Trying out the keto diet for the first ? I know exactly how you feel.
My first keto attempt sucked.
I had the dreaded “keto flu,” this weird metal taste in my mouth, and zero social game at group runs. Imagine finishing a long run in the heat and turning down banana pancakes and a beer for… bacon and water.
Awkward.
But once I broke through that fog? It was like flipping a switch.
My energy evened out. My long runs felt smoother. I stopped bonking at mile 10. It didn’t happen overnight, but the shift was real — and I started to feel like my engine was running on rocket fuel made from coconuts.
If you’re a runner thinking about going keto, you’ve probably asked yourself:
- What does ketosis actually feel like?
- How do I know if I’m in it?
- What’s normal, and what’s just plain weird?
That’s exactly what this guide is about. I’ll walk you through the signs and symptoms of ketosis — the good, the bad, and the “why do I suddenly smell like nail polish?” stuff. I’ll keep it honest, share what I’ve lived through, and throw in tips to help you get through the rough patches.
Let’s get to it.
What Is Ketosis?
Alright, before we break down the symptoms, let’s clear up the basics.
Ketosis (say it like “key-tow-sis”) is just your body flipping the fuel switch.
Normally, your brain and muscles run on carbs — think rice, bread, pasta, sugar. But when you cut carbs way down, your body gets hungry for fuel and starts breaking down fat instead.
That fat turns into ketones — and those ketones become your new fuel source.
In short? Ketosis = running on fat, not sugar.
Technically, you’re in nutritional ketosis when your blood ketone levels hit about 0.5 mmol/L or higher.
But let’s be real — most runners don’t have a ketone meter in their sock drawer. And the good news? Your body gives you plenty of clues (we’ll cover them all).
Important: This isn’t the same as diabetic ketoacidosis — that’s a serious medical condition. Nutritional ketosis is safe for healthy people and happens naturally when we fast or eat super low-carb.
In fact, it’s been around since humans were chasing antelope barefoot and going days between meals.
Keto Diet: The Short Version
So, how do you get into ketosis?
Simple: eat a ton of fat, moderate protein, and almost no carbs. That usually breaks down to something like:
- 70–80% fat
- 15–25% protein
- 5–10% carbs
In real-life food terms? That means saying goodbye to bread, rice, fruit juice, pasta, and pretty much anything that lives in the snack aisle. You’ll be eating things like avocado, steak, eggs, nuts, olive oil, and spinach.
For most runners, this means keeping carbs under 30 grams a day — which is roughly one banana or a slice or two of bread. Yeah, it sounds brutal at first. And trust me, your body will complain for a few days.
Here’s a list of what to eat on the keto diet.
How long does it take to get into ketosis?
In my case, I started seeing signs around day 3. Stronger symptoms kicked in after about a week. Research backs this: most people enter ketosis within 2–7 days of seriously cutting carbs.
But adapting — like, really teaching your body to perform on fat — takes longer. Most experts say it takes 4 to 6 weeks to become truly fat-adapted. Some athletes need up to 8–12 weeks to see real endurance benefits.
I’d compare it to building aerobic base. Getting into ketosis is like jogging a 5K — quick. Fat-adaptation is like marathon training — it takes time, consistency, and patience.
Why the Heck Would a Runner Go Keto?
Great question.
For me, there were three big reasons:
- Steady Energy. I was tired of hitting the wall in long runs. I’d run out of glycogen and feel like my engine died. With keto, I knew I’d be tapping into fat — and fat stores are pretty much endless, even for lean runners.
- Mental Clarity. I’d read about how ketosis helps some folks feel sharper, calmer, more focused. As someone juggling coaching, training, and writing, that sounded like a win.
- Weight Loss Curiosity. Yep, I was curious. Lots of runners drop weight on keto, mostly from losing water and eating fewer calories overall. One Reddit guy said he lost 7 pounds in a month and shaved nearly a minute per mile off his pace. That’s not nothing. (Just remember: a lot of early weight loss is water, not fat.)
But I won’t sugarcoat it — the transition was rough. I had no kick. My intervals felt like I was running in sand. And socially? It was weird turning down beer and bananas after a long Sunday run.
Still, I kept at it. Tracked my runs. Watched the symptoms. Adjusted. And eventually, my body started firing on all cylinders.
