Training while fasting

Can you train hard and still fast? Yes, and several trials show it. But there's one important catch: fasting on its own, without strength training and enough protein, can cost you some muscle along with the fat. Here's what the research says about getting the combination right.

Muscle is the thing to protect

When you lose weight, some of it is almost always muscle. That matters: muscle keeps you strong, supports your metabolism and helps you stay independent as you age. Fasting doesn't cause this by itself, but it can make it easier to fall short on protein and to skip the training that tells your body to keep muscle.

Strength training and fasting work together

The same holds for women. In an 8-week randomized trial, resistance-trained women ate within a window of about 7.5 hours or across about 13 hours, with the same calories and 1.6 g of protein per kilo of body weight a day. Both groups gained a similar 2–3% of lean mass, built the same muscle and got stronger by the same amount ². As long as protein and energy are matched, the length of the eating window doesn't seem to hold back your progress.

Fasted cardio: more fat burned, same result

Exercising before breakfast is popular because it burns more fat. That part is true: a review of 27 studies found that aerobic exercise in the fasted state burned a few more grams of fat during the session than the same exercise after eating ⁴.

Fitting training into your fasting day

• Schedule strength sessions near your eating window, so a meal with protein comes soon after.
• Short or easy sessions, such as walks, yoga or a 30-minute run, are fine at any point in your fast.
• For long or very intense sessions, eat beforehand or move your window so you're fed.
• Spread your protein across your meals and aim for about 1.6 g per kilo a day if you're training for muscle.
• Drink water before and during training, and add a pinch of salt on hot days or long sessions.

MYTH

Training fasted makes your body eat its own muscle.

FACT

In trials where people followed 16:8 alongside strength training with enough protein, muscle and strength were maintained or increased ^1,2. The risk to muscle comes from too little protein and no strength training, not from training on an empty stomach.

Sources

1. Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290. doi.org/10.1186/s12967-016-1044-0
2. Tinsley GM, Moore ML, Graybeal AJ, et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr. 2019;110(3):628-640. doi.org/10.1093/ajcn/nqz126
3. Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med. 2020;180(11):1491-1499. doi.org/10.1001/jamainternmed.2020.4153
4. Vieira AF, Costa RR, Macedo RC, et al. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. Br J Nutr. 2016;116(7):1153-1164. doi.org/10.1017/S0007114516003160
5. Schoenfeld BJ, Aragon AA, Wilborn CD, et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014;11(1):54. doi.org/10.1186/s12970-014-0054-7
6. Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018;28(5):1476-1493. doi.org/10.1111/sms.13054