IF YOU ask what’s most effective, the carrot or the stick, most people will unhesitatingly say it’s the carrot. We instinctively understand that persuasion not punishment is the way to influence people and change their behaviour. So I’m puzzled how doctors and academics have managed to convince themselves that the best way to counter obesity and diabetes is to slap 20p tax on a bottle of Coca Cola. They really are missing the point. Sugar tax won’t work.

The high cost of cigarettes doesn’t stop people smoking any more than the high cost of alcohol stops people drinking. Smokers and drinkers cut down on something else instead to fund their lifestyle (or addiction, depending on your point of view).

What does influence them however is education about the harm they are doing themselves and others by over-indulging. This awareness generates social pressure that gently but effectively nudges them towards healthier behaviour.

  Tell a young person that smoking kills and his reaction will be denial: “I’m sixteen so why should I care about dying when I’m 60?” Tell him that smoking makes him smelly and unattractive to girls and his reaction will be: “OMG! I’m giving up right away.”

The tragedy about the sugar tax debate is that the real culprit – the hidden sugar in processed food and ready meals – largely goes unnoticed and that’s where UK Diabetes Week ( June 14-20) has a massive role to play in raising awareness not just of the consequences but of the causes of a rapidly-growing disease which can so easily be prevented.

Of course there’s sugar in fizzy drinks like Coke and Pepsi – as much as nine sugar cubes in one 300ml can. That sugar is simple carbohydrate that is of little nutritional value to the body; it gives a short burst of energy which is followed by a horrible low. It’s one of the reasons children who down fizzy drinks on the way to school or in their lunch break find they can’t concentrate in class.

But too few people realise that sugar is all-pervasive in their daily food – and in remarkably high quantities. Educating families on what they’re eating and guiding them towards making healthier choices – voluntarily not by hitting  them with sugar tax – is in my view the only way we’re ever going to tackle obesity and diabetes.

  At ThinkingSlimmer.com we use cutting edge techniques in unconscious persuasion to help people form a new relationship with food and in many ways the methods we use are much the same as the ones advertisers use to persuade people to buy their products.

How often have you been persuaded to buy a low-fat product because the colouring and style of the packaging makes it look super-healthy? Here’s the grim reality: research presented at the European Congress on Obesity showed that 40 per cent of low fat foods contained more sugar than the standard products, as sugar is often used as a replacement for fat to ensure the taste remains acceptable.

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Dr Matthew Capehorn, of the Rotherham Institute for Obesity, said: “Weightwatchers was the biggest disappointment of this study. People who are trying to lose weight are drawn to Weightwatchers’ products, and will be even more so now that the National Institute for Health and Care Excellence has recommended their programme.

“People don’t realise that their products may contain more calories than standard products. “It is very naughty of Weightwatchers, they advertise as helping with weight loss so you would expect them to have fewer calories.”

 Dr Capehorn’s study of four major supermarkets produced results that may surprise you and make you think twice about sugar tax.

* Asda natural low fat yoghurt had more calories than Asda natural yoghurt.

* Birdseye light and crunchy breaded chicken had more calories that Birdseye crispy chicken.

* Sainsbury’s low fat custard had the same calories as Sainsbury’s custard.

* Weightwatchers wholemeal thick slice bread had more calories than any own brand wholemeal thick slice bread.

* Weightwatchers sliced cheese had more calories than any own brand sliced cheese.

* Asda own brand low fat Italian dressing had more fat than its own brand Italian dressing.

No one’s saying food retailers and diet companies are lying to the public. But they’re certainly not telling the whole truth about the foods they persuade you to buy.

Honest education about healthy food should start in the early years at school, which is why I’m such a fervent fan of Jamie Oliver’s marvellous Food Revolution Days. Knowledge is power – and we all need the power to be able to fight back against the food and drink manufacturers who are doing us so much harm.

I’d love to know what you think about sugar tax.