How to Tell If You’re in Ketosis (Without a Blood Test)
So, you’re wondering if you’ve actually hit ketosis — without needing a lab coat or pricking your finger? I got you.
Here are the clearest signs I’ve noticed (both in myself and in the folks I’ve coached) when your body finally flips that metabolic switch.
Quick Signs You’re in Ketosis:
- Keto Flu: Like catching the flu without the germs. Headache, crankiness, and low energy in the early days.
- Keto Breath: Fruity, sometimes like nail polish remover. Not cute, but a solid sign.
- Short-Term Fatigue: You’ll probably feel weak in the gym or on your run the first week or two.
- Digestive Drama: Constipation or sudden trips to the bathroom as your gut adjusts to the fat load.
- Appetite Drop: You might start forgetting to snack. Ketosis can naturally quiet your hunger.
- Stable Energy & Focus: Once you’re past the misery, your brain lights up. No sugar crashes.
- Testing Confirms It: Ketone strips or breath testers can give you the hard proof.
Each one of these is like your body tapping you on the shoulder saying, “Hey, we’re switching gears here.”
Let’s unpack the big one first…
1. The “Keto Flu” (aka Your Carb Withdrawal Hangover)
This one’s a beast. Most people — myself included — get slammed with the keto flu sometime in the first week (usually between days 2 and 7). I like to think of it as your body’s version of a sugar tantrum. You’ve been feeding it carbs your whole life, and now suddenly you’ve yanked away its favorite treat. It freaks out.
How it feels (from my own foggy nightmare):
Day three hit me like a truck. My head throbbed. I was dizzy, confused, snapping at everything, and dragging myself up the stairs like I was 90. One moment, I was trying to journal — the next, I forgot why I even opened the notebook.
Legit brain fog.
My throat felt scratchy like I was about to get sick. I even had night sweats. I wasn’t alone — a friend told me that his first keto run felt like “the hardest 10K of my life” with full-body cramps and a splitting headache.
Headaches. Weakness. Grumpiness. Nausea. Sleep troubles. It’s all part of the package.
Why it happens:
Your body’s dropping insulin fast, which tells your kidneys to flush out water and electrolytes. Suddenly you’re low on sodium, potassium, magnesium — the trio that keeps you from feeling like a dried-up raisin.
At the same time, your brain hasn’t figured out ketones yet, so it’s running low on fuel. Hence the fog and fatigue. It’s basically a system reboot.
Here’s how to survive it:
Rehydrate Like It’s Your Job
Drink water like it’s race day, and salt your food a little extra. I kept a shaker of Himalayan salt nearby and even added a pinch to water. Broth was my go-to. Aim for:
- Sodium: 3,000–4,000 mg/day
- Potassium: Around 1,000 mg/day
- Magnesium: Roughly 300 mg/day
Replenishing these helped kill my headaches and leg cramps.
2. Don’t Starve Yourself (Yet)
Now is not the time to slash calories. If you’re hungry, eat. Fat is your friend here — almond butter, cheese, eggs. Your body needs to see the new fuel to use it.
3. Ease Up on the Training
As a coach, I rarely tell people to scale back — but during keto week one, you’ll thank yourself. I swapped my runs for brisk walks and kept heart rate low. Going hard too soon will just make the flu worse. Even Healthline recommends easing off the intensity until you feel more human again.
4. Taper Off Carbs (If Needed)
If cold-turkey feels brutal, you’re not weak — you’re human. Some folks do better slowly trimming their carb intake over a couple weeks. Fewer symptoms, same end goal.
2. Bad Breath (Yep, “Keto Breath” is Real)
Let’s just call it what it is—keto breath stinks. Literally.
It’s one of those weird little side effects that shows up early when your body flips the fat-burning switch. Your breath starts smelling… off. Some folks say it’s fruity. Others say nail polish remover. My girlfriend? She said I smelled like rotting mangoes. Romantic, right?
That smell comes from acetone—one of the ketones your liver cranks out when you’re in ketosis. It’s the same stuff you’ll find in nail polish remover. Your body doesn’t really use acetone for fuel, so it just dumps it—mostly through your breath and pee.
So yeah, if your breath suddenly smells like a high school chemistry lab, congrats—you’re burning fat.
