How eating or skipping a pre-workout meal can impact your body — and performance
Forcing yourself to scarf down breakfast before a sunrise jog or a 5 a.m. class can, at times, feel harder than your workout itself. But is eating a couple pieces of toast ahead of your early morning training session actually necessary for performance?
Here’s what to know.
The Science of Fuel
First things first, a quick breakdown of how your body generates energy to power through a Precision Ride class or heavy-lifting session. Your body relies on three metabolic pathways to create energy: the phosphagen pathway, glycolytic pathway, and oxidative pathway.
The phosphagen and glycolytic pathways are anaerobic, meaning they take place in the absence of oxygen. The former system utilizes a substance called phosphocreatine stored in your muscles for immediate energy (think: seconds), while the latter creates quick energy from glucose (the sugar that comes from carbohydrates) for activities lasting a couple of minutes. That’s because anaerobic pathways can produce a lot of ATP (adenosine triphosphate, the immediate usable form of chemical energy) quickly, but the total amount of ATP produced is low, according to research published in Nature Metabolism.
Then there’s the aerobic oxidative pathway, which takes place when oxygen is available. This energy system kicks in when activity lasts several minutes to hours, using carbohydrates (specifically muscle glycogen and blood glucose) and fat (specifically fatty acids derived from both muscle and adipose tissue triglyceride stores) to create ATP, according to Nature Metabolism.
Those carbs are your body’s preferred source of energy, says Robyn Smith, N.D., a COACH X and master instructor at Equinox Bay Street in Toronto. “Whatever the body has available, it will tend to use, particularly if it’s that preferred source of glucose, readily available for that quick energy,” she explains.
Still, your energy source during the aerobic pathway changes based on your intensity. For example, carbs are used more heavily during high-intensity activity, while fat is the key source at lower intensities, according to Nature Metabolism. The oxidation of carbs produces ATP quicker than fat, but the total amount of ATP the process generates is lower than fat, per the research.
Fuel vs. Fast: Which Is Better Before a Workout?
The answer is complicated.
If you walk into the Club solely with last night’s dinner in your stomach, there’s a chance you still have some readily available glucose in the bloodstream, but not much, says Smith. Without a plethora of glucose available for energy, your body may start to shift to primarily using fat for fuel, she says. Research suggests that exercising in a fasted state may allow for higher levels of fat oxidation than exercising after eating carbs, particularly during low- to moderate-intensity exercise, according to information published in Nutrients.
This shift in energy sourcing could impact body composition. A 2022 meta-analysis suggests engaging in high-intensity interval training while fasted can improve body composition, waist circumference, and fat mass in individuals with overweight or obesity. However, diet plays a significant role in weight loss, and many of the studies analyzed combined this fasted HIIT training with a low-carbohydrate diet in general, which could have influenced the results. In a 2017 review analyzing studies on overnight-fasted vs fed aerobic exercise, the effects on body mass were trivial.
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Another snag: As your workout’s intensity increases, the fat-oxidizing effects of fasted exercise diminish, to the point where there isn’t much difference in fat “burning” when you’re training on an empty stomach compared to training with carbs in your system, according to Nutrients.
There’s also a risk of a performance blunder. The oxidation of fat doesn’t produce ATP as quickly as that of carbs. And that can translate to a decrease in power output when you deplete your carbohydrate stores during prolonged, strenuous exercise, according to Nature Metabolism. If you were to attempt to do a HIIT workout while using fat as your main fuel source, your performance may take a hit, says Smith.
Still, research is mixed. Pre-exercise carbohydrate consumption has generally been found to enhance prolonged (think: more than 60 minutes) aerobic exercise performance but not shorter duration movement, per Nutrients. In studies investigating the performance impact of carbohydrate intake before HIIT of either short or long durations, benefits and negligible effects have both been observed.
Especially if you’re a high-performing athlete engaging in a high volume of training, exercising after an overnight fast could lead to a negative energy balance, when energy expenditure exceeds your intake (aka a calorie deficit), per Nutrients. Among athletes, low energy availability could lead to performance declines and an increased risk of injury, among other concerns.
The Bottom Line
Exercising on an empty stomach could lead to greater fat oxidation, especially if you’re training at lower intensities, but that doesn’t automatically equate to body fat loss or improvements in body composition. Though there isn’t a clear-cut answer on whether or not it benefits performance, eating carbohydrates before your workout probably won’t do any harm.
To Smith, it’s a smart idea to eat something before a workout. Generally, you’ll want to eat carbohydrates about 30 minutes before exercise to see potential performance gains.
Opt for simple carbohydrates over complex ones (think: whole grains, beans, veggies), as the latter takes longer for your body to break down and use for energy, says Smith. Similarly, save the protein for after a workout, when your body needs it to rebuild muscle, she notes.
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To make getting those nutrients easier, Smith suggests drinking a glucose- and electrolyte-rich beverage before your workout. “You could be sipping that as you're starting to train so that you cover the hydration, you cover some good mineral content, and then you also get that little kick of glucose, so your body can start fueling effectively,” she says.
That said, remember that each person is unique. Consider how your body feels when you eat before a training session or when you hold off on a meal. Take a look at how your performance is stacking up in each of those situations and how you’re progressing toward your fitness goals. When in doubt about your fueling practices, chat with a Coach at your Club.