Is fasted cardio better for fat loss?

No. Cardio before breakfast makes your body use a little more fat as fuel during the session.^[1] But in the few trials that ran for weeks, people lost about the same amount of fat whether they ate first or not.^[2][3] So do your cardio at the time you'll keep doing it, fed or fasted.

Fasted cardio means aerobic exercise, like walking, jogging or cycling, done after an overnight fast and before breakfast. Fed cardio is the same session done after a meal or a carbohydrate drink. Walking counts as cardio too, and daily steps covers how much of it to aim for.

Why isn't the answer obvious? Two different things both get called "fat". One is the fat your body uses as fuel during a single session. The other is the body fat you lose over weeks. Fasted cardio changes the first one. The trials found no clear difference in the second.^[2]

Why it matters

Getting this wrong costs you in two ways.

The first cost is your schedule. You might bend your week around a 6 a.m. session on an empty stomach, for an edge the weeks-long trials didn't find.^[2] If that slot doesn't suit your life, it may be the one you skip. And long sessions on an empty stomach may go worse: in one meta-analysis, eating first helped performance in longer cardio sessions.^[4]

The second cost is attention. Fat loss follows your calorie balance over days and weeks.^[2] A calorie is a unit of food energy. Eating fewer calories than you use is called a calorie deficit, and calories and energy balance explains how it works. The fuel mix of one session is small next to that.

If you also lift, cardio and lifting covers whether cardio costs you muscle. This article sticks to fat.

What the research says

A few terms first. A meta-analysis pools the results of many studies on the same question into one estimate. A trial here is a study that splits people into groups at random and compares them. Fat oxidation is the scientists' term for using fat as fuel. It means how much of the energy your body uses comes from fat.

The studies look at three time spans: one session, one day, and several weeks. Only the last one tells you about fat loss.

During the session: fasted uses a little more fat

A 2016 meta-analysis compared single cardio sessions of up to 2 hours, done fasted or fed.^[1] In the 11 studies (117 people) that measured fat use, the fasted sessions used more fat as fuel.^[1] Moderate evidence. The gap was clear in easy to moderate sessions, but not in hard ones.^[1] Most studies in the review used men who were already active.^[1]

The gap was small: about 3 to 3.5 g of fat per session.^[1] A gram of fat holds about 9 calories, so by our maths that's roughly 30 calories' worth. These studies measured the session itself. They say nothing about body fat over weeks.

Over the whole day: the studies disagree

Mixed evidence. The studies disagree. Here's what each side found.

• The body may balance it out. In one small study, 8 young men ran on a treadmill for 36 minutes, once fasted and once after breakfast. They ate the same food afterwards. After the fed session, they used more fat in the hours that followed, and the gap was still there 24 hours later.^[5] Limited evidence. It's one tiny study.
• Or fasted may come out ahead. A lab in Japan put 10 young men in a metabolic chamber for 24 hours. That's a sealed room that tracks every breath, so it can measure the fuel you use all day. When the men cycled for an hour before breakfast, their fat use over the day went up: about 717 calories from fat, against 456 on a day without exercise.^[6] The same ride after lunch or after dinner didn't raise it.^[6] The authors said their results can't be stretched to long-term effects.^[6] Limited evidence. One small study, one day, young men.

The authors of the weeks-long meta-analysis below suggest why the extra fat used in a fasted session may not show up as fat lost. Your body may use less fat later in the day, once you eat.^[2] Expert consensus. This is the authors' view, not a tested finding. It fits the treadmill study, but not the chamber study. Both were small, both lasted one day, and they point in opposite directions.

Over weeks: fat loss came out about the same

Limited evidence. Only a handful of small, short studies have tested this. They all point the same way. The authors themselves warn that there isn't much data yet.^[2]

One trial put 20 young women on the same kind of diet for 4 weeks. Each was told to eat about 500 calories a day below her estimated needs.^[3] Each did one hour of treadmill cardio three times a week, mostly at a steady pace.^[3] Half trained before breakfast and had a shake afterwards. The other half had the same shake before training.^[3]

Both groups lost weight and body fat. The study couldn't detect a difference between them.^[3] The authors added that, with so few people over so few weeks, they can't rule out a small benefit for either side.^[3] Limited evidence. It's one small trial. The women did cardio regularly, and several were college track athletes in their off-season. They weren't lifters.^[3]

A 6-week trial in men with overweight or obesity shows the gap between "fat used" and "fat lost" most clearly. Thirty inactive men were split into three groups.^[7] Two groups cycled three times a week, building from 30 to 50 minutes a session.^[7] One of them had a carbohydrate drink before each ride. The other rode fasted and had the drink afterwards. The third group didn't exercise.^[7]

The fasted group used about twice as much fat during their rides.^[7] The rides used about the same total energy in both groups.^[7] And body weight changed about the same in both.^[7] By our maths, the extra fat used came to under 900 calories over 18 rides, about 100 g of fat in six weeks. Limited evidence. The groups were small, and the study measured weight and waist-to-hip ratio, not body fat.

