What happens to your body when you work out before you eat
Exercising in a fasted state—usually in the morning—is often touted as a way to get better results. But experts argue it’s not as simple when it comes to long-term benefits.
Fasted workouts—exercising on an empty stomach, often first thing in the morning—have become a defining habit of modern fitness culture. Scroll through social media and you’ll see them promoted as a way to burn more fat, boost metabolism, and accelerate results. The idea is simple: If you haven’t eaten since the night before, your body must rely more on stored energy when you run, walk, or lift weights.
Physiologically, there’s some truth to that since, following an overnight fast, insulin levels drop and the body has less immediate fuel from food, which can push it to use stored fat as energy during exercise.
(What intermittent fasting really does for your body and brain.)
But "there’s an important distinction between what happens acutely during a workout and what actually matters over time,” says Nicholas Tiller, a co-author of related studies and a research associate at the Lundquist Institute at Harbor-UCLA Medical Center. While fasted workouts can alter how the body fuels exercise, he explains, “the short-term physiology is clearer than the long-term performance story.”
Here’s what to know.
What happens in your body when you work out before eating
After an overnight fast of 8 to 12 hours, the body is in a different state than it is after eating.
For one, insulin—a hormone that helps store and use calories or energy—is lower, and liver glycogen—stored carbohydrates that help maintain blood sugar—is partly used up as well. When glycogen levels are reduced, the body has less readily available glucose to maintain blood sugar, which is why blood sugar levels decrease during exercise. Together, these factors affect how the body powers exercise by increasing reliance on fat compared to a fed state.
Hormones like adrenaline and somatotropin are also released during exercise. “Lower insulin concentration can increase your body's response to these hormones and therefore increase fatty acid availability to be used as fuel,” explains Shuhao Lin, a nutrient scientist and exercise physiologist at Mayo Clinic. This can further mobilize fat from storage for use as energy during a workout.
But these effects depend, in part, on the level of your workouts. “During lower-intensity exercise, your body tends to burn more fat for energy, but as the intensity increases, it relies more on stored carbohydrates or glycogen,” explains Brad Schoenfeld, a professor of exercise science at Lehman College in New York.
The most evidence-backed benefits of fasted workouts
Despite the hype, only a few benefits of fasted training, in addition to the above, are consistently supported by research—and even those come with caveats.
The strongest evidence is for increased fat burning during the workout itself.
(Scientists identify one fitness habit that may matter most for longevity.)
For instance, in a fasted state, lower insulin levels “lead to higher rates of lipolysis,” Tiller says, referring to the breakdown of fat for energy. He compares this to having two fuel tanks: one readily available (recent food) and one in reserve (stored fat). “Exercising while fasted effectively drains the first tank, forcing your body to rely on its backup fuel source—fat,” he says.
Fat oxidation—the process of using fat as fuel—then increases, which helps provide a greater share of the energy your muscles use during exercise.
Research from 2024 and 2025 show that “rates of fat oxidation will be higher when the same exercise session is done in the fasted state compared to doing it after a carbohydrate-rich meal,” says Louise Burke, chair of sports nutrition at Australian Catholic University. Repeating this pattern may also help the body adapt to become more efficient at using fat during exercise.
But burning more fat during a workout doesn’t necessarily mean you’ll lose more body fat overall. This is, in part, because your body may compensate later by burning less fat at rest or increasing hunger. So total fat loss still depends on overall calorie balance.
“What matters for fat loss is the total amount of fat burned across days and weeks; not the amount of fat burned during an exercise session,” says Schoenfeld, whose research found no significant long-term difference in fat loss between fasted and fed training.
There’s also some evidence that fasted workouts “do improve insulin sensitivity more than fed workouts,” says Lin, likely because exercising when insulin is low helps muscles take in sugar from the blood more efficiently. This could also be beneficial by improving blood sugar control and potentially reducing the risk of type 2 diabetes, though such findings remain mixed, says Schoenfeld.
Researchers have also explored whether fasted training improves metabolic flexibility—the body’s ability to switch between burning fat and carbohydrates depending on what’s available. But while it may support metabolic health, the real-world impact is still uncertain. Such findings "are scientifically interesting,” but the evidence is “promising rather than definitive," says Stuart Phillips, a professor of kinesiology at McMaster University.
And Burke emphasizes that these effects are relatively small compared to broader lifestyle habits associated with exercising on an empty stomach. For example, some people feel less hungry after fasted workouts and eat fewer calories, while others feel hungrier and eat more later. In some cases, fasted training simply pushes back when people eat, “and a shorter eating window often leads to lower calorie intake overall,” says Tiller.
Benefits that are often overstated
While fasted workouts do change how your body uses fuel, several other popular claims don’t hold up.
One is that fasted morning workouts improve circadian rhythms—the body’s internal clock that regulates sleep, hormones, and performance. While Lin notes that muscles have their own internal clocks, there’s little evidence that fasted training meaningfully improves them. In fact, "most studies show better performance strength and high-intensity training in the afternoon and evening compared to early morning,” she says.
Claims about muscle growth are also unsupported. If anything, Lin says exercising without enough fuel may reduce workout quality.
Performance benefits are similarly overstated. Many people feel more fatigued while exercising in a fasted state, especially for longer or higher-intensity sessions, because carbohydrate stores are lower. Not having enough carbohydrates can work against you, and “being under-fueled may compromise how well you function,” Phillips says. Some people compensate with caffeine to boost performance, which can help, but this doesn’t replace the actual fuel your body gets from food.
(Can fasting help you live longer? Here’s what the science says.)
More broadly, fasted training isn’t a shortcut to better health, as Tiller notes that research shows weight loss will be modest to nonexistent if overall diet doesn’t also support it. “You can’t outrun a bad diet,” he says.
Who fasted workouts may help—and who should avoid them
Whether fasted workouts are helpful also depends on the individual. For example, “the effects of fasted training may be noticeable in a previously sedentary individual who has insulin resistance,” says Burke.
But trained athletes may see reduced performance or slower recovery. In fact, fasted workouts are generally thought to be less suited for high-intensity interval training, long endurance sessions, or strength training, which rely heavily on carbohydrates.
And certain groups should be especially careful. For instance, people with diabetes or those taking blood sugar–lowering medications may risk dangerous drops in blood sugar, since exercise lowers blood glucose and fasting reduces baseline levels.
Those with a history of disordered eating or low energy availability may also be negatively affected. In these cases, "what sounds like a harmless exercise hack could quickly become physiologically or behaviorally destructive,” Tiller warns.
Research limitations also matter. “Many studies are short, small, and done in young men alone,” says Phillips. “We still need longer trials in women, older adults, and people with obesity, prediabetes, or type 2 diabetes.”
How to approach fasted workouts safely and effectively
For those who choose to work out before eating, how you approach it may matter more than the timing itself. One key is matching your nutrition to your workout. “If the workout is easy and short, fasted exercise is a reasonable option,” Tiller says. “But if the workout is long or hard, eating beforehand is the superior choice.”
Hydration is also important, as it’s easy to overlook fluid intake when skipping out on calories. And refueling afterward matters just as much. “I always recommend eating right after a fasted workout, so that you can replenish nutrients right away,” says Lin.
Most importantly, pay attention to how you feel. Though fasted workouts are generally considered safe, Burke notes that individual fatigue, dizziness, irritability, or declining performance are all signs that fasted training may not be a good fit.
While fasted workouts remain an area of scientific interest, their overall impact remains modest for now—and far less important than other exercise fundamentals. “For most people,” Phillips notes, “the big rocks are exercising at all, doing it well, and eating in a way that supports their goals.”
