The 45,000 or so runners who will gather on Staten Island for this Sunday's NYC Marathon have put in the training. They've run and stretched and strengthened and covered their finely honed machine in technical fabrics. The only question that remains is this: What should they put in the tank?


Sport scientist Ross Tucker is one of the most respected people in endurance sport research, so I asked him about marathon fueling and the difficulty of cutting through the crap to find something that actually works. If you want to cut to the main course, be my guest, the questions are below in the bold type. Otherwise, here's a quick history of how we came to the giant spaghetti dinners.

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A Brief History Lesson

Prior to 1960 or so, chariots of fire were fueled by a big juicy steak. Boxers, cyclists, endurance athletes of all sorts were advised to fortify their strength with red meat just hours before their contest. Reportedly, famed Boston Marathon director Jock Semple followed that advice, ingesting a 2-inch-thick steak just before heading to the start line of the 1934 race. Problems ensued.

That poor, misguided, primitive idiot. Now we have science; now we know that a mountain of pasta the night before the big race is the dinner of champions. ...Right?

Carbohydrates had long been thought to enhance endurance activities but the science wasn't understood until, in the late 1960s, Swedish researcher Gunvar Ahlborg found that carbs were stored in muscles and the liver in the form of glycogen. Though these stores were limited, Ahlborg found that glycogen storage capacity could be increased for marathon-length energy demands by a week-long regimen of depleting the tank and then superloading it. So the human gas tank was both enlarged and filled up.


Ahlborg's protocol started with an exhaustive 90+ minute workout done one week prior to the marathon and three or four days of extremely low (10%) carb intake. This effectively drained muscles of all stored glycogen. The three or four days leading up to the goal marathon were spent eating a very high carb diet (90%) with small percentage of fats and protein and very light training.

While this method of carb loading did provide energy for a longer period of time, many athletes found the long hard workout one week before the marathon and the carb depletion days too stressful, almost negating the benefits of extra glycogen stores.


Subsequent theories of carb loading have become shorter and more pleasant. One calls for a long, but not exhaustive, workout one week before the main event with three or four days of normal diet (50%-60% carbs) and three or four days of increased carb intake (70%).

An even more condensed version calls for a normal diet and tapering activity for six days leading up to the marathon. The day before the race, the athlete blasts a very short intense workout—two-and-a-half minutes run at mile pace, followed almost immediately by a 30-second sprint—and spends the 24 hours prior to the start line on a carb binge (90%), cutting back on fats and protein so she/he doesn't explode, like a Beagle.


Enter fruitarians, paleos, fatsos (that's my term for acolytes of Tim Noakes' high fat Banting diet) and others, including many ultramarathoners who eschew carbs and fuel their long-distance pursuits on kale, eggs, nuts and oneness with the earth. Carbs are bad. Carbs are addictive. Carbs give you cancer. Carbs make you fat. Wha…?

Now is the time to go to an expert, in this case, Ross Tucker, PhD, one of the scientists behind The Science of Sport. I put some carb loading questions to Tucker and he was kind enough to answer.


Is carb loading still valid? Was it ever valid?

Errrr, well, now, that depends! There's no doubt that when the carb stores run out, fatigue is the outcome. And there's no doubt that carb loading increases these stores, in the same way that you fill up the tank in your car before you hit Route 66 for that trans-America road trip. So in theory, carb loading is valid.


The catch is this—just like you don't plan to drive all the way across the USA without filling up again, you don't run the NYC marathon without planning to take any carbohydrates during the race. And so when we have the opportunity to constantly refuel, and provide the body with carbs [in-race drinks, gels, bars], then the loading phase becomes rather more redundant/unnecessary.

There are downsides to carb loading too: You store water with carbs, and so if you increase the carb content by say 2 pounds, you probably gain 6 to 8 pounds of mass. Give a runner an 8 lb backpack on the start line, and they'll throw it in your face. But carb loading could easily add that same mass in the days leading up to the run. That could just as easily cancel out the performance benefits.


So overall, my opinion now is that, provided the athlete is not carb-depleted, a normal diet, perhaps slightly higher in carbs, in the three days prior to the race, is probably sufficient. Then race day takes over as more important because that's where you can lose or gain the most, by having a plan to replace energy on the go.

While I'm rambling, a word about paleo diets. People who have followed them for a while, and are adapted to them, need fewer carbs than those who aren't. The body is a clever and amazing machine—it adapts to 'stress' and the removal of carbs is a stress. The response is to get better at using fats, and so these fat-adapted runners can probably get through the NYC race with very little carb replacement. But anyone who has recently taken up paleo, and is still in that phase of adjusting is heading for disaster.


There is a lot of evidence now that athletes can achieve some nice results if they go on a depletion diet while they train, and then switch to a higher carb diet in the last few days leading up to a race. Best of both worlds, right? You get the fat adaptation which makes the body less reliant on carbs while you are in the low carb/paleo phase, and then you still top up the tanks just enough that when you race, you're in good metabolic shape! Pre-race and in-race nutrition is highly individualized.

Is there data as to how much performance a super-loaded glycogen tank buys you over just eating normally?


No, it's a very grey area. In the lab, there are studies showing that time to exhaustion increases, but they haven't translated across to the "real world" too often. This is a common problem with science, incidentally. Part of the reason for this is probably the weight issue: you can gain 8 lbs from carb loading, and in the lab, when you're cycling, it's irrelevant. When you're hauling yourself up a little hill in Central Park after 35km of running, it's not so innocuous anymore.

The consensus seems to be that carb loading is worth 2-3 percent. But I'm not sure whether this is based on JUST carb loading and not taking onboard carbs DURING the run, or whether a combination effect exists—carb loading before the race plus carbs during. I actually don't know. I suspect there are big individual differences.


