Under-feeding your exercise eats up muscle
Glycogen is the storage form of glucose in cells, with most of it in our liver and muscle. When glycogen is low (see figure B, from reference 1), there is an increase in adrenalin (to increase fat availability), cortisol (to break down muscle), and an increase in the rate of sugar uptake into cells. The opposites are true when glycogen in muscle is full (figure A). Cortisol and other signaling pathways lead to a loss of about 40 Calories worth of muscle protein during each hour of exercise with low glycogen (reference 2). This implies that working out first thing in the morning without consuming any calories, or not eating right after exercise, or restricting your calories to lose weight when you exercise, will lead to body-fat reduction for only a few weeks or months before it backfires. Muscle tissue is the primary fat burner in the body, so not protecting it by eating enough to recover from you’re your workouts can cause you to lose not only body fat but also your primary fat burner, which will eventually keep you from burning any more body fat. It is ironic that the very exercise you are doing to lose fat can be what keeps you from losing it.
WHAT TO DO: If you are exercising for performance only, eat enough total calories and carbohydrate to recovery your exercise losses and recovery needs. If you are training for performance and body-fat reduction at the same time, under-eat calories and carbs by 10%. If you are exercising strictly for body-fat reduction, under-eat calories and carbs by 20%. Do not cut calories or carbs by more than this since it backfires.
HOW MANY CALORIES YOU NEED: Since they cannot all be scientifically measured, you need to guess how many calories your workouts burn and then see how your body does when you use your estimate to determine your recovery. Low, medium, and high intensity workouts for the average healthy fit person are 200, 400, and 600 Cal per hour (as much as twice this for a top athlete in peak condition). Roughly 75% (60-90% depending on relative intensity) of exercise calories burned will be carbohydrate. That means you will be burning 150-450 Cal of carbohydrate per hour. Replace most of this in the hours after exercise, taking advantage of the recovery window within the first 10 minutes after exercise by consuming 1/3 of the carbs you lost in the last hour. For workouts much longer than one hour, put up to 1/3 of your hourly carb losses into your fluids and consume them during the workout itself. These carbs should be glucose, a disaccharide of glucose (dextrose), a chain of a dozen glucoses (maltodextrin), or starchy foods that are high in glucose (potato, yam, rice, pasta, corn, bread, crackers, pretzels). Maltodextrin causes a little less stomach upset than glucose or dextrose, so the best sports fueling aides are those whose first caloric ingredient is maltodextrin. Such products include HammerGel, Gu, and many others, but maltodextrin is also sold in its pure form in any home-brew supply shop. Personally, I just stick to real food sources, but if you have a sensitive stomach after exercise, go with powder in water e.g. a gel or sports drink focused on glucose. If you consume too much of it, your insulin response will crash your blood sugar, which is worse than if you consumed no carbohydrate at all. So pace your carb intake to avoid the crash (low blood sugar), but make sure you eat replace most of your carb losses (otherwise you will, again, have a low blood sugar). Fully-stocked glycogen means a stable blood sugar and therefore low cortisol.
1. Scientific review titled “More than a store: regulatory roles for glycogen in skeletal muscle adaptation to exercise” by Andrew Philp, Mark Hargreaves, and Keith Baar, Am J Physiol Endocrinol Metab 302 2012 E1343–E1351
2. “Effect of muscle glycogen on glucose, lactate and amino acid metabolism during exercise and recovery in human subjects” by Eva Blomstrand and Bengt Saltin, J Physiol 514 1999 293