Fasted cardio for weight loss: Is it really superior?

Men running

When it comes to losing weight, popular opinion supports various forms of cardio as being most effective. Whether HIIT workouts for continued energy expenditure post-exercise, long workouts at a low intensity (LISS), or moderate workouts in the ‘fat burning zone’; each form of cardio has its advocates and each, arguably, could be shown to help with weight loss alongside an energy deficit.

One method of cardio, though, stands out as being a favourite of both slimmers and bodybuilders looking to cut weight for competitions: fasted cardio. Fasted cardio is exercise performed after a period of fasting, and is thus usually performed in the morning on an empty stomach. The argument goes that exercising on an empty stomach forces your body to turn to stored fat to fuel the workout, rather than relying on glucose recently eaten.

But does fasted cardio actually work that well, and is it superior to exercising after a meal? Do the benefits outweigh the downsides, which may include: weakness, shorter workouts, hunger, tiredness and nausea?

If fasted cardio really did deliver superior weight loss results, it might be worth overlooking a grumbling stomach, but is there any evidence to prove it does?

What does the research say?

In one 2011 study, researchers compared the fat burning response of six individuals in a fed versus a fasted state, and at different training intensities. Participants cycled for two hours at different intensities on four separate occasions. On two occasions they ate before their workouts and for the other two trials they fasted for 12-14 hours. The study found that only after 90 minutes of low intensity cycling was there a higher fat oxidation rate (the use of fat as energy) in the fasted trials. However, it was also noted that “during moderate-intensity cycling, fat oxidation was no different between trials at any time”.

The research concluded that training on an empty stomach was no better than training after a meal for promoting fat loss, and that, actually, doing so may produce “inferior results”. Furthermore, for those of you who are concerned with muscle strength and growth, the study warned that training fasted had been shown to increase proteolysis – the breakdown of protein for energy – which could hinder muscle maintenance and repair.

In fact, this warning is reiterated in a 2017 study, which suggested that participants could experience a “significant loss” in lean mass (muscle) if fasted cardio was performed over weeks or months. The research found that exercising while fasted did not influence weight loss, “[supporting] the notion that weight loss and fat loss from exercise is more likely… through creating a meaningful caloric deficit over a period of time.” In fact, the article highlighted a number of other studies that have shown that eating food before exercise led to greater energy expenditure after the exercise compared to exercising in a fasted state. This suggests that eating before exercise may actually be more effective for weight loss.

A third study from 2014 also supports these findings. In this study, 20 women took part in a four week trial consisting of three workouts per week lasting one hour, and including a customised diet plan. Half of the women exercised after fasting and the other half exercised after eating. According to the researchers, both groups showed a “significant loss of weight and fat mass… but no significant between-group differences were noted in any outcome measure” revealing body composition changes are similar regardless of whether you exercise fed or fasted.

It’s probably worth noting here, as the research did, that all these studies have their limitations and that further research should really be undertaken before conclusive evidence can be drawn.

So, what should I be doing?

Despite the research, fasted cardio isn’t all bad – in fact, the research seems to suggest there is very little difference between eating or fasting, and it also has a number of key benefits. Generally, it means you are getting your workout done early, leaving the rest of the day free to enjoy, it will also help create the calorie deficit that you need to lose weight, and, for some people, exercising on an empty stomach is preferable, particularly early in the morning.

If I’ve learnt anything about success in health and fitness, it’s you’ve got to do what works for you. If you’re an early bird and enjoy starting your day off with a run, then I wouldn’t stop now. If the morning is the only time you have to exercise, I would continue doing so, but I would also experiment with eating a bowl of porridge or a banana pre-workout, to see if this increases your ability to exercise harder.

When it comes to weight loss, your primary goal will always be to create an energy deficit – to consume less energy than you are using (normally, but not always) over the course of 24 hours. Exercising can help you increase this deficit rather than relying on food alone, but what you eat will determine your success.

Essentially, don’t feel that fasted cardio is your only (or, in fact, the best) option for weight loss. It would be far better for you to eat a healthy snack before hitting the gym and perform at your best, rather than sluggishly slaving through a half-hearted workout for half the time. Create your calorie deficit, and the weight loss will follow.

What form of cardio do you prefer? Let me know in the comments below, or email me at info@split-fitness.co.uk with your questions.

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