According to Healthline and research noted in the Journal of Strength & Conditioning Research, this breath change is actually used to measure ketosis in clinical settings. Some breathalyzers can even estimate your ketone levels just by sniffing that sweet, weird air you’re exhaling.
What it feels like socially:
It’s awkward. No way around it.
As a coach, I’m around people a lot—especially on runs. I remember one morning, telling a story mid-run, super animated, and my buddy kept leaning away from me. I knew right then: the keto dragon had escaped my mouth.
Waking up with a dry mouth and that metallic-sweet aftertaste became normal. Water didn’t do much. I’d brush, rinse, chomp gum—but it stuck around. It wasn’t forever, though (thank God).
Here’s what helped me manage it:
- Double down on mouth care. I brushed after every meal. Not just twice a day—every time I ate. I also added a tongue scraper. Trust me, that thing scrapes off more gunk than you think. Sugar-free mints and gum helped too—just make sure they’re actually sugar-free or they might kick you out of ketosis.
- Stay on top of hydration. Dry mouth makes it worse. I kept a water bottle on me all day. The more hydrated I stayed, the better my breath got. Plus, keto can make you dehydrated in general, so it’s a win-win.
- Try natural rinses. I found a drop or two of peppermint oil in water made a solid DIY mouth rinse. Just make sure your mouthwash isn’t full of alcohol or sugar.
- Wait it out. Honestly, the best solution? Time. As my body got better at using ketones, the smell faded. After about a month, it was barely noticeable. Or maybe we just got used to it—hard to say.
3. Short-Term Fatigue & Sluggish Performance
Let’s be real—your energy might tank during the first few weeks of keto. Especially if you’re a runner. And if you’re trying to train for a marathon on keto, be ready to slow the heck down.
This isn’t just regular tired. It’s “why do my legs feel like wet logs?” tired. That 5K that used to feel easy suddenly feels like a half marathon.
The first few weeks of switching to fat for fuel are rough for a lot of us.
Why it happens:
Your body is learning to run on fat. That’s it.
In the beginning, you burn through your stored glycogen fast—and along with it, a ton of water. For every gram of glycogen you lose, you also lose about 3 grams of water. That’s why you drop water weight so fast.
But here’s the kicker: with no glycogen left and your fat-burning engine still in warm-up mode, your muscles are running on fumes. You lose that explosive power, especially for sprints, lifts, or anything high intensity. (Healthline confirms this, by the way.)
Also—electrolytes? Gone with the water loss. That messes with muscle function, too. Cue the cramps, sluggishness, and that heavy-leg feeling.
Here’s what worked for me:
- Ease off and train smart. Don’t expect to hit PRs in week 2. I backed off hard intervals and just focused on easy base runs. Think Zone 2 stuff—where fat can actually fuel you. Skip races, tempo work, and time trials for now. This is your adaptation window.
- Sleep like it’s your job. I aimed for 8–9 hours, plus naps if I needed them. Stretching and low-stress activities helped me feel more human. Stress makes the fatigue worse, so I even threw in some walking meditation to chill out.
- Dial in your electrolytes. Sodium, potassium, magnesium—all crucial. I took magnesium before bed to help with sleep and muscle twitches, and loaded up on potassium-rich foods like avocado and spinach. In week 2, I added an electrolyte powder, and that was a game changer.
- Eat enough. Seriously. Keto doesn’t mean you have to eat less. If anything, you need more fat and protein in the beginning. I bumped up my calories a bit with things like coconut milk, nuts, olive oil—just to make sure I wasn’t under-fueling. Starving yourself during keto adaptation is a one-way ticket to crash town.
What I’ve learned since:
Once your body adapts, the steady energy is awesome. I could knock out 10+ miles fasted with no crash. That never happened on a high-carb diet—I’d bonk hard after 6–7 miles without fuel.
But it’s not all roses. Sprinting? 5K races? I felt slower. That’s where I learned to blend strategies.
On most days, I stick to keto-style eating for endurance. But if I’ve got a race or hard interval session, I’ll throw in a bit of carb pre-workout. It’s called “targeted keto,” and it works. An energy gel just before starting usually does the trick for me.
Read more about the impact of keto on runners here.
