During the day, the new wave of juice bars serve another purpose, to redress in an instant the damage done by doing, drinking and eating what's not good for you. Farmacia, in Covent Garden, which takes the most clinical approach, doesn't even call them juices. They're "organic liquid health solutions".
And the extremists of the freshly squeezed juices movement claim a great deal: longevity, recovery from terminal illnesses, grey hair turned back to its original colour are just some of the wishes that they insist juices can make come true.
Whether we really believe this or not, we're still prepared to swallow these juices when they're as refreshing and invigorating as a Mint Zing (orange, apple, mint and melon), a Red Planet (carrot, beetroot and celery), Pink Passion (pineapple, grapefruit and ginger) or Fatigue-Fighter (carrot, spinach and avocado). In the space of a few days, I have been cleansed, revitalised, detoxed, destressed, energised, boosted and just a bit gutted for pounds 2 to pounds 4 a throw - all in the line of research of course.
The Planet Organic supermarket, which opened in 1995, featured the first of the new wave of London's juice bars. Since then have come Fluid, Crussh, Squeeze, Juice, and Fresh 'n' Smooth. Each one squeezing and crushing and juicing fresh fruit and veg for liquid consumption.
Outside London, the choice of juice bars includes Mango & Stone in Edinburgh, which opened some 18 months ago. "We squeeze to please" is the slogan in front of which Gordon Brown, perhaps wisely, refused to stand to be photographed on a visit to promote small businesses in Scotland.
Gordon Watt, who started Mango & Stone and is planning to make it a national chain, like South Africa's Juicy Lucy's or California's Jamba, has a marketing and PR background and previously worked for the Champagne company Moet & Chandon. "We're funky first and healthy second," he says of his concept for Mango & Stone, which is aimed at stressed executives. Watt plans to open outlets in airports, shopping centres and hotels. "We didn't want to be seen as a green, wholefood-style shop," he adds.
Although Watt has researched the supposed benefits of the various combinations, and the blends are sold as "drinks for reasons", he doesn't make specific health claims. It sounds, however, like a natural anti-depressants is what consumers in Edinburgh are after: the most popular combo is the "Don't worry be happy" blend of tropical fruits.
At Planet Organic the blends that slip down fastest are carrot, apple and ginger, and mixed vegetable: celery, spinach, beetroot, carrot and fennel. Carrot alone is considered passe, and more and more customers at the juice bar are seeking something more challengingly health-giving to drink.
Now, it seems, we cannot live, or at least revive ourselves, by juice alone. The latest trend is the addition of nutritional supplements. These occupy that generally unappetising - and often unproven - area between food and medicine where extraordinary benefits are claimed but backed up only with anecdotal evidence.
At least squashed fruit and vegetables taste delicious; the same cannot be said for wheatgrass extract. Along with bee pollen, echinacea, ginseng, guarana, spirulina, blue-green algae, flax seed oil and lecithin, this is at the cutting-edge of supplements, and shots of the extract can be drunk on their own if you're prepared to risk a headache the first time you try it.
Christopher "Do you have to use my second name?" is a photographer who has been growing wheatgrass for 20 years, and using the juice in his personal health programme. His Green Seed Company now supplies many of the juice bars that crush it as fast as he can grow it, which takes about 10 days from seed.
He is evangelical about the astonishing powers which he says the substance possesses. The effect of wheatgrass juice on the system is instantaneous, apparently. It is said to boost the immune system and is chock-full of chlorophyll, which cleanses and neutralises the toxins in our systems. It also, he claims, enhances the blood's capacity to carry oxygen and nutrients to every cell including the brain, and other organs, plus the skin.
"We are all in a toxic state, by being on this planet," says Christopher. He grows around 150 trays of wheatgrass each week, seeded in compost with the addition of seaweed for additional minerals. Wheatgrass, he adds, is already one of the richest sources of vitamins A and C, and contains calcium, iron, magnesium, sodium phosphorus, potassium, sulphur, cobalt and zinc.
Not all are quite so evangelical about the supplement. Henri Davies, a former personal trainer who switched from dispensing nutritional advice along with his exercise programmes, to starting Juice in Covent Garden, is not convinced. His priority is juices which people simply like to drink.
"We live fast lives, we need to be able to have healthy food very quickly. But I didn't want to preach to the converted, I wanted to expose more people to the idea," he explains. When he started Juice less than two years ago it was the first place where you could sit and enjoy a freshly squeezed juice, eat, drink coffee, even smoke.
Smoothies, with banana, low fat yoghurt and fruits are almost as popular at Juice as "remedial juices" because they're nice and sweet. "No matter how healthy a drink is we don't sell it if it doesn't taste good," says Davies. Which generally rules out wheatgrass. Anyway he's sceptical about its benefits. Spirulina and blue grass algae "are more potent, have the full spectrum of vitamins, a rich source of protein and have 70 per cent more chlorophyll than wheatgrass", he claims. Nevertheless, "if people want wheatgrass," and it seems more and more do, "we'll give it to them."
Since Juice opened, regular users have become more adventurous. Now the Flu Fighter, with beetroot, carrot, apple and ginger, is beaten only by the Mega Detox, with beetroot, carrot, celery and apple.
As Gordon Watt says, "How many people have time to eat the recommended five pieces of fruit a day?" Juices save the time it takes to chew through a salad, and give us the power of fresh fruit and vegetables in one refreshing, colourful gulp
Farmacia, 169 Drury Lane, London WC2 (0171-831 0830); Green Seed Company e-mail: firstname.lastname@example.org; Juice, 7 Earlham Street, London WC2 (0171-836 7376); Planet Organic, 42 Westbourne Grove, London W2 (0171-221 7171); Mango & Stone, 165 Bruntsfield Place, Edinburgh (0131 229 2987); Ranoush Juice, 43 Edgware Road, London W2 (0171-723 5929); Riverside Juice Bar, 18 The Podium, Bath (no phone).
Top juicers: Fast Forward, page 51