Our bodies are “flex fuel” vehicles. They have a small fuel tank containing “higher octane” fuel (glycogen), and a much larger tank containing a slightly “lower octane” fuel (fat). I’ll explain why fat can be called “lower octane” below.
The body wants to use the abundantly available fuel, fat, at lower intensities so it can go farther. At higher intensities, which tend to be shorter activities, it wants to burn more carbs because it lets you go somewhat harder with the same amount of oxygen. By “carbs”, I’m really referring to glycogen in the muscles and liver, which is how the body stores carbs.
The figures shown below are for hypothetical athletes with varying levels of fat adaptation. Some examples from actual athletes are given in Dr. Grant Schofield’s book What The Fat? Sports Performance: Leaner, Fitter, Faster on Low-Carb Healthy Fat.
The first figure shows how carb and fat burning varies with exercise intensity, for a person with a well-functioning metabolism. Zero percent means at rest while 100 means all out sprint. At low intensities, mostly fat is burned and very little carbs. At high intensity, the body has to work hard to supply enough oxygen to working muscles, so more carbs are burned and less fat:
Next we see a person that is poorly fat adapted. Even at rest and for low intensity activities, a significant amount of carbs are being burned:
This is probably typical for a person with a dysfunctional metabolism. Taking dietary measures to reset your metabolism should go a long way to fix this. It also helps to get aerobically fitter by doing more training. A specific training trick that helps is not taking in calories while working out at low to moderate intensities, especially in longer sessions. We’ll revisit that in more detail below.
So in my opinion, everyone should strive to at least achieve the level of fat adaptation in the first figure. This is not just for athletic performance, but for everyday life. If you can’t burn a lot of fat a low levels of activity, you are more likely to get hungry in between meals and crave snacks.
But fat adaptation can be taken to another level, as shown in the next figure. Here we see a person that is highly fat adapted, and on a low-carb diet, so this person burns more fat even at higher intensities:
At first glance this seems to be advantageous, as is claimed by low-carb authors. Our bodies can only store about 2000 calories of glycogen, but even the leanest people store more than 10 times that amount of fuel as fat. That means you can exercise a lot longer if you are a “fat burner”, even at higher intensities.
But there’s a tradeoff. On a low carb diet the body actually down-regulates the ability to burn carbs, so at high intensities, it is as much “poorly carb adapted” as it is “fat adapted”. This means there is less fuel available for fast twitch muscle fibers, which can only burn carbs. Also, it requires 7½% more oxygen to burn fat than glycogen , so at a given amount of oxygen intake, performance is a bit less burning fat, which is where I got the “lower octane” from.
The question is whether it is a good trade-off to burn more fat a higher intensities like in the chart above. If you try to do a longer event at higher intensity, like the marathon and beyond, it might be good to burn more fat and less glycogen, even if it’s lower octane fuel, to keep from running out of glycogen.
For many people, enough glycogen is being burned at marathon pace to empty out the small tank by around mile 20. So we “hit the wall” (also known as “bonking”, although I just found out that has an alternate meaning in the UK 😉 ) at this point, slowing down considerably (a lot more than 7½%) and feeling awful. The feeling is because neither the brain nor the muscles are getting enough sugar. I only had this happen once: On a long group bike ride, I misunderstood how long we’d be going before stopping for lunch so didn’t bring any food. After several hours, and still about an hour away from lunch, I started feeling awful, lightheaded and grumpy and like I was pedaling through molasses. A good friend said “you don’t look too good, rich”, and came to my rescue with half of her last Clif bar. That was probably a bit over 100 calories, enough “fumes in my tank” to limp to lunch. Even short of bonking, having low glycogen stores can make us feel grumpy, or “hangry”, as some people describe it (for the combination of hungry and angry).
One way to stave this off is to “carbo-load”, or eat a higher carb diet leading up to the event so your body’s glycogen tanks are full to the brim. The problem with that is that your body stores water along with glycogen, so this makes you weigh more at the race and you can feel a bit bloated. And extra weight is payload, slowing you down. A second way to fight the dreaded “wall” is taking in calories during the event by using energy drinks like Gatorade or energy gels, and it can be a trade-off between bringing in enough fuel and not causing gastric distress.
Being adapted to using more of a lower octane fuel at marathon pace, so that you don’t run out of glycogen, avoids the need to carbo-load and risk gastric distress by taking in calories on the run. That sounds like not such a bad bargain, which is why participants in longer events are becoming interested in low-carb eating.
Bonking is even more of an issue for ultra-marathon running or longer events like ironman triathlons because it’s difficult to take in enough calories during the event. On recreational outings like century rides or long hikes, lasting several hours, it’s relatively easy to take in enough food. If you’ve watched long bike races like the Tour de France you’ve probably seen the “feed zone” where the cyclists are handed bags full of food to keep them going.
The first glimmer I heard that a nutritional approach to fat adaptation might work for longer events was the performance of Mark Allen, 6 time winner of the Hawaii Ironman. He was following the nutritional strategy of Dr. Phil Maffetone, which at the time included a diet of 40% carbs, 30% fat, 30% protein. By eating only 40% carbs, there is no way he could take in enough carbs to finish an 8 hour event without bonking, according to conventional nutritional wisdom. The logical answer is he didn’t have to, because even at the elite pace needed to win an Ironman, Allen was getting a higher percentage of energy from fat than carbs- he was more fat-adapted than his competitors.
