Many people choose to exercise in a fasted state in hopes of speeding up body fat loss, Julie Stefanski, a registered dietitian nutritionist, told The Epoch Times.
However, while this strategy may sound appealing, the evidence is limited.
“Unfortunately, the studies on this remain small, and it has not been proven to be the gold standard for body fat loss or athletic performance,” Stefanski said.
Fasted Versus Fed Exercise
For example, one small study found that healthy people who exercised after fasting burned more fat and fewer carbs compared to when they ate before exercise. Even though they felt hungrier, their overall energy intake was lower. However, they didn’t perform as well during exercise and felt less motivated and less enjoyment.
It’s not necessary to avoid eating before exercise to achieve fat loss goals, and it may even affect your ability to finish an activity strongly, she noted.
Even though fasted exercise may be helpful for some, research shows that it could also increase hunger hormones, making people feel hungrier and burn less energy over time.
Hunger tends to be higher after fasted exercise compared to fed exercise, and skipping breakfast is often linked to lower physical activity levels during the morning to conserve energy, which could reduce total daily energy use.
That extra hunger might also drive people to reach for higher-calorie foods later in the day, canceling out any calorie deficit created by the workout. The effects of fasted versus fed training aren’t just about hunger in the moment—they also influence food choices for the rest of the day, and sometimes even beyond.
One study examined how bodyweight interval training—or high-intensity workouts that use your own body weight for resistance, such as push-ups, squats, lunges, and burpees, performed in short bursts—before or after breakfast, affected women’s blood sugar, fat burning, and appetite. Twelve women, with an average age of 23, completed two workout sessions. In one session, they exercised five minutes before eating; in the other, they exercised 10 minutes after eating. Each session involved 10 one-minute bodyweight intervals with one minute of rest between each.
Heart rate, effort, and fat burning were similar in both sessions. However, when participants exercised after eating, they had a reduced appetite, lower peak blood sugar levels, and a reduced overall blood sugar response.
Performing bodyweight intervals after eating, therefore, reduced blood sugar and appetite more effectively than doing them in a fasted state.
“Having low blood sugar levels can not only impact stamina, it can contribute to confusion, poor balance, and reduced coordination, which may lead to falls or other injury,” Stefanski said.
People on a ketogenic eating plan who fast before working out may have ketone bodies available as an alternative energy source. However, while being in ketosis may support exercise, performance can still suffer compared to athletes who are fully fueled with carbohydrate-rich foods, she said.
The body also needs nutrients for recovery. Ignoring those needs can kick off a cycle of overriding the body’s natural signals of hunger, discomfort, or pain, potentially increasing the risk of injury over time, according to Stefanski.
Be Cautious With Fasted Workouts
Beyond its effects on hunger hormones and blood sugar, fasted exercise isn’t suitable for everyone. Some groups may need to approach it with more caution.
For example, when someone with Type 1 diabetes fasts and exercises, their blood sugar can drop too low or rise too high because they don’t produce insulin naturally. During fasted exercise, the amount of insulin administered needs to be lowered beforehand to prevent low blood sugar. If insulin isn’t adjusted correctly, blood sugar can become unsafe. So careful planning and monitoring are important.
“Exercising in a fasted state may place added strain on the adrenal glands, potentially impairing thyroid function and other anabolic processes,” Ian Craig, a nutritional therapist and exercise physiologist, told The Epoch Times.
Anabolic processes help the body build and repair tissues, such as muscles and hormones. When you exercise fasted, it can stress the adrenal glands, causing them to release more stress hormones such as cortisol. High cortisol levels can interfere with the thyroid gland’s ability to make hormones properly. Since thyroid hormones control metabolism and energy, this can slow down the body’s repair and growth functions.
Craig noted that most fasting studies have been conducted on young, active men who are physiologically resilient and adapt well to short-term stressors. Young, active men are typically the best candidates for fasted training, but for others, especially women, the story is different.
Women’s hormone profiles are more complex than men’s, Candace Vermaak, who holds a doctorate in sport science and is a lecturer at Stellenbosch University in South Africa, told The Epoch Times.
“The menstrual cycle, and in more severe cases, amenorrhea, the absence of a menstrual period, can make women more vulnerable to the hormonal effects of under-fueling,” she said.
Cortisol, a stress hormone, naturally peaks in the morning, according to Vermaak. When combined with intense fasted workouts, this spike can become more pronounced. Chronically elevated cortisol may interfere with the production of sex hormones such as estrogen and testosterone, potentially affecting not only athletic performance but also overall hormonal health.
Effective Alternatives
If fasted training leaves you feeling depleted, overly hungry, or out of sync with your body, there are other ways to approach exercise that support both performance and hormonal health.
Everyone has different preferences when it comes to how much to eat before a workout, what types of foods or fluids work best, and how close to training they feel comfortable eating. The goal isn’t just to fuel the workout itself—it’s also to avoid rebound hunger, support energy levels, and keep your stress and hormone systems in balance.
“If you find that you end up overeating later in the day because you’re famished, a better pre-workout nutrition plan may be needed,” Stefanski said.
Vermaak recommended a measured approach, suggesting that if you choose to include fasted training, it’s best to limit it to no more than two to three sessions per week or reserve it for low-intensity activities.
“When training fasted, finding the balance is key—using fat for energy without breaking down muscle or upsetting hormone levels. Too much fasted exercise can stress your body and hurt both performance and health,” she said.