Backpacking on a Low Carb Diet

I have been doing low carb and intermittent fasting for over 2 years. M-F I only eat from 7-9pm, weekends from noon-8pm. It’s really changed my life. Years of blood sugar spikes and crashes are gone. I did 3 days on the JMT recently with 10+ mile, 4-5K gains at 9-11K feet elevation eating the same low carb diet and I did not feel any ill effects compared to lower elevation hikes. Typical day is coffee with butter in morning, cheese and nuts at noon, tuna, cheese, peanut butter and low card bar at dinner.

Even during long day hikes I don’t feel the need to eat before noon. I think it’s worth pointing out though you can outpace the conversion of fat to energy. Just because you are fat adapted doesn’t mean you can run/hike at very high output indefinitely. During two hikes with some pretty big elevation gains where I was attempting to see how far/fast I could hike fasted I hit the wall really hard. First hike was 5 hours in, 10 miles over 5.5K gain with 40lb pack. It comes on fast. One minute you feel fine the next it’s as if you are submitting Everest. Each step seems nearly impossible. Breathing, thinking all become very difficult. Second time was recently, about 4 hours/12 miles in on a fast hike during a steep climb-the wheels just fall off. It’s a horrible feeling.

That happened to me once.

I was not and am not low carb or intermittent fasting, but

Long day of hiking.  It was raining so I didn’t stop to eat.  All of a sudden I hit a wall, could barely lift each leg to take a step.  I got to a shelter out of the rain and ate some food.  After a short while I was back to normal.

It could have been water though, or both.

Yes, I’ve experienced that as well.  While the exact numbers vary from person to person, a number of sources have generalized that when in a state of ketosis, you can convert fat to energy when output is 60% or less of aerobic threshold – defined as 220 minus your age multiplied by .6 for maximum heart rate to help avoid the bonk, hitting the wall, etc. This has generally been a pretty good guide for me.

@ John Vance  Thanks. I couldn’t find much information on fat to energy conversion rates. I did see people training fasted for marathons/ultra running is somewhat popular but the consensus on whether you can improve your threshold didn’t seem clear.

defined as 220 minus your age multiplied by .6 for maximum heart rate to help avoid the bonk

That’s only 102 bpm for a 50 year old. That’s not going to get me up a steep hill. Pass the instant mashed potatoes please.

You know, in 50+ years of serious backpacking, neither Sue nor I have ever paid any attention to all these diets. We just ate till we were ‘fed’ (ie full) or until the pot was empty.
Cheese and salami has always helped, and lots of carbs at all meals.
But neither of us has any spare fat rolls to supplement our carried food.

Cheers

“First hike was 5 hours in, 10 miles over 5.5K gain with 40lb pack. It comes on fast. One minute you feel fine the next it’s as if you are submitting Everest. Each step seems nearly impossible. Breathing, thinking all become very difficult.”

That’s a pretty high level of output for the altitude you mentioned.  I suspect you probably exhausted your muscle glycogen and dietary carbs, then hit the wall when you had to depend on fats with inadequate O2 to metabolize them fast enough to sustain the pace.

“But neither of us has any spare fat rolls to supplement our carried food.”

I’ll wager you have a lot more body fat than you think, most likely visceral.  I know I did prior to testing.  The only way to know for sure is to get it measured.  The most accurate method is still the hydrostatic method, but bio-impedance gives a pretty accurate result, too.

Cheers