Fuel

Fasted vs Fed Training: What the Research Actually Shows

Cameron Usher · Founder, px

8 October 2026 · 10 min read

Twelve years in competitive sport, trained alongside the Australian Olympic squad. px is the coach he needed after ten coaches, every wearable and every protocol.

It is 6.05am. You have forty minutes before the day starts, a banana on the counter and a half-remembered line about fasted cardio burning more fat. So you leave the banana.

Here is what the research on fasted vs fed training actually shows. Training on an empty stomach burns a little more fat during the session. It does not lead to more fat loss over weeks when what you eat is matched. Eating first helps sessions longer than about an hour, and barely matters for short, easy ones. The session decides, not a rule.

Does fasted training burn more fat?

During the session, yes, by less than the name suggests.

A 2016 meta-analysis of 27 studies (273 participants) found fasted aerobic exercise burned 3.08 g more fat on average (Vieira et al., 2016). A 2023 crossover in eight men and eight women found almost the same: 3.25 g more across 30 minutes of cycling after a seven-hour fast (Slater et al., 2023). Three grams of fat is roughly 28 kcal (at 9 kcal per gram). Real, and small.

In a metabolic chamber, ten young men who exercised for an hour before breakfast oxidised 717 kcal of fat over 24 hours, against 446 kcal when the same session came after lunch (Iwayama et al., 2015). That is the strongest case for fasted cardio: one day, ten people, fat burned. Not fat lost.

Does fasted cardio lead to more fat loss?

Not when food intake is matched.

• The controlled trial. Twenty young women trained for an hour, three times a week for four weeks, on diets below their energy needs, half fasted and half after a shake. Both lost fat, with no difference between groups on any measure (Schoenfeld et al., 2014).
• The meta-analysis. Across five studies (96 participants), fasted training's effects on body mass, fat and lean mass were trivial, on a small evidence base (Hackett and Hagstrom, 2017).
• The newest review. A 2025 review traced body composition changes after skipping breakfast to "the concomitant reduced kilocalorie intake", not session timing (Stratton et al., 2025).

One 2025 meta-analysis of four lifting trials did find more fat lost fasted, with a confidence interval running almost to zero, and its own authors still concluded fasted and fed training "appears to have similar effects on body composition" (Vieira et al., 2025a).

More fat burned does not become more fat lost, because the session is not the whole day. In a 2026 randomised crossover of 35 young adults, 17 of whom also ran after eating, food before the run lowered fat oxidation but did not reduce how much people ate afterwards; time of day mattered more (Höchsmann et al., 2026). Appetite has more than one driver, and a short night pulls it towards easy, rewarding food too. Being hungrier after a fasted session is not a failure. It is your body balancing the account.

When does eating before training change performance?

Length and intensity decide it.

The largest analysis we found, a 2018 meta-analysis of 46 studies, showed a small but significant benefit of eating before aerobic sessions longer than 60 minutes (g = 0.3) and none for shorter ones (g = 0) (Aird, Davies and Carson, 2018). The joint position stand of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine agrees: carbohydrate is "not needed" during sessions of 45 minutes or less, and before sessions of an hour or more its range is 1 to 4 g per kg of body mass, 1 to 4 hours out (Thomas, Erdman and Burke, 2016).

Hard work shows the gap sooner. After a seven-hour fast, riders did 3.8 per cent less work in a hard 15-minute effort, rated their motivation and energy lower beforehand, and enjoyed the session less (Slater et al., 2023). That matters, because a session that feels bad costs you next week.

Lifting and hard intervals

The lifting evidence is closer than either camp admits.

• Long sessions favour carbohydrate. A 2022 meta-analysis of 21 crossover trials (226 participants) found carbohydrate increased lifting volume, most clearly in sessions past 45 minutes (effect size 1.02) or after fasts of 8 hours or more (0.39) (King et al., 2022).
• Short fasted strength work held up. Trained men lost no strength, jump height or reps after fasts of 12 or 16 hours, and were hungrier after 16 (Drummond et al., 2024).
• Part of the breakfast effect may be belief. Twenty-two trained men managed 38 squat reps after water, 44 after a carbohydrate breakfast and 43 after a near-zero-calorie placebo they were told contained energy (Naharudin et al., 2020). A 2025 taste-matched placebo trial found no difference in reps over 90 minutes (King et al., 2025).

