Why Low Carb Hurts Your Gut Health
Hey runners- team dietitian Erika here! Let's chat about something often overlooked but crucial - your gut health during those long endurance efforts. If you've ever wondered whether a keto diet could be the answer for your next race, this new research might have you thinking twice.
Your Gut Has a Say in How You Fuel, And It's Worth Listening To
If you've ever wondered whether going keto might help you power through a long race without needing to constantly refuel, you're not alone. It's a tempting idea, your body has way more fat stored than carbs, so why not just teach it to burn fat instead? A group of researchers put this to the test with elite race walkers, and what they found is something every endurance athlete should know about.
What they did:
Twenty-eight elite male race walkers completed a 25-km race walk after eating a high-carb diet for 6 days (the baseline). Then they were split into three groups for another 6 days before repeating the same walk:
One group stayed on the high-carb diet (the control)
One group switched to a ketogenic low-carb, high-fat (LCHF) diet (less than 50 g of carbs per day — that's roughly one banana and a slice of bread)
One group ate a low-energy diet (same carb percentage, but significantly fewer total calories)
Before and after each walk, the researchers drew blood to check for signs of gut damage and tracked any stomach issues the athletes reported.
What they found and why it matters for you:
The keto group showed clear signs of increased gut stress after their second race walk. Specifically, a marker of intestinal cell injury (called I-FABP) doubled compared to their baseline walk. Two other markers that indicate bacteria or bacterial toxins leaking from the gut into the bloodstream also jumped significantly — by 53% and 36%. None of these changes showed up in the high-carb group or the low-calorie group.
In plain terms: just 6 days on a keto diet appeared to make the gut lining more vulnerable to the stress of hard endurance exercise. That's the kind of thing that can set the stage for cramping, nausea, or worse during a race, even if it didn't cause major symptoms in this particular study.
The keto diet group didn’t just show signs of gut stress; they also had slower times, higher heart rates, and found the race more strenuous despite covering the same distance. Yikes! So not only was the gut taking a hit, but performance suffered too.
What about cutting calories?
The low-energy group, who were eating far fewer calories but still getting adequate carbs, showed no increase in gut damage markers at all. This suggests that having enough carbs on board matters more for gut protection than simply having enough total calories, at least in the short term.
The bottom line:
This study involved elite athletes in a tightly controlled environment, so the results are strong even though the group sizes were small. For recreational athletes tackling long runs, rides, or races, the takeaway is straightforward:
Carbs aren't just fuel for your muscles, they help protect your gut during hard efforts. Restricting them dramatically before or during endurance events may increase your risk of gut problems.
If you're curious about lower-carb eating for other reasons, that's a conversation worth having, but race week is not the time to experiment with keto.
Fueling with carbs before and during your event is one of the simplest, most evidence-backed things you can do to feel better and perform better on race day.
Your gut works hard when you're out there pushing limits. Let’s fuel it right to keep you in the race and feeling great. If you're considering dietary changes or have questions about optimal fueling strategies, reach out! We're here to support every step of your journey!