You can’t get fitter without stress. I’m not talking about the psychological stress of, say, finding yourself stuck in a long airport security line while your flight is already boarding. Rather, I’m talking about the physical stress running imposes on your body, which prompts performance adaptations.

For the most part, the controlled stress you experience in the process of executing a sensible training plan does the job of getting you race ready. But you can gain extra benefit by adding the additional stress of making the conditions you run in more challenging in particular ways.

Training at high altitude is one example. When you run (or, more accurately, live) in an oxygen-poor mountain environment, you make running a little more stressful to your body, which responds by producing more red bloods cells, increasing your aerobic capacity. A second example is running in the heat. Studies have shown that training in a hot environment for a short period of time (around 10 days) boosts blood volume, another way of getting more oxygen to your muscles and boosting aerobic capacity

Then there’s carbohydrate-restricted training, a method that has received a lot of research attention lately and has become widely practiced at the elite level of running and other endurance sports. The most common form of carb-restricted training is a long endurance session undertaken first thing in the morning on an empty stomach and fueled by water (or water plus electrolytes).

It is generally believed that the main purpose of this practice is to increase the fat-burning capacity of the muscles, but most of the research on carb-restricted training has focused on a completely different benefit. That’s right: increased aerobic capacity.

What scientists have found is that muscle glycogen depletion (glycogen being the muscles’ preferred carbohydrate-derived energy source) is a major trigger of some of the physiological adaptations that enhance the body’s ability to use oxygen for muscle work. Normal training results in a degree of glycogen depletion, but carb-restricted training augments this stressor and the resulting benefits. The specific idea behind this practice is to start workouts at low muscle glycogen levels instead of merely finishing them there.

Scientists, coaches, and athletes are currently experimenting with various protocols, and no one yet knows the best way to practice carb-restricted training. But one protocol that is proven to work is what I like to call the depletion double. In a 2016 study published in Medicine and Science in Sport and Exercise, a team of Australian, British, and French researchers found that the following sequence, repeated a number of times over a period of three weeks, improved both cycling and running performance in a group of 21 triathletes:

1) Afternoon high-intensity interval workout

2) Low-carb dinner

3) A full night’s sleep

4) Low-intensity morning workout on an empty stomach

That last step might sound rather tortuous given all that comes before it—and I would not recommend the depletion double to beginners. The subjects of the study who benefitted from it were far from elite, however, so if you’re in pretty good shape and you’re looking for ways to take your fitness up another notch, don’t be afraid to give this cutting-edge method a try.

photo: 101 Degrees West

The Workout

Just about any high-intensity interval workout format will serve the purposes of the depletion double—provided it’s done in the late afternoon. The running workout that was used in the aforementioned study (there was also a bike workout) consisted of:

6 x 5 minutes at 10K race pace, with a minute’s rest after each repetition.

Be sure to warm up with a mile or two of easy jogging and (ideally) a few drills and strides before you launch into this challenging interval set, which can be done at the track or on any other flat, smooth route that’s conducive to steady, uptempo running. Cool down with another mile or two of jogging.

After you’ve cleaned up and changed, eat a low-carb dinner such as baked salmon, a cheese omelet, and a green salad with oil-based dressing. The point here is not to go hungry. Eat enough to satisfy your appetite—just keep the carb content as low as reasonably possible.

Go to bed at the usual time. Note that you may find it easier to sleep, and also feel better when you wake up, if you drink a sugar-free protein drink before you lie down, as the subjects in the study did. In the morning, suit up again before breakfast. It’s okay to drink water before and during the run, but don’t take in any calories

Run for one hour at a comfortable pace on an empty stomach.

When you’re done running, refuel with your favorite high-carb breakfast, whether it’s oatmeal, pancakes, or yogurt and berries. You’ve earned it, and your body will thank you as it rebuilds stronger.