Athletes often use a simple trick to boost performance: swishing a sports drink in their mouth for a few seconds without swallowing it. Researchers wanted to know if this trick could help cyclists who had been eating a ketogenic (keto) diet for 5 days. The keto diet is very low in carbohydrates, which are normally the body’s main fuel for exercise. Eight trained cyclists tested this mouth-rinsing trick during a simulated race after eating either their normal diet or a keto diet. The results showed that swishing the sports drink didn’t help cyclists recover their normal performance when they were on the keto diet. This suggests that the body needs actual fuel in the stomach and bloodstream, not just the taste of fuel in the mouth.

The Quick Take

  • What they studied: Can swishing a carbohydrate drink in your mouth for a few seconds help cyclists perform better when they’ve been eating a very low-carb (keto) diet?
  • Who participated: Eight trained endurance cyclists who regularly compete in cycling events. They completed the study under two different eating conditions: their normal diet and a strict keto diet lasting 5 days.
  • Key finding: Swishing a 6.4% carbohydrate drink in the mouth did not help cyclists maintain their normal performance after 5 days on a keto diet. Cyclists on the keto diet were about 4.8% slower (roughly 2.5 minutes slower on a 33-mile race) compared to when eating their normal diet, and the mouth-rinsing trick couldn’t fix this problem.
  • What it means for you: If you’re thinking about trying a keto diet while training for endurance sports, swishing sports drinks won’t make up for the performance loss. Your body appears to need actual carbohydrates in your stomach and bloodstream, not just the sensation of them in your mouth. This is most relevant for competitive cyclists and endurance athletes.

The Research Details

This was a carefully controlled experiment where eight trained cyclists completed four different test conditions. Each cyclist tested two different eating patterns (their normal diet and a 5-day keto diet) combined with two different mouth-rinsing conditions (a carbohydrate drink or a placebo that tasted similar but had no carbs). The researchers tracked what the cyclists ate, measured their blood sugar and ketone levels (which show how well the keto diet was working), and had them complete a simulated 33.6-kilometer (about 21-mile) cycling race. During the race, cyclists rinsed their mouths with either the carb drink or placebo every 7 kilometers (about 4.3 miles).

The researchers measured how long it took each cyclist to complete the race and checked their blood sugar levels before and after exercise. This design allowed them to see if the mouth-rinsing trick worked differently depending on whether cyclists were eating normally or following a keto diet.

This research design is important because it tests a real-world scenario that athletes might actually try. Many endurance athletes use keto diets to lose weight or for other reasons, and they might also try mouth-rinsing tricks to maintain performance. By testing both conditions together, the researchers could see if the trick works when the body is in a special metabolic state from the keto diet. This is more realistic than just testing the trick in isolation.

This study has some strengths and limitations worth knowing about. The strength is that it was carefully controlled—each cyclist did all four conditions, which means we can compare results within the same person rather than between different people. The researchers also measured actual blood markers to confirm the keto diet was working. However, the study only included 8 cyclists, which is a small number. This means the results might not apply to all athletes, especially those who aren’t trained cyclists or who have different body types. The study also only tested one specific concentration of carbohydrate drink (6.4%), so we don’t know if a stronger or weaker solution might work differently.

What the Results Show

The main finding was clear: the keto diet made cyclists slower, and swishing a carbohydrate drink didn’t fix the problem. When cyclists ate their normal diet, they completed the 33.6-kilometer race in about 54 minutes. When they followed the keto diet for 5 days, they took about 2.5 minutes longer (roughly 56.5 minutes). This 4.8% slowdown was statistically significant, meaning it’s unlikely to be due to chance.

When cyclists rinsed their mouths with the carbohydrate drink during the keto diet condition, their performance didn’t improve compared to rinsing with the placebo. Both the carb-rinsing and placebo-rinsing conditions during the keto diet were equally slow. This suggests that the mouth-rinsing trick, which sometimes works in other situations, simply doesn’t help when the body has been running on very few carbohydrates for 5 days.

The researchers confirmed that the keto diet was actually working by measuring ketone bodies (specifically β-hydroxybutyrate) in the cyclists’ blood. These levels were significantly higher on the keto diet, confirming that the cyclists’ bodies had switched to burning fat instead of carbohydrates. Interestingly, the level of ketones in the blood wasn’t directly related to how slow the cyclists were, suggesting that the performance problem isn’t just about ketone levels.

