Two days after arriving at Grayshott, I am shaky and sobbing. I have been deprived of caffeine, sugar, refined carbs and calories. I've yomped across the South Downs at dawn, struggling to keep up with the fitness instructor striding ahead of me. I've sat through complex lectures on nutrition. My stomach has been poked and prodded until my internal organs feel jumbled up. But what tipped me over the edge was my own folly: trying also to work, to meet a hellish deadline, with haphazard Wi-Fi.

Grayshott (grayshottspa.com) is a long-respected, proper old-fashioned health farm set in a Victorian manor house in Surrey, just an hour's drive from London. The estate, which was briefly owned by Alfred, Lord Tennyson, has 47 acres of peaceful and extensive grounds, with the glorious National Trust-owned Hindhead Commons just outside the gates.

Last year, it launched a strict, seven-day health programme, the first UK spa to offer anything like the hardcore regimes popular in countries such as Austria and Germany. I'd previously survived a week at the notorious Viva Mayr spa in Austria, which specialises in extreme fasting. The Grayshott regime is run with similar military precision, but without the need for Ryanair flights, and with a gentler, less antiseptic feel.

This is very much a residential programme – you must stay for the full seven nights, you will rarely venture out of the grounds during your week and you can't dip in and out of treatments. The programme kicks off on a Wednesday (this is non-negotiable), with the same core schedule of treatments for every participant (as well as the above, there were castor oil compresses, frequent blood tests, abdominal massage, and hydrotherapy baths).

The food is not plentiful but it is delicious and imaginative. Stephanie Moore, the nutritionist who helped design the regime, is a keen advocate of bone broth – a cheap and natural way of getting glucosamine for the joints – and a superfood I'd never heard of before: omega 3-rich chia seeds, which are soaked in water to make a paste like tapioca. It hardly sounds the stuff of holiday food dreams, but becomes a new breakfast favourite for me. There is no snacking, and on Mondays and Fridays everybody fasts: no breakfast, and only thin, watery soup in the evening.

The point of it all is to whip your digestion into shape. Stephanie tells us on our first morning: "If your digestive system is not functioning well, you can't feel well." The aim, she says, is to rest the gut, allowing the lining of the intestinal tract to heal and regenerate, to restore the complex balance of gut flora and eliminate sugar from the diet.

The four of us on the programme all eat together, with either a nutritionist or a fitness instructor – possibly so they can make sure we aren't cheating. (Not entirely successful: one fellow guest sneaks to the local town one afternoon to buy us all chocolate.)

Every meal starts with a shot of digestive bitters, and sauerkraut so sharp it sets my teeth on edge. Instead of a sugar bowl on the table, there's a little pot of probiotics: we're encouraged to take some at every mealtime, all the while discussing the state of our insides or whether omega 3s are all they're cracked up to be.

I do find it all a bit exhausting. By the last day, I'm longing for a bit of space to eat on my own. But there's an enthusiasm and honesty about Grayshott that's compelling. The warm, unstuffy atmosphere, super-comfortable bedrooms and air of faded grandeur make for a comforting stay – you can slump around in your bathrobe all week if you want and no one will care. There are several other programmes beside the health regime, and though exercise is encouraged, it's not compulsory.

Staff are kindly, well-informed and wholeheartedly believe in the programme. And it works. By the end of the week, I feel much healthier. I'm lighter and my energy levels have soared. And I'm sufficiently impressed to introduce at home two new staples in the household diet: chia seed pudding, and a nutritious bone broth.

• Accommodation was provided by Grayshott (01428 602020, grayshottspa.com), where the seven-day regime costs from £1,495pp and includes consultations, monitoring and support, lectures, meals, therapeutic treatments, and an exercise programme