You sweat. You gasp. You even make it to those early-morning park workouts when your bed seems so much more inviting. But is all that hard work actually doing anything? Not as much as you might think, according to an article published in the British Journal of Sports Medicine by a trio of public health experts – Drs Aseem Malhotra, Tim Noakes and Stephen D Finney – which argues that exercise is firing blanks in the war on weight.

But is that really true? In the economics of energy expenditure, you know that a session on the treadmill or supersets in the squat rack shifts your calorie count into the black. So long as you can hold off on that Big Mac on your way home, your body dips into your fat stores to make up the difference. The result? A looser waist band.

After a particularly arduous session surely you can even enjoy an extra helping at dinner. Maybe even a mid-afternoon something sugary as a reward for a lunch break spent groaning under a barbell. So long as you ensure that calories in doesn’t equal calories out, your scales will shift the right way.

But it’s this mind-set, the article claims, that’s undermining efforts to slim our increasingly blubbery society. “Members of the public are drowned by an unhelpful message about maintaining a ‘healthy weight’ through calorie counting,” the authors write, “and many still wrongly believe that obesity is entirely due to lack of exercise. This perception is rooted in the Food Industry’s Public Relations machinery, which uses tactics chillingly similar to those of Big Tobacco.”

As the article explains, junk food manufacturers spend billions every year to link themselves with healthy activities. More than 10% of London 2012 sponsors –including Coca Cola, McDonald’s and Cadbury’s – manufactured products associated with obesity. Half as many were associated with health and fitness.

“[Coca Cola] associates their products with sport, suggesting it is OK to consume their drinks as long as you exercise,” the article points out. “However, science tells us this is misleading and wrong. It is where the calories come from that is crucial. Sugar calories promote fat storage and hunger. Fat calories induce fullness and satiation.” As we’ve explained before, a calorie is not a calorie.

Thermodynamically, there’s no doubting food’s outgunning of exercise in its impact on your waistline. An hour of jogging sees off a comparatively paltry 400-odd calories. That’s less than a Pret BLT sandwich. It’s far more effective to focus your efforts on limiting what goes in, rather than raising what goes out. Particularly since the harder you work in the gym, the more tempting that high-calorie gunk looks when you’ve left.

So should you tear up your gym membership and steer that money you’re saving into kale? Not exactly. Despite what headlines would have you believe, the BJSM article isn’t suggesting exercise has no firepower in the war on fat. Just that it’s the weaker of your weapons. Getting to your 10,000 steps doesn’t buy you a Krispy Kreme reward.

That doesn’t mean exercise is impotent in building a newer, slimmer, healthier you. Firstly, shed weight with diet alone and you’ll soon stall. “You see programmes like The Biggest Loser work phenomenally at first,” says Dr Scott Harding, a lecturer in King’s College’s diabetes and nutritional science division. “They’ll lose a stone in a couple of days. But towards the end, you get people who are still huge and they can’t lose any more than a pound over the course of the week. It’s because the body’s resisting the weight loss.”

As intake drops, so does your metabolism, your body protecting itself from what it sees as starvation by dropping energy expenditure and making efforts to store the scant resources you’re offering it. Exercise is how you kickstart it again; how you conserve energy-hungry muscle rather than letting your body burn it up as fuel; how you get through weight loss plateaus you’d otherwise have to breach by cutting out yet more calories – and making your life miserable.

Secondly, exercise offers a host of health benefits independent of your weight. As little as half-an-hour of moderate activity every day slashes your risk of cardiovascular disease, type-2 diabetes, dementia and cancers by up to a third. It’s an effect offered even if you’re not moving up holes on your belt.

So no, this isn’t your excuse to stop hitting the gym if you want to shed that spare tyre. Rather, it’s a wake up call that you can’t rely on sweat to make up for your predilection for sweets. As Malhotra and his colleagues so succinctly put it: “You cannot outrun a bad diet.”