The fitness expert behind the viral 'Insanity' workout told us his favorite 'powerhouse' food

• Shaun T. is the mind behind the viral "Insanity" workout.
• In his new book, "T is for Transformation," Shaun talks about what motivates him to exercise and what he does when he's feeling low on fuel.
• Shaun told Business Insider that his favorite "powerhouse" food is the sweet potato, but he doesn't always eat them before a workout.

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Getting ready to work out is often the hardest part of exercising. You need a plan, a place to do it, the right kind of clothes, good shoes, and enough energy.

Caffeine can help you feel more energetic - but so can certain foods.

Shaun T. is the fitness guru who came up with the viral "Insanity" workout, a grueling 60-day fitness plan that uses interval training - an equipment-free workout approach that involves breaking up quick bursts of sweaty movement with short rest periods. Shaun told Business Insider that while he prefers to exercise first thing in the morning on an empty stomach, he sometimes needs a snack to keep him fueled before and after a workout.

His favorite food, sweet potato, is what he calls a "powerhouse food." On days when he's feeling low-energy, he eats half of one about 45 minutes before a workout.

"A half a sweet potato is my best friend," Shaun said. "I call it the powerhouse food. It's an amazing carb, and you don't need to add sugar or anything like that. Just bake it and eat it just like that."

Sweet potatoes might be a healthy pre- and post-workout food for many reasons, but one of them is that they're a great source of carbohydrates - the most easily accessible form of fuel for your muscles.

After a heart-pounding, leg-shaking workout, carbs can speedily nourish the parts of your body that need it most.

Plus, despite packing a high dose of carbs, sweet potatoes are also rich in several other nutrients which can help balance out their impact on your energy levels - meaning you won't get a spike in energy and then crash a few hours later. Half of a baked sweet potato gives you 3 grams of fiber, 2 grams of protein, more than three times your daily requirement of vitamin A, and a third of your daily dose of vitamin C. It's also only about 80 to 90 calories, fewer than you'd get from eating a piece of toast.

But don't take my word for it - try it next time you work out.

"Just eat that and you'll be like, 'Oh my goodness,'" Shaun said.