It was the sentence, we were assured, that torpedoed the referendum debate. Asked about Britain’s chances of securing a unilateral trade deal with the United States after leaving the EU, Barack Obama declared: “The UK is going to be in the back of the queue.”

The comment was catnip to the Remain side: the Brexiters have long conjured up the image of a newly divorced Britain taking her rightful place in the “Anglosphere” without the rest of the EU dragging us down. Instead, the US president was telling us, we would be left out in the cold.

But here’s a question for you: what’s so great about a US trade deal, anyway? For the past three years, the acronym “TTIP” has been floating across my vision. I’ve always had the sense it was a Bad Thing, without ever really understanding why. So what is the Transatlantic Trade and Investment Partnership, and should we be against it?

My first port of call is my nerdiest friend. “The first rule of TTIP is, anyone who thinks TTIP matters is a douche,” he tells me briskly. It’s safe to say that’s very much not the opinion of Mark Dearn, a senior trade campaigner at War on Want, who gives me a quick run-through of why the agreement has attracted such widespread protests, including a march by 150,000 people in Berlin last October.

“It’s the biggest trade deal in the history of the world,” he says. “It’s negotiated in secret: all the EU currently publishes is its offers. They don’t publish the US offers and they don’t publish the consolidated text – the legally binding documents.”

Such secrecy – which is, to be fair, not unusual in delicate negotiations – does make TTIP look sinister. Very few people are allowed to see the full set of documents, and they must do so in special reading rooms, after signing a non-disclosure agreement and handing over their electronic devices.

There are two areas that particularly alarm campaigners: food and health care. Last year, Alan Beattie of the FT summarised the objections as fears that TTIP will “gut public health-care systems and force American Frankenfoods down European gullets”.

War on Want’s Mark Dearn echoes this, and suggests that removing barriers to trade – the stated aim of TTIP – will lead to Europe lowering its food hygiene and additive standards to match those of the US.

“Eighty per cent of US beef is full of growth hormones or antibiotics that are banned in the EU,” Dearn says. “Forty per cent of US grain uses banned pesticides.” The US also permits “acid washing” of meat to remove contamination. “The EU views that as a form of moral hazard; it makes you think it doesn’t matter what you do [in the factory] up to that point, because you’re killing microbes at the end.”

Many campaigners also want the NHS exempted from TTIP. They worry its provisions on “indirect expropriation” will encourage private companies to sue governments for restricting their ability to do business. That could penalise any state that nationalised a failing industry or cancelled a planned project. Or, perhaps, ran a public health service.

The National Health Action Party has warned that TTIP could deliver a “fatal blow to the NHS”. I ask the party’s campaign manager, Deborah Harrington, what changes patients will experience if TTIP is implemented. “Nothing,” she answers, to my surprise. “But people don’t notice what’s different now, because it’s all behind the NHS logo. It will take people time to realise how the private sector has reshaped the NHS. There’s no big bang.”

Finally, I call the Adam Smith Institute, the country’s best-known libertarian think tank, reasoning that if they’re for it, then I’m probably against it. The ASI’s executive director, Sam Bowman, confirms that he backs TTIP in principle, “although it’s hard trying to predict what’s in an agreement we haven’t seen”. He tells me that the picture of the US as a food hygiene Wild West is not completely accurate: American producers can’t label beef from cows fed antibiotics as organic, for example, but Europeans can. He also doesn’t find the acid-washing of meat as alarming as it sounds. “It sounds gross – basically you’re dipping a chicken in swimming- pool water – but it’s done to comply with antimicrobial laws. And in the US, people find the idea of unpasteurised cheese horrifying.”

Bowman believes that TTIP, like the European single market, will increase GDP by increasing trade. He points out that the UK parliament will get a veto on the final text, and worries that campaigners “are taking the lack of transparency as an excuse to promote a conspiracy theory – that EU governments are colluding to deregulate”. He laughs. “As a libertarian, I wish that were true.”