‘Britain needs to go on a diet,” said Duncan Selbie, the chief executive of Public Health England, in March, announcing the latest tranche in the state’s offensive against obesity. Parents had already been advised in January to limit their children’s two permitted daily snacks to less than 100 calories each, which made some of them quite cross. Now they were being told that their lunches and dinners should not exceed 600 calories each, or 400 for breakfast. Many were left scratching their heads about why the plan leaves men with 900 spare calories – and women 400 – within their daily recommended allowance.

But, furthermore, you could live off booze-sodden bacon butties and Haribo and still stick within these calorie counts – and that almost certainly wouldn’t save the NHS any money. Could this strategy end up backfiring as badly as abstinence-based sex education policies – which lead to an increase teen pregnancies? Or the old advice that low-fat products trump foods naturally high in saturated fat, which led to everyone ditching butter for margarine full of transfats. Eventually, of course, the latter was outed as the least healthy type of fat a person can consume.

No matter how much groundwork policy wonks do, the risk of unintended consequences is always lurking. To be fair, the government is taking a multipronged approach with obesity, asking food companies to reduce calories by 20%, either by reformulating products or changing portion sizes. And the sugary-drinks levy now in effect means any high-sugar beverage-makers who haven’t reduced levels will have to swallow the tax. In response, AG Barr, makers of Irn Bru, has devastated fans by halving added sugar, although – unintended consequence alert – the drink now contains sweeteners, too, which are also linked with weight gain, diabetes and heart disease.

With drinks companies passing some of their high-sugar levies on to consumers, there is yet another potential unintended consequence studied by researchers: more boozing. A study published in the Journal of Epidemiology and Public Health has found that if prices of high-sugar drinks rise, people could end up buying more beer, cider and wine, along with juices (some as sugary as fizzy pop) and diet drinks (sweeteners again). Oddly though, alcohol sales were projected to decrease if “medium sugar” (as opposed to “high sugar”) drinks go up in price. The researchers, who used economic modelling techniques based on reams of consumer data, don’t as yet know why any of this would be. It’s complicated.

Meanwhile a second research paper has spotted a flaw in the government’s “responsibility deal” with drinks makers. The industry agreed on reductions in alcohol levels by 2015, but while more low-alcohol products are on offer, things aren’t panning out as expected. Cambridge University researchers have scrutinised how low-alcohol drinks are being promoted by Tesco, Asda, Sainsbury’s and Morrisons and found that consumers are being prompted to buy these beverages in situations where they wouldn’t ordinarily hit the bottle. The drinks are presented as a perfect lunchtime treat, and as healthy options next to pictures of fruit. “Lower-strength wines and beers appear to be marketed not as substitutes for higher-strength products, but as ones that can be consumed on additional occasions with an added implication of healthiness,” wrote the researchers. Make that an “irresponsibility deal” then.

The trouble is that industry, the wider world and people themselves present an unpredictable landscape, says Gaby Judah, a researcher in behavioural change at Imperial College London. “You don’t know how drinks companies are going to pass on that price rise in the longer term. There are actually a number of different tools that can be used and you need to look at everything that’s available, like restrictions in portion sizing, advertising, price promotions, better health labelling, not having unhealthy products at the checkout or end of aisles.”

At least researchers such as Judah have back catalogues of failures in the literature to inform them, but sometimes, when an idea feels like a no-brainer, initiatives can be rolled out unchecked. She cites an unfortunate example: “One university campaign tried to highlight how terrible it was that students were binge drinking, but in fact this seemed to normalise it, and the campaign actually increased binge drinking.” Research, testing and monitoring for unintended consequences are key, she says. “They could have predicted that [outcome] from the literature, which shows that making a behaviour seem like a social norm encourages more people to do it. You need to try to normalise the behaviour you want – in this case that would be that people don’t actually drink that much.”

Perhaps this is a failing in obesity messaging, too, in which it is hammered home just how increasingly common being overweight is. Not that you can fake it to make it in a health campaign and pretend that most people live on vegetables and pulses alone, or that sobriety is more common than binge drinking at certain universities. “You can’t lie,” says Judah. “It may just be that a social-norm intervention is not right in this situation.”

And yet, it worked wonders for late taxpayers. When HRMC trumpeted the fact that most people settle up on time, the stragglers started coughing up. “Also with handwashing, in hospitals and in the general public,” says Judah. “Making it seem like everybody is washing their hands when they’re supposed to is likely to have an impact on handwashing behaviour. You have to work out what’s an appropriate approach for the population.”

The blindingly obvious stumbling block with lifestyle-related public health campaigns is that consuming unhealthy foods and drinks is, of course, still the social norm, and is often the cheaper and more accessible option. Half of the UK’s shopping baskets currently contain “ultra-processed foods” – the nation’s tastebuds are attuned to them. As it stands, as far as vegetable consumption goes, we are living in the 1970s. Intended consequences, it seems, are a long way off.