Does ‘fasted’ cardio help you lose weight? Here’s the science

Every few years, the concept of fasted exercise training pops up all over social media.

Fasted training refers to exercising in the morning, before eating breakfast.

Fans will claim it’s the most efficient way to lose body fat. Opponents say it’s a terrible idea and will cause you to gain weight.

Who’s right and what does the research evidence say?

Where did the idea come from?

Fasted exercise, proponents say, will cause better changes in body composition – the proportion of lean mass (muscle), bone and fat. In particular, they say fasted exercise leads to fat loss.

Positive changes in body composition can occur through losses of fat mass, while either maintaining or gaining lean mass. Or even through gaining lean mass in the absence of any loss of body fat. All of which may be considered positive.

The idea fasted exercises leads to such positive body composition changes stems from research that shows exercising after eating versus exercise before eating affects metabolism differently.

Aerobic exercise in a fasted state causes you to burn more fat as a fuel (what researchers would call “fat oxidation”) when measured at a single point in time.

So it wasn’t a big leap to assume this would translate to longer-term fat loss.

However, a 2017 systematic review from my team demonstrated that a fasted exercise training program doesn’t seem to translate into long-term differences in body fat loss.

This discrepancy between fat burned as a fuel during exercise, and changes in body fat in the long term has often been misunderstood.

This apparent contradiction may come down to the fact the body seems to find ways to compensate. Fat burning seems to reduce once you eat, and people who have exercised hard may end up expending less total energy over the course of the day.

In exercise science, it’s actually pretty common to find that short-term effects don’t always translate to longer-term impacts.

For example, intense short-term exercise can negatively affect your immune system in the moment, but doing regular exercise can actually affect it positively in the longer term.

What does eating soon after or just before your workout do?

Eating a meal featuring carbohydrates and protein close to when you exercise is likely to help with performance during your next exercise session.

However, whether that meal is before or after your workout seems to have limited impact.

Interestingly, research has shown that increasing the proportion of the food you eat in the morning – and in particular, eating more protein – may help to improve body composition and enhance weight loss.

However, this timing is not in relation to exercise, rather in relation to when in the day you eat.

What about sports performance?

It’s fairly clear eating before exercising improves performance in activities lasting over 60 minutes but has little effect on performance of shorter duration activities.

This is also evidenced by the lack of elite athletes supporting fasted exercise. A survey completed by almost 2,000 endurance athletes showed non-professional athletes are more likely to exercise fasted compared to professional athletes.

What about strength training?

So do you get differences in muscle strength, size, and body composition changes in response to doing resistance training (such as weightlifting) when you’ve fasted versus when you’ve eaten? Unfortunately, the research is limited and low quality.

This limited evidence so far suggests it makes no difference.

One recent randomised controlled trial also found no difference in strength, power, or lean body mass when resistance training was done twice a week for 12 weeks either after fasting or after eating.

What are the potential drawbacks?

Fasted training can make you feel really hungry after exercise, which can lead you to make poorer food choices.

Some people may even get headaches and nausea when trying to exercise fasted. This isn’t universal experience, though; social media is full of people who say exercising while fasting makes them feel great.

In summary, there is no clear winner.

The evidence doesn’t support the superiority of fasted exercise for weight loss, or sports performance.

However, the evidence also doesn’t show it causes a problem in many scenarios (except perhaps elite sports performance).

So if you’re short on time and skipping breakfast is going to allow you to get out and get that run or workout in, then go for it. Don’t worry too much about the consequence.

But if the idea of exercising on an empty tummy makes you want to avoid the gym, then grab some breakfast before you go. Rest assured it won’t be working against your goals.

Exercise fads and wellness hacks come and go but the thing backed by solid and consistent evidence is exercise.

Simply doing it matters the most.

Not the time of day, not the exact exercise choice, not even the exact amount – and definitely, not if you have or haven’t eaten before you exercise.