This question is not a straight forward one. I would say it depends on what your goals are and how you feel.
Let’s look at the benefits of eating both fasted (before eating anything) and fed (after eating) workouts.
Studies show that fasted cardiovascular exercise can improve insulin sensitivity and glucose and fatty acid transport. This means that if you have diabetes, pre-diabetes or metabolic syndrome, obesity or insulin resistance, fasted workouts can help to correct the problems that your body has with too much insulin and/or sugar (glucose).
We also know that fat oxidation (fat burning) is greater in Low Intensity Steady State (LISS) cardiovascular exercise when performed in a fasted state than when it is in a fed state.
Something to remember here, however, is that a lot of the fat oxidized comes from intramuscular triglyceride stores (fat in the muscle), and not all from adipose tissue (stored body fat). This means that if body fat loss is your goal, the occasional fasted cardio session won’t really get you the results you want, but if you are consistent with it, it will get you more results.
Exercising fasted also might improve the glycogen replenishing effect and anabolic effect (meaning muscle growth) of a post-workout meal. This probably is due to the above-mentioned improvements in insulin sensitivity and a more robust glycogen depletion that occurred during the workout, just before the meal.
People training in a fed state can workout at higher intensities for longer periods of time than fasted subjects. When you are able to reach greater intensities in your workout, this can lead to greater calorie burning that extends into the time after your workout. This is called EPOC (excess post-exercise oxygen consumption). EPOC is higher in those that work out fed over those that workout fasted.
It could also be argued that the higher the workout intensity could lead to more calorie burning during the workout itself and better muscle gain. This is theoretical however, as some studies show similar muscle gains with those working out fasted or fed.
A caveat to this was for those in a calorie surplus. A study showed that people who trained fasted, didn’t gain as much body fat as those that exercised after eating despite them both maintaining a 30% calorie excess.
Something very interesting is that some studies have shown that while working out fasted might cause greater fat burning during the workout, it may lead to more of a reliance on glucose (sugar) the rest of the day. The opposite was true of fed workouts; while they relied more on glucose during the workout, fat burning was increased the rest of the day. This is interesting for sure, however I question if this would apply to those on a low carbohydrate diet where glucose would not be available throughout the rest of the day.
You can see that there are reasons to work out fast or fed. So how should someone decide for themselves? You have to consider what your goals are.
1. If you are healthy, and you are working out to stay healthy or optimize your health, I would suggest alternating between some fed and some fasted exercise. Exercising fasted challenges, the body’s metabolism and helps to maintain insulin sensitivity. It forces the body into making adaptations that can be associated with longevity. While you can build muscle in either state, most people perform better, lift heavier or go longer and therefore get better results in the fed state for muscle building.
Use food for power/muscle building.
Use fasted exercise for metabolic health.
2. If your goal is to optimize for power or performance that takes quick bursts (anaerobic) of energy, then mostly train in the fed state (power is glucose driven) with some, but less often fasted exercise to keep some metabolic fixability and keep insulin sensitive.
3. If your goal is to optimize for endurance, which is defined as aerobic exercise for more than 60 minutes, training fasted ensures good fat adaptation (meaning you can tap into stored body fat for fuel) for steady energy, however use the fed state for competitions.
4. If you are metabolically impaired, meaning you have insulin resistance, type 2 diabetes, pre-diabetes, obesity or metabolic syndrome (1 in 3 Americans are metabolically unhealthy) or if you are eating high calorie, then use fasted exercise to help correct the metabolic derangements and avoid excessive fat gain while eating over your calorie needs.
5. If your primary goal is to improve body composition, then it doesn’t seem that there is a significant difference from exercising in the fed or fasted state.
There are different goals and needs for each person. None of us are starting at the same place nor do we have the same goals. Everyone is unique and there is no one size fits all equation. I might want to exercise fasted, but I can only find time to work out after dinner. The main thing to remember is that exercising versus not exercising and consistency is the key. Don’t get lost in the weeds.
How do you feel your best when working out and when can you work out? Do what you can and your body will reward you!
How do you like to work out? Fasted or Fed? Let me know in the comments below!
Forever yours in Health,