What it is
Water is a molecule composed of two hydrogen atoms bonded to a single oxygen atom.
What it does
Water accounts for about 60% of the total mass of the human body, and virtually every cell of your body needs to be wet at all times. The aqueous solution in and around your cells provides the necessary medium for molecules to be able to come in contact with each other and interact. If they couldn’t float around, they wouldn’t be able to link up and carry out all of the interesting chemical reactions that are necessary for life.
Water is also necessary for most catabolic chemical reactions in your body (reactions where a larger molecule is broken down into a series of smaller molecules). When your cells break proteins or glycogen down into their constituent amino acids or sugars, they do that through the process of hydrolysis, which involves using a water molecule to sever the bonds between two other molecules.
Asking what water does for your body is like asking what the atmosphere does for complex life on earth. You could list a virtually limitless number of discrete things water does, but the most basic answer is that it provides a necessary precondition for life (as we know it) to even exist.
The US National Institutes of Health recommends a daily intake of 3.7L of water for men, and 2.7L of water for women. This recommendation accounts for all sources of water intake, including both foods and beverages. Water accounts for about half of the total mass of most foods you eat, and about 30% of total water intake. So, that means men should aim to drink about 2.6L of fluids per day (about 11 cups), and women should aim to drink about 1.9L of fluids (about 8 cups).
Of note, these are extremely inexact intake targets. Individual fluid needs vary, and are influenced by activity levels, atmospheric conditions, and losses of water from sweat.
Likelihood of tracking completeness: Very low
Water is not a nutrient that food manufacturers are required to disclose on nutrition labels. The vast majority of food and beverage manufacturers do not voluntarily list water content on nutrition labels, so most branded products in the MacroFactor database lack information on water. So, if you’d like to accurately track your water intake, you’ll need to make a point of mostly tracking “common foods,” which come from research-grade databases that have full nutrient reporting.
For more on when you can track using branded foods versus common foods when you’re trying to accurately monitor your intake of particular nutrients, you should check out this article.
Likelihood of insufficient intake: ???
The most immediate consequence of insufficient water intake is dehydration, and there’s surprisingly little data on rates of subclinical dehydration. Some researchers and public health officials hold the perspective that thirst signals are adequate to prevent dehydration, while others are of the opinion that sensations of thirst are an indication that you’re already dehydrated. Under the first perspective, dehydration would be very rare, since it’s generally quite easy to obtain fluids when you’re thirsty in most developed countries. Under the second perspective, virtually everyone is dehydrated, at least sometimes, because virtually everyone at least occasionally experiences sensations of thirst. Part of the problem is that there’s not a single agreed-upon definition of dehydration.
Excessive water intake coupled with insufficient sodium intake can cause problems as well. Sodium levels can get dangerously low, leading to a condition called hyponatremia. Mild hyponatremia is characterized by muscular weakness, confusion, and headaches. Severe hyponatremia can be fatal. While excessive water intake in isolation can cause hyponatremia, hyponatremia is far more common when exercising in the heat. Water loss via sweat increases fluid intake demands, and additional sodium is also excreted in sweat. So, when exercising in the heat, it’s a good idea to stay hydrated with beverages that contain some electrolytes (like sports drinks).
For more on nutrients with a greater likelihood of insufficient or excessive intake, you should check out this article.
Signs of deficiency/insufficiency
Signs of mild dehydration include thirst, urine that’s darker yellow than normal, a decrease in cognitive performance and feelings of general malaise, dry eyes, and a dry mouth.
Virtually any beverage, except for beverages with a high alcohol content (since alcohol increases fluid losses through urine), should have a net hydrating effect.