FACT: Exercise alone can’t create a big energy deficit.

The process should be fairly straightforward. Exercise more, burn kilojoules, develop an energy deficit and lose weight.

Except it isn’t quite that simple.

While your food intake accounts for 100 per cent of the energy that goes into your body, exercise only burns off less than 10 to 30 per cent of it. Your basal metabolic rate (energy needed for basic body functions) accounts for 60 to 80 per cent of total energy expenditure.

If a 90-kilogram man added 60 minutes of medium-intensity running four days per week while keeping his kilojoule intake the same, and he did this for 30 days, he’d lose just over kilograms, calculated Dr Kevin Hall of the US National Institutes of Health.

“If this person then decided to increase his food intake, or relax more to recover from the added activity, then even less weight would be lost,” says Dr Hall.

If you are overweight or obese and trying to lose a large amount of weight, it would take lots of time, effort and willpower to make a real impact on kilojoule deficit through exercise alone.

FACT: You can’t outrun a bad diet.

For many years we’ve believed that lack of exercise and excess kilojoules are equally to blame for the current obesity crisis. Many researchers disagree, with some claiming in the British Medical Journal that “you can’t outrun a bad diet.” They blame our expanding waistlines primarily on our food choices, stressing that where the kilojoules come from is as important as the number.

Poor diet does more than add weight. According to The Lancet global burden of disease reports, poor diet is responsible for more disease than physical inactivity, alcohol and smoking combined. You can be active every day, but it won’t negate the effects of an unhealthy diet.

FACT: Exercise is the world’s best drug.

This doesn’t mean you should give up exercise. There are many reasons to move more beyond weight loss. “It’s probably the single best thing you can do, other than stopping smoking, to improve your health,” says Dr Hall.

Upping your activity levels will reduce your risk for chronic diseases including heart disease, stroke, type 2 diabetes, dementia and certain cancers. It will support your immune system so you can better fight off illness; improve your sleep quality, fitness and mood; and protect against depression, anxiety and stress.

And while exercise may not help us lose a lot of weight on the scales unless we address diet too, studies have shown it’s essential for keeping weight off and preventing weight regain.

As a bonus, regular exercise changes our dietary habits, which means we’ll have an easier time making healthier food choices.