Failing the taste test: Celebs like Jennifer Aniston, left, favour juice detoxes but Janice Dickinson looks unimpressed by her green smoothie
Last month's news that Jennifer Aniston has become a veg juice devotee, regularly slugging back a concoction of cucumbers, beetroot, spinach, kale, garlic, ginger, carrots and celery, will only fuel the juicing fad.

Because look around any high-end gym or yoga studio these days and you'll see women swigging something that looks like it's been scooped from a pond. This is the juice cleanse, or the juice detox - don't ever refer to it as a diet, that would sound like you're doing it to lose weight, whereas, of course, this is all about purifying your body.

Popular among A-listers including Salma Hayek, Gwyneth Paltrow and Sarah Jessica Parker, juice detoxes have gone mainstream, with legions of women pulping Tesco's entire fruit and veg aisle themselves, or paying up to £40 a day - yes, really - to have supplies delivered to their door.

It sounds terribly good for you - just masses of fruit and vegetables all juiced together, filling you full of vitamins, minerals and anti-oxidants. But while if you're doing it Aniston-style and eating normally for most of the week, it probably won't do you any serious harm, the truth is that a juice regime that lasts several weeks - or even just several days - could actually be wreaking all sorts of havoc on your health.

To start with it's unnecessary. Your body doesn't need to detox, your digestive system doesn't need to rest, and if you're seriously worried about your diet and your health, you need to make long-term changes.

Comment: Considering the fact that our air, water and much of our food is polluted, detoxification IS recommended. However there are better ways of doing so without filling up on fructose laden juices.

'Juice fasts are simply not sustainable,' says Natalie Jones, a spokesperson for the British Dietetic Association. 'So if you're doing it for health reasons, there's simply no point. Any weight you lose, you'll put straight back on again, possibly with extra because short-term, low-calorie crash diets like this mess around with your metabolism.'

You'll undoubtedly feel hungry as you're not consuming any of the fibre that helps fill you up, and don't even think about exercising.

'Have a juice as one of your five-a-day but any more than that won't give you extra benefits, and could do more harm than good'

'You might be getting a quick sugar rush,' says Natalie, 'but you're not consuming any carbohydrates, so exercising, or even normal daily life is going to be almost impossible. You'll feel light-headed and exhausted.'

Stomach aches are also a common side-effect of the juice fast.

'With no fibre in your diet, even after a couple of days, constipation will become a problem, and in the long term, your cholesterol levels could be affected as fibre helps keep them low. So if you're not getting enough fibre, your cholesterol could shoot up.'

If that's not miserable enough, you could find that your net calorie intake is higher than if you were eating normally. Because while juicing gives you the same level of vitamins as you'd get from eating the whole fruit or vegetable, your body isn't using any energy to break down the food so calories from the natural sugars in the juice aren't offset by those usually used in digestion.

And while you're undoubtedly getting lots of vitamins, Natalie says that's not as great as it sounds.

'Vitamin C is, of course, good for you, but beyond a certain point, more isn't any better for you. And, if you're only drinking veg and fruit juices, you're missing out on a lot of other nutrients such as calcium, protein, vitamin D, essential fats and so on.'

This lack of vitamins and nutrients can have a knock-on effect on your appearance, too.

According to renowned trichologist Philip Kingsley, if you plan to juice for a couple of weeks, you should also plan to see your hair fall out about two to three months later.

'I've seen it many, many times,' he says. 'Women come to see me with what appears to be unexplained hair loss, and then, when you trace it back, it turns out that they were on some extreme juice fast a few months before.

'It's quite simple, if your body isn't getting the nutrition it needs, it powers down the processes that it considers as being not essential to life, and one of those is hair production.'

But it's not just your hair that will suffer.

'Juicing for anything longer than a couple of days will have a profound effect on your skin,' says cosmetic dermatologist Dr Sam Bunting. 'Not only might you find that your skin dries out as you're not getting any of the essential fatty acids it needs, but if you've already got a tendency to dry skin anyway, you may find that you start to develop patches of eczema as the barrier function is compromised.'

And there are long-term issues too.

'Low-calorie diets like this cause the body's insulin levels to spike and crash which initially causes break-outs, but over time, this insulin cycle alters the structures of collagen and elastin in the body, making them stiffer, and causing skin to look prematurely old.'

Damage to collagen will also affect your teeth, as it's collagen fibres that hold them in place. But, even if you manage to retain your gnashers, juice diets are seriously bad news for them, as Dr Uchenna Okoye of London Smiling points out.

'Juice from vegetables and especially from fruits, which tend to have a higher acid content, can damage the enamel of your teeth in exactly the same way that a fizzy drink would. We consider the acids in fruit and vegetables to be "good" but that's only in the context of eating the whole thing, not when you're drinking a super-concentrated juice.'

And the same holds true for the sugars in your juice.

'Fructose is a natural sugar, but to the body it's still just a sugar, so too much of it will cause cavities as the bacteria in the mouth feed on it.'

Comment: Just because fructose is natural does not mean it is healthy, it speeds up the process of becoming obese and promotes candida among other evils:

If you are going to drink juices, even just as part of a balanced diet, Dr Okoye recommends always using a straw and never ever brushing your teeth straight after drinking as the sugar and acid softens the enamel of the teeth so you could actually be doing more damage. Ideally she suggests brushing teeth before drinking a juice, and using a fluoride toothpaste which will strengthen the teeth.

So, a lack of energy, a messed-up digestive system, prematurely aged skin, rotting teeth and hair loss - hardly the healthy, cleansed body that juice devotees are aiming for, is it?

'By all means have a juice as one of your five a day,' says dietitian Natalie Jones. 'But any more than that simply won't give you extra benefits, and could actually end up doing you more harm than good.'