Dietitian explains the nutritional facts behind bottled waters

It's sweltering outside and you've worked up a big thirst, so you duck into a shop to pick up a bottle of water. But these days there are so many types on the shelves that you could drop from dehydration before working out which one to buy. In fact, says market research firm Kantar Worldpanel, sales of bottled water boomed by more than 14 per cent in the year to January.

Regardless of their prices or promises, all the waters on the market hydrate you equally well, and no better than tap water does. So if that is all you care about, buy the cheapest. Even better, fill up a bottle at home. But if you want something that tastes different or has the possibility of added health benefits, here’s the lowdown.

Water, plain and simple

Bottled water is simply water fit for human consumption that is bottled safely. It could be packaged tap water. But beyond that baseline there are official definitions for terms such as “purified,” “spring,” “artesian” and “mineral”.

“Purified” means the water – from any viable source, even, say, a municipal water supply – has been filtered or distilled to remove impurities, such as chlorine, that affect taste. Spring water comes from an underground formation that flows naturally to the surface, and artesian water is tapped from an underground aquifer that’s under pressure.

Mineral water must originate from a protected underground source. Most of these waters are so mineral-rich that drinking them can significantly boost your intake of the nutrients, especially calcium and magnesium, that many people lack. Plus, mineral water tends to be alkaline, which may help your bones. (More on that later.) Depending on the brand, one litre a day can cover you for 20 to 58 per cent of calcium and 16 to 41 per cent of magnesium needs.

Flavoured waters

For those who don’t care for the taste of plain water, drinks companies have come up with flavoured options – and we’re lapping them up. Flavoured waters now account for 30 per cent of all bottled water sales by volume in the UK. Some are just treated with a hint of natural fruit and/or herbal essences. Others have sweeteners, colourings and artificial flavours, making them, to me, more like soft drinks. Many sweetened waters contain a lot of calories (one popular brand contains nine teaspoons of sugar in a 420ml bottle). I suggest going for one that is as much like actual water as possible, unsweetened and with minimal additives.

Plant-derived waters

When coconut water first burst on to the scene, it promised better hydration than water, but marketers have since backed off on that unjustified claim. What it can claim to be is a lightly sweet liquid with a nutty taste that hydrates as well as water and provides a significant dose of potassium. Coconut water comes from the inside of the young, green fruit and, unlike coconut milk, has no fat. But there is nothing magical about it. You can hydrate and replenish your potassium (plus get other nutrients and fibre) by drinking a cup of water and eating a small banana.

A new kid on the shelf in health-food shops is maple water, the liquid (sap) from the maple tree that is usually boiled down to make a syrup. In its unconcentrated form, it is clear and has a subtle sweetness, plus some minerals, for about 20 calories per serving. There isn’t enough research to back the many claims about its health benefits, including the “cleansing” power I was told it has by the woman providing tastes of it at my local supermarket. (I hope she didn’t see me rolling my eyes.) But I thought it was delicious, if expensive.

pH alkaline waters

A growing trend is the emergence of “pH-balanced” alkaline waters – boasting a pH greater than 7. They stem from a popular but unfounded theory that if we consume too much water that is on the acidic side (which tap water often is), we wind up acidifying our bodies and compromising our health. The fact is, the body’s pH is kept in a tight range, thanks to our kidneys and other buffering systems, and there is no substantial research to show that drinking more acidic water does any harm per se. But there is one well-documented pro to drinking water that is more alkaline: it could benefit your bones.

Several studies show that drinking water that is more alkaline because of its electrolyte and mineral content (whether naturally occurring, as with mineral water, or added) can help preserve bone by reducing the kidney’s need to tap into calcium reserves to balance acid in the body. So, not only do you ingest more important minerals and nutrients such as potassium, you also help keep calcium in your bones instead of breaking it down. Skip brands made alkaline through ionisation; they won’t give you the nutrient benefit that minerals and electrolytes do. Also keep in mind that more alkaline (a higher pH) is not necessarily better. Aim for a pH between 7.5 and 8.5 – above that, water tends to have a slippery feel and a less appealing taste. Bottoms up!

A version of this article appeared in the The Washington Post’