Every now and then in even the most complacent of lives, a turning point arrives, or perhaps a moment that will at least prove to be a turning point.

My Paulian moment occurred early one morning going through security at Heathrow. I was sleepy, but cheerful, with the prospect of a short but sweet visit to Rome ahead of me. I was pondering which museums to visit; what shopping I might do and what I might have for lunch (a risotto maybe?). And then what I might have for dinner (lamb cutlets alla romana?). And what I'd drink with them (most likely slightly too much). With these happy if vacuous thoughts bubbling away, I daydreamed my way through departures and into security, first placing my bag on the conveyor belt of the X-ray machine, then my coat, then walking towards the metal detectors, only to be hailed by an official standing to one side.

“Can you take your bumbag off, too, please?” he asked me. I turned to him with as much dignity as I could muster and, patting my tummy with an instinctive, protective gesture, quietly told him, “That's not a bumbag. That's me.” He looked away, as well he might. Me? I boarded my plane, feeling slightly less buoyant than I had been, flew to Rome and then had just about the loveliest 24-hour trip I've ever had. But I came back to Blighty with the firm conviction that something had to be done.

Shiny vision of a whole new you

But why adopt a Japanese diet, why go Super- Bonsai Me, as I quickly dubbed my new regimen? Partly because I've always been a sucker for a fad and, while I've learnt to take my fads with a pinch of salt (découpage, for example, who was I kidding?), the shiny vision of a whole new you can be a useful cognitive tool, especially when we're attempting a life change.

Think of yourself as a warrior, the Taoists tell us, beginning an heroic struggle. Besides, going Japanese for a month is hardly a sacrifice, I said to myself, pondering my new, sashimi-fuelled life. When I lived in New York, I ate Japanese four or five times a week; in the UK it often feels like the healthy option if you are eating out. And there are good reasons for thinking that a month spent eating only Japanese food might make me healthier and, yes, slimmer. The arguments in favour of a traditional Japanese diet have been well reported: lots of fish and vegetables, with very little, if any, red meat, which adds up to less heart disease and more longevity; plenty of antioxidants in all that green tea and seaweed; grains and rice, rather than bread; very little fat; small amounts of many different foods in a single meal...

Some of the benefits are less obvious. Before launching into my Super-Bonsai month, two experts gave me some tips. Kimiko Barber, the author of two excellent recipe books of Japanese food, The Japanese Kitchen and Japanese Pure and Simple (both Kyle Cathie, £14.99), pointed out that “eating with chopsticks slows you down” - and true enough, you can't shovel in food using chopsticks. Silla Pjerrum, the Danish-born co-founder of the Feng Sushi chain of sushi restaurants, pointed out an even more unexpected fact, that as Japanese rice is rounder than other rice grains, it has a lower GI rating.

A Japanese diet isn't all good news, though: it can be short on protein and calcium; stocks such as dashi are light and healthy, but soy sauce packs in the sodium; and all that lovely raw fish can add up to lots of mercury (found, tellingly, in high levels in the sushi-mad population of Manhattan).

One of the appeals of the Japanese diet was that it also offered the prospect of a culinary adventure, which began with a trip to the Japan Centre, on Piccadilly, Central London, where I had a bit of a Lost in Translation moment, struck by the alien-ness of Japanese food; the weird-looking dried mushrooms, strange pieces of seafood, bright pink pieces of ginger, all packaged in equally exotic-looking packaging.

To be honest, a slightly lost feeling never entirely left me, no matter how many Japanese suppers I cooked myself. Perhaps I was trying too hard. I soon got bored of spending a couple of hours every night poring over a recipe book and found myself living off teriyaki chicken, made with readymade sauce, or a kind of improvised yaki udon, using wok-readyudon noodles.

The full Japanese is a red-letter day event

Breakfast was also tricky. Thankfully Pjerrum had already pointed out that the full Japanese breakfast - involving smoked fish, rice and pickles - was a red-letter-day event, like a full English. More normal was fruit, which wasn't hard to prepare or enjoy, or miso, with some cooked rice thrown in, which kept me going but which I wouldn't eat again in a hurry.

