I am an endurance athlete (running and sometimes triathlon). The reason I tried keto initially was to become fat adapted so I could avoid having to use glucose as fuel for longer efforts and to be able to “tap into” body fat stores during exercise. The language in relation to utilizing fat as fuel in athletics makes it sound like body fat is an "unlimited " source of fuel and once you become fat adapted you are theoretically able to use body fat as energy even if you are very lean, since even lean individuals have a large amount of stored energy. HOWEVER!! This is where my confusion sets in. If we are theoretically able to tap into body fat for energy during endurance efforts, how does this square away with the physiological limitations of our body to be able to use a small amount of stored energy each day from body fat?? This is the issue Richard has discussed many times, that our bodies can only use something like 33 calories per day of energy from stored body fat. SO…as a very lean athlete, my BF is about 12% which means I only have about 10 pounds of fat in my body. This means I can only generate about 400 calories per day of energy from body fat. And I need the rest of my energy to come from outside sources to even meet my basal metabolic needs. So it is FALSE that body fat is an unlimited source of energy, at least on a daily basis. Why is it marketed to athletes that keto gives you an unlimited supply of energy from body fat? Ironically this is directed at a population that tends to have relatively little body fat compared with the general population. I kind of feel a little like the wool is pulled over my eyes in this regard. A very lean runner cannot go out on day, a twenty mile run without taking in extra fat calories from food. I don’t know why this math is not made clear when discussing the benefits of keto for athletes! Not that i don’t love keto or that I’ll change anything…this just frustrates me.
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@Emacfarland I know very little about this subject but it does interest me. It does seem though that being fat adapted is still an advantage as you can carry fat calories much more economically than glucose calories as fat has a 9 cal/g advantage over glucose 4 cal/g.
only 33 cals per day from body fat? That doesn’t sound right. I’m sure when I’m on an extended fast my body needs more than 33 calories a day to operate.
Interesting article about how many calories are in body fat. https://authoritynutrition.com/calories-in-a-pound-of-fat/
It would seem that it would be easier to keep fueled with fat for endurance events, as fat has 9 kcal/ g whereas carbs only have 4. So if you need 4000k cal for an event. you would need to take in 1000g of carbohydrates, if you are fat adapted you would only need to takes in (4000-400)/9 or 400g of fat
@Pilotbob, It’s 33 calories per pound of body fat.
See here for additional details:
As to endurance athletics, I wonder what might happen if you added MCT oil, or maybe coconut fat along the way. I’ve ridden several 100 mile centuries on a bike and for sure, you need to eat something as you go along. The other thought is what about pre-loading fat near the start with saw 100 grams of a reasonable fat, that would be 900 calories. In the end, I’m still skeptical that you can do an endurance activity that requires 80% or greater output on just fat. i.e. Breathing hard and sweating for 3+ hours.
Polar explorer ate mostly butter, but I would think most calories went for heat production and slow walking. That’s different than pushing a bike at 18 to 20 MPH (aprox 10 cal / min) for 6 hours. Anything where you’re breathing hard seems tricky to do for hours on end without glycogen, aka carbs. Does anyone have any details on high output exercise with ketones?
You can only use aprox 33 kcal per lb of fat that you have, so for me who still has a substantial gut, i could expend 3000 cal a day and not have to worry about refuelling. for those with much lower body fat, can’t pull enough out of reserves so would need to ingest the difference.
I’ve seen this one! Awesome . I’d love to have my fat oxidation rate measured but it’s $$
I was thinking that maybe the exercise itself allowed access to more fat burning than normal because of the drop in insulin during activity. And the fasting rate is after the first couple of days, which is when you draw down on resources available in your bloodstream and digestive tract.
I do think some of the claims are exaggerated though. I reckon most keto endurance athletes use additional nutrition, sometimes supplemental carbs for long events.
@ianrobo does a lot of long rides and may know more. And @richard of course.
well I have gone on fasting rides of 4 hour plus including some efforts that would use carb stores on top. The FASTER study approaches some of this and use of fat is higher than previously thought I believe.
But remeber on top of what you get from fat use per hour the body also produces carbs as well as a top up plus the 2K of calories in your carb stores which can be used. So it is a mix of things.
If you have not come across endurance planet podcast BTW it is one where Jeff Volek goes through this, two points from the show notes
How to burn fat for fuel by restricting carbs,
What we’re seeing in fat-adapted athletes and the benefits, including the ability to run ultras on <2,000kcal vs. traditional 6,000kcal,
At 7-14% body fat one still has 20,000-30,000 kcal of adipose tissue reserves.
Previously fat oxidation rates were seen at 1g per minute prior; seen in this study. But the FASTER study showed up to 1.8g per minutes of fat oxidation – first to be documented in a lab setting!
Another useful example - http://zachbitter.com/blog/2014/04/takeaways-from-the-faster-study.html
and then of course from VESPA - http://www.vespapower.com/the-emerging-science-on-fat-adaptation/
the key being in fat adapted state you use less calories. This study is so good because 1) the methods were proper science and 2) these athletes they tested on were at the peak of the sports it is unmatched by anything else out there and easily survives peer reviews that pro carb people when asked to argue against FASTER can not.
should add that one key takeway of FASTER was how we used to see the fat burning zone. It used to be defined in HR zones of Zone 2 and some of Z3. FASTER shows that it is much greater of that and goes into Z4 where you see the higher levels of fat oxidation.
In other words you dig into your carb stores at a much later point in the power/HR curve leaving you much more for later in rides/runs etc
Interesting thread on the Ketogenic Athlete about this and Chris Froome, note the pictures
Oh!! I had no idea! That’s fascinating! I forgot about the glycogen production by the body! Wow, I learn so much from all these amazing people!
ha ha thats the beauty of forums etc all the info is here.
but as I found the best way is to give it a go and see what happens. plan a course for 20 miles that loops close to home so if you feel like you need something just go home … never carry just in case as more likely to use it !
I’ve found that I can easily run for 3+ hours on just water, and that’s with starting out in a fasted state. Granted, I’m not the speediest person, so I’m probably not burning as many calories as a faster runner would.