More recently, fat adaptation has become a hot topic, but the emphasis is often on keto-adaptation , which is having your body adapt to being in a state of nutritional ketosis. Some significant results have been obtained: most notably keto-adapted runner Timothy Olson won the Western States 100 mile ultramarathon twice, and set the course record in 2013 of just under 15 hours. That is impressive, but Scott Jurek won Western States a record 7 straight times and was a previous record holder, and from his book Eat to Live, it is clear his diet was whole food plant based and not low carb. Olson’s record was broken by Jim Walmsley in 2018, who was also not a low-carb eater.
The disadvantage of eating a low-carb diet is that high end performance is not as good, as mentioned above, making it seem a better strategy for ultraendurance events than for shorter events. But the strategy of “train low, race high” is used to address that- use the ketogenic diet while training but take in extra carbs for your event, boosting high end performance , which seems to work well for some people. I haven’t heard how short of an event you can be competitive in using this approach.
One thing I haven’t seen explained satisfactorily is what the advantage is of going all the way to a ketogenic diet to get fat-adapted. We started with having to drink a lot of Gatorade and eat little gel packets just to get through a marathon, then flipped all the way over to an extremely low-carb diet. Isn’t there a happy medium in between? We know that Mark Allen was fat-adapted but not on a ketogenic diet so the answer appears to be yes. What benefit does the ketogenic approach have over this more moderate approach? Dr. Grant Schofield is an MD in New Zealand, and also a triathlete, who advocates a restricted carb but not ketogenic diet . He claims that while the ketogenic diet does lead to fat adaptation, you can also get fat adapted, but more slowly, without going into ketosis. And there can be some unpleasant side effects of transitioning into ketosis that are avoided with a more moderate approach.
We already discussed the “lower octane” effect of fat, that it takes 7½% more oxygen to get the same amount of energy from fat as from carbs, so while cruising at the same percentage of max oxygen uptake, the more fat-adapted athlete is slower. That is the downside to low carb eating and fat adaptation for athletic performance, as explained on nutritionist Jeff Rothschild’s website and in exercise physiologist Dr. Owen Anderson’s book Running Science. This is not just a theoretical result, there is evidence of degraded race performance in athletes who improve their fat adaptation by eating low carb . The “train low, race high” strategy doesn’t fix this because low-carb diets have a negative effect on the ability to use carbs after fat adaptation .
But there are some techniques to becoming somewhat more fat adapted without eating a low carb diet. Eating a diet of alkaline producing foods like fruits and veggies (scientific term is “low Potential renal acid load” or low PRAL) enhances fat adaptation . Meat, dairy, and grains are pro-acidic, so cutting back on one or more of these helps. In vegan or wfpb, meat and dairy are eliminated or drastically reduced, while in paleo, dairy and grains are, so either of diets these can be alkaline.
You can also avoid taking in calories during long training sessions. In a comment to his post about low carb and fat adaptation, I asked Jeff Rothschild whether doing this type of technique while eating higher carb the rest of the time would have the same negative performance effects mentioned above, and he replied “Doing a few sessions per week with little or no fuel in the tank won’t have negative effects on carb utilization”. He and Conrad Earnest discuss various dietary manipulations and how they affect fat adaptation and athletic performance in detail in ref. .
I eat a diet with very few refined carbs but plenty of carbs from whole foods as discussed in “my eating story”. I don’t usually measure, but I estimate I eat about 50% carbs, higher than the 40% of Mark Allen but less than the 60% often recommended in nutrition suggestions for endurance athletes. I can go at least 4 hours at a brisk cruising pace without “bonking”. When on a group bike ride or hike, whenever there’s a break, I see others reaching for power-bars, trail mix, etc., which I don’t find necessary. And at the end I am not “hangry” or desperate to “carb reload”. So I must be pretty well fat adapted. Aside from eating an alkaline diet, the main trick I’ve used is that I often don’t take in any carbs during my workouts, including my longest workouts of the week, so my body gets used to not getting its sugar tank topped off.
Four hours is about the longest activity I do these days so this is enough for me. I would guess that before doing these adaptations I probably could only cruise for a little over 2 hours without bonking unless I took in carbs along the way.
- Anderson, O, Running Science, Human Kinetics, 2013
- Maffetone, P, Training For Endurance, David Barmore Productions, 1996
- Volek, J , and Phinney, S, The Art and Science of Low Carbohydrate Performance, Beyond Obesity LLC, 2012
- Greenfield, B, The Low Carb Athlete, Ben Greenfield Fitness, 2015
- Schofield, G, Zinn, C, Rodgers, C, What The Fat? Sports Performance: Leaner, Fitter, Faster on Low-Carb Healthy Fat, The Real Food Publishing Company, 2015
- Burke, L, et al, “Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers”, The Journal of Physiology, 2016
- Stellingwerff, T, et al, “Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration.”, Am J Physiol Endocrinol Metab., 2006
- Caciano, S, Inman C, Gockel-Blessing E, Weiss E.,”Effects of dietary Acid load on exercise metabolism and anaerobic exercise performance.”, J Sports Sci Med., 2015
- Rothschild , J, and Earnest, C, “Dietary Manipulations Concurrent to Endurance Training”, J. Funct. Morphol. Kinesiol., 2018