PUTTING on weight during a stay at Chiva-Som isn’t quite the outcome most visitors are looking for, but after a week there I can understand how it might happen. I expected a health resort to serve unappetising meals in small portions, but the food at this chic, 57-room retreat in Hua Hin, on Thailand’s Gulf coast, is quite sensational. Disastrously, you can eat as much as you like. You can even drink wine, though few do.

During a cookery class to teach us his healthy recipes, head chef Paisarn Cheewinsiriwat told me: “People think if they come here they may have to eat hospital food, but it’s not true.” Too right. The buffet tables for breakfast and lunch groaned with tropical fruits, freshly baked breads (some wheat-free), dozens of exotic salad ingredients, many from the resort’s own organic garden, fish dishes and tasty clear soups flavoured with coriander and lemongrass.

In the evening, à la carte dinners included a spicy and sour kingfish soup with green papaya and a wonderful chicken and potato curry. Astonishingly, the nutritional information told me that this generous, filling dish had just 292 calories and 10g of fat. Fellow guest Sarah, a stockbroker from Singapore, confessed that on one of her previous visits she had even put on weight.

Before you form the impression that Chiva-Som is all about eating (and with the option of joining a communal table, meals are definitely a sociable high point), there is also the teensy matter of spa treatments and exercise classes.

I had signed up for a week’s detox programme — the resort has a bewildering choice of what it calls “retreats”, including “body tuning”, golf, weight management and yoga — and soon had a packed timetable. Every guest has a daily 50-minute massage, and on top of these I had specific detox treatments including chi nei tsang (a Chinese stomach massage), a “cocoon” body wrap designed to draw out impurities, and colonic irrigation. I’ll spare you the details other than to say that the atmosphere of the resort is so relaxed that near strangers are soon inquiring after the state of your bowels over breakfast. Which might not suit everyone, I admit.

It was tempting to spend hours in the enormous spa, one of the best organised I have visited. A series of freeform spaces lead you from changing area to the steamroom, sauna and rose-petal- covered plunge pool, eight powerful showers, and a relaxation area with daybeds.

Although the resort was full, at no point did I see more than one or two others in the spa, though with more than 40 treatment rooms and dozens of blue-uniformed Thai staff no one has to wait long for their massage. Chiva-Som’s masseuses were terrific, with a strength that belied their delicate build, and I was glad that my treatments were at the end of the afternoon so I could collapse in a heap for an hour before supper.

But taking exercise is essential if you’re to really benefit from the retreat, and so focused is everyone on getting fit that a sort of collective brainwashing came over us. I rarely find myself in a gym, yet by 7.30am I’d be pounding the treadmill for 20 minutes, running a little farther each morning. I’d also join in the daily classes, such as t’ai chi or aqua-aerobics; my favourite was a stretch class, held in a pavilion where we could survey the resort’s lush grounds to take our minds off the harder positions.

Later I’d swim in the huge outdoor pool or go for a power walk along the beach. Hua Hin is not Thailand’s prettiest resort, and when Chiva-Som’s general manager, Paul Linder, told me that it was “the Hamptons” of Thailand, he was referring to the property prices (boosted by the fact that the popular monarch, King Bhumibol Adulyadej, spends most of his time here), not the rather grubby beach, ugly skyscraper apartment blocks and girlie bars in the town centre.

So it is to Chiva-Som’s credit that its peaceful gardens and landscaping make it easy to ignore the resort on its doorstep.

I did have a few gripes. While the rooms are comfortable, some of the cheaper ones are too close to the restaurant for real privacy. The more expensive Thai pavilions are bigger and set around a private courtyard, but the bathrooms are tired and a much-needed refurbishment is promised. Mineral water is handed out in waste-creating small bottles, instead of being provided by water coolers. Paul Linder said he was looking into changing this system: and as water is served from communal jugs in the restaurant, I don’t think guests would object.

Many of the guests come alone, though they may have partners at home, as the resort has a reputation for being welcoming to single travellers. There are slightly more women than men, but the gap is narrowing. Most come with a specific goal: over dinner one night I discovered that Mark from London had come to stop smoking, Henry from Frankfurt had been sent by his wife to lose weight, while Dawn Dixon, 41, from Cumbria, said it was to save her health and sanity.

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