The Mathematics of Race Fueling

(This popular article has been revised and reprised twice since its original publication, once in 2002 and again in 2007.)


In the 1995 Hawaiian Ironman Paula Newby-Fraser skipped a few pit stops and ran out of gas on the bell lap. She sat down on Alii Drive, literally within sight of the finish line. Talk to her boyfriend/coach (now husband) Paul Huddle and there’s an edge in his voice which suggests that although he’s long ago emotionally processed and shelved this race he still, just a little bit, kicks himself over it. During the last eight miles of the run Newby concentrated and was focused but was still getting caught by Karen Smyers. She took a calculated risk that passing up a few aid stations would buy her some precious seconds. She thought she could get to the finish on desire.

Desire and a quarter will buy you a cup of coffee, though, when you’ve got no gas in the tank.

Mathematics is not psychology or history. There is no room for interpretation. It is an exact science, and when you’ve got no feed in the stable you can’t ride your horse to town. There is a brutal reality to the fueling considerations you must make during a long-distance race. You’ve got to know how much to take in, where to take it in, what to take and in which form it should be taken. You need to figure out a plan in advance for getting fuel into yourself. You’ll know more about how to make that plan after reading what's below.

Hawaiian Ironman competitors are the lab rats for all long-distance athletes in all disciplines. They boldly go where no athlete has gone before—except those who’ve raced the same race on the same course in prior years. Hawaii differs from other races in that things often don’t go as planned. Elsewhere, you figure out your fueling plan in advance and you’re usually able to execute it. The problem with Hawaii is its heat and humidity. Your stomach doesn't cooperate. Then your plan is shot to heck.

I'll draw your fueling roadmap. Bringing Hawaii into the discussion, though, highlights a problem: You can comport yourself properly on the course and still have things go haywire. Hawaii is the quantum physics that bends the linear math described below. But when you experience a hitch-in-the-getalong during your race, all is not lost. There are some workarounds that can save your race, and we’ll cover those, too.

Now, about that reference to one’s stomach "not cooperating" and even described as having "shut down." It is only fair to say that there is some question about how, when, why, and if the stomach can or does "shut down" in its ability to continue to absorb and digest water and nutrients. Certainly the stomach receives less blood flow while the body is at work because blood is shunted to the skeletal muscles, reducing the rate of digestion.

Also, ironically, absorption of water through the stomach and intestinal walls will slow if you are dehydrated—just when you need water the most, you can’t get it. This much is known. What is in question is whether the heat of stomach contents will also slow or stop digestion. If this is the case, one might surmise that cooling the stomach might help. But medical experts with whom Slowtwitch has spoken disagree with the notion that you can cool the stomach by taking in cold liquids. They say that most liquids, hot or cold, reach body temperature by the time they get to the stomach—so it only matters that you provide liquid, not what the temperature of that liquid is.

It is a matter of fact, though, that digestive problems often do exist during a race like the Ironman, and it is quite common to see athletes with bloated stomachs or to see them expel their stomach contents during the event. Obviously, when that happens, it’s a waste of a lot of dutiful eating and drinking. Whether an athlete is having problems digesting because the stomach is too hot, the body is too dehydrated, or because the blood is at work doing other chores, the remedies are similar. One must slow down or stop and let the stomach "catch up". Because most digestive problems during an Ironman or other long triathlon occur during the ride, it seems unimaginable for an athlete to voluntarily dismount and stand by the road. But, as remedies go, you could do worse.

Like a jet airliner, your body has several "fuel tanks." Much of your fuel is stored as glycogen, and at least half of your glycogen resides in your muscles ready for use; another quarter to a third waits in your liver. Then there are your fat reserves, and these will be called upon during a long-distance race as well. You’ll always burn a certain amount of fat during exercise, but the rate doesn’t rise as the intensity rises. It also, by the way, doesn’t go down. There is a misconception that high-intensity effort isn’t "fat-burning." Actually, it is. It’s just that your rate of glucose consumption increases as your intensity increases, which means that fat, as an overall percentage of fuel used, goes down.

"Fat burns in a carbohydrate fire," it is often said, and sugars are a necessary component of fat metabolism. When you bonk you are, of course, out of the sort of fuel most efficient to metabolize: sugars. What’s more, in the absence of sugars your body will not be able to metabolize fat and will then seek out other sources of fuel. As was discussed in an earlier Slowtwitch piece on overtraining, the body’s cortisol levels increase during exercise—and cortisol is catabolic, meaning it breaks down tissue. One of the body’s uses for cortisol is to catabolize lean body mass (i.e., muscles) for use in a process called gluconeogenesis, in which the body converts protein to a useable source of fuel.

