Energy Balance During Endurance Events

We are generally strictly recreational runners. Cynthia ran a few marathons some 20 years ago; the longest races that David ever entered competitively were a few 10K events (plus a sprint triathlon), though we’ve also both done quite a bit of race organizational work. On something of a whim, we completed our first 50K race last weekend. The race was in the Marin Headlands, north of San Francisco. It was probably not the best choice for a first 50K, since it involved serious hill climbing (about 7000 ft of total climb spread out over seven “hills”) and rough trails. We were probably not as well prepared as we should have been for such an event, but figured if we could do 18 miles over similar terrain, we should be able to struggle through another 13 miles if determined and prepared for blisters.

Our basic aim was to complete the event in reasonably good condition, and we succeeded admirably, if not speedily, coming in at about half the pace of the first finisher, and very near the back of the pack. Not surprisingly, the last half was more an exercise in keeping going than trying to achieve any speed at all—we walked up all the hills and jogged pretty slowly on the way down. However, we were still mobile at the finish line and had no major injuries or problems. We took two days off and were able to run one of our standard hour-and-a-half hilly runs, feeling, if anything, stronger than usual the third day. On balance, except for the slow pace, we were pleased with our success.

Just so as not to blame our low carb diet for any difficulties, we had a pasta dinner the night before in the traditional “carb loading” style of runners everywhere, and even ate oatmeal for breakfast before the race began. (As it turns out, Cynthia felt worse than usual, whether that was due to the early hour and lack of sleep the night before or to the change in diet, we don’t know.) And of course we tapered our training the couple days before. One advantage of running in an organized race is the periodic aid stations, where you can load up on water, electrolytes, and food such as potatoes, cookies, chips, PB&J sandwiches, so you don’t have to carry liters and liters of fluids and snacks with you.

There were many with the usual runner physiques, slim and wiry, and quite a few older runners, some significantly older than we are, some distinctly overweight, but in strong physical condition nonetheless (better than us it turned out). Certainly many of the people interested in these long events tend to be those that are good at it, with some inherent natural speed and talent. But you don’t have to be inordinately talented to enjoy a long walk/jog through the woods, communing with nature, or challenging your own talents.

The course was every bit as difficult as promised, and about half way through, after the first three monster hills, Cynthia felt “bonked”—low on energy and wondering how to eek enough effort out of her tired legs to get through the rest of the event. In fact, Cynthia thought it was far more difficult than any of the street marathons she ever did—those resulted mainly in increasing pain in the legs and feet, and less of a sense that she could not continue due to the low energy or the weight of her legs. And it took more than twice as long as any marathon she had ever run! A serious testing indeed. Even her lungs feel fried days afterwards.

Description of the event can be found in an article in the Marin Independent Journal, as well as Jean Pommier’s fartherfaster blog.

David clearly was better prepared to maintain a higher level of energy output and likely could have gone significantly faster, but we stayed together for safety, support, and companionship. The fabled Headlands fog never lifted and the cool sea breeze was a blessing. Cynthia doesn’t think she could have made it in normal summer heat. The volunteers were wonderful and supportive: Many thanks!

But the interesting story is really our observations on energy intake and expenditure. Cynthia was reluctant to start eating at the aid stations, because the lump of breakfast hadn’t even passed yet, and why should she need more carbs when the oatmeal was still digesting? She did go for juice and chicken broth (a favorite for cutting hunger pangs and preventing nausea) and a bit of sandwich, but she didn’t feel that anything coming in made much of a difference, except perhaps to allow her to keep going, albeit at reduced pace. And perhaps that is enough, to keep going when fatigue, nausea and assorted pains would otherwise make you stop. We estimate that Cynthia’s total caloric intake during the race was only about 800 kcal—She has never been good at eating while running. We think she drank enough to stay hydrated—not too difficult in the cool conditions.

David’s approach was quite different: he’d quickly down a couple of gels (at 100 kcal each), a couple pieces of boiled potato or bit of banana at each aid station, and drink a couple of pints of sports drink and/or water over the distance between aid stations (about 4 miles apart). His total intake during the race is estimated at more like 3000 kcal.

