Review of Knuiman et al. 2015

Knuiman, P., M.T.E. Hopman, and M. Mensink. 2015. Glycogen availability and skeletal muscle adaptations with endurance and resistance exercise. Nutrition and Metabolism 12:59.

There isn't anything special about this article by Pim Knuiman and collaborators, but at some point I printed out a copy and read it because I thought it would be useful. It's a review article. It starts with background on what effect starting exercise with low amounts of stored glycogen (while fasting, weight loss diets, or just many hours since a meal) had on muscle adaptations following the exercise. In endurance exercise, studies have shown that mitochondrial biogenesis increases to a greater degree in this condition, but only when glycogen is only mildly depleated.

Also in the background material I found some interesting facts about the distribution of glycogen within muscle. There is glycogen stored within the sarcomeres (5-15%), within the myofibrils (5-15%), and between the myofibrils (75%). Each location uses the glycogen to fuel different molecular processes.

I learned that in general, moderate intensity endurance exercise (30-65% VO2 peak) is predominantly fueled by fat, whereas exercise exceeding that degree of intensity is fueled more by carbohydrate. I had heard that before, but somehow in my mind I reversed it until reading it again here. An interesting note is that this is just a general rule, and that well-trained individuals will have different abilities to use fat and carbs at different levels; they have "greater metabolic flexibility."

The article later addressed resistance exercise and confirmed the general rule that exercise + protein ingestion = muscle hypertrophy. This is not as true for endurance exercise, only because the protein gain (which is real) is simply replacing the protein lost during the exercise (known as the "interference effect"). In any case, protein consumption with a small amount of carbohydrate is important following exercise of all kinds, either to increase or simply maintain muscle mass, strength, and efficiency.

There is a general understanding that when one does both endurance and resistance exercises in one day, the endurance exercise should be conducted in the morning at a somewhat depleted glycogen state, and the resistance exercise should take place in the afternoon (since depletion wasn't shown to be important), followed by a high-protein meal.