A 6-week trial of interval training found the same. Interval training means short, hard efforts with rest in between. Sixteen women with overweight or obesity, new to training, did 18 sessions of ten 1-minute hard efforts on a bike.^[8] Both groups lowered their body-fat percentage. The study found no clear difference between fasted and fed in anything it measured.^[8] Limited evidence. It had 8 women per group.

Does skipping breakfast help you eat less?

Mixed evidence. The studies disagree. Here's what each side found.

• In a one-day study, 12 active young men skipped a 431-calorie breakfast and cycled for an hour. At lunch they ate about 166 calories more than on the day they ate breakfast first. That's far less than the breakfast they skipped.^[9] They ended the day about 400 calories short. When they ate breakfast first, they ended the day about even.^[9] Limited evidence. One small study, one day, men only.
• In a four-day study, 12 active adults (10 men, 2 women) ran for 75 minutes, fasted or fed. Over the four days, there was no difference in how much they ate, how much energy they used, or their appetite.^[10] Limited evidence. One small study over four days, mostly men.

If skipping breakfast does help someone eat less overall, that's the calorie deficit at work. It isn't the fat used during the session.

Performance: eating first helps longer sessions

A 2018 review of 46 studies looked at eating before exercise.^[4] In its pooled results, eating first improved performance in longer aerobic sessions. In shorter ones, it made no clear difference.^[4] Moderate evidence. The summary doesn't say how long "longer" is.

How to apply it

Steps 1, 2 and 4 follow from the studies above. Step 3 is coaching practice, not a tested finding.

1. Pick the time you'll keep. Before breakfast, at lunch or after work. Fasted or fed, fat loss came out about the same in the trials.
2. Eat something before long sessions. Eating first helped performance in longer cardio sessions. In short sessions it made no clear difference.
3. If fasted sessions feel flat, eat something small first. In the weeks-long trials, eating first made no clear difference to fat loss.^[2]
4. Keep the calorie deficit. That's what moves body fat, whatever time you train. How big a deficit to aim for is covered in cut, bulk or recomp.

A worked example

Sam weighs 90 kg and wants to lose fat. He rides an exercise bike for 40 minutes on three mornings a week. Does he need to skip breakfast first?

On a fasted ride, a bit more of the energy comes from fat. In the pooled studies, the gap was about 3 g of fat a session.^[1] Fed, more of it comes from carbohydrate. In the 6-week trial, the rides used about the same total energy either way.^[7]

So the choice comes down to how Sam feels. If he rides better after a slice of toast, he eats the toast. What decides his fat loss is whether he eats fewer calories than he uses over the week. Over several weeks, his weight and waist trend should show which way it's going.

Common mistakes

Mixing up fat used and fat lost. A fasted session does use more fat as fuel.^[1] People hear that and assume more body fat comes off. Over weeks, it didn't.^[2] The "fat-burning zone" idea (go slow, because slow cardio runs on more fat) makes the same mix-up. Fix: judge by your weight and waist trend over weeks, not the fuel mix of one session.

Forcing a session you dread. An empty-stomach 6 a.m. ride sounds disciplined, so people pick it. The weeks-long trials found no fat-loss edge for it.^[2] Fix: train when it suits you, and eat first if you want to.

Going long on an empty stomach. People stretch the fasted session to make it "count" more. But eating first helped performance in longer sessions.^[4] Fix: eat something small before a long ride or run.

Ignoring the rest of the day. It's easy to treat the fasted session as the whole plan. But in one study, men who skipped breakfast before cycling ate some of it back at lunch.^[9] Fix: keep an eye on what you eat across the day, not just before training.

How Gymbers fits

Fat loss comes down to the deficit, not the clock. The Gymbers app logs calories, protein, carbs and fat against daily targets. Steps sync from Apple Health (iPhone) or Health Connect (Android). You can log body weight and waist measurements, each with a chart, to watch the trend over weeks.