There are some nifty studies that have found, for example, that simply tasting a carb-containing drink and then spitting it out (in other words, rinsing your mouth) improve performance compared to not doing it. So it may be that the performance benefit from carbs DURING exercise outweighs any loading benefit because there are sensors in the mouth that make a difference.

Is it possible to overload on carbs the night before? What would the result be?

Yes, for sure. Bloatedness, stomach problems, diarrhoea the next morning. Especially in people who don't regularly eat that much of them. If you are typical fit person, you probably eat about 4 to 5 grams of carbs per kilogram body weight per day, so maybe 300 to 400g per day. You look in a textbook and find some expert has said you need as much as 10 g per kg per day, and so you go from 350g to 700g. You're going to feel like you've been force-fed to bursting point. Not a good way to prepare. So again, I really do think that a person's habitual diet, maybe increased slightly, is the way to go, PROVIDED they get the IN-RACE nutrition right. Car analogy again: Don't overfill the tank, just make sure it's not empty and then make sure you don't skip that gas station because next thing you know, you're hoping for a kindly traveller to pick you up in Death Valley.


Why did carb loading/glycogen fueling become the preferred method for marathon fueling when it is so limited? Why didn't burning our very adequate fat stores become the standard?

Precisely for that reason. Like a chain is only as strong as its weakest link, our fuel supply is only as effective as its most limiting source. It was also partly a question of what could be measured—muscle glycogen was measurable because in the 60s and 70s, biopsies became possible. Suddenly we could see inside the muscle, and glycogen was measurable and it became a candidate perhaps with more weight than it deserved. Same thing happened with lactate and fatigue, incidentally. You're only as good as your eyesight, and that was all scientists could see, it had to matter, right?


But because glycogen was limiting, it was the candidate. And in fairness, many studies confirmed this. If you cycled or ran for as long as you could before the lights went out, that fatigue coincided with a critically low level of glycogen, either in the muscle, or in the liver, which affects the blood. So logic said it was the limiter. And it is, when you set up a relatively contrived exercise situation. In the real world, it's a little more complex, but still true that if we exceed the body's capacity to provide energy, we crash. Carbohydrates are excellent at doing that. Because they're so readily available, they're the best source of energy for relatively high intensity exercise. Fat may be limitless, but it's hard to access, it's slower, and just unable to meet the demands for muscle contraction. Unless, of course, you're a very highly trained athlete or you eat a diet super high in fat and low in carbs. Then you adapt and become so good at using fat that you can afford to use less carbs.

Why do people bonk? Why don't their bodies just switch over to burning fat mid-race when the glycogen runs out?


Partly for the reason I explained above. Fats just can't meet the RATE of energy required. The muscle is using all this energy, and it has to be replaced. So is the brain, and at some point, the fats can't provide the energy at the rate required. The brain is also pretty lousy at using any source of energy other than glucose, especially during exercise. It has to have it. And so the bonk happens when the fuel tanks run dry. The car runs out of gas, the body runs out of fuel. And yes, it has another source of energy [fat], but that source isn't enough. The poor brain is by now struggling so much that the person becomes dizzy and weak and confused, tunnel vision, loss of coordination—all symptoms of brain 'failure', in the sense that the brain can't function without at least some glucose in the blood. Glucose in the blood comes from glycogen in the liver, so you have to defend it. That's why constant refuelling on the go is so important—it's adding to the blood glucose from OUTSIDE, so that you can spare what is inside.

Mid-pack runners seem excessively concerned with fueling, sometimes more so than with training. If your long run has been 15 miles, can carb loading carry you through the remaining 11 miles on race day?


Yeah, the average runner eats and drinks too much. This is partly the result of a fear doctrine that exists, I have to say, more in the USA than elsewhere. It's why the USA gets so many cases of hyponatremia, which is a condition where people drink so much fluid that they basically drown themselves. Very common, but happens almost entirely because people drink more than they need to. The same can happen with fuel, especially because most of our energy during exercise comes in liquid form.

The truth is that you need some kind of plan, but you can't plan rigidly. You can't create a timetable that says XXX ml per 15 minutes, because on a hot day, that may be too little, and on cool day, too much. For energy, we can make suggestions for topping up on the run, and the idea here is to aim for around 500 to 800 ml of a sports drink PER HOUR. Not all at once—you feel too bloated—but in smaller doses. So the plan might be 150 to 200ml every 15 minutes. An athlete who does that will be fine. There really is no need for more, especially in people running four hours or more, because their intensity is low and they're not really burning up the carbs like the elite athletes.


So practical tip—150ml to 200ml of whatever sports drink the race provides (Gatorade) every 15 to 20 minutes. Or to thirst. If that feels like too much, then it is. The body is smarter than we give it credit for.

Does carb loading the night before a marathon make sense if you've been following a high fat/low carb diet?


Yes, but with restrictions. There's quite a bit of evidence that people on this low carb diet become fat adapted, and thus really efficient at burning fat as fuel. They don't need the carbs, and so in theory need not carb load. However, there's also evidence that if they do, they get the best of both situations—good fat burning ability from their normal diets, and more carbs from their last few days. Practically, it's not always that easy, because if you give carbs to someone who hasn't eaten them, you're inviting trouble—headaches, nausea, bloating. So you have to gauge for yourself where the upper limit is. One potato? One bowl of oats or pasta? Different people, different responses.

Why carb load if you're going to be downing gels and sports drinks all along the 26.2 miles?


Exactly. That's why the advice is probably to make sure you don't start empty, but don't obsess over starting super full either. Unless you have an aversion to eating and drinking on the run!