Over weeks, the other popular myth, that training fasted costs you muscle, fails too. In a 12-week randomised trial of 28 young adults lifting twice a week, with daily food guided to be equal, fasted and fed groups gained muscle and strength with no difference between them (Vieira et al., 2025b). For one long, heavy session, eating first is still the safer bet.

So should you train fasted or fed?

The evidence by session type, as a guide rather than a rule:

If food before training sits badly with you, that is a fair reason to keep easy sessions fasted, not long ones.

What is "train low", and who is it for?

Train low means deliberately starting some endurance sessions short of carbohydrate to push the muscle's adaptation: a periodisation tool for endurance athletes with a specific aim, not a way of life.

A 2018 review counted stronger cell signalling in 73 per cent of 11 studies and larger rises in oxidative enzymes in 78 per cent of 9, but better performance in only 37 per cent of 11 (Impey et al., 2018). Its framework is to "fuel for the work required": carbohydrate matched to each session, so the hard work is still done well.

• When it helped. 21 trained triathletes ate the same daily carbohydrate for three weeks, timed differently. The sleep-low group cut 10 km run time by 2.9 per cent, the control group by 0.1 per cent (Marquet et al., 2016).
• When it did not. 26 elite male endurance athletes added low-carbohydrate sessions three days a week for four weeks. Both groups improved equally (Gejl et al., 2017).
• When more fat burned meant slower. Elite race walkers on a ketogenic diet burned far more fat and lost economy. The high and periodised carbohydrate groups improved 10 km times by 6.6 and 5.3 per cent; the ketogenic group did not improve (Burke et al., 2017). A whole diet, not fasted sessions, but it breaks the idea that more fat burned means faster.

The position stand calls it recognised but open to misuse (Thomas, Erdman and Burke, 2016). If you are training for something dated, plan it around your hard days rather than drifting into it.

Is fasted training different for women?

Honestly, the evidence is thin. In a 2022 network meta-analysis of fasted exercise, only six of 23 studies included any women (Frampton et al., 2022).

What exists matches the data in men. The four-week body composition trial above was run entirely in women (Schoenfeld et al., 2014), and the 2023 crossover included eight women among its 16 riders (Slater et al., 2023).

We found no trial on whether cycle phase should change the answer, and we will not pretend one exists. What is well documented, in women and men, is that regularly eating too little for the training you do harms performance (Mountjoy et al., 2023). A fasted easy session inside a well-fed week is a different thing from a pattern of under-fuelling.

Will a fasted session show up in your HRV?

We found no good evidence that it does.

The only trial we found that measured heart rate variability around a fasted session tested 12 adults, average age 59, walking after a 12-hour fast, and found no difference in it over seven minutes of recovery (Rodrigues et al., 2019). Nothing we found links fasted sessions to next-morning HRV or resting heart rate.

HRV is the most useful single recovery signal we have, and also the most misread, so do not pin a low reading on yesterday's empty stomach. A fasted session shows up in the session: output on the hard part, how ready you felt (Slater et al., 2023) and hunger (Frampton et al., 2022). If your morning signals already say today should be trimmed or switched, eat first: a fuel shortfall on top of a sleep shortfall is how one flat day becomes several.

How px decides whether you eat first

px has no blanket rule about fasted training. The session decides.

The coach treats fasted training as a legitimate choice for easy or short work, and as something that costs you on long or hard work. Under about an hour, water is the answer. Before hard work, it points to a carbohydrate meal 2 to 4 hours out, or something small and low in fat and fibre inside the last hour. Ask px what to eat before a 7am session and you get that logic back, with the reason; it does not place a snack on your morning for you.

Meals you book land on My Day inside an eating window, twelve hours by default. That window only spaces the day's food. There is no fasting setting, and px does not run fasting programmes. It will not coach fasting for someone it has reason to think it would harm.

Food changes training too. When a training day runs well short on food late into the day, px trims that day's Move and Mind sessions by ten minutes and tells you why: you have not replaced what you used. That is what a training nutrition plan built around your actual load looks like. If you train early, the next question is whether you need protein before a morning workout. If fasting appeals because it feels disciplined, read why "clean eating" is not the same as fuelling well.

On a Thursday before a long ride, it reads like this (illustrative):

"Today's run is easy and under an hour, so water is enough and breakfast can wait; Saturday's two-hour ride is where fasting costs you, so eat a carbohydrate breakfast two to four hours before you set off."

That is Direction on the smallest decision of the morning.

Eat for the session you have, not the rule you heard. px reads what you already track and builds one plan across training, mind and food before your day starts. Try px

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