I have made the mistake of buying a litre of mineral water thinking it would be flat, then opening it in a quiet waiting room. It was not flat. The confusion between mineral water and sparkling water is common, partly because labels differ across borders and partly because lots of mineral water is also carbonated. So before asking which one is better, it helps to know what is actually in the bottle.

Here is the short version: for hydration, the research says they are tied. For teeth, plain sparkling water is mostly fine, but citrus-flavoured sparkling water is a different story. For minerals, natural mineral water has an edge if the label shows calcium or magnesium you actually need. The rest is nuance, and nuance is where the useful stuff lives.

What the labels actually mean

Mineral water is not just water with minerals added afterwards. Under European Union rules, natural mineral water has to come from a protected underground source, be microbiologically safe at the spring, and keep a stable mineral composition. The United States Food and Drug Administration sets a standard of identity requiring at least 250 parts per million of total dissolved solids for a product to call itself mineral water. That phrase is a bag of salts — calcium, magnesium, bicarbonate, sodium, sulphate — picked up as the water moves through rock.

Sparkling water is simpler: water plus carbon dioxide gas. The fizz comes from carbon dioxide dissolved under pressure, forming weak carbonic acid in the bottle or can. Seltzer is usually carbonated tap water. Club soda is carbonated water with added minerals for taste, often sodium bicarbonate or potassium sulphate. Tonic water is not part of this comparison; it contains quinine and often as much sugar as a soft drink.

Hydration: the tie researchers actually measured

The most direct evidence comes from a 2016 study in the American Journal of Clinical Nutrition. Researchers gave volunteers one litre of different drinks and measured urine output over four hours to build something called the Beverage Hydration Index. Still water and sparkling water scored nearly the same, and both hydrated better than coffee and sugary cola in that specific setup.

My family has asked whether the gas somehow makes you retain less water. It does not. The fluid is absorbed the same way; carbon dioxide mostly comes back up as a burp. For ordinary daily hydration, choose the one you will actually drink. The best hydration beverage is the one that does not sit unopened on the kitchen counter.

Teeth: the acid question no one explains on the label

That weak carbonic acid means carbonated water has a lower pH than still water. Dental researchers often use 5.5 as the rough threshold below which enamel can begin to lose minerals. Plain sparkling water usually sits below that, but its buffering capacity — how hard it is for the acid to keep attacking enamel — is nowhere near that of sugary cola or citrus squash. The practical risk is small if you are sipping unflavoured fizzy water in a normal way.

Flavoured sparkling water changes the arithmetic. Many brands add citric acid, sometimes malic or phosphoric acid, to make lime or grapefruit taste bright. Those can push pH down to the same acidic neighbourhood as some sodas. The American Dental Association advises checking labels for citric acid and suggests plain sparkling water rather than citrus-flavoured ones if you are concerned.

For anyone who sips flavoured fizzy water across the whole workday, swishing with still water afterwards and not holding each mouthful are small habits that matter more than people expect.

Stomach, burps and reflux

That carbon dioxide comes out somewhere. Sparkling water stretches the stomach briefly, which some people read as fullness and others read as find me somewhere private for the next five minutes. If you have gastroesophageal reflux disease, the gas can increase pressure and send acid where it does not belong. Clinical advice for reflux often lists carbonated drinks as a trigger. Mineral water with bicarbonate can be soothing for some people with sour stomach, but that is a different mechanism than the fizz.

The mineral argument

This is where mineral water earns its keep. Magnesium and calcium do not appear in meaningful amounts in plain sparkling water, and neither does bicarbonate. They do appear in natural mineral waters, sometimes enough to count toward daily intake. The trick is reading the label. A water with 100 milligrams of calcium per litre can help someone who does not eat much dairy. A water with more than 200 milligrams of sodium per litre may be a problem for someone watching blood pressure. Mineral content varies more between brands than between mineral water and sparkling water as categories.

Here is a two-line label check I use:

• Natural mineral water has mineral levels on the label — look for calcium and magnesium.
• Club soda and seltzer rarely list calcium, but they do list added sodium or potassium.

The 2025 weight-loss paper everyone misread

In January 2025, a short paper in BMJ Nutrition, Prevention & Health set off headlines about carbonated water and weight loss. The author, Akira Takahashi, described a possible mechanism: carbon dioxide in the blood might make red blood cells take up a little more glucose, as suggested by a small haemodialysis study. Takahashi himself wrote that the effect would be too small to matter on its own, and no trial has shown people lose weight by swapping still water for sparkling. The fizzy-water cure is not a thing.

Choosing without overthinking

For most healthy adults, neither side wins in a way that changes daily life. If you are choosing by hydration alone, flip a coin. If you are looking for calcium or magnesium, read the mineral label on a natural mineral water and ignore the fizz. If you drink citrus-flavoured sparkling water all day, your teeth may notice before your waistline does. If you have reflux, flat mineral water is probably the safer bet.

Here is a decision shortcut I actually use: if I want minerals, I check the label for at least 100 milligrams of calcium or magnesium per litre. For fizz, I pick unflavoured and save flavoured cans for meals rather than desk sipping. If my stomach already feels like a half-shaken bottle, I go flat. That is it.

Photo by Denes Kozma on Unsplash