Water Needs

  • 13 Minutes Read

Water Needs

How many times have you read that you are supposed to drink at least eight glasses of water every day? Is this true? Well, the answer is both "yes" and "no." In fact, your need for water depends on many factors, including:

This article will explain why we need water and approximately how much of it we need. The article will also list various sources of water and explain how water intake relates to your level of activity.

Why We Need Water

Although we can survive for weeks without food, we cannot survive for more than a few days without water. Body water allows us to regulate our core temperature so that it gets neither too hot nor too cold. Body water is also a critical medium for blood and cells so that:

Given the same weight, leaner people have more body water (because of greater lean body mass), and people with more body fat have less body water. Body water also decreases with aging. Water accounts for about 75% of an infant's weight, 60% of a young adult's weight, and 50% of a 50-year-old adult's weight.

Will I Lose More Weight If I Drink More Water?

If you replace a caloric beverage with water, then you will consume fewer calories. That could help you lose weight gradually, even if you changed nothing else in your diet. Example: You stop drinking one 20 fl oz bottle (the most common size in vending machines) of root beer and replace it with 20 fl oz of water. Savings = 250 calories per day = 1750 calories per week = 7500 calories per month = 2 lbs per month.

Even if you simply add water to your dietary intake, you might benefit from enhanced weight loss if drinking water prevents you from overeating. I sometimes wonder if people who do not hydrate properly before, during, and after exercise, later find themselves craving succulent foods. Although I am all for encouraging increased intake of fruits and vegetables, exercising hard or sweating a lot requires efficient rehydration. So, quench your thirst first with water before diving into high calorie smoothie drinks, fruit juices, or frozen dairy treats.

Estimating Water Requirement

The Academy of Nutrition and Dietetics uses two figures to estimate basic water requirements: a person's calorie intake or body weight. The term "fluid" is used interchangeably with water, so if you see the phrase "fluid intake needs," you can assume this refers to water, whether it is in the form of actual water, another beverage, or from a food source.

Method 1: Based Upon Calorie Intake

1 milliliter (ml) of water for every 1 calorie intake

Example: My Weight Maintenance Calories is 2022 calories, so my water intake from all sources should be approximately 2022 ml (about 2 quarts).

This is probably not a good method to use if you are on a weight loss diet and/or are exercising or sweating a great deal. For instance, if I consume only 1500 calories, then I would still aim for 2022 ml of water rather than 1500 ml. If in addition to a restricted calories intake I am also exercising, I might further increase my fluid intake to account for water loss through sweat.

Method 2: Based Upon Body Weight and Age

Age: Young adult 16 to 30 years

Water per unit of body weight: 35 - 40 ml/kg or 0.54 - 0.6 fl oz/lb

Age: Adult 31 to 54 years

Water per unit of body weight: 30 - 35 ml/kg or 0.46 - 0.54 fl oz/lb

Age: Adult 55 to 65 years

Water per unit of body weight: 30 ml/kg or 0.46 fl oz/lb

Age: Adult > 65 years

Water per unit of body weight: 25 ml/kg or 0.38 fl oz/lb

Example: I am a 47-year-old adult who weighs 60 kg (132 lbs), so my water intake from all sources should be about 1800 ml - 2100 ml (about 2 quarts). Below is the math.

30 ml /kg x 60 kg = 1800 ml or 1.8 liters
35 ml / kg x 60 kg = 2100 ml or 2.1 liters
Intake range = 1.8 liters - 2.1 liters

0.46 fl oz / lb x 132 lbs = 60.72 fl oz or 1.9 quarts
0.54 fl oz / lb x 132 lbs = 71.28 fl oz or 2.2 quarts
8 fl oz = 1 cup, 4 cups = 1 quart
Intake range = 71/2 cups - 9 cups

Sources of Water

The good news is that many foods and beverages contain a fair amount of water, so you do not need to consume quarts of plain water all day long! Commonly consumed beverages such as milk, juice, tea, and coffee all provide a significant amount of water. Count one cup of these beverages as one cup of water. If you are sensitive to caffeine, then use non-caffeinated beverages if you are counting these towards your water intake. Do not count alcoholic beverages towards your water intake since alcohol acts as a powerful diuretic. Consider it "dehydrating." In fact, for every alcoholic drink I imbibe, I drink one extra glass of water.