Stable Energy Levels and Mental Clarity
Here’s the part of keto that doesn’t get hyped enough: the clean, stable energy and sharper focus you get once you’re over the initial keto flu. It’s not just about weight loss. Once you’re fully adapted and running on fat, your energy feels level all day.
You don’t crash, you don’t get hangry, and you don’t need a caffeine IV to survive the afternoon.
In fact, I’m a big coffee addict but I’d rarely finish my Latte when I’m on keto – as if my body saying “ah we don’t need that”.
The old 11 a.m. and 3 p.m. snack attacks? Gone. My brain was firing clean, and it felt like I was cruising all day.
This isn’t just personal hype either. Research published in Frontiers in Psychology found that ketosis may help fuel the brain more steadily, reducing energy dips and mental fog compared to a high-carb diet. Think of it like switching from a rollercoaster to a bullet train.
How it feels for running:
As a runner, the big win here is no more hitting the wall. When I was training for half marathons while on keto, I could go 10–15 miles without a gel or sports drink. No bonk, just steady energy.
One ultra-runner on Reddit mentioned doing 15-mile fasted runs needing only water and electrolytes. I’m not an ultra guy, but I can back that up for anything under 2 hours. It builds a kind of endurance confidence. You stop worrying about fuel and start trusting your body to just go.
Why it happens:
Once you’re in solid ketosis, blood sugar stays low and stable. You’re not riding that sugar spike and crash cycle anymore. Ketones, the byproduct of fat metabolism, are excellent brain fuel. Some studies even call them a “superfuel” for the brain. They’re being researched for everything from epilepsy to Alzheimer’s.
But for everyday life, they just make you feel more focused and stable.
Mood often gets better, too. For me, I became more chill and patient (my wife noticed it before I did). Once the early keto fog lifted, I found myself sharper, more upbeat, and far less reactive.
How to keep it going:
- Stick with it. Don’t cheat yourself out of this clarity by yo-yoing in and out of ketosis. Save carbs for when they matter, like pre-race or big workouts.
- Stay hydrated + salted. Sometimes you think you’re tired, but you just need salt. I always keep water with a pinch of sea salt handy. If I get sluggish, it’s usually hydration, not hunger.
- Caffeine can be a rocket booster. I love a cup of coffee in keto mode. No crashes, just smooth focus. Bulletproof coffee is still in my rotation on big writing or coaching days.
- Use carbs smart. If I’ve got a hard interval session, I’ll have a banana 30 minutes before. Doesn’t kick me out of ketosis long-term but gives that extra kick when needed.
Digestive Changes
Alright, let’s talk gut. Keto can mess with your digestion at first. It’s common. Some folks get backed up. Others are sprinting to the toilet. It’s just your gut adjusting to a radical shift in how you eat.
When I started keto, I had a rough Week 2. My usual morning runner’s ritual? Gone. I felt bloated, sluggish, and kind of off. It wasn’t fun. But I knew it was part of the transition.
One of my buddies had the opposite problem – high-fat meals would send him running to the bathroom within 30 minutes. The gut needs time to rewire itself.
Why it happens:
- Fiber drop: You ditch bread, grains, beans – and with them goes a lot of fiber. If you don’t replace that with low-carb veggies or chia seeds, things slow down.
- Dehydration: Keto flushes water out fast. Less water = slower stool movement.
- Fat overload: Your body isn’t used to high fat meals, so things can either get sluggish or move too fast.
- Gut bacteria shift: Your gut bugs change based on what you eat. Fewer carbs = new bacterial balance. That shift takes time.
How to handle it:
- Get your fiber in. Load up on leafy greens, avocados, chia seeds, flax, zucchini, etc. I started adding ground flax to smoothies and eating big salads daily.
- Hydrate like a boss. Half your body weight in ounces of water – minimum. More if you live somewhere hot like Bali or sweat a lot (me = both).
- Watch dairy and fake sweets. Some folks get clogged from cheese. Others get the runs from sugar alcohols in keto snacks. For me, packaged keto treats were a gut bomb, so I ditched them early.
- Add magnesium. I take magnesium glycinate nightly. It helps with sleep and keeps me regular.
After a few weeks, things leveled out. I wasn’t as frequent as I was on a grain-heavy diet, but I found my rhythm. Don’t freak out if things change – your body is learning a new routine.
And if your gut feels fine? That’s great too. Everyone reacts differently. The key is to listen and adjust.
