The following article was written by Dr X
Today I will briefly discuss the pump, where the blood comes from and the relation it has to what is eaten and or drank prior to working out.
When skeletal muscles are at rest, only 20% to 25% of the capillaries are open. During exercise 100% of the capillaries are open due to the demand for both oxygen, nutrients and for the removal of waste products. The flow to an active muscle changes depending on exercise intensity, contraction frequency and rate. A power-lifter and a bodybuilder even though both lifting weights will have different demands thus blood flow.
Take a look at two charts below, so you can get a reference from where that blood comes from once exercise begins. During exercise, blood flow is dramatically increased to muscle, the respiratory tissues, the heart and the skin. Blood flow is decreased to all inactive muscle, the GI tract and kidney. The GI tract usually can get by with less blood flow during exercise because most of the blood flowing to the digestive system is for absorbing food molecules rather than for keeping the organs alive. The small intestine is 6 meters long, about 20 feet so the massive surface area requires tremendous amounts of blood! About 90% of what we eat enters the blood from the small intestine. Also blood vessels to unnecessary systems directly related to exercise such as urination and reproduction are constricted, thus limiting the amount of blood to these organs too.
So you can see what you eat prior to and what you drink may interfere with two simultaneous demands on the body. Exercise automatically decreases blood flow at and around the gastrointestinal tract slowing digestion and absorption for the working muscles. But for those using insulin that ratio may change depending on dose. As blood sugars are drawn down so must the need for absorption thus a new equilibrium of flow has to be maintained. Blood is thus shunted back from muscles to aid in absorption. This is why that drink and pre-workout meal must be simple in design and easily moved through. You want as much of the blood flow to remain in working muscles not rededicated to digestion. Remember this next fact, blood flow to the GI tract increases gradually and markedly after a meal, and more so after a big meal than after a small one. The increase in blood flow reaches its maximum after 20-40 minutes and lasts for 1.5-2 hours. Exogenous Insulin use is a balancing act of needed carbs, and current blood sugar levels. The higher the blood sugar levels prior to training the lower the threshold of sugars that may be needed from a drink. This last point gets difficult to nail down and standardise day in and day out for me personally. Sometimes I have a reading of 60mg/dl, 30 minutes prior to entering the gym and others 130mg/dl. Both having the exact same diet intake and physical activity to the prior days. This is why a blood glucose meter is so important when using insulin. Daily adjustments must be made to prevent both poor training sessions and excessively low blood sugar levels.
Now for another fun fact. Why do some bodybuilders throw up during super intense legs days? It is because of that blood flow mechanism explained above. The body simply cannot digest, absorb food and exercise in the most intense of situations. Legs require the biggest demand of blood flow. Thus the body has to expel undigested food as it cannot concurrently do both simultaneously at a 100% efficiency.
My last important point is do not fall for all the additional supplements and additives some claim is needed in that workout drink. That sales pitch is purely for their profit .
Keep your hard earned money and keep that drink and pre-workout meal as simple as possible!