Does diet or exercise have the biggest impact on your health?

Earlier this year, I finally made the decision to stop complaining about being out of shape and do something about it. I said goodbye to sleep ins and signed up to bootcamp.

After three weeks I couldn’t see any difference. I hopped on the scales to find out I’d actually put ON weight that couldn’t just be explained by muscle.

My problem? I’d thought that because I was going from doing nothing to something reasonably full on, that simply doing exercise would be enough. I didn’t alter my eating habits – if anything they’d probably gotten a bit worse because I’d “reward” myself for working out.

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According to nutritionist Melanie McGrice , it’s a common health mistake people are making.

“I often see clients who are frustrated because they’ve joined a gym but haven’t achieved much weight loss. Exercise provides a lot of additional benefits, but most people can’t lose weight without modifying their dietary intake as well,” she explains.

So does the saying being healthy is 80 per cent diet and 20 per cent exercise really stand? Or can you do one without the other?

“I believe that focusing on 80 per cent diet and 20 per cent fitness to lose weight is a good balance,” says McGrice.

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“About 65 to 85 per cent of our energy expenditure results from our metabolism, so although exercise increases the number of kilojoules we burn, it only impacts our energy equation by a relatively small proportion,” she explains. (Post continues after gallery.)

Clinical dietician and exercise physiologist Gabrielle Maston agrees.

“People start attending gym everyday, then start eating more because ‘they exercise’. It’s almost like exercise gives them permission to over-indulge. For long term success you need to change your diet and start exercising.”

In fact, a healthy diet could even be more important than exercising – and whether you believe this or not could actually be affecting your outcome.

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According to a study published in the journal Psychological Science, people who think that diet is the most important factor in controlling their weight tend to have a lower body mass index (BMI) than those who believe that exercise takes first priority.

More than 1200 people were asked about the main factor that makes people overweight and had their BMIs calculated.

Researchers also offered participants unlimited chocolate, and interestingly the people who said they thought staying active was the secret to maintaining a healthy weight ate more.

"Our beliefs guide our actions," says study co-author Assistant Professor Brent McFerran.

"While exercise can support weight loss, people tend to overestimate the amount of calories they burn while working out and compensate for the extra activity by eating more."

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And while you might worry less about exercise if you believe diet is key, you're more likely to watch what you eat, which McFerran says is smart because most people greatly underestimate the amount of calories they consume. (Post continues after gallery.)

"For example, going for a half hour walk may burn off an additional 500 kilojoules, but there’s more than 500 kilojoules in a bottle of Gatorade that you may drink after your walk."

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For long term results, all agree a combination of both is best.

"When you just diet you can lose weight, however you're more likely to gain the weight back after two years. This is because when you diet and don't exercise, you lose a lot of muscle mass," she explains.

Which do you think is more important - diet or exercise?