Low Energy During Workouts on Keto

Quick answer: It depends on what kind of tired. If a walk, a jog or a light gym session leaves you flattened, the likely cause is low sodium and potassium, not missing carbs. If it's the last sprint, the heavy set or the third round that falls apart, that's the kind of work keto is weakest at, and a small, timed carb dose is a reasonable experiment. And if you're in your first few weeks, some flatness is just your body still learning to run on fat.

On this page

• Two kinds of tired
• If light exercise wipes you out: check electrolytes first
• If hard efforts fall flat: the carb-dose experiment
• If you're in month one: give it time
• What the research says
• KetoDick's take
• When to involve your doctor
• Key takeaways
• Related

Two kinds of tired

Your muscles have two broad fuel routes. Steady, easy work (walking, gardening, an easy bike ride) runs mostly on fat, and on keto you're getting better at that every week. Short, all-out work (sprints, heavy lifts, a hard round on the bag) leans on stored carbohydrate, called glycogen, because it can be burned fast.

Keto drains glycogen. That's the point of it, and it's also why the all-out stuff can feel like it's missing a gear.

So the first question is which one you're feeling. "I can't get through my 30-minute walk" and "my last two sprints are gone" are different problems with different fixes.

If light exercise wipes you out: check electrolytes first

KetoDick is blunt about this one. Explosive sports aren't keto's strength, he says, but if you're tired from walking or doing chores, it's probably electrolytes (video -dHjt_Bpruk).

The mechanism he gives: once processed food and starches leave your plate, most of the salt in your diet leaves with them. In ketosis your body also holds on to less sodium, and more water passing through flushes more out (video -dHjt_Bpruk). You lose salt from two directions at once, then you go sweat on top of it.

So what you do is boring and cheap:

1. Add sodium on purpose. His own routine is a chicken bouillon cube in a cup of hot water, three times a day (video -dHjt_Bpruk). In his basics video he suggests a cup of salted broth or bouillon twice a day (video UygKGoP_BtA).
2. Add potassium the gentle way. He stirs about a quarter teaspoon of light salt (potassium chloride) into each cup, which he works out at roughly 640 mg of potassium a cup (video -dHjt_Bpruk).
3. Don't count on sports drinks. He points out that sports drinks and flavor drops carry far fewer electrolytes than people assume; one glass a day doesn't cover it (video -dHjt_Bpruk).
4. Give it a morning. He says electrolytes act fast: a drink in the morning should help before lunch (video -dHjt_Bpruk). That makes it an easy test. Have broth before your walk and see.

Back-of-envelope, using his numbers: three cups a day comes out to about 3,300 mg of sodium and 1,800 mg of potassium (video -dHjt_Bpruk). That's a real dose, not a sprinkle, which is the point.

If hard efforts fall flat: the carb-dose experiment

First, ask whether it matters to you. If you're doing keto to lose weight and your exercise is walking, jogging, the elliptical or light weights, he says you don't need workout carbs at all (video XhEJ9x5sP0g). He did an hour-plus of kickboxing on keto without them (video XhEJ9x5sP0g).

Where he would experiment is fully anaerobic, competitive work: CrossFit competition, HIIT, sprinting, wrestling, heavy lifting, team-sport camps (video XhEJ9x5sP0g). For those, here's the shape of what he describes.

Two rules go with it. Start low, near 10 g, and only go up if it helps (video XhEJ9x5sP0g). And try it in training, never for the first time on game day (video XhEJ9x5sP0g).

Notice how small those numbers are. This isn't "carb loading"; it's a targeted dose for one session.

If you're in month one: give it time

Your body needs a few weeks to get good at fat. KetoDick puts adaptation at about 2 to 4 weeks for thinner people and 4 to 8 for heavier people (video NUJNFG6lLWw), and elsewhere says fat adaptation needs at least 2 to 3 weeks for liver enzymes and mitochondria (video cTGYIFOTPl8).

In that window, flat workouts are normal. It's also the worst time to cut calories on top of cutting carbs; he says doing that in month one will leave people fatigued (video NUJNFG6lLWw). Eat to full, salt your food, and keep the workouts easy until the fog lifts. See fat adaptation.

What the research says

The evidence pages behind this site don't include a trial of exercise performance on keto, so here's what they do support:

• Losing sodium early is real physiology. Insulin reduces how much sodium, potassium and phosphate your kidneys let go of; an insulin infusion cut urinary sodium and potassium excretion in healthy men (DeFronzo et al., 1975). Low-carb eating lowers insulin, which runs that in reverse. No highly cited trial directly measures this under a ketogenic diet in adults; the mechanism evidence comes from fasting and insulin studies.
• The early-weeks slump is common and usually passes. An analysis of online posts found reported "keto flu" symptoms clustered in the first weeks and were mostly described as transient (Bostock et al., 2020).
• The body does shift fuels. After weeks of fasting, ketones replaced glucose as the brain's main fuel (Owen et al., 1967). That's the adaptation you're waiting on.
• Hard training in a deficit needs protein. Young men on a roughly 40% energy deficit with intense training gained lean mass and lost more fat on 2.4 g/kg/day of protein than on 1.2 (Longland et al., 2016). If you're training hard and eating less, don't let protein slide.

The specific carb-timing numbers above are his training experiments, not figures from a trial.

KetoDick's take

He trains MMA and kickboxing on keto and is honest that he gasses out sooner; he says he'd add carbs in a fight camp (video cTGYIFOTPl8). He also keeps the priority straight: for weight loss, weight is mostly "made in the kitchen" (from the video), so get shopping, meal prep and carbs right first, then add exercise (video f119qGyJ7AY).

Outside fight camp, he's fine with that trade: he's training to keep his skills up, so when he gasses out in class he works tired, using footwork and range (video cTGYIFOTPl8). If you're not competing, a slightly flatter sprint may be a fair trade for you too.

When to involve your doctor

Talk to your doctor before you add potassium or magnesium supplements or make big sodium changes, which KetoDick says too (video lbRPlGZ3Rvw). That goes double if you have kidney disease or take blood pressure medication, since both change how your body handles potassium and sodium. If you're on diabetes medication, check before adding workout carbs or cutting them, because your doses may need adjusting. And dizziness, chest pain or palpitations during exercise are a doctor question, not a forum question.

Key takeaways

• Tired on a walk: think sodium and potassium before carbs. Broth before exercise is a cheap one-morning test.
• Flat on sprints and heavy sets: that's keto's weak spot. Try 10 to 20 g of carbs around the session, in training first.
• Weight-loss keto with light exercise doesn't need workout carbs.
• Month one is an adaptation period. Keep workouts easy and don't cut calories yet.
• Keep protein up if you're training hard in a deficit.

Related

• Guide: Workout carbs · Exercise on keto · Electrolytes · Sodium
• Blog: Why am I so tired on keto? · Keto for MMA and combat sports · Can you build muscle on keto? · Broth for keto flu · Why exercise still matters
• Videos: Electrolytes are absolutely critical · Dosing carbs for hard workouts · Do you need exercise?