How much water do I really need to drink per day?
There is no magic number: for a healthy adult who is not very active, thirst is a reliable guide, with a total fluid intake of roughly 2 to 3 litres per day from all sources as a reference point. Drink more as soon as you exercise, when it is warm, or as you get older.
The well-known advice of eight glasses of water per day has no scientific basis. Reviews involving thousands of healthy adults show that such amounts of pure drinking water are not necessary for people who are sedentary in a temperate climate.
How much fluid you actually need depends on everything you drink and eat. The American institute for nutrition set total daily fluid intake at 3.0 litres for sedentary men and 2.2 litres for sedentary women. A European review arrives at an average of 1.5 litres of drinking water per day for a sedentary adult, on top of the fluid obtained from food. Coffee and tea count just as much as moderate amounts of beer: the popular belief that caffeine dehydrates you is not supported by the research.
Drinking more than those recommended amounts offers no additional benefit for healthy people, with one exception: people who suffer from recurring kidney stones may benefit from a higher fluid intake. For everyone else, more is not better.
During exercise, sport, or hot weather, the thresholds are considerably higher. Sweating greatly increases the need for fluids, and in those situations the standard recommendations are no longer adequate. This also applies to older adults and young children: their fluid regulation is less precise, making them more prone to dehydration. Dehydration can cause muscle weakness, dizziness when standing up, and an elevated heart rate, and there are indications that it also negatively affects concentration and mood.
For healthy adults, the body already regulates fluid balance very precisely through thirst, a hormone that directs the kidneys, and the kidneys themselves. A small fluid deficit is generally corrected by the body on its own within a day. Drink when you are thirsty, keep your urine light yellow, and drink more when you are active or it is warm. That is enough.
All sources are reviews (not randomised trials or meta-analyses). The recommendations are based on observational data and physiological models, not on controlled experiments with clinical endpoints.
How many fruits and vegetables do I actually need per day?
Five portions of fruit and vegetables per day (two pieces of fruit, three serving-spoon scoops of vegetables) is the number with the strongest evidence base; eating more does not provide additional benefit, but most people do not even reach this minimum.
How many vegetables do you really need per day?
Five servings of vegetables and fruit per day offers the best protection, with non-starchy vegetables counting and juice and potatoes not. More than five servings adds nothing.
How many cups of green tea do you need per day?
Two to three cups of green tea per day is the amount at which research shows the most health benefits, though cause and effect has not been proven. Let the tea cool a little before drinking; tea that is too hot increases the risk of oesophageal cancer.
How much lithium do you need each day?
There is no official daily recommendation for lithium; scientists cite 1 mg per day as a cautious guideline, but evidence for benefits in healthy people is largely absent, so supplementation is not yet substantiated.
How much iron do you need per day?
For healthy adults without a deficiency, 1 to 2 mg of iron per day through food is sufficient. When a deficiency or anaemia is confirmed, requirements are much higher; discuss that with your doctor.
How much vitamin D should you take per day?
For healthy adults under 75, extra vitamin D is probably not necessary. For children, adults aged 75 and over, pregnant women, and people at high risk of diabetes, there are indications that supplementation helps, but discuss this with your doctor.