Our bodies are around 60% water, give or take.
It is commonly recommended to drink eight 8-ounce glasses of water per day (the 8x8 rule).
Although there is little science behind this specific rule, staying hydrated is important.
Here are 7 evidence-based health benefits of drinking plenty of water.
If we do not stay hydrated, physical performance can suffer.
This is particularly important during intense exercise or high heat.
Dehydration can have a noticeable effect if you lose as little as 2% of your body's water content. However, it is not uncommon for athletes to lose up to 6-10% of their water weight via sweat (, 2).
This can lead to altered body temperature control, reduced motivation, increased fatigue and make exercise feel much more difficult, both physically and mentally ().
Optimal hydration has been shown to prevent this from happening, and may even reduce the oxidative stress that occurs during high intensity exercise. This is not surprising when you consider that muscle is about 80% water (, 5).
So, if you exercise intensely and tend to sweat, then staying hydrated can help you perform at your absolute best.
Bottom Line: Losing as little as 2% of your body's water content can significantly impair physical performance.
Your brain is strongly influenced by hydration status.
Studies show that even mild dehydration (1-3% of body weight) can impair many aspects of brain function.
In a study of young women, fluid loss of 1.36% after exercise impaired both mood and concentration, and increased the frequency of headaches ().
Another similar study, this time in young men, showed that fluid loss of 1.59% was detrimental to working memory and increased feelings of anxiety and fatigue ().
A 1-3% fluid loss equals about 1.5-4.5 lbs (0.5-2 kg) of body weight loss for a 150 lbs (68 kg) person. This can easily occur through normal daily activities, let alone during exercise or high heat.
Many other studies, ranging from children to the elderly, have shown that mild dehydration can impair mood, memory and brain performance (, , , , 12, ).
Bottom Line: Mild dehydration (fluid loss of 1-3%) can impair energy levels and mood, and lead to major reductions in memory and brain performance.
Dehydration can trigger headaches and migraines in some individuals (, ).
Several studies have shown that water can relieve headaches in those who are dehydrated ().
However, this appears to depend on the type of headache.
One study of 18 people found that water had no effect on the frequency of headaches, but did reduce the intensity and duration somewhat ().
Bottom Line: Drinking water can sometimes help relieve headache symptoms, especially in people who are dehydrated.
Constipation is a common problem, characterized by infrequent bowel movements and difficulty passing stool.
Increasing fluid intake is often recommended as a part of the treatment protocol, and there is some evidence to back this up.
Low water consumption appears to be a risk factor for constipation in both young and elderly individuals (, ).
Carbonated water shows particularly promising results for constipation relief, although the reason is not entirely understood (, ).
Bottom Line: Drinking plenty of water can help prevent and relieve constipation, especially in people who generally do not drink enough water.
Urinary stones are painful clumps of mineral crystal that form in the urinary system.
The most common form is kidney stones, which form in the kidneys.
Higher fluid intake increases the volume of urine passing through the kidneys, which dilutes the concentration of minerals, so they are less likely to crystallize and form clumps.
Water may also help prevent the initial formation of stones, but studies are required to confirm this.
Bottom Line: Increased water intake appears to decrease the risk of kidney stone formation. More research is needed in this area.
A hangover refers to the unpleasant symptoms experienced after drinking alcohol.
Alcohol is a diuretic, so it makes you lose more water than you take in. This can lead to dehydration (, ).
Although dehydration is not the main cause of hangovers, it can cause symptoms like thirst, fatigue, headache and dry mouth.
A good way to reduce hangovers is to drink a glass of water between drinks, and to have at least one big glass of water before going to bed.
Bottom Line: Hangovers are partly caused by dehydration, and drinking water can help reduce some of the main symptoms of hangovers.
Drinking plenty of water can help you lose weight.
This is due to the fact that water can increase satiety and boost your metabolic rate.
In two studies, drinking half a liter (17 ounces) of water was shown to increase metabolism by 24-30% for up to 1.5 hours (, 27).
This means that drinking 2 liters of water every day can increase your total energy expenditure by up to 96 calories per day.
The timing is important too, and drinking water half an hour before meals is the most effective. It can make you feel more full, so that you eat fewer calories (, ).
In one study, dieters who drank half a liter of water before meals lost 44% more weight, over a period of 12 weeks ().
It is actually best to drink water cold, because then the body will use additional energy (calories) to heat the water to body temperature.