Chris van Tulleken has suggested we meet at his local pizza place, Sweet Thursday, in Islington, London. If the choice seems counterintuitive for a man with a mission to improve our national diet, he puts me right when we sit down. “Pizza has become emblematic of junk food,” he says, “but proper homemade pizza is very healthy.”

At Sweet Thursday, purist Italian chefs work their fresh sourdough bases in an open kitchen (rumour has it they are so purist in this vocation that they draw the line at making salad). But it is not just authenticity that counts, it is also community. Van Tulleken lives around the corner; the owner grew up nearby and this is where local families tend to come to catch up or to celebrate. “Above all, a restaurant should never be just a way of extracting money in exchange for nutrition,” Van Tulleken says. “Or for paying dividends to offshore investors. And I think these things are actually obvious even if you don’t live, like me, in a world of nutritional studies.”

The distinctions Van Tulleken makes go to the heart of his research into the damage that ultra-processed foods (UPFs) are causing to our physical and mental health. The contention of his bestselling book Ultra-Processed People is that food engineered by corporations with additives and emulsifiers and modified starches essentially “hacks our brains”, disrupting the normal regulation of appetite. It tricks us into eating more by being softer, slicker, saltier, sweeter than whole foods and it is that trillion-dollar fact, his evidence suggests, which is driving the obesity epidemic. In the course of his deep research, he acts as a guinea pig for these theories (with the occasional help of his twin brother, Xand, also a doctor and, because they share a genetic makeup, his built-in control group). His months of eating badly served to show that what he was consuming was not food, it was, as one academic colleague kept insisting to him, “an industrially processed edible substance”. Or “food that lies to us”.

As we order at Sweet Thursday – rustic chicken liver crostini and fried zucchini to start, spring risotto for him and pizza of the month – with artichoke and asparagus – for me, he makes a little prediction: “You won’t be able to finish your pizza here.” That is to say, it won’t slide down like a Domino’s deep dish and leave you wanting more. It may take some proper chewing and digesting, and fill you up.

As we share our starters, Van Tulleken apologises in advance for any fuzziness there might be in his thinking – his third child is six weeks old, and sleep is a memory. As well as promoting the paperback of his book, there’s the day job to consider – he’s a specialist in infectious diseases at University College London – and episodes of his latest podcast series (with Xand) to think about. He has spent the morning writing his submission to the Lords select committee on food, diet and obesity.

In the year since the book came out there has been strong pushback against his claims. In an afterword to the paperback edition, he offers a pretty devastating rebuttal of that criticism, a significant proportion of which, he reveals, comes from academics whose research has been sponsored by various multinational food conglomerates. “Tentacular” is the word he uses to describe the involvement of those companies in the committees devoted to debating their regulation.

He has had first-hand experience of that reach. “When the book came out, I half-imagined I might be on the witness stand against Nestlé or whoever,” he says. “But the way they do it is more subtle.” One large food company, for example, asked if he would be interested in giving a half-hour talk to its senior team, for a fee of £20,000. He said he would, but he’d pay his own expenses and give the money to a food charity.

When the contract came through, he changed his mind. Within it was a clause binding him not to disparage the firm in public statements, “throughout the universe and in perpetuity”.

In order to counter the detrimental effects of UPFs, Van Tulleken makes two recommendations. First, outlaw conflicts of interest on UK scientific and advisory bodies. And second, create effective warning labels on food products.

Much of the criticism of the idea of UPFs is that they are hard to define and therefore slippery to regulate. Van Tulleken argues that better enforcement of existing UK dietary guidance on fat and salt and sugar would “catch 95% of UPFs”, that a black warning octagon on those foods would mean that they could neither make health claims nor target children in their marketing. “Take Coco Pops,” he says. “Pick up a box in a supermarket and there will be half a dozen health claims on it. But if there was a warning octagon it couldn’t make those claims; it couldn’t put a jolly cartoon monkey on the front; it couldn’t be sold in hospitals or schools.”

The route that brought him to this evangelism is instructive. He grew up in Hammersmith, in west London. His early plan was to be a fighter pilot – he had watched Top Gun – but his first solo flight put him off. He then trained as a surgeon, but eventually followed Xand into the research of tropical diseases.

Working in central Africa he saw lots of kids dying of infections. “And the reason they died,” he says, “was not because we lacked antibiotics. It was that they were being fed baby food made up with filthy water … milk formula was directly marketed to families as aspirational.” The more he witnessed of this tragedy the more it became clear that “the solution should be to try to limit that corporate [marketing] power, rather than needing more antibiotics. What we now call the commercial determinants of health.”

His investigations have led him to expose how food multinationals work hard to make us eat more and more of ingredients that have less and less nutritional value. Their testing, he shows, concentrates on speed and volume of consumption. It is no accident, he suggests, that tobacco giants Philip Morris and RJ Reynolds acquired, respectively, Kraft and Nabisco in the 1980s: “They knew they had a set of technologies they could apply to food.”

In recent weeks much of the argument about UPFs has come down to our daily bread. I wonder what he made of the Guardian article by Giles Yeo, the Cambridge genetics professor, who, while acknowledging the evidence linking UPFs to 32 adverse effects on health, also appeared to argue that “taste aside, supermarket bread is no worse for you than fancy bread”.

“I’ve got a lot of time for Giles,” he says. “But for me, that article was very confused. If we look at a loaf of basic supermarket bread, versus a loaf of real bread, nothing fancy, the supermarket bread will be extremely high in salt and generally high in sugar, above the recommended level. It will have high energy density because it’s very dry, to promote shelf life – and we know energy density, the number of calories per 100 grams of food, is really, really important for weight gain. And then the supermarket bread will be extremely soft meaning you eat it quicker and consume the calories before you become full.”

Yeo’s wider argument against the imprecision of the UPF label touched on that other persistent claim, that whole food is an elitist concern; a four quid loaf is all very well if you can afford it. Van Tulleken has two rejoinders to that. The first is that the obesity crisis costs the NHS billions of pounds a year – why not tax elements of UPFs and use that to subsidise healthier, local food production? And second that much of the “snobbery argument” about proper food is, he believes, “industrially generated” by vested interests.

“I mean, the British Nutrition Foundation [whose members include corporations such as McDonald’s, British Sugar and Mars, with funding from firms including Nestlé, Mondelēz and Coca-Cola] had that quote: ‘We think it’s important not to stigmatise people in poverty [by advising them what not to eat].’ I completely agree! The real source of shame and stigma should be directed toward governments refusing to regulate this stuff…”

While he has been talking me through all of this, I’ve been making my way slowly through my fabulous pizza of the month. It is far from a double blind trial, but his prediction is right; it is, in fact, so very satisfying that I can’t finish it. But how about a scoop of homemade gelato, he asks.

Oh, go on then.

• Ultra-Processed People by Chris van Tulleken (Cornerstone, £10.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.

• This article was amended on 23 May 2024 because an earlier version located the restaurant Sweet Thursday in Hackney, east London. In fact is it in the borough of Islington, close to the border with the the borough of Hackney.