Leading sports nutritionist and exercise physiologist, Dr. Stacy Sims, answers your most pressing nutrition questions.
Q: I wake up at 5 a.m. and begin training within 15 minutes. Should I fuel before my workout?
A: There are several factors to consider when it comes to fasted training, primarily around sex differences and the type of session you are going to do. If you are a guy, then no, you can skip fueling before your workout, provided your session is low intensity, not over 90 minutes, and you refuel well after to facilitate recovery. If you are trying to hit higher work output (such as intervals, VO2 max efforts, or HIIT), then you need to have some food prior to or during your session to ensure you are able to hit the intensities you need to for optimal training adaptations. If you are a woman, then yes, you need to fuel first. We know exercising in a fasted state can slightly increase fat burning in women (less so than in men), but fasted exercise over the longer term causes dysfunction of the endocrine system, specifically sex hormones.
Regardless of gender, the final consideration when thinking about fueling before a workout is your energy availability: You want to avoid being in a low energy availability state. As triathletes, you may have limited recovery time between sessions and fasted training is known to decrease appetite hormones and daily energy intake, so it is critical to really think about fueling for each session and recovering well. Every now and then it’s OK to do a short, low intensity session in a fasted state, but for long-term health and performance, it is best to eat something small beforehand to reduce the overall negative stress on hormone balance and avoid running yourself down.