How much water can you expect to get from food? Well, it really depends upon your food choices. Fruits and vegetables contain mostly water by weight, so a diet with plenty of fruits and veggies will also provide a significant amount of water. Listed below are approximate values for water content (based upon data from MyNetDiary's database):

Water Source: Non-starchy vegetables

    95% water
The foods highest in water content and lowest in calories are the non-starchy vegetables (e.g., leafy vegetables, cucumbers, celery, tomatoes, herbs, zucchini, or peppers). As a general rule, non-starchy vegetables are those that contain 5 grams of carbohydrates or less, per 1/2 cup cooked or 1 cup raw serving.

Water Source: Fresh fruit

    90% water
Fresh fruit is also very high in water, with melons and strawberries being the highest (91% water) and bananas being the lowest (75% water).

Water Source: Starchy vegetables and grains

    65% - 75% water
Believe it or not, cooked starchy vegetables (e.g., potatoes, beans, and peas) and grains are quite high in water. Think of it this way - where does all the water go when you cook brown rice?

Water Source: Lean meat, fish, and poultry

    65% water
Low fat cuts without skin are high in moisture since most of the weight comes from muscle. Fatty cuts are lower in water (e.g., bacon is only 12% water).

Water Source: Fresh Cheese

    60 - 80% water
Ricotta, fresh mozzarella, and cottage cheese fall into this category. Aged, firm, or hard cheeses are much lower in moisture - usually between 35 - 40% water.

At room temperature, the weight of water is roughly the same as its volume (i.e., 8 oz of water is about 8 fl oz). I average about 1100 grams (about 1.1 liters or 1.2 quarts or 41/2 cups) of water every day from my food and beverage intake. This does not include the amount of plain water I drink. Since my goal is about 7 1/2 - 9 cups of water from all sources, I need to consume about 3 - 41/2 cups of plain water to meet my goal. That is much more manageable than drinking 8 cups of plain water!

MyNetDiary Tip

To track water intake, simply tap the water glass icons in MyNetDiary's app or web program. Unless you specify otherwise, the default size of the glass is 8 fl oz (237 ml). You can count all non-alcoholic beverages towards your water requirement. For example, if you drink a cup of tea, tap 1 glass icon. Note that when you sync MyNetDiary with iOS Health app, the total volume of water comes from the sum of all filled water glass icons.

Hydration for Physical Activity

The ideal approach is to be well hydrated before activity, minimize dehydration during activity, and fully rehydrate after activity. The fluid intake guidelines described above are appropriate for activities of daily living, but could easily be too little for those of you who exercise, especially in high heat or at high altitudes. If this describes your situation, then please read Mary Nadelen's " Preparing for and Playing in the Heat", published in the "American College of Sports Medicine's Fit Society" newsletter. Ms. Nadelen, a certified trainer, recommends that athletes consume about 7 - 10 oz of water about 20 minutes before exercise, 8 - 10 oz of water every 15 minutes during exercise, and within 2 hours after practice, consume 24 oz of water for every pound of weight lost during activity. Weighing yourself "dry" before and after activity allows you to estimate your sweat loss and fluid intake needs to replenish those losses.

For activities that last less than 90 minutes, barring extreme sweat loss, drinking water is the best way to insure adequate hydration before, during, and after activity. Replacement of salt will occur naturally when you eat your next meal or meals unless you are on a sodium or salt-restricted diet. Please ask your doctor for help if you sweat excessively during exercise and are on a low sodium diet.

For endurance activities (continuous aerobic activity that lasts 90 minutes or longer), an electrolyte replacement beverage is often preferred over water. These beverages typically contain a low concentration of salt and glucose, so that the athlete can hydrate, replace electrolytes, and delay onset of muscular fatigue during activity. Use of these beverages minimizes dehydration and electrolyte loss during activity; they do not fully replenish losses. If you are training for an endurance event and are new to the event or unused to the climate in which you will be competing, then I strongly recommend that you seek guidance from a sports medicine clinic on proper hydration and diet while you are still training (that is, well before the event).

Signs That You Might Be Dehydrated

If you are dehydrated, then start drinking! You might find it easier to tolerate drinking smaller volumes of water every 15 minutes or so, rather than trying to guzzle a huge volume of water all at once. For large sweat losses, you will want to replace both fluid and salt.

Of course, if you have any concerns regarding possible heat illness (high body temperature related to dehydration), then please get medical help right away!

Good luck! Please remember that you can ask questions about this topic in the Community Forum.

Last Updated on May 15, 2018

May 15, 2018
Katherine Isacks
Katherine Isacks, MPS, RDN, CDE - Registered Dietitian and Certified Diabetes Educator (CDE)

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