Fasting for Ultra Runners- Does It Help or Hurt Performance?

The Story

Fasting & Ultra Running: Does It Help or Hurt?

What happens when you combine fasting with one of the most demanding forms of exercise on the planet?

In this episode of Running Miles with Heart Disease CHD, we take a deep look at fasting and ultra-endurance running and ask a question that doesn't have a simple yes-or-no answer: Is fasting helping an ultrarunner become healthier—or could it actually interfere with training, recovery and long-term health?

Fasting has become increasingly popular, with people using everything from 12- or 16-hour eating windows to 24-, 30-, 45-hour or even longer fasts. But what happens when you're also asking your body to train for a 50K, 50-mile, 100K or 100-mile race?

We explore the potential benefits of fasting, including weight management, metabolic health and increased reliance on fat for fuel. Research suggests that exercising in a fasted state can increase fat oxidation, particularly at lower and moderate intensities, but whether those metabolic changes actually translate into better endurance performance remains uncertain.

We also look at the other side of the equation: fueling enough to support the demands of ultra training.

Could restricting your eating window make it harder to consume enough calories, carbohydrates, protein and micronutrients? Could repeated under-fueling interfere with muscle maintenance, glycogen restoration, recovery, sleep, hormones, bone health or immunity?

That brings us to Relative Energy Deficiency in Sport (REDs). The International Olympic Committee recognizes that problematic low energy availability can affect both male and female athletes and can impair multiple body systems, increase injury risk and decrease performance.

We also tackle one of the biggest questions for ultrarunners:

How long is too long to fast?

Is a 12-hour overnight fast fundamentally different from 24 hours? What about 30 or 45 hours? Is there actually scientific evidence that tells an ultrarunner when fasting crosses the line from a potentially useful nutritional strategy into something that could compromise recovery or health?

And what about after the race?

Should an ultrarunner immediately prioritize carbohydrates, protein, fluids and recovery nutrition after a 50- or 100-mile race—or could there ever be a reason to fast during the post-race recovery period?

Current research in athletes does not establish one universal fasting duration that is safe or harmful for every athlete. Studies of intermittent fasting combined with exercise have generally found that performance can sometimes be maintained, but some research also finds reductions in fat-free mass, and the evidence is limited when it comes specifically to elite endurance and ultramarathon athletes.

Ultimately, this episode isn't about declaring fasting good or bad.

It's about asking a more important question:

Are you giving your body enough energy to handle the training you're asking it to do?

Because becoming a better ultrarunner isn't just about how much stress you can tolerate. It's also about how well you can fuel, recover, adapt and stay healthy enough to keep running for years to come.

Listen in as we separate the science from the hype and explore whether fasting has a place in the world of ultra-endurance running.

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