6. Reduced Appetite
Once ketosis kicks in, your hunger dial shifts. A lot of people report that their cravings ease up, and that constant urge to snack? It fades.
This isn’t magic—it’s one of the keto diet’s biggest perks when it comes to fat loss. You’re still eating tasty meals, but the random “gotta raid the pantry” moments start to disappear.
I’ll be honest—I didn’t buy it at first. I thought, “No way I’ll stop being hungry all the time.” Then I lived it. And yeah, I had to eat my words… and fewer snacks.
How it feels (my take):
Before going keto, I was the guy who was always hungry. I’d eat a full breakfast, go for a run, and by 10 a.m., I was already thinking about my next snack.
On long-run days? Total fridge bandit. I was shoveling down food just to stay ahead of the hunger beast.
But something shifted a couple weeks into keto. I started skipping lunch by accident—not because I was trying to fast, but because I legit forgot to eat.
I’d have a big breakfast—eggs, cheese, avocado—and then suddenly it’d be 3 p.m. with zero cravings, no hanger, no brain fog. That was wild for me.
Even during marathon training, I could stick to two or three solid meals and feel fine. Smaller portions naturally felt “enough.” That steady energy—no crashes—was what hooked me.
Let me explain why does this happen so you won’t freak out:
- Hormones shift gears. Research shows keto lowers ghrelin, the hormone that screams “FEED ME.” Normally, when you diet or lose weight, ghrelin shoots up and makes you ravenous. But in ketosis? Ghrelin chills out. At the same time, hormones like CCK (the “you’re full” signal) go up. Even leptin sensitivity may improve. One study even tied high ketone levels (specifically BHB) to lower hunger and higher satiety peptides [MDPI].
- Blood sugar stays steady. Without the carb rollercoaster, you avoid those big crashes that make you want to devour a whole pizza. Think about it—how many times have you eaten a pile of pancakes only to feel starving again in two hours?
- More protein, more fullness. Keto isn’t high protein, but it’s not low either. Most folks end up eating enough meat, eggs, and dairy to benefit from protein’s filling power. It slows digestion and signals your brain that you’re good. Honestly, bacon and eggs at 8 a.m. kept me fuller than any cereal or granola bar ever did.
- You stop fighting your food. When you stop counting every calorie and just eat to satisfaction, your body starts playing along. With fat and protein doing their job, most people fall into a mild calorie deficit without even trying. I didn’t obsess over numbers. I just ate real meals, and over time, the fat came off naturally.
Here how to manage it:
- Listen to your body, but don’t under-eat. If you’re never hungry, great. But don’t push your intake so low that you’re undernourished—especially if you’re training. I had days where I only ate twice, but I made sure those two meals were loaded with veggies, fats, and protein. Just skipping food isn’t the goal—fueling smart is.
- Match food to your training. I noticed that after big workouts—especially long runs—my hunger kicked up a notch. So I planned for that. If I wasn’t super hungry right after, I still made sure to get in something small—like a protein shake or cheese and nuts—within an hour. That helped recovery. Then later, when hunger snuck back in, I was ready for a real meal.
- Use the freedom to ditch the junk. One of my favorite things about reduced appetite on keto? I wasn’t pulled toward crap food. I stopped needing nightly snacks and didn’t miss them. That made space for better choices—whole foods, different protein sources, and way more veggies. Just don’t fall into the trap of eating the same thing daily. Mix it up and keep those nutrients coming.
- Fasting, if it fits. A lot of keto folks slide into intermittent fasting without even trying. I sometimes did a 16:8 schedule (skipping breakfast), and it felt natural. But don’t force it—especially if you’re training hard. I often run fasted in the mornings, but I always eat a solid brunch afterward. Find what feels right. Keto gives you flexibility—that’s a tool, not a rule.
7. Testing for Ketones: When You Want Cold, Hard Proof
Let’s be real—sometimes, you want more than just a “feeling” to know you’re in ketosis. You want proof. That’s where testing comes in.
You’ve got three main ways to check:
- Blood meters: Measures beta-hydroxybutyrate (BHB). This is the big one—your cells actually use it for fuel.
- Breath analyzers: Detects acetone—yep, the same stuff behind keto breath.
