Switching to a high-fat, low-carb lifestyle isn’t all abs and glory. There are real perks, no doubt—but also some serious trade-offs. Especially for us runners. So let’s break it down from both sides—what keto gave me, what it took away, and how to know if it’s worth it for your running goals.
Why I Gave Keto a Shot—and What It Actually Did for Me
1. Burning Fat Like a Furnace
Once I dropped my carb intake low enough, my body had no choice—it started eating into fat stores for energy. And it worked. I lost 11 pounds in about 6 weeks, and I wasn’t starving myself or skipping meals. Just swapped out the rice, pasta, and sugar bombs for meat, eggs, nuts, and green stuff.
There’s a Reddit guy who said he lost 16 lbs in a month—10 of them just fat. That tracks. When you’re carrying less weight, especially fat, running gets easier. Your joints take less of a beating. And honestly? You feel lighter on your feet—mentally and physically.
2. Endless Energy on Long Runs
This was the big win for me. Once my body got used to burning fat, my long runs felt steady—no more mid-run energy crashes or that sudden bonk that hits like a truck. I wasn’t relying on gels or sugary drinks anymore.
One fat-adapted runner said, “I’m slower on keto, but I never bonk.” That’s exactly it. You don’t run out of fuel because your body has a backup generator: body fat. That’s a game-changer in marathons or ultras. You stop worrying about stuffing your face mid-race and focus on the run.
3. No More Constant Snacking
Before keto, I felt like I was always one skipped snack away from a meltdown. After switching? A breakfast of eggs, cheese, and avocado kept me full for hours. No crashes. No zombie walk to the fridge every two hours.
Mentally, it was a relief too. Food didn’t control my day. I wasn’t obsessing over the next bite. That freedom is hard to explain unless you’ve been there.
4. My Brain Was on Fire (in a Good Way)
This one surprised me. After a week or two, I noticed my mental focus getting sharper. Writing training plans, reviewing footage—I was locked in. No brain fog. Studies suggest ketones might be a cleaner fuel for the brain. I believe it.
Someone on Reddit said they didn’t even go keto for weight loss—they did it for mental health and gut issues—and just felt better across the board. I felt that too. Moods were steadier. No afternoon crashes.
5. Lean Body Without Muscle Loss
I wasn’t lifting much during my keto stretch (dealing with a foot issue), but I didn’t lose muscle. My legs stayed solid. My calves actually looked more defined once the fat layer thinned out.
As long as you’re getting enough protein, research shows you can maintain muscle just fine on keto. That’s key for runners—less fat, same strength = better power-to-weight ratio.
6. Fuel Flexibility Is a Superpower
After I reintroduced carbs later, I noticed something cool: I could run fasted with no issues or crush a hard workout after a bowl of oats. My body had learned to handle both fuels. That’s called metabolic flexibility.
Some ultra runners do this on purpose—train low-carb to build fat-burning skills, then throw in carbs on race day for a turbo boost. “Train low, race high.” I didn’t invent that, but I’ve felt the power of it.
7. Way Fewer Gut Problems
Let’s be real—mid-run bathroom emergencies are every runner’s nightmare. On keto, mine vanished. No gels, no sugary pre-run meals. Just steady digestion and calm guts. Maybe it was the lack of fiber overload or the diuretic effect of ketosis. Either way, it worked.
I’m not promising it’ll fix everyone’s runner’s trots, but for me, the difference was night and day.
But It Wasn’t All Smooth Running: The Downsides of Keto
1. The Dreaded Keto Flu
The first couple weeks sucked. I’m talking headaches, crankiness, and dead legs. My easy runs felt like death marches.
That’s the infamous “keto flu”—your body losing its glycogen stores, shedding water like crazy, and scrambling to figure out how to burn fat. You lose electrolytes fast. That’s what causes those pounding headaches and zombie runs.
One Reddit runner said it took them 6 to 8 weeks to fully adapt. That’s a long time to feel like garbage. Most people quit here. But if you hydrate aggressively, up your salt, and keep your training light, it does get better. I felt more human again by week three or four.
2. Speed Took a Hit
Endurance held up, but anything high-intensity? Brutal. My intervals were slower, and they hurt more. Without glycogen, your top-end speed suffers. No sugar = no turbo.
Even after a month, I still couldn’t hit my usual 400m paces. Some people never fully bounce back on strict keto. A Reddit post I read said, “My running performance tanked on keto.” And they weren’t wrong.
That’s why I don’t recommend keto during peak training or race season. Save it for base-building or recovery phases. Or tweak it—use carbs before your hard sessions (“targeted keto”) so you’re not running on fumes.
3. Muscle Loss Is a Risk
Go too low on protein or calories and your body might start eating muscle. I kept my protein high (about 0.8 grams per pound of body weight) and did some bodyweight strength stuff to keep my muscles from checking out.
But I’ve seen others lose muscle on keto—especially if they’re too aggressive with cutting calories. One Reddit user said they lost 6 pounds of muscle in a month. That’s a lot. So yeah—lift something, eat enough protein, and don’t treat keto like a starvation diet.
4. Social Life Takes a Hit
Let’s not pretend keto is easy socially. Pizza night? Out. Beer? Nope. Brunch with pancakes and fruit? Forget it.
In Bali, I was lucky—lots of healthy cafes that cater to keto or low-carb eaters. But even there, I got tired of being “the guy asking for a burger with no bun and a side of avocado.” It’s doable, but it takes planning—and sometimes it just feels lonely.
I missed cereal. I missed bananas. I missed spontaneity. Keto can feel like a full-time job when life gets busy.
5. Missing Nutrients, Weird Side Effects
Cut out whole food groups and things can get wonky. Less fiber = potential constipation. Less fruit = lower potassium. Less variety = higher boredom risk.
I dealt with it by eating lots of leafy greens, nuts, and seeds, plus magnesium and salt supplements. Still, I had to stay on top of it.
Also… keto breath is real. That metallic smell from ketone production isn’t pleasant. I carried mints when coaching face-to-face. Clients never said anything—but I wasn’t taking chances 😅.
Final Take
Keto isn’t magic—but it is a tool. For me, it worked like a focused training block. I leaned out, learned a lot about fueling, and built a fat-burning engine that still helps today.
But it’s not a forever diet, and it’s definitely not ideal for peak performance phases. I wouldn’t recommend trying it right before a race or during heavy interval training. Too risky.
Treat it like you’d treat hill training or tempo blocks—use it with intention. Know what you’re getting into. And don’t be afraid to adjust or step back when it stops serving you.