Obesity and other diet-related illnesses are easily the greatest public health problem of our time. But losing weight and keeping it off is incredibly difficult; it is not what we are evolved to do.

Over the past 20 years, my research at the University of Cambridge's MRC Metabolic Diseases Unit has focused on the genetics of why some people get fat and some don't. Science is set up to get to the truth — eventually. It does not provide quick answers.

As a result, there are many desperate people looking for a way out, a silver bullet. Over recent years, a proliferation of, by and large, skinny and attractive food gurus such as the Hemsley sisters and Deliciously Ella have emerged, armed with dietary advice that is not based on any serious scientific evidence.

Much of this new advice goes far beyond healthy eating, and in some instances argues that food can actually make you well. Welcome to the world of "clean eating", which I have spent the last few months investigating for a BBC Horizon documentary, to understand just how scientific these claims really are.

It became clear that many hundreds of thousands of people are more likely to believe the advice of these food gurus - buying their books and following their social media feeds - than listen to scientists and other experts who are taking an evidence-based approach to nutrition.

Something of a backlash began last year, when Bake Off finalist Ruby Tandoh warned against the clean-eating "evangelism" she said such gurus propagate. She warned that it was based on "bad science" and gave "false hope" to vulnerable people, adding: "Food is not medicine."

I put this to one of the key drivers of the clean phenomenon, Ella Mills, otherwise known as Deliciously Ella, who I interviewed (and cooked with) for this programme. She told me it had lost its way. "My problem with the word 'clean' is that it's become too complicated, become too loaded," she says. "Clean now implies dirty and that's negative — I haven't used it, but as far as I understood it when I first read the term, it meant natural, kind of unprocessed, and now it doesn't mean that at all. It means diet; it means fad."

And just this week, the Hemsley sisters, Jasmine and Melissa, whose claims for the benefits of grain-free cooking I sifted through in the programme (but who declined our invitation to appear), followed Ella in distancing themselves from the "clean-eating" trend.

Speaking at Selfridges in Oxford Street, where they opened their Hemsley + Hemsley cafe last year, Jasmine said: "As with any trend there is always a backlash, which in a way is a good thing as it gets people talking about food. It is a media-coined term. We have never, ever used the phrase 'clean eating'."

But for healthy-eating devotees, Instagramming everything that passes their lips, the term #clean still reigns supreme. Clean eating is not one way of eating, but encompasses many different dietary approaches. In the documentary, we focused on three of the big beasts: giving up gluten, an alkaline diet, and a plant-based diet.

What rapidly emerged was that "post-truth" science permeates the entire culture. This troubling narrative was best illustrated by my surreal visit with so-called doctor Robert Young, father of the "alkaline diet", which is endorsed in the UK by food brand Honestly Healthy.

Its co-founder, Natasha Corrett, one of the food "gurus", has hundreds of thousands of followers on Instagram and four best-selling cookbooks. Honestly Healthy recipes are vegetarian and, having tried out a few myself, are very good. If you ignore the pseudo-babble, then what an "alkaline diet" encourages is lots of vegetables with little to no meat. Where's the harm, you might think? Well, when I drove into the hills outside San Diego to see Young on his "pH miracle ranch", I found out.

A manicured millionaire's paradise, preposterously set behind a moat, the ranch sits at the epicentre of his "alkaline" empire, and was built on the proceeds of his "pH Miracle" book series, which has sold more than 4 million copies.

As Young welcomed me in, he began to share his alkaline view of the world: "The human body in its perfect state of health is alkaline in its design." The pH of our blood is 7.4, which is slightly alkaline, so Young is broadly correct, although different body parts, such as our stomach, function at very different pHs. Everything else about "alkaline living", however, is complete fantasy.

Young believes that in order to maintain the alkaline pH of our blood, we have to eat alkaline foods. The problem is, there is no evidence that your blood's pH is influenced by what you eat. Your stomach, at around pH1.5 (akin to battery acid), is the most acidic environment in your body. So whatever you eat, will arrive in your intestine at the same acidic pH. In fact, nothing, apart from almost dying, will change your blood's pH. So if it does change, you're probably about to die.

Young's food categories also don't make any sense. Meat is considered "acidic", yet all mammals have alkaline blood. Equally, certain citrus fruits are considered "alkaline", when they are by any measure acidic. Don't waste time trying to wrap your head around this nonsense. It doesn't stand up to even casual scrutiny.

Yet Young goes further still, claiming that disease emerges from acidity - which he believes transforms blood cells into bacteria by a process he calls pleomorphism - and that, by extension, disease can be reversed with alkalinity.

Can I be crystal clear? Young's view goes against all current scientific understanding. When I posed this to him, he said this is "a new thought, a new consideration". In other words, this is his "post-truth" fantasy.

A large part of the pH Miracle ranch has been set aside as a "clinic" to treat cancer. Young uses "cancerous" as an adjective, to describe a state of acidity, rather than as a noun. He brings terminally ill patients to stay for months at a time and intravenously infuses them with an alkaline solution of sodium bicarbonate. He does not have a medical degree, and there is no evidence whatsoever that infusing baking soda into your bloodstream will do anything against cancer.

In 2011, Young's activities at the ranch attracted the attention of the Medical Board of California, which began an undercover investigation. Investigators established that at least 81 cancer patients had been treated at the ranch since 2005. None of the 15 whose prognosis could be documented outlived it.

One patient, Genia Vanderhaeghen, died from congestive heart failure - fluid around the heart - while being treated. An invoice, which we obtained, documented 33 intravenous sodium bicarbonate drips, each charged at $550 (??450), over 31 days; some administered by Young himself.

Another patient was Naima Houder-Mohammed, a young captain in the British Army. After she left the military academy at Sandhurst, she was diagnosed with breast cancer, and told she had six months to live. Naima came across Young on the internet and he encouraged her to come to the ranch for a "healing programme".

Young insisted Naima pay for her care up front, and tens of thousands of pounds were raised so she could travel to California. Naima's treatment cost more than ??60,000. After three months at Young's facility, her condition worsened and she was taken to hospital. Naima died in the UK with her family. She was 27.

Last year, Young was convicted of two charges of practising medicine without a licence, and faces up to three years in prison. I asked him if he felt remorse. He said: "I don't have remorse because of the thousands, if not millions, of people that have been helped through the [alkaline-diet] programme."

When pseudo-science goes beyond advising people to eat more vegetables, and is used to manipulate the vulnerable and most ill in society, it becomes a true problem.

Natasha Corrett told the Telegraph that while she had been "interested in Young's findings - as were thousands of other people", Honestly Healthy is "absolutely not" comparable to his theories.

The gurus of clean are doing nothing wrong encouraging healthy eating, but they have a responsibility to ground their promises in proof. The NHS advises us to eat a balanced diet including fruit, vegetables, wholegrains, and dairy, while limiting meat. And the simple - if unfashionable - truth is that science has, so far, discovered nothing to prove otherwise.

The Telegraph, London