What happens if I work out on an empty stomach?
For decades, sports nutritionists have been devising ever more sophisticated ways to ensure your body is perfectly fuelled before, during and after every workout. With gels, bars and belt-mounted drink bottles, you can have calories within reach no matter where you are.
But what if quaffing fewer carbs and calories - or even none - resulted in a better workout?
At a recent sports nutrition conference at the Australian Institute of Sport in Canberra, researchers and coaches were buzzing about an emerging practice they refer to as "train low, compete high." The idea is to do some of your workouts in a carbohydrate-depleted state - the nutritional equivalent of training while wearing a weighted vest - then race with a full tank of carbohydrates.
With initial research showing the technique boosts fat-burning, as well as other metabolic responses to exercise, elite athletes aren't the only ones taking note. It remains a controversial approach - but it's relatively easy to give it a try.
There's no doubt that being properly fuelled, particularly with carbohydrates, allows you to go faster and farther during sustained exercise. The philosophy behind "train low" is that attempting to make your workout easier is counterproductive: The whole point, after all, is to stress your body so that it adapts and becomes stronger.
"The idea is driven by new interest in 'cell signalling,' " explains AIS sports nutritionist Louise Burke. Exercise stimulates the production of specialized proteins that signal your body to adapt to new demands. These proteins are locked up in your body's carbohydrate stores, and released as those stores are burned; starting with low carbohydrate stores means the proteins are already free to do their signalling as soon as you start exercising.
Researchers have experimented with different protocols to empty out your carbohydrate stores. One is to exercise for 30 to 60 minutes at 70 per cent of maximum effort, rest for an hour or two without ingesting any calories, then proceed to your "real" workout.
"It's not for the faint of heart," admits John Hawley, an exercise metabolism researcher at the Royal Melbourne Institute of Technology.
A more practical approach involves doing your workout first thing in the morning, before breakfast. The overnight fast won't fully deplete your muscles, but it will leave your liver - the other key location for carbohydrate storage - about half-empty.
Researchers in New Zealand recently put 14 cyclists through a four-week training program with half training before breakfast and the other half after breakfast, five mornings a week. The "fasted" group increased the amount of carbohydrate they were able to store in their muscles by 54.7 per cent, while the "fed" group only increased it by 2.9 per cent.
Another study, from the University of Birmingham in Britain last year, used muscle biopsies to show that "training low" taught the body to burn fat instead of carbohydrate. But in a test of actual performance (a 60-minute cycling time trial), the subjects failed to show any improvement compared with controls.
"There's often a disconnect between changes in cellular, 'mechanistic' variables and real-world athletic performance," Dr. Hawley says.
Still, athletes have been experimenting with this type of approach for decades, he adds. Frank Shorter, the 1972 Olympic marathon champion, and five-time Tour de France winner Miguel Indurain are two famous examples rumoured to have experimented with long, hard training sessions in a depleted state.
For those of us who are more interested in getting fit or losing weight than racing, the findings about increased fat-burning suggest we should pay less, not more, attention to fuelling our workouts.
For example, a University of Massachusetts study found that overweight subjects who walked on a treadmill for an hour a day improved their insulin sensitivity by 40 per cent - unless they replaced the calories burned with a sports drink after the workout.
The lesson here is not that you shouldn't eat - that's a tactic doomed to fail - but that making a special effort to fuel your workout with extra calories from sports nutrition products may backfire unless your workout is particularly intense.
And if doing the occasional workout on an empty stomach seems to make things a bit harder - well, that's the point.
Pros and cons of "training low"
It's unclear whether the apparent benefits of training with low carbohydrate stores outweigh the disadvantages, says Louise Burke, a nutritionist at the Australian Institute of Sport. Here are some of the factors researchers are debating:
- Building more endurance without increasing training could reduce stress on joints and minimize injury.
- Reduction in body fat.
- Teaching the body to rely less on carbohydrate during exercise could mean consuming less sports drink or gel and experiencing fewer gastrointestinal problems during long races.
- You can't train as hard or fast as you would with full fuel stores.
- Training in a depleted state may increase the risk of injury and illness.
- Teaching the body to rely less on carbohydrate during exercise could make it harder to respond to surges and finishing sprints.
- Training after fasting puts extra stress on your body, so it's not something you can do every day. At most, try it twice a week at first.
- The time between meals isn't enough to deplete your carbohydrate stores, so first thing in the morning is generally the only practical time to "train low." Eating a low-carbohydrate dinner the night before may enhance the effect.
- Training low may help boost endurance, but it interferes with your ability to develop speed and power, so athletes should "periodize" their nutrition. Try training low several months before an upcoming race, but switch to fully fuelled training as your competition approaches.
Alex Hutchinson blogs about research on exercise at sweatscience.com. His new book, Which Comes First, Cardio or Weights?, will be published this month.