See Venice and diet

There must have been long faces at McDonald's head office last week. The fast-food chain's announcement that it intends to scrap supersize portions backfired, interpreted as an admission of its role in an epidemic of obesity. The company denied it, rejecting any link between huge portions of chips and burgers and the weight problems of its customers. But it is hard to credit that the company's bosses thought the announcement - that they are going to sell people a bit less rubbish, in effect - would help its image. Junk food is still junk food, and fiddling with portion size cannot compensate for the ubiquity of the burger chain or the way it has changed the world's eating habits for the worse.

I even came across a McDonald's in Venice last weekend, although why anyone would order a Big Mac when they could be eating fegato alla Veneziana is beyond me. Fast-food chains have been extraordinarily successful in weaning people away from their traditional diets, persuading them to eat food that is cheap, loaded with calories and dreadful for their health. Minor changes to the McDonald's menu may be front-page news but they are largely cosmetic, just as Starbucks' boast that it is paying a minimum price to coffee growers diverts attention from its sugar-laden muffins. And while the McDonald's announcement was failing to have the desired effect, Coca-Cola attracted derision for the launch of a pricey bottled water that turned out to have come from a tap in Sidcup. Do they think we were born yesterday? Apparently so. It was hilarious to hear a spokeswoman for the company trying to justify this blatant attempt to fool the public.

I have to say that if this is the food and drink industry's notion of corporate responsibility, I don't know why the Government is so nervous about regulating it. More than 100 organisations have called for statutory controls to protect children from the promotion of unhealthy food and drink, which is done by a combination of advertising, vending machines in schools and special offers. The prospect of regulation and the suggestion of a "fat tax" have produced predictable complaints about the "nanny" state, along with claims that if huge numbers of people want to eat themselves to death, it is their business. This is nonsense, for the NHS is spending vast amounts of taxpayers' money on weight-related conditions such as heart disease and diabetes, while increasing numbers of obese people are too sick to work. Only last month a Treasury report suggested that the NHS could save £30bn a year by 2022 if public health were improved.

Today's children are the first generation in this country whose life expectancy is likely to fall, compared with that of their parents, as a direct result of terrible eating habits acquired while they are still at school. Ministers would be failing in their duty if they did not tackle obesity, especially when the evidence demonstrates that it is the poor who eat junk food and suffer as a result. The notion of consumer choice does not mean much when people on tight budgets are targeted by an industry with huge resources, sophisticated promotional techniques and as little sense of moral responsibility, it appears, as the tobacco companies. Most of us accept that the Government is right to tell smokers about the consequences of their addiction, through warnings on cigarette packets, and it should not shy away from doing the same with fast food and sugary drinks. The alternative is a new class system in which people's income, social status and life expectancy are evident from their weight.

Indeed, it hardly seems controversial to suggest that poor people need protection from fast-food chains and companies which market sugary drinks. The good news is that adverse publicity and the threat of statutory regulation are already working, as the behaviour of McDonald's proves. But the sceptical public reaction suggests a harsher lesson. Global corporations have entered a phase where they cannot do anything right, and it is going to take a lot more than cosmetic changes to regain our trust.