An important preparation for Yoga is avoiding meals prior to the Yoga practice. Generally, we are encouraged to not have eaten 1-2 hours prior to performing Yoga and Breathing (Pranayama) exercises.
Having food in the digestive system can disrupt one’s overall practice. The digestive process requires energy. When food is in the digestive tract, the body requires a full flow of blood to these tissues to nourish the gastrointestinal cells that are processing food material. Energy is required for the contraction of gastrointestinal walls to move food through the system. Energy is required for absorption of nutrients and for the production of hormones, enzymes and other chemicals to manage digestion.
When we perform Yoga poses, these poses and transitions take energy away from the digestive system and, if food is in the digestive tract, this system becomes limited in its’ capacity to digest.
Another consideration of having food in your system while practicing Yoga is the hydrostatic pressure that can occur. Food moving through the system is generally fluid in consistency. When one twists, bends, and compresses the torso, portions of the gastrointestinal tract can be pinched off (like a balloon filled with water). With further compression from the pose, the food can push outwards (hydrostatic pressure) against the lining of the tract. For some people who are already prone to digestive disorders like diverticulosis, this pressure on the tract linings can lead to damage of theses tissues or aggravate existing conditions.
Important note: if you have health concerns like diabetes or hypoglycemia, you should consult a health professional to determine how eating should be approached with Yoga exercises so you can manage your blood sugar levels properly.
Beyond these health issues, other considerations with not eating before practice should be observed:
*food in the digestive system may cause nausea, bloating or gas during practice
*often one needs to go to the washroom while practicing if the digestive system is processing food, thus disrupting one’s practice and connection to the flow
*if practicing on an empty stomach makes the practice difficult (ie low energy), eat a small portion of easily-digestible food like apple sauce or yogurt 30-45 minutes before practice. This small amount of food will make its’ way through the system quickly and will not have adverse effects during the practice.
Kreg Weiss 
Great post. Thanks for sharing. I have always wondered exactly why it is suggested you don’t eat before class. I knew it can make you feel sick, but I have never sought out a deeper explanation.