Why you should eat your water

But, surprisingly, in his new book The Water Secret, Murad dismisses the notion that we need to drink eight glasses of water a day. "What I really want you to do is eat your water," he explains.

He has no argument with the 1948 study which showed we need 2.5 litres a day of water to function properly: "It's just that no account has been taken of how much of that water we can get from our food.

"Cucumber is 97 per cent water, tomatoes 95 per cent and carrots 88 per cent; even so-called dry food contains water. Water is nearly two-thirds the content of baked salmon and chicken, and one-third of a piece of bread."

The point, he says, is that eaten water is better value than that which is simply drunk. "It's structured water; it carries vitamins and other nutrients, unlike the water in our glass.

"There's nothing wrong with drinking water, but if we don't improve the capacity to get it into our cells and keep it there, it goes straight into the toilet without doing us any good." Or worse, floats between our cells, ageing us and making us feel fat and sluggish, he warns: "Puffy eyes, swollen ankles and a bloated stomach are signs of a body that is not handling water efficiently."

The method, which dovetails with government health guidelines, is to eat lots of fruit and veg. "Buy the most vividly-coloured you can find for the greatest nutritional value," he says.

Murad, who is a qualified chemist as well as a skincare guru, recommends buying organic versions of varieties which may contain high pesticide residues, like cherries, grapes, peaches, pears, strawberries, lettuce, spinach, celery and potatoes.

The message he has for his Jewish kinfolk is less palatable: "Pure fat, pure salt and refined bread are evils to health… and that makes me think about salt beef, chopped liver with schmaltz, challah and bagels, none of which are good."

Although not a vegetarian, Murad points out that red meat creates the acidic environment in which cancer cells flourish; he recommends getting protein mostly from chicken, fish and eggs. "Eggs, like seeds, are embryonic food - they are packed with vital nutrients for growing organisms. And it's good to eat as many vegetables as possible raw." Feel free to throw olive oil over them - it is one of the good fats he embraces: "And if you have to cook them, either steam them or stir-fry briefly in rapeseed oil rather than other oils which undergo potentially harmful changes with heat."

Murad is not using this book to plug his skincare range which has earned a "beauty genius" endorsement from Elle magazine. As far as skincare goes, he says it comes down cleansing, wearing a moisturiser with sunscreen by day and a richer one without by night.

While he recommends facials and massages, it is as much for their emotional benefits as physical: "The healing power of touch is grossly underestimated in our society, yet it's one of the most potent tools for emotional care."

'The Water Secret' is published by Wiley at £12.99