We attempted to estimate the energy expenditure (as excess over basal metabolic rate) required to complete the race. We did this a couple of ways based on published values for running and walking on the level, and some information on the food energy intake required to sustain a certain level of mechanical work output (you need about 4 kcal of food energy in for every 1 kcal of mechanical work). Our rough estimates are that Cynthia burned about 3500 kcal, and David burned about 4500 kcal during the race. (It felt like a lot more!)

Of course, in addition to the food consumed during the race, we ate more than usual before and after the race. All told, Cynthia consumed about 1000 kcal more than typical, and David consumed about an extra 3000 kcal. If these numbers are anywhere close to correct, Cynthia ran a net deficit of about 2300 kcal, and David a net deficit of about 1500 kcal. Conventional wisdom is that it takes a net surplus/deficit of 3500 kcal to gain/lose a pound of fat, so a strict energy-in-minus-energy-out-equals-energy-stored-as-fat model would suggest that we should each have lost some measurable weight (but under a pound) once we were back to steady-state hydration levels (i.e., by the next morning or so).

So what really happened? Our weights were up a pound or so on race morning (presumably from carb loading the night before). The next morning, David’s weight was up another four pounds; Cynthia’s weight peaked the second morning after, up about three pounds from pre-race levels. Thereafter, the weight came back off over 4–5 days, returning to previous trendlines. The gain was particularly rapid, although the loss was also very rapid if looked at out of context.

Even though we don’t believe in the energy-in-minus-energy-out-equals-energy-stored-as-fat model, we were still surprised to see such a dramatic weight effect and especially to see a significant temporary weight gain as a result of the event. So why did our weight increase? Maybe muscle and ligament damage from overuse results in temporary swelling and fluid retention due to inflammation? Maybe the body fears that you’re going to go out and get dehydrated again, so compensates with some hormonal signals to retain additional fluids? Our appetites were elevated substantially for a couple of days, but not enough to account for the weight gain. Is it psychological or physiological? What about growth hormone and insulin levels after endurance events? Or some other hormone levels? Or were we just seeing the results of a spike in carbohydrate consumption and the consequent increase and decay in carbohydrate-induced fluid retention? (The correlation between carb intake and some amount of increased fluid retention is well-known, particularly in the context of initial weight loss when starting a low-carb diet, although it is less well understood exactly where and why all the increased fluid retention occurs or what the time-constants associated with diet changes should be.)

A week later we ran one of our longer routes—about half the length and time of the race—consuming much more modest total calories in general and carbs in particular. Knowing what to look for, we saw a modest increase in weight the next morning (a pound or so, and more or less within the noise of normal daily fluctuations).

The bottom line? We clearly can’t recommend endurance running as a weight loss aid. One, the energy consumed is just too small!!! It may feel like you’re burning off huge amounts, but the fact is, our bodies are quite efficient, and can perform large quantities of work using modest amounts of fuel. Two, while it is easy to end up with some calorie deficit, it is difficult to do so in a measured and controlled way—the tendency is to overcompensate for a perceived expenditure and negate any losses. Three, with more strenuous expenditures, there is a need to replace not just calories, but also vitamins, protein and electrolytes, etc., to heal injuries and strengthen bones, ligaments and muscles, and restricting food can lead to exhaustion if not serious injury and illness. Four, at least in the short run, there appears to be compensatory fluid retention and weight gain, so don’t think you can go out and burn off a pound of fat in a day and thereby fit into that dress you want to wear!

While, in general, serious endurance athletes report being able to eat more or less unlimited quantities of whatever they feel like eating, more casual or occasional endurance efforts aren’t going to have much effect, and may even tend to show weight gain instead of loss, especially if one erroneously believes one is burning off fuel in excess of actual needs. That said, we did find the experience to be a positive one overall, given our incoming level of fitness (i.e., we weren’t attempting anything too far outside of our level of fitness). We recovered quickly and felt, if anything, stronger and fitter afterwards. We stayed within the recommendation that, when first attempting longer and harder events, one should first just aim to finish and become “comfortable” (or at least, familiar) with the new distance, then try to do it faster. We're still working on it, but further gains may have to wait until we’ve dropped down to more of an optimal weight, and until we’ve undone various metabolic and/or structural damage due to poor eating habits over many years (assuming our present diet is doing so)!