Let’s start with the tales of two metabolisms. We will start in the context of intermittent fasting, then go run a marathon.
First to define intermittent fasting, this is consciously eating your daily food within a set window of time, usually 6 hours, and not ingesting anything but water during the other 18 hours. Fasts can be any duration, but effective daily fasts are usually 18 hours. Periodic 24 hour, 72 hour, and even longer fasts can also be done for varying reasons and additional benefits.
The first case we’ll consider is the carbohydrate eater. They hear about fasting and its benefits and want to try it. The problem with fasting on a carbohydrate based diet is the insulin roller coaster that the body gets into, with massive spikes and troughs and it is anything but a stable sustainable strategy. These ups and downs of insulin and blood glucose cause major hormone fluctuations and put many of the other body’s systems into a roller coaster as well, including bouts of extreme hunger, maybe even “h-anger”, and hitting troughs of fatigue mid day. In fact, the hormone roller coaster stresses the homeostatic processes in the body to the extent that it can release a significant amount of cortisol. Cortisol is the anti-stress hormone released when you are, well, stressed. Its job is to reduce inflammation, which will be present from the high blood glucose, and it does this effectively when secreted in short bursts. But chronic high levels of cortisol are horrible. They begin to cause adrenal dysfunction, and cortisol is antagonistic to testosterone, so testosterone levels will plummet and stay low in a chronically stressed, i.e. a chronically carbohydrate fed state.
Moreover, if you are fasting, you can very well put yourself into a caloric deficit. Some folks think this is great. They want to lose weight. A caloric deficit is where they think they want to be. But this is very problematic in this circumstance because the body will easily slip into starvation mode due to the constant spiking of insulin levels and the lack of energy available to the cells.