Much like blenders and marbles, digestion and exercise simply do not mix. When you work out, your blood wants to flow to your extremities. When you process food, your blood wants to assist your stomach. And when you try to do both, you’re inclined to do neither well. In short, your mom was right (although not necessarily for the reasons she thought): You should wait a while after lunch before you go swimming. When it comes to exercise, an empty tummy is a happy tummy. (Unless you’re a diabetic. Consult your doctor.)
Blood flow isn’t the only issue here. Even if you wanted to knock out some plyo while macking on a Royale with cheese, your body has ways to actively sabotage your efforts, and it isn’t afraid to use them. It’s primary method of action: Your nervous system.
At the risk of over-generalizing, your nervous system has two sides. Your sympathetic nervous system is responsible for “fight or flight” functions. It kicks in when you’re under stress (e.g., during hard exercise), releasing a cascade of hormones such as epinephrine, norepinephrine, and cortisol that prime your body for action. Your parasympathetic nervous system, meanwhile, is in charge of your “rest and digest” functions. It’s also responsible for healing.
The problem is that when your sympathetic system kicks in, it shuts down your parasympathetic system, including anything that isn’t mission critical for the task at hand. Have you ever had to urinate when something stressful popped up, causing you to completely forget about your need to whiz? That’s your sympathetic nervous system in action, and it treats your digestive process the same way. Since the food in your stomach doesn’t provide an immediate survival benefit—and yes, your body assumes your life is at stake if you’re exerting yourself strenuously (why else would you do something like that?)— it hits the kill button on digestion to support your attempt to fight or flee. If you’re walking or cycling leisurely, you’re fine—you’ll continue to digest what you ate. But if the going gets tough, your digestion stops going. Whatever is in your tummy will just sit there—and you’ll probably feel it.
Don’t confuse exercising on an empty stomach with training in a fasted state. An empty stomach means you’ve given yourself enough time to adequately digest your food. That can be anywhere from 10 minutes to 3 hours, depending on the size of your meal. Being in a fasted state means you’ve gone without eating for somewhere on the order of 12 hours (typically overnight). At this point, your food has not only been digested, but the fuel it supplied has been largely depleted, leaving your blood sugar low and your liver glycogen wanting. In this situation, your metabolism shifts and you’re more prone to burn fat—but the benefit has more to do with athletic performance than weight loss.
Timing your pre-exercise feeding to avoid these conflicts is easy. The general rule is to wait 3 hours after a full, balanced meal. Wait 2 hours after a lighter meal where the nutritional balance is skewed toward carbs (e.g., half a turkey sandwich and a glass of juice). Wait 1 hour after a similarly carb-rich snack, such as a glass of chocolate milk. For anything less than an hour, keep your snack below 100 calories and focus on fast-absorbing carbs (e.g., half a banana). For more information on pre-workout nutrition, click here.