Jennifer Lopez swears by fasted cardio - but what is it? Photo / AP
Jennifer Lopez swears by fasted cardio - but what is it? Photo / AP
The headline reads “Jennifer Lopez swears by fasted cardio”, and accordingly, thousands Google this magic new key to shaping their body. J-Lo has become an international icon of age-defying physicality and is currently the face of Intimissimi lingerie, so she must be right. Right?
Dwayne “The Rock” Johnson is also a fan, but emphasises that he follows the scheme in conjunction with a very controlled nutrition programme.
The world of fitness is beset by fads and supposed shortcuts. Understandably, people seek fast, efficient ways to glean the benefits of working out – there are only so many hours in the day. Something that sounds scientific and feels instinctively right is always bound to tempt us.
Dwayne Johnson is also known to be a fan of fasted cardio. Photo / AP
“Fasted cardio” is essentially going for a run, cycle or any aerobic activity without a recent meal to fuel you. Gym chain Nuffield Health’s personal training lead Sam Quinn says: “Generally, the activity is performed in the morning after a night’s sleep, but as fasting has gained popularity, it could be any time you’re in a fasted state – so whenever you’ve had a period of eight hours or so free of food.”
Quinn says a truly fasted state would be visible in blood readings such as glucose, insulin or blood lipids (fatty acids), and for this, eight hours without food would be the minimum.
The idea at the heart of the fasted cardio trend is that when your body has depleted glycogen stores, it will burn more fat as fuel while you’re working out and have you looking like J-Lo in double-quick time.
The research is somewhat conflicted. Fasted cardio necessitates a journey into human physiology, so settle in while we explore the way exercise and body fat truly interact.
First, there is the question of whether indeed we do burn more fat when fasting. There are some small studies that find that slightly more fat may be burned as fuel when exercising without food – the evidence on how much more, if any, is so far inconclusive. Quinn says a bigger study is needed. The real crunch, however, is not what you burn during exercise, but across the week.
Fasted cardio is supposed to help you lose weight faster - but what do the experts say? Photo / 123rf
“Even though you might be burning more fat at the time, achieving your body-composition goals (how much fat and muscle you are aiming for) will be more influenced by your behaviour over a 24-hour period.
“You will need to be in a caloric deficit [burning more calories than you are taking in], and whether you are fasted or not fasted as you exercise will make little difference if you’re over-consuming calories later in the day.”
Also, at extremes, fasted exercise is not without risks. Quinn points out you could prompt your body to start using your own muscle as fuel if your exercise is too intense or prolonged, causing the loss of hard-earned gym gains. Not to mention the risk of sickness, dizziness and light-headedness.
For the majority of healthy people, steady cardio before breakfast is very safe. Anyone with low blood pressure or blood sugar issues should avoid it, and it’s best to ease your way in with shorter bursts of gentle exercise and monitor yourself carefully.
For some, working out while digesting food just isn’t comfortable, and they’d simply rather work out before breakfast rather than spend half their day waiting for a meal to digest.
Brett Sizeland of outdoor exercise company Strength& says, "Fasted cardio offers no significant benefits compared to fed cardio". Photo / Getty Images
Brett Sizeland, a director of outdoor exercise company Strength&, says: “Fasted cardio offers no significant benefits compared to fed cardio, assuming all calories and nutrition are equal. For most clients, the question is not ‘Should you do fasted cardio?’, but ‘Do you want to do fasted cardio?’ Some clients that are early risers will often do fasted cardio because they simply can’t train in a fed state. But it’s really down to personal preference. My advice – focus on calorie balance, key nutrients and keeping your lifestyle as tip-top as it can be.”
Seeking to find a consensus, I called a number of experienced exercise and nutrition experts. Josh Silverman, head of education at Third Space gyms, agrees with both Sizeland and Quinn.
“The reduction in body fat is dictated by energy balance throughout the day, regardless of what energy you’ve burnt during exercise. A 2021 study by MB Harris at William & Mary research university in Williamsburg concluded that using fat as energy is not equal to lipolysis [reducing body fat].”
In other words, it doesn’t really matter what type of fuel you burn during exercise, it’s the overall calorie deficit that counts.
Similarly, research analysing the results of five different studies concludes fasted cardio and non-fasted cardio provide similar results for fat loss.
So, if fasted cardio turns out to be more of a personal preference than a magical key, what is the best way to go about losing any unwanted fat?
Nuffield’s Sam Quinn says he’d first look at a client’s overall calorie intake.
“Then, depending on how committed they are to their nutrition, we would look at their resistance training. Resistance training is the most effective approach for changing body composition because it increases lean muscle mass, which enables you to burn more calories over longer periods. Then we’d recommend cardiovascular exercise to further increase the calories they’re burning.”
The story of the fasted cardio trend appears to be as much the result of collective wishful thinking as confusing research.
“People want a quick fix,” says Quinn. “Something that’s going to act fast but doesn’t require hard work. Everybody looks amazing on social media and people want to achieve those body goals as quickly as they can. But unfortunately, this isn’t that quick fix.”
Hopefully, there will be a study large enough to sort out any ambiguity on this issue. Until then, our collective journey to be more like J-Lo will mean eating well, sleeping and lifting weights. Let’s enjoy breakfast.
