If you eat fat, your body uses what it immediately needs, then stores the rest as fat. If you need carbohydrate, there's no way for your body to convert fat to sugar. [To head off the inevitable person who says that the glycerol moiety can be converted to glucose - it doesn't happen under physiological conditions and, even if it did, would be unimportant.] On the other hand, if you eat carbohydrate, your body uses what it immediately needs, then restocks its stored carbohydrate (glycogen) and if there's anything left over, converts the remainder to fat.
There are two different stores of glycogen: liver and muscle. Liver glycogen (usually about 400 kcal.) can be used wherever it's needed, but muscle glycogen (typically about 1300 kcal) has to be used in the muscle where it is stored. Muscle glycogen thus cannot be turned into fat! If one is eating carbs and burning carbs, one is storing less of one's calories as fat.
There are four basic concerns to competitive running. 1) How fast can you run? 2) How fast can you run a certain distance? 3) How far can you run at a given speed? 4) How far can you run? These are, in order: sprinting, racing, pacing - for lack of a better word - and stamina runs.
The first few seconds of an all-out sprint, one's body is using for energy a phosphocreatine/ adenosine triphosphate (ATP) shuttle. When one's phosphocreatine runs out, glycogen supplies the energy to restock it. Sprints can deplete the Type b fast-twitch muscle fibers.
Running more than a mile at a very fast speed, whether racing or a fast continuous training (tempo) run, can deplete the Type a fast twitch fibers and/or the slow-twitch fibers, depending upon the length of the run.
It was discovered that one could run a greater distance at a given speed if one broke it into parts. Four minute milers can run 8-10 quarter miles in a minute easily, 12-15 with increasing difficulty. Interval workouts are done faster than race pace for most competitive runners and can deplete fast-twitch fibers.
Long runs burn primarily fat, yet eventually one usually reaches a point where the slow-twitch fibers get depleted.
Back to the first 7 parts
There are no fat centenarians. There are no fat elite runners. Those who live in societies that are long-lived tend to do low-intensity work continuously all day, just as the Tarahumara tend to walk or run all day. This low intensity effort over a long time burns fat and it is this type of activity that is lacking in modern American society. The best way we have to simulate this is to exercise frequently, but frequent long runs (which burn the most fat) lead to ingestion of more calories to compensate.
Frequent fast runs would use primarily carbohydrate and this, because of the methods of fuel storage and because of the body's methods of economizing fuel usage, could lead to fat loss. The problem with this is that one cannot deplete glycogen stores very frequently without leading to overuse injuries; it takes a minimum of two or three days to recover glycogen stores.
High mileage is akin to eating a low-carbohydrate diet, as one is continuously lowering glycogen stores. Long runs are akin to fasting, as one depletes glycogen and then burns one's fat reserves. Hard fast runs, because of the body's becoming more efficient, are like the low calorie "longevity" diets. There appears to be a way to mimic by training whatever people try to do by diet.
The challenge is: if one can only do occasional long runs and can only do occasional fast runs, how does one create an efficient method of training? The problem is exacerbated by the (nearly) universal need to fit everything into a 7 day week.
Let's start by putting a fast continuous run on Saturday (when most races are held). It takes until Tuesday or Wednesday to be able to run hard again. Put an interval workout on one of those days and there's a three day and a four day gap. Exercising only twice a week brings far less benefit than three days per week, so one adds another hard day in the middle of the longer gap - only to find the schedule is too hard.
So... we need two hard days and a "moderate" day. We have the hard run on Saturday, the interval workout on Tuesday and a short easy sprint session on Thursday. But wouldn't 4 days be better than 3? There's decreased benefit with each additional workout per week, but there's still a sizeable increase between three and four days. There can be no more "hard" days, but one can add a long run, which doesn't affect glycogen stores much. As most people only have time for a long run on weekends, we put it on Sunday. Then, one runs easy runs on other days, which help to burn calories and speed recovery.
This is exactly the schedule that you see almost every top runner has found by trial and error, regardless of the distance for which they train! As described, it fits best with a 5k-10K track racer, but it doesn't change much for others. For example, many hundred mile trail specialists do repeat miles on the track on Tuesday, do easy hill repeats on Thursday (which, as a resistance exercise, mimics the explosive energy usage of sprints) and run two long runs on the weekend; the first run might be 5-7 hours, which will deplete glycogen stores regardless of pace and the second might be 3-4 hours, which, glycogen not being restored, is entirely done on fat.
In trying to fathom the depths of physiology and energetics, I re-invented the wheel. But, because I think I now understand better what each workout is meant to do, I'll get more out of them.
If your eyes haven't glazed over by this point, I hope you enjoyed the ride and maybe you learned something along the way. Now... time for me to start training again.
Aid Station: Eugene Curnow
2 hours ago