Intermittent fasting, or IF, is a timed approach to eating with various periods of fasting and non-fasting, including the 16/8 or 5:2 methods. The former involves eating for 8 hours during the day — for example, 10 a.m.–6 p.m., and fasting the other 16 hours. The latter focuses on “normal” eating five days a week and eating minimally (500–600 calories or less) on the remaining two days. Proponents say it can help with weight loss by supporting your metabolism to burn more calories at rest. There is also some research on how it can help manage diabetes, possibly due to the effect fasting has on insulin sensitivity and facilitating the body’s use of stored fat for energy.
The belief fasted workouts may be beneficial to performance stems from changes in how your body uses energy when working out without fuel. The idea is, because we have no available carbohydrates to use (from food) and somewhat lower glycogen stores, the body increases reliance on fat for energy instead. Since we have much higher fat stores than carbohydrates, the theory is if we teach the body to rely on fat for energy, we can go harder, longer.
While we’ve talked about intermittent fasting and cardio, you might wonder if it’s OK to fast before a strength-training session.
Much of the available research on fasted workouts focuses on endurance activities like running and cycling instead of strength and resistance training. The conclusions on possible benefits still requires more data. One review suggested a decrease in performance by fasted athletes. This was associated with prolonged endurance activity and high-intensity activity. Although fasted workouts may help the body become more efficient at using fat for energy, research on notable performance benefits is lacking, which I believe negates any possible metabolic benefits.
Many studies have found changes in hormonal and inflammatory markers in fasted individuals, including an increase in IL-6, a known inflammatory marker, an increase in stress hormones (e.g., adrenaline), and lower levels of melatonin (which helps regulate your sleep/wake cycles). This finding helps emphasize how stressful a fasted workout can be on the body, and is an important factor to take into account before trying a fasted workout of any kind.
One study found no difference in body mass or composition among groups of bodybuilders who fasted versus those who did not before a workout. Another study found no negative effect on power output among fasted powerlifters, as long as they met daily energy and sleep needs.
Loss of muscle tissue is one concern with a fasted strength-training workout because this can occur with prolonged fasting as the body dips into protein stores for energy. This contradicts the reason why one may want to strength train. One review found a loss in lean tissue mass among fasted athletes, and this increases over a longer period of fasting (e.g., during Ramadan, a month-long Islamic holiday where many Muslims fast from dawn to dusk, in which a lot of studies on fasting take place).
In general, protein catabolism (the breakdown of muscle protein) is a common concern when the body has little available energy or energy stores to work with. To date, there is not enough research showing benefits of fasting before a weight-training workout, and it’s not something I’d recommend trying.
READ MORE > A SPORTS DIETITIAN ON THE EFFECTIVENESS OF FASTED WORKOUTS
More research on the benefits of fasted workouts is needed, so I wouldn’t recommend it at this time. That said, there are plenty of individuals who simply cannot tolerate food before exercise. Thus, waking up and working out (either cardio or strength training) for up to 60 minutes without fuel first can be OK for some people. However, the longer or more intense a workout, the more important it is to build in time for a pre-workout meal or snack to help your body handle the workout and support the recovery process.
If you need help tailoring your nutrition routine around your workout schedule, consider reaching out to a registered dietitian (especially one with a background in sports nutrition). Ultimately, the key is to figure out what works best for you and do it consistently.