The Pros and Cons of Fasted Training: Is It Right for You?

Fasted training promises fat loss and performance gains but carries risks, especially for women. Discover a balanced approach.

Nok Lek Travel Lifestyle/Shutterstock

Unleashing the power of fasted training has captivated athletes and fitness enthusiasts alike, promising a path to enhanced fat burning, metabolic flexibility, and elevated performance. However, this ascetic approach is not without caveats, particularly for women.

Navigating the delicate interplay between fasting and physiological demands requires careful consideration, as embracing this trend could undermine overall well-being and disrupt the hormonal balance that supports optimal function.

A Fitness Trend With Some Scientific Backing

Intermittent fasting and time-restricted eating have profoundly influenced our eating habits, Ian Craig, a nutritional therapist and exercise physiologist, told The Epoch Times.

Fasting is a popular strategy for various health benefits, including optimizing blood lipids, reducing oxidative stress and inflammation, facilitating cellular detoxification, promoting gut rest, managing weight, and stabilizing insulin levels. There has been a surge in fasted training—which is exercising while in a fasted state—among athletic communities, underscoring its relevance for sports performance, Mr. Craig said.

Story continues below advertisement

A 2016 study published in the Journal of Translational Medicine investigated time-restricted feeding in 34 resistance-trained men. One group consumed food within an eight-hour window (fasting group), while the other followed a standard 12-hour eating window. Both groups maintained similar calorie intake. After eight weeks, the fasting group experienced significant fat mass reduction while preserving muscle mass.

A 2009 study in the Journal of Strength and Conditioning Research showed that restricting calorie intake by up to 40 percent over three weeks, coupled with overnight fasting, could enhance cyclists’ power-to-weight ratio without compromising endurance performance.

Research in The British Journal of Nutrition found a significant increase in fat oxidation during exercise when the participants had fasted rather than been fed.

The Hidden Risks of Fasted Training

While restricting food intake before and during exercise can induce a state of cellular stress, potentially yielding benefits, Mr. Craig said that it also has the potential to compromise an athlete’s recovery capabilities and performance.

Exercising in a fasted state may place increased strain on the adrenal glands, potentially impairing the function of thyroid hormones and other anabolic processes, he added, noting that most fasting studies have been conducted on young, active males who possess physiological resilience and can adapt relatively easily to short-term challenges.

Story continues below advertisement

Each athlete is unique and may not derive the same benefits from fasted training. When working with clients, especially “over-enthusiastic amateurs or elite athletes,” the risk of adrenal fatigue must be taken seriously, Mr. Craig said. They may already experience compromised endocrine function, including the pituitary, adrenal, gonadal, and thyroid systems, which could be further exacerbated by a fasting regimen, leading to increased cortisol release in the absence of sufficient nourishment, he noted.

A 2008 study suggested that training in a glycogen-depleted state, such as during fasting, could benefit endurance athletes early in the training season due to the greater importance of metabolic capacity over muscle strength. However, as training intensity increases, replenishing glycogen stores becomes crucial. For motor endurance athletes, the potential adverse effects of glycogen depletion on muscle mass should be considered due to elevated cortisol levels, which may restrict skeletal muscle hypertrophy.

Another study found that inadequate glycogen levels or food intake before exercise could hinder post-exercise muscle protein synthesis. The study authors suggest athletes should “fuel for the work required.” Carbohydrate availability could be deliberately reduced before training to promote a “work-efficient” approach to training. However, if the objective is to accomplish the highest workload possible over extended durations, sufficient carbohydrates should be consumed within the 24 hours before and during the specific training session.

Why Women Must Approach With Caution

Fasted training is not universally suitable, especially for women, Candace Vermaak, who holds a doctorate in sport science and is a lecturer at the Stellenbosch University department of exercise, sport, and lifestyle medicine, told The Epoch Times.

Women’s hormone profiles are more complex than men’s, Ms. Vermaak said. The menstrual cycle and, in severe cases, amenorrhea (the lack of a menstrual period) could add to the hormone imbalance. All of this makes women much more sensitive to inadequate nutrient intake, she noted.

Story continues below advertisement

Cortisol levels naturally peak in the morning, potentially exacerbated by intense fasted workouts, she said. This heightened cortisol release can lead to chronic elevation—particularly impactful for women—disrupting the delicate balance of sex hormone production.

Such disruption may impede not only athletic performance but also overall well-being by interfering with the regulation of estrogen and testosterone levels.

When embarking on fasted training, it’s crucial to understand the delicate balance between using fat stores for energy and the potential risk of tapping into muscle reserves, particularly if fat levels are low or hormonal fluctuations are present. Excessive or high-intensity exercise in a fasted state can exacerbate this risk, placing undue stress on sex hormones and inhibiting their release, which can detrimentally affect overall function and performance, Ms. Vermaak said.

How to Make Your Training Work for You

Drawing from his decades of working with clients, Mr. Craig often recommends reintroducing a nourishing breakfast to those following a strict fasting regimen to support morning training sessions. This approach often helps optimize body composition, enhance training performance, and facilitate better recovery between exercise sessions.

The word “intermittent” is often missing in the interpretation of intermittent fasting, Mr. Craig said. These approaches should be used intermittently, not all the time, depending on a person’s current health state, training patterns, stress levels, and genetic predispositions.

Story continues below advertisement

Ms. Vermaak advocates for a measured approach, recommending no more than two to three fasted training sessions per week or limiting such sessions to low-intensity activities. A 2020 scientific review in the Open Access Journal of Sports Medicine suggests that endurance athletes should avoid high-intensity training while fasting because “little evidence” suggests it increases fat oxidation.

“We suggest that athletes train at relatively low intensities (and not at high intensity levels) when fasting to ensure that they recover adequately to optimize performances in competitive events,” the authors wrote.

Fasted training is an individualized endeavor requiring careful consideration of factors such as blood sugar levels, stress levels, sleep quality, and metabolic profile, Ms. Vermaak said. Prioritizing these assessments ensures optimal performance and well-being while mitigating potential risks associated with training in a fasted state.