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. Research covering thousands of healthy adults shows 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. The recommendations are based on physiological models, and no study can establish cause and effect for the clinical endpoints discussed.
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 much iron do you need per day?
For healthy adults without a deficiency, compensating for 1 to 2 mg of iron per day through a varied diet is sufficient; if you have a demonstrated deficiency or are pregnant, the requirement is much higher and it is best discussed with your doctor.
How much vitamin D should you take per day?
For healthy adults under 75, extra vitamin D is probably not necessary, but for children, people aged 75 and older, pregnant women, and those at high risk of diabetes, supplementation is worthwhile. If you are unsure whether you belong to a risk group, it is worth discussing this with your general practitioner.
How much magnesium do you need per day?
For an average-weight adult, the guideline is approximately 250 mg of magnesium per day from food; if you cannot reach that through spinach, nuts and whole grains, a supplement of up to 600 mg per day can help as a top-up.
How much omega-3 with EPA and DHA do you need per day?
There is no one-size-fits-all dose: for heart protection ~840 mg of EPA+DHA per day is probably sufficient, for an anti-inflammatory effect you need 2 to 4 grams, and during pregnancy 200 to 300 mg of DHA is already enough.