How Does the Autonomic Nervous System Affect Your Workout? (Rest & Digest Vs Fight or Flight)

How Does the Autonomic Nervous System Affect Your Workout? (Rest & Digest Vs Fight or Flight)

Let’s talk about the autonomic nervous system. Understanding the basics of this system can help you better appreciate when and what to eat before you exercise. It’s extraordinarily complicated, so I’ll do my best to boil it down to it’s simplest elements. Because this is a simplified explanation, it may not contain the level of detail you find elsewhere.


The autonomic nervous system is composed of two branches, one is called the sympathetic nervous system and one is called the parasympathetic nervous system. Similar to two scales, these two branches compete for dominant influence in the body and many of it’s functions. One or the other is never totally turned off, they just scale up or down depending on what the situation or environment demands.

Each branch is, essentially, managed by a different set of hormones and hormone receptors. The sympathetic nervous system responds to increases epinephrine, also known as adrenaline, and other hormones that act similarly. The parasympathetic nervous system responds to increases in a hormone called acetylcholine. These hormones bind to receptors on target organs from head to toe including but not limited to eyes, salivary glands, sweat glands, heart, lungs, gastrointestinal tract, liver, bladder, and blood vessels.

Famously, each of these two branches have a nickname that helps understand what they do. The sympathetic nervous system is known as the “fight or flight” branch and the parasympathetic nervous system is known as the “rest and digest” branch.

sympathetic versus parasympathetic

An example of when you’re in a sympathetic state is when you exercise. When you start making demands on your cardiovascular system and your skeletal muscle, whether thats the gym, running, playing a sport, etc; the sympathetic nervous system starts to ramp up. And again, this makes sense: your heart rate and respiratory rate go up, your body slows down food digestion, your sweat glands open up, etc. All of these things happen to facilitate this “fight or flight” mode.

Now what really matters is when we’re talking about eating and exercising. Your arteries also respond to sympathetic and parasympathetic activity. They are programmed to help your body send blood where it needs to go to be efficient. After you eat, you enter that parasympathetic state, aka rest and digest, hence the name. Your stomach, your small and large intestine get activated. There is now food in your stomach, so your gastrointestinal tract fires up to do it’s job. Part of that means that your body begins to divert blood from organs not required to digest and absorb food (mainly skeletal muscle) to your GI tract. So the arteries around your stomach, liver and intestines dilate from the surge in parasympathetic activity and allow more blood in. At the same time, the arteries that supply blood to your skeletal muscle get constricted, reducing how much blood gets to them. And this makes sense right?

We want to absorb that food in our stomach and small intestine and we need blood there to do it. If your heart is pumping too much blood to your muscle in this state, then you’re not absorbing as well. So mother nature has designed this really smart system to allow us to digest and absorb our food.

How intense of a parasympathetic state you enter mostly depends on how much and what kind of food you eat. If you have a small snack like a banana, it’s not going to be a big swing. Have you ever been tired after a big meal, for example thanksgiving? This is why you get tired, in part at least. So if you eat a big pre-workout meal too close to your workout, your body is trying to digest that food and absorb all that protein, carbs, fat, vitamins and minerals. While it’s doing that, it’s diverting blood away from your muscle. It’s also slowing your heart and respiratory rate and ultimately it can affect your workout.

Important for timing your meal before you workout

  • Big meal: 3-4 hours before
  • Small meal: 1-2 hours before
  • Small snack: <60 minutes before

This is why your mother always told you not to swim after you eat, you get cramps because now you’re exercising and moving the scales towards that sympathetic state while your stomach and intestines are still trying to digest food. Hence cramps.

Just to reiterate, your sympathetic tone is going to take over when you exercise, regardless of how much food is in your stomach. The point is that an excess or abundance of food in your stomach can divert blood flow away from your skeletal muscle and negatively impact your performance.

So timing your intake will help you maximize performance while exercising, working out, building muscle, playing sports or doing anything else physical active. And so that’s a really crude introduction into how the autonomic nervous system can influence your workout.

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