The researchers also measured blood glucose (sugar) levels after the race. Cyclists who ate their normal diet had higher blood sugar after exercise, whether they rinsed with carbs or placebo. This makes sense because their bodies had plenty of carbohydrates available. During the keto diet conditions, blood sugar was lower after exercise, which shows that the cyclists’ bodies were struggling to maintain normal blood sugar levels. This might be part of why they performed worse.

Previous research has shown that swishing carbohydrate drinks in your mouth can sometimes help athletes perform better, especially when they’re fasting or have low muscle fuel stores. Scientists thought this trick might work particularly well during a keto diet because the body would be desperate for carbohydrate signals. However, this study suggests that the trick doesn’t work as well as expected in this situation. This finding challenges the idea that the mouth-rinsing benefit works purely through taste and brain signals—it appears the body actually needs real carbohydrates available in the bloodstream and muscles.

This study has several important limitations. First, only 8 cyclists participated, which is a small group. Results from small studies can be less reliable and might not apply to all athletes. Second, the study only tested one strength of carbohydrate solution (6.4%), so we don’t know if a stronger solution might have worked better. Third, all participants were trained cyclists, so these results might not apply to casual athletes or people doing other types of endurance exercise. Fourth, the keto diet lasted only 5 days—we don’t know what would happen if athletes followed it longer and became more adapted to it. Finally, the study didn’t measure muscle glycogen (fuel stored directly in muscles), which might be important for understanding why performance suffered.

The Bottom Line

Based on this research, if you’re a competitive endurance athlete, avoid following a strict keto diet during training or competition periods. The performance loss appears to be real and significant (about 5%), and simple tricks like mouth-rinsing won’t fix it. If you’re interested in keto diets for other reasons (like weight loss), consider doing it during off-season periods when performance isn’t critical. The evidence here is fairly strong (Level 2 evidence) because it’s a controlled study, though the small sample size means we should be somewhat cautious about applying it to everyone.

This research is most relevant for competitive endurance cyclists and other endurance athletes (runners, swimmers, triathletes) who are considering keto diets. If you’re exercising casually for fitness or health, this probably doesn’t apply to you as much. Athletes who use keto diets for weight management should know about this performance cost. Coaches and sports nutritionists should be aware of this finding when advising athletes. People with diabetes or other medical conditions should consult their doctor before making any diet changes.

The performance loss from a 5-day keto diet appears to happen relatively quickly—within the first 5 days. If you stop the keto diet and return to eating carbohydrates normally, your performance should recover within a few days to a week as your muscles rebuild their carbohydrate fuel stores. However, this study didn’t specifically test recovery time, so this is an estimate based on general sports science knowledge.

Want to Apply This Research?

  • If you’re an endurance athlete considering a keto diet, track your actual performance metrics: time to complete a standard distance, average power output (if you have a power meter), or perceived effort level. Record these weekly and compare them to your baseline performance before starting any diet change. This gives you concrete data about whether a diet change is helping or hurting your training.
  • Instead of trying mouth-rinsing tricks during a keto diet, focus on timing your carbohydrate intake strategically. Eat carbohydrates before and after your hardest training sessions, even if you’re generally following a lower-carb approach. This ’targeted keto’ approach might give you better performance than strict keto while still providing some of keto’s potential benefits.
  • Set up a simple tracking system that logs: (1) your diet type each day (normal, keto, or mixed), (2) your workout performance metrics, and (3) how you felt during exercise (energy level, fatigue, ability to maintain pace). Over 4-6 weeks, you’ll see patterns showing how different eating approaches affect your actual performance. This personal data is often more useful than general research findings because it’s specific to your body.

This research discusses how diet affects athletic performance in trained cyclists. It is not medical advice. If you have diabetes, heart disease, or any other medical condition, consult your doctor or registered dietitian before making significant dietary changes, especially restrictive diets like keto. This study involved only 8 trained cyclists, so results may not apply to all people or all types of exercise. Always consult with a sports medicine doctor or certified sports nutritionist before making major diet changes related to your training or competition. Individual results vary significantly based on genetics, training status, and other factors.