Running on Keto at Ultra Distance: When It Works, When It Doesn't

I'm not a keto advocate. I'm also not a keto skeptic. The honest answer on low-carb diets for ultra-distance is: it depends heavily on the race, the runner, and the intensity, and the research is more nuanced than either camp's talking points.

What keto actually does physiologically

On a normal endurance diet (5-8 g carbs/kg/day), runners oxidise a mix of fat and carbohydrate at any given intensity, with carb share rising sharply as intensity climbs. At marathon pace, most trained runners burn 60-80% of energy from carbohydrate. Glycogen stores (400-600 g total) cover about 2 hours at that intensity; past that, you bonk unless you're taking in gels.

On a ketogenic diet (under 50 g carbs/day for weeks), the body adapts. Glycogen stores are smaller (200-300 g). But fat oxidation rates at submaximal intensity climb dramatically — from ~0.5 g/min in carb-fed athletes to ~1.5 g/min in keto-adapted ones (Volek et al. 2016, the FASTER study). In theory, at true aerobic pace, a keto-adapted runner can metabolise fat at a rate that sustains effort indefinitely, without the sugar-and-gut failure modes of carb-based ultras.

In practice, it's more complicated.

Where keto actually works

1. Very long, very slow races

100-milers at 14:00-16:00 mile pace. 200-milers. Multi-day adventure racing. When effort is clearly below aerobic threshold for the whole event, fat oxidation can supply energy. Runners report fewer GI issues, more stable energy, less sugar fatigue. Zach Bitter's 100-mile treadmill world record (2019) was run largely on keto principles, though he added carbs late.

2. Runners prone to sugar-induced GI issues

Some athletes simply cannot tolerate 60-90 g/h of carbs at ultra intensity. For them, keto-adapted racing sidesteps the problem entirely — they eat 50 kcal of fat plus sips of salt water and just keep moving. Not fast, but they finish.

3. Specific body-composition goals

Low-carb diets often lead to modest weight loss in the adaptation phase. For runners with clear body composition goals (not universal), this can be useful. Separate question from performance.

Where keto fails

1. Any race with intensity

Marathon. Half-marathon. 50K at competitive pace. Anything where you're spending significant time at or above lactate threshold. Fat oxidation tops out around LT1 or LT2 depending on training. Above that, you need carbs. A keto-adapted athlete racing at threshold will hit a performance ceiling that carb-fed athletes don't.

Practical consequence: someone who runs a 3:30 marathon carb-fed will likely run 3:40-3:45 keto-adapted, even after full adaptation. Not a disaster, but not a win either.

2. The adaptation period

The first 6-8 weeks of keto adaptation, training quality drops. Hard sessions feel terrible. Runners frequently report they "can barely jog" during this phase. For someone in a race build, this is catastrophic. Keto should be started 3+ months before any race that matters.

3. Low energy availability risk

Keto is naturally satiating, which means many runners under-eat on it. During high-volume training, a chronic calorie deficit develops, and low energy availability (LEA) follows. This is a particular risk for female ultra-runners. Missed periods, stress fractures, and chronic fatigue are signals to stop immediately.

4. Social and practical difficulty

70 mpw of training plus a ketogenic diet means eating 2,800+ kcal/day with under 50 g of carbs, which means a lot of butter, oil, cheese, meat, and eggs. Travel, group meals, and normal restaurants become hard. Most runners who try strict keto long-term drop off within 6 months.

The FASTER study, honestly

Volek et al. 2016 tested 10 keto-adapted vs 10 carb-fed elite ultra-runners. Key findings:

• Keto-adapted runners oxidised fat at peak rates of ~1.5 g/min, vs ~0.5 g/min for carb-fed
• At 64% VO2max (roughly a comfortable ultra pace), both groups maintained similar performance on a 180-min treadmill test
• Keto-adapted runners had higher muscle glycogen than expected (roughly 80% of carb-fed values)
• Carb oxidation was higher in carb-fed athletes at any given intensity

Careful read of the paper: keto-adapted runners matched, didn't beat, carb-fed runners at submaximal intensity. At higher intensity, keto-adapted athletes would almost certainly lose. The study is often cited as "keto is better" — it isn't what the data show.

The middle path: "train low, race high"

This is what most research-informed coaches actually do. It's not keto. It's periodised low-carb work within a normally carb-sufficient diet. The idea:

• Do some easy sessions in a low-glycogen state (before breakfast, after an evening carb-restricted meal). This triggers fat-oxidation adaptations.
• Keep hard sessions fully carb-loaded.
• Carb-load normally before races.
• Fuel normally during races.

The result: slightly improved fat oxidation during easy miles (useful for ultras), zero performance penalty on hard days, no months-long adaptation drama. Studies by Louise Burke and colleagues support modest benefits of this approach for ultra-distance runners.

If you want to try keto for a 100-miler

Some rules of thumb from runners I've coached who've tried it:

1. Start at least 4 months before the race. Three months absolute minimum.
2. Monitor ferritin, thyroid, and periods (for female runners). Drop keto immediately if any red flags.
3. Keep protein moderate-high (1.5-1.8 g/kg/day) to protect muscle and support immune function.
4. Race with some carbs. Most keto-racing protocols add 30-50 g of carbs per hour during the final third of a 100-miler. Research supports this.
5. Don't evangelise. What works for one runner's physiology is not universal advice.

Who shouldn't try keto

• Anyone with a history of disordered eating or LEA
• Anyone racing at marathon pace or faster
• Anyone in the final 3 months of a race build (bad time to adapt)
• Anyone without access to diverse, whole foods (travel, camps)
• Anyone whose main motivation is weight loss rather than performance

Bottom line

Keto isn't magic and isn't poison. It's a tool that works for a narrow set of ultra-distance use cases. Most runners will do better with periodised carb intake — fully carb-fueled hard sessions, occasional low-glycogen easy runs, normal carb loading and race fueling. The evidence for that approach is stronger than for strict keto, and the practical downside is much smaller.

Related

• Ultra Running Fueling Strategy — standard approach
• High-Mileage Training Nutrition — baseline diet
• Gels vs Real Food — mixed fueling