AM Cardio?

Skeletal Muscle Glycogen Content
Diurnal Variation and The Effects of Fasting
RK Conlee, MJ Rennie, and WW Winder

To test whether skeletal muscle glycogen concentration is related to food consumption, glycogen content was determined in red (R) and white (W) vastus lateralis and in soleus (S) muscles from six groups of ad libitum-fed rats killed at 4-h intervals and from 24-h-fasted animals killed at 0800 and 1600 h. The animal quarters were illuminated between 0700 and 1900 h. Glycogen values exhibited a peak at 0800 h and a nadir at 2000 h. These changes bore no relationship to blood glucose and lactate or plasma free fatty acids, glucagon, insulin, and corticosterone concentrations. Fasting resulted in reductions of glycogen content of 49% (S), 47% (R), and 29% (W) in animals killed at 0800h, but at 1600h changes were only 23% (RY), 17% (W), and 8% (S). The smaller changes at 1600 h were apparently due to lower glycogen levels in the tissues of the fed animals. It was concluded that skeletal muscle exhibits a diurnal variation in glycogen content, and that, contrary to accepted belief, fasting significantly alters muscle glycogen concentration.

Commentary From The EvilGenius

The above abstract is one of the more interesting pieces I’ve found lately from the world of academia. The time-honored idea that muscle glycogen is lowest in the morning “because you’ve not yet eaten” may be starting to show its age.

The data as presented suggests to me (and many others) that activity is the deciding factor in muscle glycogen content, in normally fed subjects. Keep in mind that wasn’t the expressed intention of this study but may well be the lasting legacy of the work of these three researchers

Relevance To The Athlete

The fun side note to this train of thought is that it may not change anything! For decades now athletes have been doing cardio in the early am before their first meal of the day. The idea firmly entrenched that low am glycogen allows for a higher proportion of expended energy to be derived from fatty acids.

In truth, it looks more and more like the real value of am cardio, just maybe that it merely expedites the inevitable and “normal” diurnal depletion of muscle glycogen later in the day. True to the misguided theory of days past depleted glycogen stores will force the metabolism to seek the additional needed energy from secondary sources (stored body fat), just not proximally to the time frame of the cardio performed.

Food for Thought

With the concept and acknowledgment of diurnal variations in fuel stores perhaps the most superior positioning of fat burning exercise in a daily scheme is at the end of the day. The day’s end model offers many advantages to my eye, it holds up to scientific scrutiny (a damn big plus), and it limits the possibility of “re-eating” the energy expenditures made.