What's in your bottle?

Most of us buy it because we think it's purer and healthier. But bottled water from the supermarket could be up to two years old and have a lower mineral content (and therefore fewer positive properties) than tap water, a report claimed last week.

So what are you really drinking when you buy bottled water? Twothirds of that sold in the UK is natural mineral water, which is a legally protected term. To qualify, the water must come from a known and protected source, be bottled at source, and have a consistent mineral analysis - the health-generating properties - with that analysis printed on the label.

Most of the other bottled water on sale is spring water, which also comes from an underground source. This can be treated to remove impurities and need not be consistent in its mineral composition from bottle to bottle.

Most natural mineral waters and many spring waters have filtered through rocks for millennia before surfacing. "This makes 'freshness' a non-issue," says Dr Sara Stanner of the British Nutrition Foundation."A year or two in the bottle makes no odds."

So what are we actually getting in our bottle?

Spa: With the very lowest mineral content, it contains no more calcium (good for growth) and magnesium (excellent for the heart and energy) than the softest tap water. But it's a good choice for people with kidney disease, who may require a water with low dissolved solids. Volvic and Highland Spring also have a low mineral content.

Buxton: From one of the deepest underground sources in Europe. Some calcium, but only half that of London tap water.

Evian: A reasonable source of calcium and magnesium. Also quite rich in bicarbonate, which aids digestion and creates an alkaline environment in the body that boosts bone strength.

Badoit: Very high in bicarbonate, so good for digestion. One litre also contains nearly a quarter of the Recommended Daily Allowance of calcium and 28 per cent of the RDA of magnesium. It's one of the higher-sodium bottled waters with six per cent of the recommended daily limit per litre, so watch your salt intake.

San Pellegrino: A litre contains 23 per cent of the RDA of calcium and 17 per cent of the RDA of magnesium, so good for heart and bones. Less sodium than Badoit.

Perrier: Reasonable calcium (18 per cent of the RDA per litre) and rich in alkaline bicarbonate, so good for digestion.

London tap water: Contains on average 108mg of calcium per litre, just over an eighth of the RDA. Insignificant amounts of magnesium. The nitrate content (average 28mg/litre) is higher than in most natural mineral waters (which in large amounts is potentially toxic to humans), but is still well below the safe limit of 50mg per litre. It is low-sodium.