Natural Mineral Water is always
the most recommendable, though
within that class of water, waters
featuring low mineral content
may be more suitable to certain
age groups (babies) and waters
with medium or high content to
others, for example, elderly
people with osteoporosis problems
may find calcium-rich water more
suitable. Very often, which water
to recommend depends more on
the state of one's health than
on one's age.
This depends on the state of one's body at the time: in situations where one is losing salt (doing sports, for example), water with a high mineral content is more suitable, while for replenishing water lost through ordinary metabolic processes (urination, defecation, evaporation etc.), water with a low mineral content may be more appropriate.
Tap water, water from public fountains (unless otherwise stated on the fountain itself) and bottled water are all fit for drinking.
Water from other sources must be analysed, or be known to be subject to regular analyses by the health authorities.
Salt water can be made fit for drinking, though it is very expensive to do. Even so, in places that are short of water such as the Canary Islands, salt water is one of the main sources of drinking water.
Properly stored bottles of water (i.e. in a cool, dry place out of direct sunlight and free from aggressive odours) could remain in perfect condition indefinitely, but the regulations demand a ''Best before'' date even so.
Water with gas is neither better nor worse than water without gas ? it's just different. Which one chooses is a matter of taste. In certain very particular cases, doctors may see fit to recommend one type rather than the other.
If the water is in perfect condition, it is unaffected by direct sunlight. There may however be a problem with certain plastic bottles, since the action of ultra-violet rays may affect the taste, which is unfortunate but harmless. In any event, this problem has been solved through the latest generation of plastic bottles. Another case is water that contains microspores of plant origin (which should never happen with properly bottled natural waters): if it is left exposed to direct sunlight for a while, a green slime can appear, which is simply a build-up of organisms of plant origin.
It all depends on the mineralization processes that occur from the time the water comes down as rain to the time it emerges or is tapped from the earth. There are some very special processes that lead to carbon dioxide gas being captured, resulting in waters as authentic as Vichy Catalan.