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Fasted training has been shown in the literature for years to potentially aid in metabolic efficiency. The theory behind this is that exercising in a glycogen-depleted state (i.e. after fasting overnight) improves the body’s ability to oxidize fat for fuel at higher intensities, therefore sparing our glycogen stores for when we really need it, toward the end of a long, sustained effort, such as a marathon.
For a little more background on why this might be advantageous: at lower exercise intensities (30-65% of VO2 max), fat is the primary fuel source. At intensity levels above 65% VO2 max, muscle switch to utilizing carbohydrates, in the form of glycogen, or the carbohydrates that are stored in the muscles. Running out of glycogen is affectionately called “hitting the wall” or “bonking” by endurance athletes. Bonking is the decline in performance precipitated by low energy availability, and results in that heavy, cramping, I-don’t-want-to-take-another-step feeling. Therefore, if we are able to train our bodies to use fat for fuel instead of precious glycogen, we could potentially exercise longer and harder without running out of fuel. Studies testing this theory have found that while fasted participants do not perform better during their exercise bout, their muscles exhibit enhanced metabolic adaptations to exercise after a fasted training program (1)
Seems simple and makes sense, right? Well unfortunately, it’s not. Especially for women.
In the book Roar: How to match your food and fitness to your female physiology for optimum performance, great health, and a strong, lean body for life, Dr. Sims makes the case against fasted training to female athletes with research conducted on only female athletes, an attribute missing in many of these physiological studies, where men or a mixed group of men and women are used. In the morning, cortisol (the body’s stress hormone) levels are at their highest. Exercise is an additional stressor to this system, meaning even more cortisol is produced. However, the body needs the correct building blocks to do so, which are the sex hormones: testosterone, estrogen, and progesterone(2). In the long-term, a female athlete that is in a chronic state of cortisol elevation due to fasted training can develop hormonal imbalance, potentially risking reproductive health and paving the path to amenorrhea and other symptoms of RED-S and low energy availability. Dr. Sims’ research points to the possibility that men may respond better to training in a fasted state, as the muscle adaptation following fasted training programs is greater in men than in women. This could be because women naturally have a higher fat oxidation, thus have less room to improve through fasted training.
The health impact of fasting for women casts a wide net. Hormonal disturbances brought on by low glycogen states can cause fatigue, increased risk for bone stress injuries, and even potential weight gain due to decreased metabolic rate. Additionally, time spent in a fasted state, independent of total energy consumption in a 24 hr period, can cause reproductive dysfunction(3). In other words, even if you are eating enough food to meet your energy needs, if you spend a significant amount of time fasted throughout the day, it can affect your menstrual cycle and reproductive health. While there are benefits to intermittent fasting for certain populations, a female athlete should take extra scrutiny before deciding that it is right for them.
So what’s a gal or guy to do before that 5:30am run? Physiology is so unique and individualized that this is really something that has to be taken on a case to case basis and the risks and benefits must be weighed. This is where a sports dietitian would be beneficial, to make sure you’re fueling yourself right for training and life.
See you on the roads (after my morning snack),
Kacy Seynders, PT DPT
Yeo WK, Paton CD, Garnham AP, Burke LM, Carey AL, Hawley JA. Skeletal muscle adaptation and performance responses to once a day versus twice every second day endurance training regimens. J Appl Physiol (1985). 2008;105:1462–70.
Roar: How to match your food and fitness to your female physiology for optimum performance, great health, and a strong, lean body for life, Dr. Stacy Sims, PhD
Fahrenholtz, IL, Sjödin, A, Benardot, D, et al. Within‐day energy deficiency and reproductive function in female endurance athletes. Scand J Med Sci Sports. 2018; 28: 1139– 1146