I read a lot of blogs and listen to a lot of podcasts that at least touch on the idea of fueling for endurance events with fat. In a nutshell, and leaving out the health effects of a carb/fat/protein focused diet, the idea is that our bodies can only store so much energy in the form of glucose in the bloodstream and glycogen (the stored form of glucose) in the tissues to fuel our body. From what I understand, we can store about 500 grams of glucose and glycogen in our bodies at any one time, which we can burn through pretty quickly – a few hours just sitting around. Much faster when exercising. Eating carbohydrates (ie, sugar) is the quickest way to replenish them, which explains the quick boost of energy we can get after eating a Snickers bar (I love Snickers). That 500 grams of glucose is awesome when we need to do a 100 yard dash, but not so good to fuel us on a long, multi-hour bike ride. So, it would stand to reason that we could, and should, continually replenish our bodies with carbohydrates so we don’t “hit the wall” during exercise. In fact, the sports energy drink/gel industry tells us that we need to continually take in more sugar to replenish what has been used up. At least one brand goes so far as to tell you, based on your weight and level of exertion, how much of their gel to eat per hour of exercise.
On the other hand, a lean, 150ish lbs man will have about 18 lbs. of fat on board. That’s 100’s of miles or several days worth of energy. That fat isn’t glucose yet, of course, so it needs to be converted to glucose by the body. Two points here to note. First, our bodies aren’t going to readily burn fat if they aren’t already in the habit of burning fat. What I mean is, if our body is accustomed to fueling on sugar, it’s going to be expecting that next dose of sugar rather than choosing to burn fat.
Most advocates of a fat burning approach call it being “fat adapted” when our bodies are readily burning fat rather than sugar. The trouble is, becoming fat adapted doesn’t happen overnight. We can’t rely on sugar to fuel our runs today, then wake up tomorrow and decide to be fat burner – you can’t fuel your car with gasoline today and then decide you want to use diesel tomorrow. And there’s no “Am I fat adapted?” test that we can take to find out. Mark Sisson says that one indicator is “If you can handle missing meals and are able to go hours without getting ravenous and cranky (or craving carbs), you’re likely fat-adapted.” After we’re fat adapted, we can utilize our fat stores for sustained energy.
But, the second thing to note is that the process of turning fat in our bodies into glucose happens at a much slower rate that turning sugar (sucrose, fructose, etc.) that we eat into glucose. So, I can easily go from dinner at around 7p until breakfast at 7a without eating. Or, go from my small 7a breakfast until 12:30p or so when I have lunch. That’s plenty of time for the fat to be converted to glucose to meet my energy needs while sleeping, sitting at my desk, etc. That’s easy and I do it all the time. The problem comes when I try to workout. If I get my heart rate too high I can definitely feel the drain and I start losing energy fairly quickly. When I go at an easier pace, though, I can keep the exercise up for a much longer time. That just makes sense to me, but I think what’s happening is that my body is able to replenish the glucose my brain and muscles are using with my stored fat at a sustainable rate. If I go too fast, the rate isn’t sustainable and I start burning out. Which, the sports drink industry would tell me, means that I need a shot of sugar to keep going. And then another, and another, etc. My approach is more that I need to slow my pace so that I’m utilizing the stored energy (fat) I already have.
This whole fat adapted area is interesting and, at times, a little confusing to me. And, it’s where I see a lot of people not seeming to grasp, or caring to learn, how the idea of fueling with fat works. In the past few months I’ve read two articles on a major running magazine’s website talking about how carbs are the best source of fuel. The same author wrote both articles and I think she has some vested interest, other than health or fitness, in pushing that argument, as she only cites very small studies, with non-fat adapted subjects, that show how useful sugar is. When I read the articles, I thought of similar headlines that could have read, “Study shows that gasoline engines still run best on gasoline!” Well, stop the presses right there! Who knew?! But, to be fair, she may have been only focusing on the runners going more for speed than endurance. That wasn’t clear to me in the articles.
I haven’t seen much information focusing on fat adapted athletes and how they fare during endurance events. Here’s an article that does, though, in Ultra Running Magazine – The Emerging Science of Fat Adaptation, which seems to show that our bodies can adapt to be fat burners and convert fat at a faster rate than the rate at which a sugar burner converts fat. Here’s another one at Runner’s World – Adapting to Burn Fat as Fuel. It talks about the ability of fat adapted runners to burn fat faster than carbohydrate focused runners, but it also mentions how we can’t burn fat at as fast a rate as we can burn carbs. This seems to be in line with other things I’ve read as far as using fat as the endurance fuel, sugar as the sprinting fuel.
Speaking from my experience, as N=1 in my own experiment, since I’ve drastically cut my carb intake (usually well below 150 grams per day, I think), I can exercise on an empty stomach and exercise for longer periods than I could before. Where I used to have a small snack before a run after work, I can easily start a run 4-5 hours after my last meal and run further than I could before. Likewise, on longer bike rides, I don’t feel like I need to take along a sports drink to keep the sugar train running. I’m fine with water for 2-3 hour easy-to-moderate intensity rides. I do have some work to do in the balance department, though. Converting fat to glucose takes time, so it isn’t realistic to think that I can count on fat only to fuel an intense ride or a run. I need to experiment more to get a better handle on the sugar I need on tap to be able to get to the top of a climb or sprint across traffic. Maybe I do need to include a small amount of sugar in my water to make sure my stored glucose (glycogen) level doesn’t dip too much, that I always have a little something in the tank for a short burst. And, I need to work more at keeping my heart rate lower, because again, fat doesn’t burn as fast as carbohydrate. So, I need to make sure I’m not working at a rate that is faster than my body can fuel itself with my considerable supply of fat. Otherwise, I’ll have to fuel with sugar again, which I want to avoid. And what about protein? Holy crap, there’s a whole other discussion on whether protein during exercise does any good. But, I’m not against running my own experiment on that to find out.
All that said, I do miss Watermelon Gu Chews! Yum! I gave away a lot of gel and sports drinks, but held onto a small stash of these as emergency candy!