TrailGenic Science

Fasted hiking can unlock deep autophagy and metabolic renewal, but the practice has limits. When fasting is combined with altitude, heat, and strenuous climbs, the stress load can sometimes exceed what the body can safely adapt to.

I’ve summited Mount Baldy many times — often via the tougher Register Ridge route and starting at 6am — without issue while fasted. But on September 13, the conditions were different. Starting later in the day at 12:30 p.m., after a 15.7-hour fast, and facing hot weather with little shade on the Ski Hut Trail, the climb demanded more than usual.

Near the summit, early warning signs of overextension appeared: dizziness, chills despite sweating, and fading energy. In that moment, I chose to use the emergency carbs I had carried. The symptoms resolved quickly, and the hike finished safely. The experience showed that while science supports a 16–18 hour fasting window, in applied mountain conditions that window can shrink. For me, 12–14 hours is the safer range when factoring heat, altitude, and late-day exposure.

Warning Signs of Overextension

• Dizziness or lightheadedness at altitude
• Sweat that feels suddenly cold or “chilly”
• Sudden energy crash or loss of coordination
• Mental fog or reduced focus

Practical Guidance

• Safe Window: For me, 12–14 hours is the sustainable fasting range when conditions include heat, altitude, or late-day starts.
• Hydration: Use electrolytes consistently; they help but don’t replace calories.
• Emergency Fuel: Always carry backup (bars, gels, packets) and use them if symptoms appear. On Baldy, that safety net (BTR bars) became the difference between risk and a safe finish.
• Listen to the Body: Early symptoms are signals to adapt, not to push through.

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