Intermittent fasting and exercise: train without losing performance
Training fasted burns more fat but can dent your performance. When to train, what type of exercise, and how to hold on to your muscle.
Part of the complete guide Intermittent fasting and exercise: training fasted without losing muscle
What the science saysFasted training: work out without losing muscle
You can exercise while fasted. Fasted aerobic work raises fat oxidation during the session, but it does not produce more body fat loss than training fed when calories are matched. Eating beforehand improves performance in long aerobic efforts; in short or strength sessions, no meaningful difference shows up.
- Meta-analysis of 27 studies (273 people): fasted aerobic exercise oxidized 3.08 g more fat during the session than fed (Vieira 2016). [1]
- Eating before training improves long aerobic performance, but not short-duration efforts (Aird 2018, review of 46 studies). [2]
- On a calorie-restricted diet for 4 weeks, fasted and fed cardio produced similar weight and fat loss (Schoenfeld 2014). [3]
Can you work out fasted?
Yes, and plenty of people do. With glycogen low, training fasted favors using fat as fuel. The catch: in very intense or long efforts, performance can dip because there is less sugar available.
Fasted cardio
Low to moderate cardio (brisk walking, easy jogging, relaxed cycling) works great fasted and nudges fat oxidation up a bit. For very intense cardio or hard intervals, you will probably perform better having eaten.
Fasted strength training
You can lift fasted with no problem on normal sessions. The key to not losing muscle is not training fed — it is (1) enough total protein in your window, and (2) keeping the muscle stimulated with real loads. If you are chasing peak performance or high volume, eat first.
When to train, based on your window
- Train at the end of the fast, right before you eat: you get the fat-burning upside and eat after to recover. Very popular.
- Train inside the window (fed): better for peak performance and hypertrophy.
- Train deep into the fast: save that for low to moderate intensity.
Protein and recovery
Spread your protein across your eating window and make sure a solid dose lands in the meal after your workout. Hydration and electrolytes count too, especially if you train fasted and sweat a lot.
How Ayúname helps
With Ayúname you set the window so your workout lands where it suits you: fasted to burn fat, or right before you eat so you recover fast. The phase timer tells you which fuel you are bringing to the gym.
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Frequently asked questions
Is it better to work out fasted or fed?
Do you lose muscle if you train fasted?
When is the best time to train on 16/8?
Scientific sources (4)
- Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition, 2016.
- Effects of fasted vs fed‐state exercise on performance and post‐exercise metabolism: A systematic review and meta‐analysis. Scandinavian Journal of Medicine & Science in Sports, 2018.
- Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 2014.
- Time-restricted eating effects on performance, immune function, and body composition in elite cyclists: a randomized controlled trial. Journal of the International Society of Sports Nutrition, 2020.