This is the weakest link in this series of posts and I expect it'll probably be the most often read and commented on.
A person's energy stores are in the form of glycogen and fat. A typical runner has enough glycogen to last 15-20 miles, but enough fat for several hundred to several thousand miles; except for a few pounds, this is just dead weight hindering every step. For the most part, the less fat carried the better, and faster runners tend to carry less than slower runners.
Many people take up running as a method of losing weight and it often works, at least temporarily. Typically, one runs 15-20 miles per week and finds one has lost a few pounds; then one might increase to 35 miles per week and another couple of pounds are lost. Similar (usually decreasing) amounts are lost with each increase in mileage, but eventually one reaches a plateau and one cannot run any further; after staying at this level of mileage, one's weight often starts to increase again. The reason for this is exactly the same as why diets work only temporarily: one's body adjusts to the deficit in calorie expenditure.
Still, runners generally weigh less than their sedentary counterparts and this too needs to be explained. When I look back at my records, I find that my weight does not correspond well with my mileage, but the faster I ran, the less I weighed. The standard belief is that the less one weighs, the faster one can run (up to a point - eventually one loses muscle mass and slows), but I think the opposite is also true: the faster one runs, the more efficient one must be and one way to become more efficient is to lose excess weight. Weight loss is therefore a byproduct of fitness; losing weight is not the way to fitness.
One often sees that top runners manage very high mileage (the extreme: in his prime in the 1960's Gerry Lindgren averaged 300 miles per week) and top runners have low amounts of body fat [Elite male marathoners may be down to 3-4%. Occasionally, one sees lower numbers, but the best methods of testing are only accurate to at most +/- 2%. I once had skinfold measurements that said I was at 17.6% fat at 6 feet tall and 132 pounds - I was probably at 5%]. Each mile run brings less benefit than the one before it; not only is there a law of diminishing returns, but there comes a point where additional miles in one run becomes detrimental. Those who do very high mileage invariably run more than once per day.
For a very long time, I thought that the only benefit of high mileage was that one can only race a small percentage of one's miles, so the more miles one ran, the more frequently one could race. Then, upon entering the world of ultrarunning, I discovered that there is an advantage to being comfortable running at more than one time during the day (if one is going to be running all day) and that one learns to become comfortable running after eating, out of necessity.
There is one other benefit to running twice a day that has recently occurred to me and that comes from the fact that one burns primarily carbohydrates (glycogen) early in a run, so one may lower blood glucose levels by burning sugars more frequently - but that's the subject of the next post.
Intermittent high mileage?
If one becomes used to high mileage, one's body adjusts by eating more to compensate. The solution would seem to be to run high mileage perhaps one week each month, so that one does not become used to it. This, however, inevitably leads to overuse injuries.
The further one runs, the more fat burned and the higher percentage of calories in the form of fat are burned for fuel. The solution for losing weight would seem to be to run as far as possible, as often as possible; this, however, is just the high mileage approach in a different guise. If one runs very long often enough, one again becomes accustomed to it and one eats more to compensate.
The successful method is to run very long only occasionally. One burns a large number of calories, almost entirely as fat, and one eats enough to maintain one's weight at the number of miles one usually does. The only downside to this is that fat loss by this method is glacial; a 30-40 mile run every other week would lead to a weight loss of 20-25 pounds in a year - if one made no other changes for that entire year.
There is a place for resistance training in running. The more specific it is, the greater the benefit and that is why I am a proponent of hill running. Many take up weight training for the specific reason that increased muscle mass means a lowered percentage of body fat. By lowering one's percentage of body fat, one will look leaner, but it does not mean that one will run better. In fact, one will not only have the same amount of body fat to carry (unless one makes dietary changes as well), but also muscle mass that is not specific to use in running.
Many top ultramarathoners are more muscular than their marathoning counterparts, but this is for other reasons. First, ultras tend to be on hilly and technically difficult terrain, which requires strength and stability. Less importantly, if one runs out of glycogen, the body burns protein and the larger one's muscles, the less a detrimental effect that will have. There is also the possibility that, in races of 50-100K, larger muscles can accommodate larger amounts of glycogen, giving an edge, but that's unlikely to be important.
Running faster to weigh less
If, as I suspect, the body sheds excess body fat as a method to become more efficient, then the best way to lose fat would be to increase running speed. Here the problem is obvious: not only is there a limit to how fast one can run, but the faster one runs, the less distance one can run (and hence, the fewer calories burned).
Runners very often run the same distance at about the same speed every day. They become more and more efficient at running that one distance and they slowly improve their speed of running that distance until they reach a plateau where they no longer improve and their weight stabilizes. It is amazing how many runners I've met who have followed this pattern and wondered why they improved rapidly for a while, only to get stuck running races in the same times for years. When I've suggested trying to run a little faster in training, they invariably refuse.
Instead of running a bit faster in one's everyday training, one could run much faster on occasion. This is speed training - again, it's the next post. It sounds like an odd way to lose fat, because one is burning glycogen almost exclusively, but it is practical on the basis that weight loss is simply a matter of calories in and calories out. The current fad in weight loss by exercise is circuit training, incorporating weight training and interval training, which is based on this idea of burning fat by burning glycogen.
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