Intermittent Fasting and Training
Introduction
Few eating practices have generated as much enthusiasm and as many attractive metabolic explanations. And few hold up as well in the evidence for a reason far more boring than the one usually given.
Intermittent fasting works for plenty of people. What's rarely said is why it works.
It isn't a metabolic trick. It's a schedule that makes eating less easier for some people.
What it is
Several different things under one name:
- Time-restricted eating (16:8 and similar): you eat within an 8 to 10 hour window.
- 5:2: two days a week with very reduced intake.
- Alternate-day fasting.
All share the idea of concentrating intake into less time.
What the studies show
It's fairly consistent and fairly unspectacular: when calories are equated, intermittent fasting produces fat loss similar to continuous restriction. Several meta-analyses reach that conclusion.
That is: there's no metabolic advantage. What there is, is an adherence tool. For many people it's easier not to have breakfast than to control portions across five meals, and that's a perfectly valid reason to choose it.
Effects on health markers — insulin sensitivity, blood pressure, lipids — tend to follow the weight loss rather than the schedule itself.
Where there is a nuance for people who train
Here's the part that interests this book and gets little mention.
Protein distribution gets compressed. If you eat within an eight-hour window, you have fewer opportunities to spread protein out. It's manageable — three meals fit in eight hours — and it requires deliberate attention, because the real risk is not hitting the daily total.
Lean mass. Some studies comparing fasting with continuous restriction find slightly lower lean mass retention with fasting; others find no difference. The evidence is split, and the factor that seems to explain it most is whether protein was sufficient and whether strength training was done.
The practical conclusion: if you do intermittent fasting and train, protein is the variable to watch.
Training fasted
This is a different question from the above, and worth separating.
For low-intensity work, training fasted is fine. Walking, easy jogging, comfortable cycling: no problem, and many people find it convenient.
For high-intensity work, performance drops. Heavy sets, intervals and long sessions go worse without available fuel.
And on "fasted cardio burns more fat": it's the most persistent myth on the topic. It's true that during fasted exercise you oxidize proportionally more fat. What doesn't follow is that you lose more body fat, because the body compensates across the rest of the day.
Schoenfeld and colleagues' study directly compared fasted cardio with fed cardio, with diet equated, over several weeks: there was no difference in fat loss.
If it works for you, how to do it well
1. Prioritize total protein. Aim for 1.6-2.2 g/kg and spread it across three meals within the window. 2. Put training near the eating window. Ideally just before your first meal or between meals, not at the end of a long fast if you're training hard. 3. Save intense sessions for when you're fed. Intervals and heavy strength perform better that way. 4. Watch total energy. The risk of fasting for someone training a lot isn't overeating: it's not eating enough, and that limits recovery. 5. If performance drops persistently, the schedule isn't serving you, however good it sounds.
Who shouldn't
Worth naming clearly:
- History of disordered eating. Fasting can reactivate restrictive and binge patterns.
- Pregnancy and breastfeeding.
- Type 1 diabetes, or type 2 on insulin or sulfonylureas, because of hypoglycaemia risk. It requires medical adjustment.
- Underweight or low energy availability.
- Adolescents.
- And generally, if you take medication requiring food or strict timing.
If you have a diagnosed condition, discuss this before starting.
Common mistakes
Expecting a metabolic effect. There isn't one beyond the deficit it produces.
Compensating within the window. Eating in eight hours doesn't prevent overeating.
Neglecting protein. The most common error among those combining fasting with strength training.
Doing hard sessions at the end of the fast. They perform worse for no reason.
Sticking with it though it makes you feel bad. It's an adherence tool; if it doesn't suit you, it isn't doing its only job.
Conclusion
Intermittent fasting is a schedule, not a mechanism. It works the same as other forms of restriction when calories match, and its real value is that it simplifies eating less for some people.
If it suits you, keep it and do one thing differently: calculate your total protein and make sure it's spread across three meals within the window. That's the point where fasting and strength training can interfere with each other, and it's the only one you need to watch.
References
- Schoenfeld, B. J., et al. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11.
- Tinsley, G. M., et al. (2019). Time-restricted feeding plus resistance training in active females. The American Journal of Clinical Nutrition, 110(3).
- Cienfuegos, S., et al. (2020). Effects of 4- and 6-h time-restricted feeding on weight and cardiometabolic health. Cell Metabolism, 32(3).
- Templeman, I., et al. (2021). A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss. Science Translational Medicine, 13(598).
- Levy, E., & Chu, T. (2019). Intermittent fasting and its effects on athletic performance: A review. Current Sports Medicine Reports, 18(7).