We all know that the activities of muscular contractions call for increased blood flow to the working muscles, but do you know that digestion also calls for an increased amount of blood flow? I wanted to take a moment to examine the physiological changes that occur as the demand for blood increases in any particular area of the body. This understanding will lead into the sports nutritional aspects of meal timing as well as how to enhance the performance for both aerobic and anaerobic activities.

We’ve heard as a kid to not eat for an hour or so prior to activity, right? I’m gonna delve into this a bit by first saying that we should not eat within at least 90 min prior to prolonged and/or intense physical activity. There is a physiological change that takes place relative to the cardiovascular system upon ingestion of foods. This change during digestion acts to redirect blood flow from the muscles to the GI tract (digestive system).

Since the body’s total blood volume desirably remains the same, the movement of excess blood to any one part of the body requires an enlargement (vasodilation) of the vessels in that specific region, and a reduction (vasoconstriction) in the size of the vessels in all other regions. When you eat, there is a greater need for blood in and around digestion for the purposes of carrying nutrients to the main vascular tree and aiding in the muscular contractions of the intestines. When vessels dilate around digestion, they constrict in muscular areas.

If you perform prolonged and/or intense exercise during digestion, the central nervous system will constrict vessels around digestion and dilate in the working muscles. This nervous input and redirection of blood flow slows digestive processes and the motility of the digesting foods in the gut. As a result you may experience not only cramping but also nausea, dizziness, and a general feeling of fatigue. The purpose of waiting 90 min prior to activity is to allow appropriate time for the digestive processes to complete.

You’ll note that in a previous post, I made mention to a 3-4 hr timeframe. This is to allow for ample time for both digestion and the assimilation of the circulating nutrients. By doing so, you will be ensuring that optimal liver glycogen and blood glucose is readily available at the onset. The majority of the digestion alone will take approx. 90 minutes depending on the composition of the meal.

Eating to Enhance Aerobic Performance

In the case of the aerobic athlete, or a cross-training athlete engaging in aerobic activities, it is of the utmost importance that liver glycogen stores are full and blood sugar is at its peak at the onset of activity. The liver can store between 300-400 calories worth of glucose as glycogen, and the blood has the capacity to carry about 80-100 calories worth of glucose at any given time. Absorption of nutrients into the blood can take as long as 3-4 hours.

Taking into account all of the above considerations, this means that to fully replenish energy stores, as much as 480 calories of complex and simple carbs should be ingested about 3-4 hours prior to an aerobic activity while remaining totally inactive. Not only will digestion be completed at this time, liver glycogen and blood glucose will be at optimum levels, thereby offsetting the exhaustion of aerobic activity. As a side note, it is not the exhaustion of fatty acids or oxygen that generally terminates aerobic activity; it is the exhaustion of available blood glucose.

Eating to Enhance Anaerobic Performance

In the case of the resistance athlete, muscle glycogen stores and blood sugar levels need to be at their optimum at the onset of resistance exercise. The majority of the energy used during resistance exercise comes from stores already compiled inside the muscle. Optimizing blood glucose levels is a precautionary measure in case of prolonged energy needs in the gym (high volume training). There are several nutritional methods for optimizing the various energy pathways for goal attainment, but the key is to eat properly during the entire recovery period to optimize replenishment of muscle glycogen stores.

From this standpoint, pre-resistance workout meals should be planned 3-4 hours in advance solely on the basis of completing digestion, which will in turn optimize blood flow to the working muscles at the onset of activity. This IMO is a crucial aspect that is often not given consideration.

Now, I’d like to add to both methods with a sports nutritional technique for say, using aerobic activity in conjunction with specific nutritional elements to maximize the fat burning processes both during and after. The common knowledge is that cardio only burns calories during the activity and will therefore cease doing so shortly following completion. It is this concept that brought to light the HIIT principle. This newer principle suggests that calories will be burned both during and after activity as the fuel sources will transition from one to the other. While both are true to an extent, there are some sports nutritional elements that can be incorporated to allow for fat to continue being burned after steady-state aerobic activity, as well as further igniting the furnace during and after HIIT sessions.

That will be another post…