- Urine strips: Looks for acetoacetate, the ketones your body dumps early on when it hasn’t figured out how to use them well yet.
If you’re in ketosis, these numbers will spike compared to a regular carb-heavy state. For example, blood BHB levels from 0.5 to 3.0 mmol/L mean you’re in nutritional ketosis, according to the Journal of Strength & Conditioning Research and other solid sources.
How It Feels
You won’t “feel” the test itself (unless we’re talking a finger prick). The real hit is mental.
The first time I used a blood meter was about a week into my third keto attempt. I poked my finger, dropped the blood on the strip, and boom—1.2 mmol/L. I was pumped. It was like seeing a gold star on a test you didn’t study for. I even did a happy little dance in my kitchen.
Urine strips were my entry point. I saw dark purple on day three and thought, “Okay, this is working.” But here’s the catch: after a few weeks, those strips didn’t show much—even though I felt more dialed in than ever.
That’s because your body gets better at using ketones, so less spills out in pee. If you’re sticking with keto long-term, blood testing is your best bet for accuracy.
Why It’s Worth Doing
Testing ketones isn’t mandatory, but it helps in a few solid ways:
- Reassurance: Not sure if that brain fog is from keto or just a bad night’s sleep? A quick test can clear things up.
- Learn Your Threshold: I found out I drop out of ketosis around 45g of carbs, but I’ve coached runners who stay in even at 60g. Everyone’s different.
- Understand What Affects You: Tough workouts bumped my ketones up. Stress and poor sleep? They pulled them down.
- Avoid Slip-Ups: When you know you’re testing tomorrow, that late-night cookie feels less worth it.
- If You’re Going Deep: For medical uses (like epilepsy), numbers matter more. But for runners or fat-loss folks, any ketosis is good enough.
Pro tip? Don’t obsess. I used to chase high numbers (like 3–4 mmol) thinking more was better—but that just led me to overeat fat and cut back protein. Bad call. Now I shoot for 1.0–1.5 mmol and feel amazing.
The Tools & How to Use Them
1. Blood Meters
These are like glucose meters. Finger prick, drop of blood, done. Brands like Keto-Mojo, Precision Xtra, and Nova Max are legit. You’ll get a clear mmol/L reading. Most folks in ketosis fall between 0.5 and 3.0. I usually landed around 1.2 to 1.5.
Downsides? Strips cost $1–2 each, and pricking your finger isn’t fun. I tested 2–3 times a week, tops. If you’re a data geek, you might go daily—fasted mornings or post-meal checks can show patterns.
2. Breath Meters
These pick up acetone, another ketone byproduct. You just breathe into the device—some show a color (like Ketonix), others give numbers (like Biosense). They’re decent, not perfect.
Studies back them up, but your breathing style affects the results.
I used one early on. Watching it shift from blue to red as I adapted was a cool motivator. Bonus: no recurring costs after buying the device.
3. Urine Strips
The cheapest, easiest intro tool. Pee on the strip, check the color. The darker it goes, the more ketones are in your system.
During week one, they’re super helpful. I remember hitting pink on day 3 and feeling like I won the keto lottery.
But they fade fast. After a few weeks, the readings drop even if you’re still in ketosis. That’s your body getting better at using ketones. Hydration also messes with the results.
Still, for beginners, they’re a great “you’re on the right track” nudge.
Do You Need to Test?
Nope.
If you’re feeling the signs—less hunger, clear focus, steady energy, maybe some keto breath—you’re probably in ketosis. Testing is optional. But if you’re a numbers person or tweaking things for performance, it’s a helpful tool.
Here’s how I usually coach it:
- Short-term keto? Grab some urine strips and call it a day.
- Going long-term or doing cyclical/targeted keto? Consider a blood meter.
- Just want peace of mind? Spot-checking once or twice a week works fine.
Just don’t let the numbers boss you around. This isn’t school—you’re not getting graded. Ketosis is a tool, not a scoreboard.
Final Thoughts
When you see those ketones show up on the meter, know this: your body is running on a different kind of fuel now.
That’s powerful. You’re burning fat. You’ve changed gears.
I still get a little spark of excitement seeing a 1.2 or 1.5 reading.
But don’t chase the number just to feel successful. The real win is feeling stronger, clearer, and more in control. That’s the stuff that sticks.
