It’s been over a decade now since researchers demonstrated that simply rinsing a sports drink in your mouth, then spitting it out without swallowing, can enhance your endurance under certain conditions. It’s a great demonstration of the brain’s role in determining your limits: trick it into thinking more fuel is on the way, and you’ll be able to access more reserves.
A new study in Medicine & Science in Sports & Exercise, from researchers at the Federal University of Pernambuco in Brazil, digs a little deeper into the conditions that determine how helpful carb rinsing is, with some interesting results.
One previous finding has been that mouth rinse seems to work best when you’re fasted or hungry, which makes sense. If your body already has plenty of fuel, your brain doesn’t really care whether more is on the way.
The new study tested cyclists under six different conditions, three with carb rinse and three without:
One pair of tests was conducted under “fed” conditions: breakfast at 6 a.m., then starting the experiment at 8 a.m.
Another pair of tests was “fasted”: after an 8pm dinner, a 12-hour fast before the 8 a.m. start. This overnight fast depletes glycogen stores in the liver, but not in the muscles.
The final pair was “depleted”: a 6 pm workout (90 minutes of cycling at 70 percent of peak power, then 6 x 1:00 hard with 1:00 rest), followed by a very-low-carb dinner at 8 pm.then a 12-hour fast until the experiment at 8am. This protocol depleted both liver and muscle glycogen.
So how did the subjects perform in a 20-kilometre time trial (lasting on average 42 minutes)? Pretty much as you’d expect:
Data on mouth rinse.
IMAGE COURTESY OF MEDICINE AND SCIENCE IN SPORTS AND EXERCISE
The carb mouth rinse, which was a tasteless beverage (the carbohydrate was 6.4 per cent maltodextrine) provided in 25 mL amounts after 5, 10, 15, and 18 kilometres of cycling, didn’t have any effect when the subjects were fed. There was a possible benefit in the fasted condition, and a big effect in the depleted condition.
It’s worth noting that swishing carbohydrate couldn’t totally erase the deficit caused by carb depletion. Your muscles really do need carbohydrate—just not quite as much as your brain pretends.
There are some other interesting details in the study. They also measured carbohydrate and fat oxidation during a steady ride, but saw no significant changes with carb rinse. In other words, the benefits aren’t because your muscles themselves start processing fuel differently.
Instead, the benefits appear to be neuromuscular. Somehow, knowing that more fuel is (apparently) on the way convinces your brain to either recruit more muscle or transmit signals from brain to muscle more efficiently. Here, for example, is a measure of muscle recruitment using EMG electrodes placed on the quadriceps muscles of the subjects:
Data on mouth rinse.
IMAGE COURTESY OF MEDICINE AND SCIENCE IN SPORTS AND EXERCISE
It’s clear that EMG activity is reduced in the depleted condition, but that effect is counteracted by rinsing with carb drink.
There are a couple of useful messages from this study. One is that, once again, carb rinses work. It’s true that no one would start a race with fully depleted carb stores. But think of the late stages of a marathon: after several hours, your carb stores will be very close to depleted, suggesting that you’d be in a position to benefit from a rinse.
Of course, you could—and should—actually swallow your sports drink and gels. But late in a race, you may have trouble stomaching as much fuel as you need. If that’s the case, swishing it around in your mouth before swallowing might add an extra boost. (In the study, the subects were asked to swish it around for 10 seconds before spitting.) That’s especially true in the final miles, when you won’t have a chance to actually absorb anything further from your stomach anyway. That’s the reason Meb Keflezighi, for example, grabs his bottle at the 40K station during marathons: to rinse and spit.
The other point that the authors mention is that carb rinse could be a strategy to help perform better during training sessions performed with depleted carb stores. Doing occasional (once or twice a week) carb-depleted training sessions has become increasingly popular as a way of spurring greater fat burning during exercise.
The problem is that these training sessions are usually very low-quality, because you’re running on empty. A carb rinse can help make the training better; and the metabolic data from this study shows that it doesn’t affect the carb and fat burning that you’re targeting with low-carb workouts. It’ll still feel like crap—just a bit less so.
