Fasted Workouts and Blood Sugar: Helpful Hack or Hormonal Stressor?

Should you work out on an empty stomach to help boost blood sugar benefits? Although fasted workouts are often promoted as a metabolic shortcut for improving insulin sensitivity and enhancing fat burning, the reality is much more nuanced.

• High-intensity workouts, such as weight lifting and sprinting, can trigger the body to produce stress hormones, such as adrenaline, prompting the liver to release glucose into the bloodstream.
• Low levels of insulin — the hormone that normally blocks glucose release — can encourage the liver to release glucose into the bloodstream more easily.

A number of other factors can also affect whether blood sugar levels drop or rise during a fasted workout, including the length of the exercise session, the time spent fasting, and your overall health, says Ricardo R. Correa, MD, an endocrinologist and adjunct assistant clinical professor of internal medicine at the University of Arizona College of Medicine in Phoenix.

“This is where people get tripped up. The problem isn’t the ‘fasting,’ per se. It’s the release of stress hormones,” he says. “Intense and prolonged exercise while fasted increases cortisol and adrenaline, which can raise your blood sugar.”

• Stress levels are low, which allows your body to safely adapt to exercise without triggering an additional stress response.
• Your baseline insulin sensitivity is strong, meaning your body is able to manage glucose efficiently without drastically spiking blood sugar levels (a key sign is consistent energy levels with no surges or crashes).
• The workout is not too physically demanding, so you can reap the benefits of exercise without risking fatigue or impairing recovery.

“For example, walking or doing yoga in the morning after seven to eight hours of sleep with no glucose issues can help improve insulin sensitivity, decrease fat, and lower postmeal blood sugar later in the day,” says Dr. Correa.

• Cortisol and adrenaline are high, as the lack of preworkout fuel adds stress to an already elevated hormonal state, which can make you feel wired or jittery and affect your ability to sleep and wake up feeling rested.
• You’re chronically underfueled or overtrained, as your body is already operating in survival mode.
• You have fluctuating hormones, such as in perimenopause, which can have an impact on how your body responds to exercise.
• The workout is high intensity or long, as both states increase the demand for glucose, which can be unsafe without proper fueling.

Keep in mind that fasted training is optional. It’s not required for metabolic improvements or weight loss, Correa says. “Sometimes fasted workouts are simply not beneficial because they cause more stress than benefit in your body.”

Whether fasted exercise works for you is highly individualized and often depends on health goals, preferences, schedule, and any underlying health issues.

• Pay attention to how you feel during and after a fasted workout. Take note of any fatigue, performance changes, or soreness up to three hours postworkout. Consistently feeling “off” may be a sign that fasted workouts aren’t ideal for you, Koutnik says.
• Monitor blood glucose levels as needed. “Blood glucose is not constant but should always stay within a normal range,” says Dr. Niebauer. Exercise-induced changes are normal, but destabilization is a warning sign, he adds.
• Opt for a partially fasted workout. “For many people, this is the sweet spot. Having protein prior to exercise helps performance and prevents some of the fasting complications,” says Correa. Consider a snack with around 15 grams of protein — for example, a half a cup of cottage cheese with some berries — before your workout.
• Prioritize postworkout nutrition. “Consuming 25 to 30 grams of a high-quality protein [such as two eggs and one egg white] after exercise helps kickstart muscle repair and adaptation,” says Koutnik.

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“Exercise itself is the most powerful daily tool for improving insulin sensitivity,” says Koutnik. “Progress doesn’t come from perfection. It comes from consistency.”

• Fasted workouts have the potential to support metabolic health by encouraging your body to use stored glucose (sugar) for fuel, but this strategy may not be ideal for everyone.
• Exercising without eating first may trigger blood sugar swings, elevated stress hormones, and fatigue, particularly during high-intensity or lengthy workouts.
• Consider factors such as your physiology, the workout type and timing, and any underlying health conditions you have to determine whether fasted training is right for you.