In the February 2015 issue of Applied Metabolics, I discussed whether the common practice of engaging in aerobic exercise on an empty stomach actually does lead to increased fat oxidation. This is one of the many exercise concepts that bodybuilding and fitness competitors take for granted: that doing aerobics without eating first will always lead to more fat-burning effects. On the surface, it makes perfect sense. The usual practice involves doing the aerobics soon after awakening before any food is consumed. Most forms of exercise are powered mainly by stored glycogen, a long-chain carbohydrate, stored in the liver and muscles. But small amounts of glycogen are always broken down by enzymatic action in the liver to maintain blood glucose levels within a certain normal range. As such, if you haven't consumed any food for over eight hours, such as when you sleep, your morning glycogen levels are lower than usual. The theory behind fasted aerobics is that this lower level of glycogen will allow you to tap into an alternative energy source, stored body fat, faster than usual.
While the theory makes perfect sense, as my article noted, the studies that have examined the effects of fasted aerobics, including that done in the morning prior to eating a meal, have largely come up emptier than morning glycogen stores. Conversely, eating a meal prior to morning aerobics results in greater or equal fat loss effects compared to doing the same exercise in a fasted state. There are various reasons to explain this, which I discussed in detail in that previous article. But this also brings to mind another common question: is there a preferred time to workout? That is, does training at a certain time tend to produce more gains in muscular size and strength than at other times? And is there a specific time that will lead to more efficient fat oxidation or "burning" during training?
To understand the temporal, or time-based effects of training, you need to know about something called chronobiology.
What is chronobiology, and why should I care?
Chronobiology is the science of biological rhythms. Your body has a built-in natural clock that governs various mechanisms in the body, including the release of hormones. Perhaps the most familiar example of a biological rhythm is the need for sleep. The body is programmed to sleep at certain times, mainly at night. You are set up to sleep at night by exposure to light, which affects the pineal gland in the brain. This gland converts a brain neurotransmitter, serotonin, which itself is produced from the essential amino acid, L-tryptophane, into another hormone called Melatonin. This conversion slowly occurs all day, although the melatonin release is blocked by exposure . . .