To eat or not to eat: The science behind running on empty

My brain loves science, but my body begs to differ.

Regarding the research I'm about to share — research about exercise and an empty stomach — my body scoffs and says, "Ha, ha! Nope!"

Let me start out by saying I possess a finicky digestive system that doesn't like food sloshing about during exercise. That's how I became a fan of running in a fasted state. In 2013, I did the first 10 miles of my Boston Marathon qualifying run without ingesting anything after the previous night except a large Starbucks medium roast. Granted, I crashed hard in the last few miles. But I made my time!

There are those who erroneously believe that running — or doing any exercise — on an empty stomach "mobilizes fat stores" and accelerates weight loss. Unfortunately, this is in direct violation of the first law of thermodynamics; caloric balance is all that matters when it comes to dropping fat from your frame. 

A 2014 study in the Journal of the International Society of Sports Nutrition compared a group of people who did aerobic workouts after a meal with another group who fasted overnight before exercising. After a month, researchers found no significant difference in body composition — weight, body-fat percentage, waist circumference, etc. — between the two groups.

But his column isn't about fat loss. It's about performance. With Boston ticked off my bucket list, it was time for me to revisit an elusive goal: running 10 kilometers in under 40 minutes. The research suggests that if I wanted to reach my goal, I'd need to give up fasting beforehand.

"You can sustain exercise for hours at a low-to-moderate intensity on an empty stomach," said nutritionist and author Susan Kleiner. But at higher intensities, she said, you need some fuel — and that means carbs.

Alicia Kendig, senior sports dietitian for Team USA, echoed Kleiner's comments, saying that stored body fat can be metabolized for energy up to a certain point. But when you get into fast-pace runs or high-intensity intervals, it's a good idea to consume something with carbohydrates in it before exercise.

"It gets the engine greased and oiled," added Kleiner.

Sometimes, it's not just having the fuel that helps, but thinking we have the fuel. Receptors in the mouth can trick the brain into believing you've eaten, Kleiner explained. She cited a 2014 article in the journal Nutrients that reviewed studies looking at the effect of carbohydrate mouth rinse on exercise performance. Participants were given either water or a flavorless carbohydrate drink to swish in their mouth and spit out, so no energy was ingested. Those who received the carbohydrate rinse experienced a "significant" performance increase.

Kleiner explained it this way: "The brain opens up the fire hose to allow full access to the body energy stores" because the receptors in the mouth told the brain that food was on its way.

This theory jibes with my experience. While running, I'll occasionally bonk — sports-speak for that sudden, low-energy feeling of utter wretchedness. (Although I understand it means something quite different in the UK and Australia.) I usually carry a fruit bar with me. Eating it creates an almost immediate sensation of bouncing back and being able to start running again.

There's no way the fruit bar gets digested and becomes bio-available fuel in under a minute. That takes closer to an hour. So it's that neural message from mouth to brain that reopens the fire hose of my body's available fuel sources.

The advice of these two sports dietitians had me re-evaluating my pre-run fasts, so I decided to try a sports drink. Rather than go for the more common maltose (sugar) variety, I bought one made of "fractionated starch" because it boasted "2.3 times faster gastric emptying," meaning it turns into fuel quicker.

I chugged what felt like a massive dose: the recommended 280 calories. The sheer volume made me feel bloated and gross. I had to wait 80 minutes before I was ready to go.

The results weren't impressive.I've had better runs the morning after an epic tequila bender. My limbs felt leaden. I bonked 2 miles in and reached for the fruit bar.

Kendig told me it can take time for the stomach to adjust to having food in it before exercise. She suggested I water down the beverage by using less of the sports drink powder. Kleiner added that I should run sooner, hitting the pavement just 30 minutes after my drink.

That's what I did, and it worked much better. But my time still wasn't quite as fast as when I fasted.

Kendig also suggested including a small amount of protein to help keep my energy levels balanced and hopefully to stave off a sharp drop in blood glucose that can lead to that bonking badness.

Her mention of protein made me realize my pre-run coffee includes a moderate amount of milk, which contains protein. Maybe that's enough for me to get the engine greased and oiled, because after several experiments with fueling, I'm still not hitting the times I can when I "fast" for a 10K. But I still haven't run it in under 40 minutes, either.

I have more experimenting to do, and that's the moral of the story: While the science is straightforward, you need to experiment to see what works best for your physiology. Just don't wait until race day to do it.

James S. Fell is a certified strength and conditioning specialist and owner of bodyforwife.com.