Can Gargling Glucose Make You Faster?

Every once in a while I read a study that completely changes the way I think about a given topic. This week I review a study that did just that.

After years of studying exercise physiology and doing my master’s thesis on carbohydrate and aerobic performance, I thought I’d heard everything.

The basic rule is that with aerobic exercise you need to take in a certain amount of carbohydrates to offset the amount you’ve used up. If not, you hit “the wall” (you can’t continue at the same pace). However, you don’t really need to ingest carbohydrates unless you’re going for more than an hour.

Now, scientists are still nailing down things like precise amounts and ratios for individual sports. But one thing we “knew” for sure: you have to actually ingest the carbs to help your exercise performance.

Right?

Well… in this week’s review the researchers find that carbohydrates don’t even have to make it to your stomach to make a difference in exercise performance.

Intrigued? I was.

Chambers ES, Bridge MW, Jones DA. Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity. J Physiol. 2009 Apr 15;587(Pt 8):1779-94.

Methods

This study is actually two studies in one – actually more but I’ll stick to the first two for now.

Study 1: Oral carbohydrate and exercise performance

Cyclists swirled either a glucose solution or an artificially sweetened (saccharin) solution in their mouths and cycled a time trial.

Study 2: Brain response to glucose, maltodextrin and saccharin

Participants again swirled either a glucose/maltodextrin solution or a saccharin solution in their mouths and had an MRI that looked at brain activity.

I’ll introduce study 1 and then go though study 2, since study 1 is the prelude to study 2.

Methods for study 1: Gargling with glucose

For study 1 there were 8 male cyclists (cycling at least 2 a week). Average age was 29 and a respectable 60.8 mL/kg/min VO_2max.

VO_2max

What is VO_2max? It is the maximum amount of oxygen your body can use.

People with high a VO_2max tend to do better in endurance events. Lance Armstrong has a reported VO_2max of 81.2 ml·kg^–1·min^–1 [1] while a VO_2max of 45 mL/kg/min and 35 mL/kg/min would be considered good for the average 30 year old Joe and and Jane, respectively [2].

So at 60.8 ml/kg/min these participants are in pretty darn good shape, aerobically speaking.

These fit cyclists came to the lab 4 times, once to test their VO_2max, once to get use to the testing procedure (familiarization), and two times for time trials with either glucose or placebo.

Normally the glucose time trial would involve the participants actually drinking the glucose. In this study the participants merely swirled the glucose solution in their mouths like you would with wine at a wine tasting. In fact, the researchers kindly included  a section in the methods describing the “mouth rinse protocol”.

Summary of the “mouth rinse protocol”

After completing 12.5% of the time trial the participants were given 25 mL (0.85 oz) of either glucose or placebo solutions.

As the participants cycled away, they rinsed the fluid around their mouths for about 10 seconds and then spit it into a bowl held by one of the researchers (aka the most junior grad student in the lab).

So NO drinking! Just rinsing.

Rinsing solutions:

• 150 mL of concentrated aspartame and saccharin sweetener in 1 L of water for the placebo
• 64 g of glucose in 1 L of the placebo solution for the glucose solution

Notice that they used the placebo solution to dissolve the glucose. That way, the overpowering sweetness of the artificial sweeteners wouldn’t allow the participants to figure out which was which.

Results: Study 1

Good news! The participants couldn’t tell which solution was the placebo and which was the glucose solution. If they could then the whole study results would be in doubt. Not knowing which is the placebo is key to having a placebo in the first place.

Now the cool part. After swirling, swilling, rinsing, gargling or whatever you wish to call it with glucose (but remember, not swallowing a single drop), there was an improvement in time trials!

Glucose rinse times averaged 60.4 minutes. Placebo rinse time averages were slower, 62.6 minutes. This means an average improvement in power of 2.0% in the glucose time trial.

At this point I’m sure you’re thinking “How the heck does swirling anything in your mouth let alone glucose improve performance?”

I’m sure that was what the researchers asked too, so they decided to do a follow-up study to figure some of this out.

Methods for study 2: Your brain on sweetness

This study tried to figure out which regions of the brain responded to caloric (glucose) and non-caloric sweetness (saccharin).

