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Generally, I keep my carbohydrates low, less than 100 grams a day if I can. I do a fair amount of running and strength training. I participate in regional amateur competitions for the same.

I'll be doing a multi-week backpacking trip, involving roughly ~15 miles a day of trail hiking. Lots of altitude gain/loss, and carrying a ~40 pound pack. In the backpacking and hiking world, protein is treated as a nice-to-have second class citizen, and carbohydrates are much lauded for their easy energy. Everything is noodles, rice, flour, and other pulverized carbs.

Anecdotally I "know" that distance runners need a lot of carbs and I'd imagine that ~6 hours a day of strenuous backpacking is in the same ballpark. Specifically though my question is:

Are there established guidelines for when a fitness activity requires significant carbohydrates in the diet beyond what can be provided via ketosis?

I don't mind eating carbs for two weeks if necessary, but I'd like to understand why they're necessary beyond just the folk wisdom of "because you need carbs for energy". I would imagine Inuit people weren't lounging about all day and managed without the carbs.

Re-asking this a slightly different way, how does someone know that they need to have carbohydrates in their diet in order to maintain a given level of aerobic activity?

  • Your muscles requires energy (Kcal) in the form of ATP, to contract. Your body can metabolise ATP from either fat OR sugars, so unless you're involved in an anaerobic activity, where the speed / efficiency of the conversion is important it's largely immaterial how the energy is stored. Have a read of the free chapter of: Nutrition for Health, Fitness, & Sport – arober11 Apr 20 '15 at 14:34
  • I don't have the expertise to answer your question directly, but I think you might find, "The Art and Science to Low Carbohydrate Performance" helpful and informative. They point out that with keto-adaptation "carbs can be reduced to an optional nutrient for athletes" as the body stores about 2000kcal of carbs compared to >40,000 kcal of fat. They also discuss the use of VESPA, a fat burning fuel catalyst useful for prolonged events. (Note - the link to the book is from our site's amazon store.) – BackInShapeBuddy May 1 '15 at 7:00
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2010 ISSN Position Stand: • Individuals engaged in a general fitness program can typically meet needs by consuming a normal diet (45-55% CHO; 3-5 g/kg/day). • Athletes involved in moderate amounts of intense training (2-3 hrs/day, 5-6 times/week) typically need to consume 55-65% CHO (5-8 g/kg/day or 250 - 1,200 g/day for 50 - 150 kg athletes) in order to maintain liver and muscle glycogen stores. • Athletes involved in high volume intense training (3-6 hrs/day of in 1-2 workouts for 5-6 days per week) may need to consume 8-10 g/kg/day of carbohydrate (i.e., 400 - 1,500 grams/day for 50 - 150 kg athletes) in order to maintain muscle glycogen levels. 2009 Position of the ADA, Dietitians of Canada, and the ACSM: • Carbohydrate need of athletes are 6 -10 g/kg/day, depending on total daily energy expenditure, type of sport, sex, and environmental conditions.

Up to 2hrs activity you should be fine as long as your daily macronutrient intake is met. Past that try dosing 50-70g per hour

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Dr Peter Attia is an accomplished athlete who has remarkable athletic endurance accomplishments performed during nutritional ketosis (due to very low carbohydrate intake). His blog is well-researched and well written. I think you'll find his answer to your question is that carbs are not necessary for the type of activity you plan.

My personal experience is that after becoming accustomed to a ketogenic diet, I was capable of four days of hiking like you describe (in the Central Cascades of Washington State, pretty much straight up and down), on 70% fat, 15% carbs, 15% protein, with ketone levels in the 1.6 - 1.9 mmol/L range. Although I had a calorie deficit each day, I was not subjectively hungry.

  • Not being hungry is not the same as being healthy. A diet of 70% fat is not sustainable long term. – JohnP Jul 6 '15 at 14:52
  • A 70/15/15 diet is absolutely sustainable long term and research suggests it may be closer to a historic human diet. – Rob May 22 '17 at 22:37

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