Why a yoga practice is done on an empty stomach
It has been said that you should not eat 3 to 4 hours prior to a yoga practice but I have never had any modern yoga teachers able to tell me why. Bikram is known to have been quite abusive in his way of yelling at people about not being able to have water in a class yet none of his teachers that i have asked have known the reasons behind the commandment.
The why here is a very simple one and lends a bigger understanding to the fact that asana is only to balance the pranas and open sushumna so you can sit in a comfortable position for the rest of the practice for extended hours.
When you eat a meal or any food, it is chewed and swallowed and the digestion has begun. It takes a person between 3 hours and 8 hours to digest a meal depending upon the quality of the substance they ate as well as the strength of their own digestive fire which is variable dependent upon the person and the season as well. In general, in the winter the digestion is strong, in the summer the digestion is weak due to the hours of sun in the day and the heat created on the planet at that local. The body is weak in the summer and the digestion is weak as well due to the long days of heat that pull the strength from the body. In the winter, the short days of sun allow the season to be cold. This cold pushes the heat of the body to the torso making the digestive fire stronger. Exercise is done minimally in the summer to maintain strength (once in 15 days) and much more (once in 3 days) in the winter.
When the food is being chewed, kapha starts to do its job. It mixes with the food pulling it together into a bolus. Swallowing occurs due to prana and then the first part of digestion is kapha again. The movement of the food through the entire digestive process is due to 1 of the 5 pranas. This movement will be disturbed if you eat then exercise as exercise stimulates prana and that digestive prana will move out of the digestive tract. The whole process of digestion is stopped and the food is then unprocessed turning into aama (toxic morbid metabolic waste) and clogs the subtle channels of the body. Asana (real asana) and pranayama function to open the subtle channels of the body so that the state of unmani is attained. The cleansing of the nadis by pranayama allows proper regulation of prana in the body. But once again, a real yogi is not going to be getting a latte before class much less eating three meals a day or snacking on gluten free kale chips so not much of any of this knowledge is applicable to what is going on in the modern world.
In the texts (HYP verse 2.3), it is said that a small amount of milk (obviously boiled) is okay to be consumed prior to the practice in the beginning. Later this is stopped for the obvious reasons stated above.
So you see, having water or food during or prior to an asana practice hinders the aspirant and clogs the channels. This toxicity that occurs through the improper digestion of the intake creates clogged channels which further imbalances the flow and proper functioning of prana. Prana itself is responsible for all the sensory and motor movements of the body and mind. This means the physiology, how the organs each work separately and in tandem and their health as well as the proper functioning of the mind with its ability to perceive, digest, follow thru with proper choices, and memory is all due to prana.