By Christian Wernstedt
It has been proposed that performing intense workouts (such as weight training) in a fasted state is a particularly efficient way to ensure that muscle glycogen reserves are emptied during training, and that achieving this state magnifies the metabolic benefit of increased insulin sensitivity in muscle tissue that comes from high intensity training.
I have found these ideas quite compelling, and I have worked out this way myself for quite some time, but I have learned about some qualifiers (from Robb Wolf and others) that one might want to take into account before rushing headlong into fasted training, particularly if one's mode of training entails frequent- and/or intense workouts added to a perhaps already somewhat stressful life situation.
The physiologic reasons for being cautions are the following (in my somewhat crude understanding):
Even in a person who has adhered to a low carb paleo diet for a long time, the brain never becomes completely independent of glucose as a fuel source, and there are also other tissues such as red blood cells that need glucose as fuel, so after muscle- and liver- glycogen has been depleted during a training session the liver must begin to convert other energy stores (fat and protein) to glucose. This process of fuel conversion is called gluconeogenesis, and in the context of a glycogen depleted state, it is tied to the release of the stress hormone cortisol.
This is all fine if it happens occasionally and acutely, however if more cortisol is added on top of an already elevated cortisol level, the result may be a cascade of unwanted side effects such as fat accumulation (typically around the waist), poor blood sugar control, and lowered sex hormones. (Cortisol is made out of the same precursor hormone as testosterone and estrogen, and thus "competes" with them for the same building material.)
Elevated background cortisol can be caused and exacerbated by chronic stress, lack of sleep, high coffee intake, and other stressors - unfortunately, few of us doesn't have at least a few of such in our lives.
Having cortisol out of control can actually lead to the paradox of a lot of sugar floating around the blood stream, even if one eats as little as zero carbs.
An example from real life is my recently measured fasted morning blood sugar of 95 after spending weeks in ketosis. This high reading might be benevolently explained by my generally up-regulated gluconeogenesis from being very low carb, elevated morning cortisol levels, and a bit of acute stress before the blood test, but I think the lesson is that the body can very well produce a lot of glucose with just the right hormonal inputs, and we don't particularly want something like this going on chronically.
Where the exact sweet spot lies in terms of fasted training vs. cortisol and stress likely (as all things health related) varies a lot from person to person, and according to each particular individual's total life situation in terms of stressors.
I think that if one lives a relatively stress free life, gets a lot of sleep, and doesn't work out intensely for more than say 45 minutes at a time, fasting both for several hours before and for an hour or so after a session of intense weight training may be the way to go. (Classic De Vany style.)
However, for many people it may be better to prioritize cortisol control before aiming for the boosted insulin sensitizing effect from fasted training, because elevated cortisol can negate many benefits from the paleo lifestyle, including exercise.
If one is already stressed out right out of the gate, yet aims for a prolonged monster work out, then eating a little something (not a gigantic meal, but perhaps a few low GI carbs - I really have no detailed idea) before going to the gym may be a good idea.
Another implication from the above is that in a person with an intense life style (career, kids, mortgage, etc) the infrequent, brief, yet intense style of workout offered by the Body By Science protocol may provide a reasonable "island of safety" in terms of cortisol, while achieving the insulin sensitizing benefits from fasted training. BBS seems to allow for fasted workouts without the risk of adding that much extra stress and cortisol. (This is also part of the strategy behind De Vany's training style.)
Note: I have no particular axe to grind when it comes to training styles, so my general point is only that the stress impact from exercise is something to consider when choosing what kind of exercise to engage in.