Aerobics classes seem like hard work and make us feel out of breath – so we believe they must be burning calories and making us slimmer.

But the science says this isn’t true. They could actually be making us fatter!

Cardiovascular exercise, like aerobics, works the big muscles of the body, for example the legs. In turn the heart works harder to pump more oxygenated blood to the muscles. And this means the lungs have to take in more air to provide this oxygen. This is why you feel out of breath.

The bad news is that while this may feel exhausting, the calories it burns are pretty pathetic. A full hour of aerobics will burn around 540 calories – that’s 50 calories less than a coronation chicken sandwich and a skinny latte.

But that may not be enough to help you lose weight. “Once you start exercising your body is likely to ask for more food, and you could easily find yourself eating more than you are burning,” explained Dr Nick Krasner, sports and family doctor.

But it gets worse. “Aerobics could actually be breaking down important lean muscle tissue as the body struggles to generate energy. Your internal organs are made of muscle so the last thing you want to do is start damaging these. And if you lose muscle your metabolism slows – in other words you’ll burn less calories on a day to day basis,” explains Zana Morris of Educogym [] in Harley Street, “So, the more muscle you have, the more calories you burn.”

The loss of lean muscle tissue can lead to the classic yo-yo dieting situation where you lose weight and look slimmer but as soon as you slacken off from the diet and the exercise the pounds pile back on again and you are bigger than when you started! This is because you have less muscle and so your metabolism is slower and you are burning fewer calories.

And it doesn’t stop there. It can also lead to hormonal changes. Exercise causes an increase in hormones, such as cortisol and growth hormones, according to Harley Street anti aging expert, Dr Jeya Prakesh.

And there’s evidence that there’s a link between an excess of cortisol and fat storage.

Cortisol is a stress hormone. It’s designed to prepare our bodies for 'fight or flight'. To do this, we need a ready supply of energy. Cortisol provides this by raising our blood sugar by producing new glucose (sugar) from protein and fat in the liver. And as we know, raised blood sugar stimulates the release of the hormone, insulin, and this can lead to fat storage. Studies at Birmingham University have shown that the roll of fat some of us store round our middle can actually manufacture cortisol, thereby promoting ongoing obesity (and the potential for type 2 diabetes).

So if aerobics could make you fat – what can you do to slim down without burning muscle? And how can you be sure the weight will stay off?

“The answer seems to lie in lifting weights – the trouble is many people believe that if they start doing that they’ll end up looking like Arnold Schwarzenegger,” says Dr Krasner. “So unfortunately this puts a lot of people off doing the exercise that would get them the results they want.”

“Resistance exercise, in other words lifting weights or working out on weight machines, builds up lean muscle tissue and boosts your base metabolic rate. With a higher BMR it is easier to maintain your shape as you’ll consistently burn more calories. This helps stabilise your weight even if you miss the odd workout or succumb to the temptation of a slice of cake!” says Zana Morris.

And even better you can do it in 20 minutes, rather than an hour in the cardio-studio. “By focusing on one muscle group at a time, working at a high intensity and pushing out just a few extra reps, even when you feel you can’t, you burn fat and build muscle – which is exactly the combination you want. And it will help to protect and strengthen your internal organs,” continues Zana.

An hour of aerobics or 20 minutes of weights? I know what I’d choose.

For more information about Educogym visit

This article is taken from a series of interviews on PASSION for the PLANET radio ( There are lots more interviews available on air and on the website.

Author's Bio: 

Chantal Cooke is a journalist and co-founder of PASSION for the PLANET - the UK's leading group of radio stations focusing on your health, your environment and you personal development. Hear inspirational interviews with world experts and great music from around the world.