There is another reservoir of fuel you carry around with you: whatever is sitting in your stomach. This is important; and, personally, I’d recommend eating as hearty a breakfast as you can tolerate on the morning of a long triathlon. You shouldn’t worry about any negative digestive effects from a nice-sized breakfast because you’re probably not going to start the race sooner than two hours after finishing your meal, and you’re not going to be going that hard during such a long race anyway. But don't fill your stomach with protein or fat. These do take a long time to digest and will impede the progress of digesting long-chain carbs.

All these fuel reservoirs will take you through three hours worth of Ironman-style work, or perhaps four in a best-case scenario. That doesn’t mean you can do a four-hour race on just water, though, because you’re going a lot harder in the shorter race and therefore will consume fuel at a higher rate. But at Ironman pace, you could probably race that far on water alone without a bonk.

That said, you don’t want to go through your glycogen stores early in the race—you’ll need them later. This is because the critical factor is not how much you ingest during the race. Yes, this is important, but more important are your burn rate and your absorption rate. Your burn rate is going to be higher if you go harder. And remember, you’re burning about the same amount of fat riding a slower pace as you are riding a faster one, and all the additional fuel required to go faster is much-needed carbohydrate.

This is important for two reasons. First, when you run out of carbohydrate, as we discussed, you really run out of two fuel sources: carbohydrate and fat. And when you’re riding harder you’re going through carbohydrates very, very quickly.

Second, you never absorb as much as you use. You’ll burn 750-1,000 calories an hour during an Ironman race. You can absorb only about 275 calories an hour, or perhaps more if you’ve got a particularly efficient system for gastrointestinal uptake. Mark Allen was convinced he could uptake more than 400kcal an hour, and as much as 500kcal an hour. Either way, you don’t want to wait until you’re out of glycogen before you replenish or, as you can see just by the math of it, you’ll be toast. Considering all this, you can see why people literally run out of fuel on the course.


If you are, let’s say, a 1:15 swimmer, then you’ll be on your bike and down the road and could conceivably put on the feed bag about 1:30 into the race. If you intend to ride, say, six hours, then you’ve just sat down to a four-and-a-half-hour lunch. You should do as good an eating job as you can while on the bike because you’re more fresh and eating is a bit easier while riding than while running. Also, what you eat during the last half-hour of the run is going to be of lesser value as you’ve got to ingest it and get it to your tissue sites before it is of any efficacy. By the time the fuel from the final two aid stations gets to your muscles, you’ll be in the massage tent. So you’ll have only four hours on the run during which you can profitably eat, and you’ll have to make the best of the time allotted.

Now you can prepare a plan for fueling yourself. It should be strictly regulated, and the food should be accessible. Easily digestible calories should be handy, and I like the idea of taping GU or other gels to your bike’s top tube. You tape down the pull-off tab so that you can rip the gel off the bike and open it up all in one motion. But don’t dilly-dally with it. Stick the open end of the package into your mouth immediately, even if you’re not ready to eat it all. If you don’t, the gel will start oozing out of the package and the wind will blow little strings of sticky gel onto your legs and arms and handlebars and you’ll never get it off.

It's obvious to me that many athletes don't think about this. I recently competed in the Wildflower half-IM and had eight gels taped to my top tube. Two competitors approached me before the race and asked, "Are you really going to eat all that?" Let's figure it out. These eight gels will deliver me between 800 and 900 calories, depending on how much of each gel I get into me. I'll be riding for about three hours, and my last gel will be eaten just before I leave for the run. That's at best 300 calories per hour, which is about right (as I take in most of my calories in gel form during the bike). So these top tubes pictured above and below aren't as outrageous as they look, since they're on bikes in the Kona bike corral.

Good fueling doesn’t start, though, when you get on your bike, or even at breakfast before the race, but days before. Much has been written about carbohydrate loading, and I’ll defer to others for recipes and protocols for this. Proper pre-race hydration is a bigger concern. You need to enter a long race fully hydrated. That means drinking a reasonable amount of water in the two or three days prior to the race, while staying away from too much tea and coffee, or any diuretics causing you to pee it right back out.

That established, you shouldn't go overboard in your pre-hydration. I did this some years ago right before a hot half and had to stop twice during the 56-mile ride to pee. (Mark Montgomery's remedy for this is described in his article on the fine art of on-bicycle evacuation.) A pee break is inconvenient, hyponatremia is more than inconvenient. Some believe that pre-race hyperhydration puts you at greater risk of becoming hyponatremic during the race.

Some people have an exceedingly low salt intake in their regular diets. Normally, of course, one would not want to take in too much sodium. But if your intake is exceptionally low you may become dehydrated early in a race. Exceedingly low sodium makes it hard to keep water at the cellular site to decent pre-race levels. So it might not be a bad idea for those who have very low salt intake to drink fluid replacement instead of clear water in the days prior to the race, to increase the sodium intake and pack the cells full of water and sodium (this will also help you from becoming hyponatremic).

