Who Gets the Blood?
Gut vs Skeletal Muscle Perfusion in the Post-Prandial Period
Main Takeaway: If you want to decrease the chances of GI-distress in your workout, try to maintain a 2-2.5 hour window between your last meal and your workout. Liquids and small snacks within that window can be beneficial without negative gastrointestinal symptoms.
Imagine this: you have a lunch meeting scheduled for noon. You want to impress your client so you choose your favorite restaurant in town -- the brazilian steakhouse downtown. You meet, share some small talk, and tell him to order whatever he wants off the menu. In an effort to impress your client, you tell the waitress to bring the most popular appetizers on the menu for the table. After multiple rounds of lamb, steak, chicken, and the seafood you didn’t even want but ate anyway to justify $200 you just dropped, you both waddle out of the restaurant uncomfortably full.
It is 1pm and you suddenly remember that you pre-registered for today’s spin class at 2pm. In an effort to evade the $15 cancellation fee, you muster up the will to go to the class despite your painfully full stomach. You convince yourself it will all be digested in an hour by the time the class starts. You continue driving in willful oblivion to the reality of the situation…. that you were very, very wrong.
You’ve changed into your new lululemon workout clothes, filled up your hydroflask, and found your bike. The all too energetic spin class instructor bounces out to the front of the room to start the class. You feel good for the warmup, and then it hits….the intense stomach cramping in your right side. You feel nauseous and your legs feel 500x heavier than they should for the first 3 minutes of the spin class.
So...Who Gets the Blood?
Here we find the battle of competing priorities. Who gets the blood - your gut or your muscles?? Your body was in digestion mode; it was shunting the blood supply to your gut to digest, absorb, and handle all those calories and protein you ingested less than an hour ago. Now, you’ve changed it up and you are asking for a sudden pivot from digestion to powering your skeletal muscles through this intense workout. It's not an either/or situation. Blood is constantly pumping throughout your body, BUT the supply and demand to certain organs varies depending on what you are doing.
"So who wins the battle for the most blood supply? In most instances, due to physiological adaptations, the skeletal muscle will win in terms of blood volume redistribution. So, expect the stomach cramps from a lack of oxygen if you eat too close to exercising."
Your legs are working hard to move the pedals and they need more blood supply and oxygen to keep working and firing those muscle contractions. They are feeling heavy earlier than normal because a majority of your body’s blood supply was shunted to the gut to support peristalsis and other digestive functions. Peristalsis is a series of muscle contractions that propel your food along the digestive tract and aid in the mechanical breakdown of your food.
When your body tries to accommodate and get more blood to your thighs (skeletal muscle), you start to cramp in your stomach. The cramping is from a lack of oxygen to your digestive system where all that food is just sitting. This lack of necessary blood to the gut is called gut under-perfusion.
So who comes out on top in the battle for the most blood/oxygen?
In most instances, due to physiological adaptations, the skeletal muscle will win in terms of blood volume redistribution. So, expect the stomach cramps from a lack of oxygen if you eat too close to exercising (De Oliveira & Burini 2011).
The picture below depicts normal arterial perfusion to the gut. This can be helpful in visualizing the circulatory system and just how much blood is required for the digestion process.
For visual learners, the graph below (2nd image) depicts blood redistribution in periods of rest versus periods of exercise. During exercise, you see that blood flow to the gut drops under the median line while the heart and skeletal muscle rise above the median line of perfusion.
Studies have shown that blood supply to the digestive tract in the postprandial period (after a meal) reaches its peak around 20-40 minutes post-ingestion and can last around 2 hours (Waaler & Toska 1999). This is all dependent on many factors like how much an individual consumes and what the macronutrient breakdown of that meal consists of. However, as a general rule, it is safe to operate within a 2 hour window in regards to exercise and your last meal.
2 Hour Window
How does this apply to me?
Here are some tips when it comes to eating before a workout:
For the average person, nutrient timing only matters to the extent that it might impair your workout. Don’t get bogged down by the timing of all your meals until you have the “big rocks” nailed down like eating enough in a day, eating mostly whole foods, and drinking plenty of water. However, if you have noticed the cramping and nausea in your workouts from eating too closely to exercise, consider implementing some of the strategies above!
References
De Oliveira, E. P., & Burini, R. C. (2011). Food-dependent, exercise-induced gastrointestinal distress. Journal of the International Society of Sports Nutrition, 8, 12. https://doi.org/10.1186/1550-2783-8-12
Waaler, B. A., & Toska, K. (1999). Digestive system's large and changing needs of blood supply. Journal of the Norwegian Medical Association: Journal of Practical Medicine, new series, 119 (5), 664–666.