Participants this time were right-handed (I’m assuming to minimize any difference that handedness could play), and on average  23 years old. This time there were 4 men and 3 women.

fMRI

This time the participants drank either a glucose solution or a saccharin solution while hanging out in a functional MRI (fMRI).

fMRI is a method of “live-action” brain imaging that can detect blood oxygenation level-dependent (BOLD) responses. Changes in BOLD mean that a given area of the brain is responding to the solutions. Basically, BOLD lets you see the brain working. (Cool huh?)

Glucose solution was a little stronger, at 90 g of glucose in 1 L of water. In this study there was no reason to try to cover up the taste, as the participants had no conscious control of the outcome.

Saccharin solution was 60 mg of sodium saccharin in 1 L of water. This was chosen because the participants thought it matched the glucose solution in sweetness.

Fake spit – yum!

And just to make sure they had a baseline – what a brain looks like when its tasting something that doesn’t taste like anything – the researchers included a tasteless control that was made up of the main components in saliva.

This artificial saliva had 25 mmol of KCl (potassium chloride, sometimes used as a salt substitute) and 2.5 mmol of NaHCO₃ (i.e. sodium bicarbonate or baking soda).

For some reason I find drinking artificial saliva kinda gross. Hopefully, the researchers didn’t refer to it as “artificial saliva” when they gave it to the participants.

Early test subjects

Results: Study 2

The insulta/frontal operculum – that is the supposed human primary taste cortex — lit up like a Christmas tree after both the glucose and saccharin solutions, but glucose also lit up the right and left dorsolateral prefrontal cortex, a part of the right caudate that forms part of the striatum.

Oh heck, just look at Figure 1A! (Top row of brain pictures)

Meanwhile, if you take a look at Figure 1B (bottom row of brain pictures; the control group), you can see a lot less activation overall and none in the right caudate and striatum.

So even though the participants couldn’t tell the two solutions apart, their brains could.

What does this mean?

The researchers believe that the differences they found between saccharin and glucose is because not only can we taste sweetness, but we can taste calories. Wow.

I’m not going to go into the nitty gritty but they did two more studies in this paper.

Study 3: Sweet vs less sweet

In a third study, the researchers compared placebo (sweet) and maltodextrin (not so sweet) solutions and lo and behold there was an improvement in time trials performance if cyclists rinsed with maltodextrin.

Study 4: Sweet vs less sweet 2

A fourth study did the “opposite” of the second study: rather than comparing solutions with the same sweetness but different calorie content, it compared sweet vs less-sweet tastes with the same amount of calories.

This study found that solutions of glucose (pretty sweet) and maltodextrin (not so sweet) with the same calories light up the same parts of the brain.

Thus Study 3 and Study 4 suggest that there is a “calorie detector” somewhere along the way from mouth to brain. It’s not just about the taste. Somehow, we’re able to tell the difference between sweet taste and actual sugar calories.

Conclusion

Swirling sugar water in your mouth without drinking it improves how fast you can ride a bike over an hour (relatively short time for aerobic performance).

Think about how crazy that is.

First, the amount of carbohydrate you could take in during an hour is pretty small (22g). So even if you did drink the sugar water it shouldn’t matter – but it does.

Second, you’re not even drinking it and that really shouldn’t matter – but it does.

Possible sport supplement advertising campaign?

After looking at pretty pictures of various brains the researchers think that this difference in performance when carbohydrate is present in the mouth is because reward and motor control regions are activated.

This study also means that there are yet unidentified oral receptors that detect carbohydrates regardless independent of sweetness. Woah.

Bottom line

After I read this study I thought holy cow, I really should be more careful about what I eat, since it looks like my body is responding to more things than just taste.

Personally, I’m still trying to wrap my head around the idea that just tasting glucose could make me cycle faster.

Looks like the body still has a few tricks up its sleeve… uhm… mouth.

References

1. Coyle EF. Improved muscular efficiency displayed as Tour de France champion matures. J Appl Physiol. 2005 Jun;98(6):2191-6.
2. Brooks GA, Fahey, TD and White TP. Exercise Physiology: Human Biogenetics and Its Applications. Mayfield Publishing Company. Mountain View, CA. 1995. 2^nd edition: 589.