While we’re on the subject of sodium: Your body regulates its sodium based on how you’ve conditioned it. When you see a person cross the finish line of a race with those white salt deposits all over their clothing it’s a fair bet they have a pretty high amount of daily sodium intake. This is especially true in America. While Europeans average about six grams of sodium a day, for Americans it’s ten grams a day.

We have a bunch of dogs in our household, and one of them loves the taste of salt. When I come in after a long run or ride I have to beat this dog off with a stick or subject myself to 20 minutes of licking -- and since I don’t beat the dogs, I have to sit for the latter. Perhaps the dog likes me better than others, but in this case it is because my normal salt intake is much greater than that of some, and I therefore throw off a lot more salt in my sweat.

In a long race, therefore, my need for salt is going to be much greater. Somebody who processes salt this quickly can go through as much as five grams of salt an hour in a long, hot race. Others will not go through nearly that much. That means two things, practically. I’d probably be a better long-distance athlete if I didn’t metabolize so much salt -- which means I’d be well-advised to cut down my regular salt intake, and perhaps some months later I would have trained my body not to sweat out so much salt.

And second, there is no possible way my body will take in as much salt as I’ll need with any fluid replacement drink. The average gel only has 25-50mg of sodium, and even a salt tablet will only have 250mg. Some pro athletes have taken up to 30-40 salt tablets during a hot race like Hawaii.

If you cramp late in long rides or if you have excessively salty sweat, you might consider supplementing with salt tablets during your race. But like everything else written here, this isn’t something you can simply try the day of the race. You should practice your fueling plan during training, and if you think you’ll need extra salt during the race, you should practice this as well.

Now for the race itself: During a long-distance event a nice plan might be to eat a gel once every 20 minutes. That takes care of 300 hourly calories. One 20-ounce bottle of fluid replacement, assuming 100 calories per bottle, gets you up to the max you’ll be able to uptake. But you shouldn’t do the whole ride on just gel and fluid replacement. What the gel people don’t tell you, because they’re trying to be politic, is that you don’t need fluid replacement with gels. Gels are instead of, not in addition to, fluid replacement. You obviously need water, and the gel folks would stress taking in a lot of water. But variety is the spice of life, and fluid replacement should not be neglected. You’ll have too much solute, though, and not enough solvent, if you don’t take in a certain amount of clear water. And you'll note as you gain experience that you’ll quickly tire of not having some clear water to wash out your palate after taking in a lot sugar-laced gels and fluid.

This solute/solvent ratio—which is what is referred to when a fluid replacement maker refers to its "percent solution," is as causal as anything for the bloated and undigesting stomach referred to above. I think a lot of people in Kona ride too hard for the heat, don't watch their heart rates, and compound it all by eating gels and drinking fluid replacement at full strength. That's just too much "stuff" and not enough water, and pretty soon not even the water will get absorbed, because of the stomach's osmotic pressure.

On the assumption that you can get down two to three bottles an hour -- about half of that fluid replacement, the other half clear water -- you should be OK. Your water intake will go up as the air temperature gets hotter and as you get hotter and more tired later in the ride. But don’t rely on that happening. Fluid is the most important component to your race. You must not allow yourself to become dehydrated -- everything else written here becomes moot when you reach a state of dehydration. So try your best to keep your fluid intake up in the earlier miles.

It sounds like a piece of cake, but just when you think you have it wired, problems begin. It’s all well and good to get the fuel down -- and with the right ratio of food to water -- but now it’s up to your stomach to process it all. It must digest its contents and pass that along to the small intestine. Eventually, after the all nutrients have been taken up here, much of the water is then absorbed by the large intestine. But if your stomach doesn’t cooperate, you’ve got trouble.


There is a school of thought that fuel uptake, like salt metabolism, is trainable, so as you practice your fueling plan in training you may also be training your body to uptake more carbohydrates during effort. Either way, you need to find out in training what your rate of uptake is so you’re not starving yourself needlessly nor riding around with a lot of stuff sloshing around in your stomach. Also, it’s not easy to eat when your pulse is 155 beats per minute. It takes some getting used to. Practicing your fueling plan will get you used to what it’s like to eat during effort.

Even when you practice and get it all down, sometimes bad things happen on the course. Which brings us to the disagreeable notion of standing down. If I found I wasn’t absorbing, meaning I absolutely couldn’t get the food down; if my stomach was sloshing with undigested food and water; if it was threatening to return from whence it came; and especially if I was falling behind my eating schedule, I’d slow down. If that didn’t work, I might even pull in to the nearest aid station and stand down. I might get off the bike, perhaps drink a little clear water to correct my stomach's osmotic pressure, empty my bottles and refill them with cold, clear water. Let's say I stood down for two minutes, maybe three. I might give my body the once-over with a cold towel, stretch some, and just relax. Perhaps I’d just gone too hard and the simple act of standing down would reduce my pulse to a rate that would make absorption and digestion easier.

