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Fasting cardio has been used by athletes who are trying to 'cut' weight for decades. Boxers use it to make weight for fights, struggling to drop those last few pounds to get into a weight category at weigh-in time. Bodybuilders often swear by fasting cardio to lose bodyfat and give them the defined look they crave. And in recent years, people who want to lose weight have eyed fasting cardio and wondered, 'can it do the same for me?'
First, let's look at what happens to your body when you do fasting cardio.
The principle of fasting cardio is not that your stomach is empty, but that your blood sugar is low. If your blood sugar's low, when you start doing cardio your body will try to use other sources of fuel, including glycogen stores, but these will quickly be depleted. Fasting cardio sessions often go on for 45 minutes, sometimes more. The purpose is to force the body to use body fat as a fuel when other fuels are exhausted.
The theory seems to make sense, but does it work?
It certainly seems to. As we saw in the first paragraph, fasting cardio is a tool boxers and bodybuilders have been reaching for for decades. But just because the boxers are losing weight, doesn't mean they're losing fat. And just because the bodybuilders are losing fat, it doesn't mean it's the fasting cardio that's doing it.
Boxers who want to get down to weight use all kinds of unsafe practices - many boxing equipment stores still sell 'sweatsuits, designed to sweat off a couple of pounds of much-needed water before a weigh-in. It might work for their purposes, but we wouldn't recommend it for actual fat loss. In the same way, boxers who use fasting cardio to lose weight are typically going to lose more muscle than fat. We'll get into how that works in just a moment.
Additionally, cortisol and testosterone-like substances bind to the same sites - the more testosterone you have, the less cortisol affects you, so if you're taking anabolic steroids you can elevate your blood cortisol without being affected by it.
Their bodies don't react like ours and we can't measure the training methods that work for a man in his 30s who weighs 275 pounds at 5'8 and has visible muscular striations, and apply the same protocols to ordinary people. Without the genetics and the drugs we couldn't sustain the crushing weight training schedules these athletes use; we shouldn't use their cardio protocols either.
The key argument against the use of fasting cardio for pretty well anyone who isn't a professional figure athlete is this:
it tends to metabolize muscle at the same pace as fat.
If you're trying to lose weight, the chances are that you're trying to look better and feel better as well as weigh less. Fasting cardio isn't your friend here.
The other reason is connected: fasted cardio will tend to reduce your capacity to lose fat further down the line.
If you're stronger you can move more load, which means more 'work,' in the physics sense of the term, which requires more energy - more calories. If your muscles are thinner and weaker that's harder and as a result your training sessions are less intense and often shorter.
Let's get a bit deeper into the reasons why fasting cardio will tend to reduce muscle mass.