the time for fasting). But if you’re metabolically flexible, your body will easily switch to burning stored fat once it uses up the sugar from your meal. As a result, you’ll have a virtually unlimited supply of energy… and that means you can leave your competition in the dust. 

What rules do I need to follow when working out during a fast?

• Schedule your workouts at least six hours after a meal
• Start with easy exercise such as walking or slow cycling, and build up to more intense workouts.
• Drink plenty of water before, during, and after each workout.
• Eat soon after doing an intense workout. I recommend refueling with an easy-to-digest protein like a bone broth protein shake or some chicken or salmon and a starchy carb such as sweet potato.

When you start working out during a fast, it’ll take about two weeks for your body to master the art of burning fat for fuel. Hang in there, even if your workouts are a little tough at first. I promise that your investment will pay off big-time when the pounds melt off, your insulin level drops, and you crush your rivals on the court, the track, or the field!

Keep thinking Big and living BOLD!