Halfway through her retreat, Sarah Vine hears a convincing theory on the cause of obesity and learns exactly what to avoid

Any products in this article have been selected editorially however if you buy something we mention, we may earn commission

Damn and double-damn. I oversleep and miss breakfast. I was so sure that my stomach would wake me up, demanding food, that I didn't bother to set the alarm. I have only five minutes to get to my 9.45am hydrotherapy session, so I sling on my dressing gown and head downstairs.

Before long I am lying back in a sea of algae, jets of hot water frantically pummelling away at various bits of me. It only lasts 16 minutes, but it packs quite a punch. Afterwards I feel rather hot and headachey, like I'm coming down with the flu. Oddly, though, I don't feel at all hungry, which is just as well because lunch is a while off.

I retire to my room to wash my hair, which the hydrotherapy has somehow turned into dried seaweed. By the time I'm done it's time for today's lecture: Carbohydrates.

Our host is Will, who looks about 28 but who, he later lets slip, is in his mid-forties. Will is an intense man, very committed to his cause, which is to fundamentally change the way we think about food.

He explains how, in the 1970s, the American government commissioned a study on diet. The findings - that vegetables and fruits should make up the base of our food pyramid, followed by grains and starches, then dairy and proteins, with sugars and oils at the very tip - were overturned by the powerful American food lobby.

They successfully lobbied for a more commercially lucrative model, in which bread, cereal, rice, pasta (cheap to produce) and so on were flipped with fruit and vegetables (seasonal and therefore less stable) so that the former, not the latter, became dominant.

This, he went on to explain in great technical detail, is why America has such a problem with obesity, and why we too have the same problem. The Americans have now recognised their mistake - which is why, in 2011, Michelle Obama  launched a new food plate initiative  designed to rectify the imbalance. But the damage had been done.

Will's basic message is: stay away from grains, all processed foods, starchy vegetables (e.g. potatoes, parsnips, carrots), sugars, cereals and other packaged foods such as biscuits; eat good carbohydrates such as leafy vegetables and nuts and seeds. Cut out artificial sweeteners (aspartame is a neurotoxin, he tells us gravely).

At lunch (chopped vegetable salad, chicken thighs, squash and soup, preceded by the usual combination of bitters and sauerkraut), I ask the question I've been longing to ask: what about wine?

A really healthy person, says Will, should have no more than just under a bottle of wine throughout the course of a single week. We all look guilty. "But my standards are pretty strict," he adds, by way of consolation.

The afternoon holds a real treat: an abdominal massage designed to, ahem, facilitate the detoxification process. It's lovely: there's something about pampering the one area of my anatomy that I hate the most that feels strangely comforting.

Supper is a lively affair - not so much for the food, which is as delicious as it is pious - as for the conversation with my fellow Regimers. But I'm flagging. It's Saturday night. I'm supposed to be at home with my family, sharing a bottle of wine with my husband and watching Eurovision with the kids. Life feels great, empty, a little joyless. I want to go home.