After only one track workout, I've thrown out my back (from yard work). So, back to weird ideas.
Since the 1930's, it's been known that cutting an animal's calorie intake by 20% from what it would choose to eat increases its healthy lifespan. There are several reasons for this, but I'm going to focus on blood glucose levels, as that bears on the energetics of training. Continuous semistarvation, though it may have been the common state of man before agriculture, is unpleasant and difficult. It's been shown that many of the benefits of reduced caloric intake can be achieved by eating only every other day; because of a satiation response, one doesn't simply eat twice as much on the feeding days. This too is challenging; it has led to the popular "5-2 diet," where one eats 25% of the usual day's calories two days per week, which turns out to be the same 20% reduction in calories as in standard caloric restriction, but in a pattern that is achievable for most people.
A second fasting pattern that has its advocates is a monthly fast of 3-5 consecutive days with 30-50% of normal calories. While this does not reduce the average calorie intake per month by as much, it seems to have a positive effect on insulin response to glucose levels. It's worth noting that the original carbohydrate loading plan for marathoners, as devised by Åstrand, had a three day glucose depletion diet; though the utility of this depletion has become suspect, the biochemistry of fasting is useful to know:
On the first day of a fast, the body uses its supply of stored glycogen to meet the needs of cells that require glucose, such as blood cells. This gets exhausted, so on the second day, the body starts making glucose from breaking down proteins in a process called gluconeogenesis. Because the major source of protein is muscle and the body needs muscles to acquire food, it then begins another process on day 3 without food, turning fats into ketone bodies, which the brain can use as fuel in place of glucose. After three days, the body starts making long-term changes to adapt to not having fresh sources of glucose and after 6 weeks [how sad it is that we have good scientific data on people not eating for 6 weeks] only 6% of calories come from protein and 94% from fat stores.
The ketogenic diet, popular among ultrarunners, duplicates the starvation described above by eliminating carbohydrates from the diet. Because even a lean runner stores enough fat to run 500 miles, they eliminate the constant need to feed that other distance runners have. As a short distance specialist that never ran a great marathon because of running out of glycogen, it might have been worth trying, but it seems senseless for short distance that require some anaerobic running. Muscles can't burn ketone bodies and they require oxygen to burn fat, so after depleting a very small store of creatine phosphate (5 seconds), they have to burn glucose if working anaerobically. On a ketogenic diet, this glucose comes from protein. It seems unlikely - and here I have no good science to back me up, it's just opinion - that the body would adapt to convert enough protein to glucose to form significant amounts of glycogen to store in muscles. The best evidence I can find to support this is that the body stores water with glycogen and the "shredded" look that comes with such diets comes from not storing water in muscles. I tried the ketogenic diet for a bout 4 months and hated every minute of it; it's not for me.
A third method of fasting is to eat only during a few hours each day (how many hours varies according to source). Again, the idea is that satiety keeps one from eating as much in those hours as one would if grazing throughout the day. Here, glucose levels drop very low until feeding, but not long enough for one to start gluconeogenesis. I'd actually done this for years before I'd ever heard of it - it just happened to fit with how I was living. I can say from experience that it's difficult to avoid occasionally binging on junk when doing this.
None of the fasting research involves athletes. It's generally acknowledged that exercise is important for a long healthy life, but no one's connected the energetics of exercise to that of fasting. I think there's a clear connection that will show fasting is immaterial if one exercises correctly.
The continuous restriction diet is the same as heavy training loads, energetically. If one's eating 2000-2500 calories per day, a 20% reduction is 400-500 calories, or 4-5 miles per day or 30-35 miles per week. The problem is that we are uncannily accurate in taking in the calories we expend. You'd have to stay at the same number of calories, but increase mileage 30-35 miles per week - which is possible, but requires diligence that few can muster.
Never taking in carbohydrates to keep glucose levels low is equivalent to constantly burning calories throughout the day. Hunter-gatherers might walk 12 hours or more every day, but few of us are capable of staying active all day. The closest we can come are two-a-day workouts (there are people who have run 3-5 times per day every day for months, but they are rare for logistical reasons).
Getting glucose levels very low by not eating for most of a day can be compared to a standard practice among distance runners of not eating until after they've run in the morning; some claim this helps train the body to use glycogen sparingly. What is certain is that carbohydrates eaten immediately after exercise tend to get stored as glycogen in muscles where it can't contribute to blood glucose levels.
The 5-2 diet depletes glycogen twice per week. That's also what two hard workouts per week - what people commonly do - accomplishes. The body routinely stores enough glycogen to run 13-15 miles, but one doesn't need to do two long runs each week. A truly hard workout - "running to failure," to the point where one has to stop or dramatically slow, depletes glycogen.
I said the monthly fast, as I described it, is similar to what's seen in carbohydrate loading, but it also has a connection to what's known as crash training, where hard workouts are done back-to-back, or in this case for 4 days every three weeks. I recommend four different workouts, to hit different aspects of training and deplete muscles differently. Day 1, I'd do an interval workout working on maximal oxygen uptake. Day 2, I'd do a fast continuous run. Day 3, I'd do a long stamina run. Day 4, I'd do a sprint workout. The order would depend upon one's strengths and goals. The latter workouts will be difficult and slower than usual, which is why several rest days are needed afterward.
If you wanted to incorporate all of these into a training plan, I suggest running in the morning and skipping breakfast, then having a second workout later in the day that is cross-training and either extremely long and easy (preferably on weekends) or short and explosive; alternately, add a 4-5 mile run each day. Run hard on Tuesdays and Saturdays each week and every three weeks also run hard on the Sunday and Monday to make four consecutive days.
4 days ago