Lunch, however, was an unalloyed pleasure. Business lunches I steered towards Japanese restaurants, while four days a week a takeout delivery of lovely salads and excellent raw fish sashimi and nigiri (a dish made with sushi rice and fresh fish) arrived from Feng Sushi. I must have been a slightly irritating friend to have for that month, and an especially annoying dinner guest, and there were notable lapses, such as the day I succumbed to a steak pie for lunch and a curry for dinner. There were also subtle renegotiations of the original plan, which occasionally embraced Japanese cosmopolitans. But by and large I kept to my plan, eating a diet which most Japanese would be familiar with.

And my bumbag? Well, it is visibly reduced. I look well, people tell me. My skin looks good and healthy and I have lost about 4lb. But perhaps the most effective thing about my Super-Bonsai Me project has been that I have broken a cycle of self-indulgent behaviour.

My new regimen is much simpler to follow and doesn't involve puzzling over recipe books and the internet wondering how long you cook buckwheat noodles or shitake mushrooms. I simply eat less, exercise more and avoid booze during the week. Dear reader, it works, but then you all knew it would, didn't you?

To order or to enroll in a Silla Bjerrum sushi masterclass, and to find out more about her book Simple Japanese, log on to www.fengsushi.co.uk

Thomasina Miers is away

A week's Japanese diet plan

MONDAY

Breakfast Pineapple, miso soup

Lunch Tempura (prawn and veg), blackened cod, rice, sashimi, sushi rolls

Dinner Somen noodles with spring onions, teriyaki and prawns

TUESDAY

Breakfast Miso soup with rice

Lunch Tuna sashimi

Dinner Autumn mushroom soup, sukiyaki beef with celeriac; Tiger beers

WEDNESDAY

Breakfast Banana and a smoothie, later miso with brown rice

Lunch Mackerel, salmon and tuna sashimi

Dinner Home-made sushi

THURSDAY

Breakfast Fruit for breakfast, followed by miso with brown rice.

Lunch Tuna salmon nigiri and one piece of soft-shell crab roll

Dinner Soused pork with salad; two Sapporo beers.

FRIDAY

Breakfast Avocado with mayonnaise

Lunch Black cod with miso, oshitashi, sashimi

Dinner Tempura, sashimi

SATURDAY

Breakfast None

Lunch Yaki udon

Dinner Teriyaki beef steak, rice

SUNDAY

Breakfast Miso soup, rice

Lunch Sweet potatoes with soy dressing (below right)

Dinner Teriyaki chicken with sir-fried noodles with onion

Roasted sweet potatoes with soy honey glaze (serves four)

1lb sweet potatoes, peeled

3 tbsp vegetable oil

1 tsp Asian sesame oil

Pinch of salt

For the glaze

2 tbsp honey

1 tbsp soy sauce

1 tsp juice from grated fresh ginger

1 tbsp toasted sesame seeds

Preheat oven to 200C/400F. Cut potatoes into cubes. Cover a baking tray with foil and spritz with nonstick baking spray. Mix potatoes with oils and salt and spread on the tray in a single layer. Roast for 30 to 40 minutes until cubes are soft. Drizzle with glaze and continue to cook until brown on the edges. Remove and sprinkle with seeds.

(Recipe from Japanese Pure and SImple, by Kimiko Barber)

Nutritionist's verdict

Sweet potatoes are bursting with vitamin C, vitamin E and beta-carotene. These antioxidants help to give our body protection against free radical attack, which can trigger anything from heart disease to cancer. One serving has 1.8g of our daily maximum salt intake of 6g and the equivalent of 4 teaspoons of sugar from the honey. The dish provides 342 calories and 13g of fat.

Sushi rice is one of the benefits of a Japanese diet. The special sushi vinegar added when creating these little works of culinary art lowers the GI to medium. Soya bean curd is used in miso soup and many traditional dishes. It is rich in protein and calcium and low in fat, saturated fat and salt, and has fewer calories per 100g than chicken, meat and most fish.

However, it is worth remembering that there is a lot of salt in soya sauce and miso soup. So, if you're following a Japanese style diet, opt for lower-salt soya sauce and have fresh rather than smoked fish where possible.

AMANDA URSELL (www.amandaursell.com )

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