For this marginal gain approach to truly work, you need to carefully program rest and proper nutrition around your fasted workout so you can recover and refuel properly for this approach to work. Additionally, training needs to be manipulated in a way that you are actually taping into fat burning during the workout - which can be difficult to quantitively measure.
My advice: Nail the basics first. A well balanced diet, good sleep (at least 7-8 hours), mobility, good recovery – get all these things dialed in before you attempt incoroprating fasted workouts into your training. Without a solid foundation of nailing the basics, you risk under performing during your workouts and the risk for injury, sickness and fluctuating energy levels increase.
Despite clear benefits of pre-workout carbohydrate ingestion on improved performance, mental focus, immune system health and longevity in sport, athletes still refuse to intentionally not eat before early morning workouts. What's the most common reason why?
"I need to burn fat for fuel to become more metabolically efficient."
In my opinion, this is the number one reason why athletes intentionally restrict/avoid consuming carbohydrates before an early morning workout. The primary science behind fasting relates to its metabolic effects - by working out on an empty stomach, fat burning is enhanced. Although there are several different approaches to dietary manipulation in an effort to train the muscles to more readily use fat as a substrate, let's put science/research aside for a second and look beyond a cellular level.
As an athlete, your training is designed to prepare you for your upcoming event. In order to do so, you need to be consistent with training and your body needs to adapt to the training sessions - some of which will be short, some are high volume, some are easy and some are very intense. Knowing that your body is utilizing different fuel sources to adapt to different training sessions, it is very difficult to know which workouts are truly metabolizing fat for fuel. In other words, you are likely underfueling and sabotaging most of your workouts. Additionally, training yourself to become a great fat burner doesn't ensure that you will excel on race day - there are many factors from mental to environmental that will affect your race day performance. It's unlikely that your body fat percentage or fat burning capabilities will be your primary limiter on race day. Being a great fat burner will do nothing for you if you aren't an efficient open water swimmer or if you don't know how to manage the terrain while riding your bike for 112 miles on rolling hills and in the wind.
As you balance work, family/kids, training and a busy life, is not eating before an early morning workout really making a significant impact in your athletic development?
Keeping in mind that most athletes nullify the effects of a fasted workout by being sedentary throughout the day, skipping meals (or overeating), being overly stressed and overworked and skimping on restful sleep, you'll do your body and mind a great favor by fueling yourself for the workout training stress. And once again, if you aren't putting effort into your sleep, eating, hydration and recovery habits, the occasional or regular fasted training session is likely hurting your performance, more than it's helping.
Karel and myself eat before every workout and consume sport nutrition during every workout - all year long. Even after a combined 29 IM performances and 9 Ironman World Championship finishes, we still choose a smart fueling approach over a marginal gain approach.
Eating before a workout and fueling during a workout gives you confidence that you are doing the small things well. And overtime, the small things will add up to something great with your performance.
Remember – there are no shortcuts, hacks or quick fixes to improving your performance. Consistent training, hard work, patience and keeping yourself in great health will help pave the way to great future performances.
For more information on nutrient timing, I dedicated three (very detailed) chapters to this topic in my book Essential Sports Nutrition.