Three minutes taken now might save you 30 minutes or an hour later on. It’s just not that consequential. Both Paula Newby-Fraser and Luc van Lierde won the Hawaiian Ironman after a mandatory three-minute stand-down, so how deleterious to your race can that be?

Later into the run I would rely more on the gels, and maybe even digestible solids like bananas and Fig Newtons, because I’m hungry by this time and I need something substantial (hunger satiety is an important component to your nutrition plan). About ten miles into the run I’m starting on the Coke. I need the caffeine because it is an aid to fat burning and it’s a stimulant, which by now I can use. It’ll give you a lift that you’ll feel within 20 minutes. But at a certain point in the race athletes report a crash from the Coke "high," so it’s not something you want to start drinking too early in the race. I’m sticking with fluid replacement and water prior to that point.

I remember watching TV some years ago and was surprised to see a major drug brand advertised in a way I’d not previously seen. But this is a trick pros have known about for a long time. The ad was for Advil (or maybe Motrin) and featured a middle-aged fella talking about how he pops a pill or two before going out to play tennis (or maybe it was golf). The prophylactic use of this anti-inflammatory painkiller seems now to be out of the bag and for the general public’s consumption. So I suppose it’s OK to mention that a couple of 200mg tablets of these per hour starting, maybe, halfway through the bike appears to have made the race a bit easier for certain athletes.

But be careful, because heavy use of ibuprofen has been linked with organ problems. This is serious stuff. As one athlete put it, you might not know what you’ve done to yourself until you start to "pee brown." That said, it has not been uncommon in years past for a pro athlete to ingest two grams or so during the race, and so far the record for the most ingested (that I've heard of) is five grams (specifically, 24 x 200mg ibuprofins). I mention this not because I recommend it, but because the practice has existed and should be openly discussed.

That established, the practice of popping ibuprofen has been falling out of favor over the past decade year. A couple of athletes I talked to say they've replaced this in favor of one or two Aleve the night before and maybe one more the morning of. Others have discontinued the practice altogether. I have heard of a study, apocryphal so far to me as I’ve not yet located it, suggesting that taking a lot of ibuprofen during an exertion is linked to muscle tiredness lasting months after the event is over. I've not so far found any anecdotal evidence among the pros to whom I've spoken to back this up.

And, as previously stated, taking high doses of ibuprofen over a long period of time have been proven to be a potential danger to one’s organs, specifically one’s kidneys (hence the remark about brown pee). Whether the hormonal cascade that results can occur over a period of time can be sped up via the rigors of an Ironman event is not anything I know about.

Finally, I should add that when I now talk to the athletes who did pop a lot of Ibuprofin during their races, they universally shudder at the memory, and wonder how close they came to doing more serious damage. To a person, none recommends the practice today to the athletes they coach or advise.

The Hawaiian Ironman has had a lot of different fluid replacement sponsors during the recent history of the race. I believe I can remember Cytomax, Exceed, Gatorade, MET-Rx and Internutria, and I might be forgetting some. Will all brands agree with you? Only you will know, and for that reason I suggest you find out which fluid replacement is going to be on the course of the long race you’re planning and ride your training rides with that. Of course, it wouldn’t hurt to occasionally practice your entire fuel intake regimen during some of your rides. If you find your stomach is quite particular about the choice of fluid replacement, there is a trick that works quite nicely even if your drink of choice isn’t going to be served on the course.

I've successfully experimented with fitting elite athletes' bikes with handlebar drinking systems filled with a ten-to-one ration of that athlete's personal favorite fluid replacement. Two water-bottle squirts of the concentrate are enough to bring 20 ounces of clear water up to the proper concentration. The athlete grabs bottles of clear water from the bike aid stations and empty them into the handlebar delivery system. If you want to eat a gel, or hard food, you drink the pure water. At the next aid station, you grab a bottle of clear water, empty it into the handlebar bottle, and squirt in the concentrate. Using this system an athlete always has cold water and your favorite fluid replacement also served cold.

Probably when you started the sport it was just a matter of riding and running. Perhaps triathlon was your escape from having to think and concentrate. Now you’ve got your pulse, cadence, perhaps power, to monitor, and you can add to that the tracking of your fuel intake. I’d probably tape down my gels with write-on tape, and I’d scribble when each gel should be eaten so I didn’t fall behind on my eating schedule.

When I raced the Ironman in 1981, none of us knew any of this stuff. We just raced. Like the early days of anything, you made it up as you went along. I remember seeing a spectating acquaintance eating an avocado and sprout sandwich about 80 miles into the ride. I needed something real bad, so I ripped it out of his hands and kept going. If I ever do that race again, I’ll do it more scientifically. I do like avocados, but they’re not the best thing during a long, arduous ride. Besides, they’re hard to tape to the top tube.