If you aren't eating before a workout, you should ask yourself if it's going to serve you well or hinder your performance. It all depends on the individual, so an experience for one person might differ from your own. However, consider you mental and physical state, as well as your goals.
For instance, if you're low in energy, you might want to rethink heading into spin class without a snack for fuel. Yet, if you're not hungry and you feel energized, you could be just fine (and maybe even find some surprising benefits!). And, if you're looking to try fasting pre-workout as a long-term strategy, you may need to work through any challenges until your body has adjusted.
Either way, here are a few possible side effects to expect when you workout without that handy pre-workout snack.
You Might Feel Sick
You might need to take more breaks than usual or sit down for a bit, if you're feeling under the weather. "Feeling lightheaded, dizzy, nauseous, and sensitivity to light are all signs of exhaustion and high blood pressure. As this point, you need simple sugars to help you get out of this zone like orange juice or a small granola bar," says James Shapiro, MS, NASM CPT, CES, & PES, a NYC-based personal trainer.
Your Warm Up Might Suffer
Your warmup, whether it be on a cardio machine or stationary in free space, will be sluggish. "We can attribute that to your potential low blood sugar levels, low blood pressure, fatigue brought on by having an empty stomach, and also less mental focus and concentration," says Shapiro.
You Might Have Fewer Good Sets/Reps
Once you start your workout you will then notice you cannot achieve the same time/sets/reps/quality of movement you had maybe during your past workout," explains Shapiro.
"A set might have a higher rate of perceived experience (RPE) than before and you might have to cut the set short. You might start to slug around and want more water (which can actually hamper the workout further as hydration happens throughout the day not during the workout)," he explains.
You Might Never Return
You might not go in for a second workout after that experience! "The serious side effects of going on empty can the adverse feelings that might tempt you to not workout afterwards," says Shapiro.
"As a trainer, I become disappointed in both my client and myself because part of my responsibility is teaching my client behaviors that lead to success. It rarely happens but learning to leave your ego at the door when coming into a workout empty can help you avoid these feelings. This means going for a yoga class or changing your usual workout for a light spin on the exercise bike," he recommends. Or, just eat something and be prepared for the challenging workout ahead.
You Might Burn More Fat
Here's an upside, if you can get through the transition and adapt to going on empty. "Carbohydrates are the body's most readily utilized fuel source. Carbohydrates are stored in the body as glycogen and these glycogen stores are depleted when we expend energy. In the fasted state, there is not much in terms of glycogen so the body must burn fat as fuel instead of carbohydrates. In order to burn fat as our main source of energy, the body enters a state of ketosis in which ketone bodies are formed and burned for fuel," explains Joey Kane, an ACE certified personal trainer and nutritionist.
"If a person becomes adapted to fasted training, meaning they choose to workout on an empty stomach regularly, the drop in performance disappears. Human beings become good at what they practice. This acclimation period usually takes around 2 weeks for men and a month for women," he says.
You Might Actually Perform Better
If you go on empty where you're hungry or tired, your workout will suffer, without a doubt. However, if you work out on empty but you're not fatigued, it could improve.
"When you work out on an empty stomach but not 'hungry' per se your body actually performs pretty well due to a couple of reasons. Blood can be diverted to skeletal and cardiac muscle more easily in the absence of food, whereas when you eat within an hour of working out blood is primarily shunted to the stomach and GI tract for digestion, which decreases exercise performance. The more blood you have available to working tissues the better your performance," says Brandon Mentore, A.C.E., CSCS, N.A.S.M., A.A.D.P., a strength and conditioning coach, functional medicine practitioner and sports nutritionist.
"In the absence of food cortisol and adrenaline have a greater surge in the blood stream, which also enhances performance. Cortisol improves training drive and energy and adrenaline enhances strength output," he says.