Water: The Most Essential Nutrient

Water is the most abundant substance on Earth, covering about 70% of the planet’s surface,^[1] and it is also the major component of the human body. Body water content varies across the life span—accounting for roughly 75% of body weight in infants, 50–60% in adults, and as little as 45% in older adults.^[2] These changes occur because the proportions of muscle, fat, bone, and other tissues shift from infancy through adulthood and into old age.

Among all nutrients, water is the most critical for survival. Unlike energy-yielding nutrients, which can be stored in the body, water must be consumed regularly. Without adequate water intake, survival is limited to only a few days.

Water performs many essential roles in the body, which can be broadly grouped into four major functions:

• Transportation vehicle – carries nutrients, oxygen, hormones, and waste products throughout the body

• Medium for chemical reactions – provides the environment in which metabolic reactions occur

• Lubricant and shock absorber – cushions joints, organs, and tissues, including the brain and spinal cord

• Temperature regulator – helps maintain normal body temperature through sweating and evaporation

Water Content of Foods: Sources of Water in the Diet

Water needs are met not only through beverages but also through the foods we eat. In fact, approximately 20–30% of total daily water intake typically comes from foods, while the remainder comes from drinking water and other beverages. Foods vary widely in their water content, depending on their structure, processing, and nutrient composition.

Foods High in Water

Fruits and vegetables are the richest dietary sources of water, often containing 80–95% water by weight. These foods contribute significantly to hydration while also supplying essential vitamins, minerals, fiber, and phytochemicals.

Examples include:

• Fruits: watermelon, strawberries, oranges, peaches, grapes

• Vegetables: lettuce, cucumbers, tomatoes, zucchini, bell peppers

Because of their high water content and low energy density, fruits and vegetables help support hydration while promoting fullness and healthy weight management.

Foods with Moderate Water Content

Some foods provide moderate amounts of water and contribute meaningfully to total fluid intake:

• Milk and yogurt (~85–90% water)

• Soups and broths (often >90% water, depending on sodium content)

• Cooked grains and legumes, such as oatmeal, rice, and beans, which absorb water during cooking

These foods can be especially helpful for hydration in individuals who struggle to consume adequate fluids through beverages alone.

Foods Low in Water

Foods that are high in fat or dry by nature tend to contain much less water, often below 40%. Examples include:

• Crackers, chips, and baked goods

• Nuts and seeds

• Cheese and cured meats

Although these foods can be part of a balanced diet, they contribute relatively little to hydration and may increase thirst due to their sodium content.

Beverages as Water Sources

Plain drinking water is the primary and most efficient source of hydration, but other beverages—such as milk, tea, coffee, and 100% fruit juice—also contribute to total water intake. While caffeinated beverages were once thought to be dehydrating, current evidence shows that they can still contribute to fluid intake when consumed in moderation.

Meeting daily fluid needs involves a combination of beverages and water-rich foods. Emphasizing fruits, vegetables, soups, and dairy products—along with regular water intake—supports hydration and overall nutritional quality across the life span

Water Balance and Electrolytes

Although water makes up the largest proportion of body volume, it is not present as pure water. Instead, body fluids contain dissolved substances called solutes, many of which are essential for life. Key solutes include electrolytes such as sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻). These electrolytes help regulate fluid balance, support nerve impulse transmission, enable muscle contraction, and maintain acid–base balance.

This is why beverages like sports drinks (e.g., Gatorade^[3] ) contain sodium and potassium in addition to water. During prolonged or intense exercise, especially when sweating heavily, the body loses not only water but also electrolytes—particularly sodium. Replacing both water and electrolytes can help maintain hydration, prevent muscle cramps, and support athletic performance. However, for most everyday activities, plain water is sufficient, and sports drinks are generally unnecessary unless physical activity is long-lasting or very intense.

Together, water and electrolytes function as a tightly regulated system that supports nearly every process in the body, highlighting why hydration is about more than just drinking water and why electrolyte balance matters for health and performance across the life span.

Sodium: Essential for Life, Easy to Overdo

Sodium often gets a bad reputation because of its link to high blood pressure, but it’s actually a nutrient your body can’t survive without. Sodium helps keep fluids balanced in your body and is essential for sending nerve signals and allowing muscles to contract, including your heart. The issue isn’t sodium itself—it’s getting far more than the body needs.

Food Sources of Sodium

Sodium occurs naturally in many whole foods, but the majority of sodium in the typical American diet comes from processed, packaged, and restaurant foods, not from salt added at the table. Food manufacturers add sodium to enhance flavor, improve texture, and extend shelf life. As a result, even foods that may not taste salty—such as breakfast cereals, breads, sauces, and condiments—can provide 10% or more of the Daily Value (DV) for sodium per serving.

Current dietary guidance recommends limiting sodium intake to less than 2,300 milligrams per day for adults, which aligns with the Tolerable Upper Intake Level (UL) and reflects evidence linking high sodium intake to elevated blood pressure and increased cardiovascular risk. Despite this recommendation, most Americans consume far more sodium than advised, with average intakes close to 3,400–3,500 milligrams per day. ^[4]

Because most dietary sodium comes from foods prepared outside the home or from packaged products, reducing sodium intake often requires choosing fewer processed foods, reading nutrition labels, and selecting lower-sodium options. The major sources of sodium in the American diet are shown below.

Sodium Deficiency and Toxicity

Deficiencies of sodium are extremely rare since sodium is so prevalent in the American diet. There is too much sodium, which is the main concern. High dietary intake of sodium is one risk factor for hypertension, or high blood pressure. For many people with hypertension, cutting salt intake can help reduce their blood pressure. However, studies have shown that this isn’t always the case. According to Harvard Medical School, “About 60% of people with high blood pressure are thought to be salt-sensitive — [a trait that means your blood pressure increases with a high-sodium diet]. So are about a quarter of people with normal blood pressure, although they may develop high blood pressure later, since salt sensitivity increases with age and weight gain.” ^[5] Genetics, race, sex, weight, and physical activity level are determinants of salt sensitivity. African Americans, women, and overweight individuals are more salt-sensitive than others.

The Dietary Approaches to Stop Hypertension (DASH) eating pattern is one of the most well-studied diets for heart health. In randomized controlled trials, DASH has been shown to lower blood pressure and LDL (“bad”) cholesterol, thereby reducing the risk of cardiovascular disease. Rather than focusing on strict rules, DASH emphasizes overall eating patterns—encouraging plenty of vegetables, fruits, and whole grains, along with fat-free or low-fat dairy products, fish, poultry, beans, nuts, and vegetable oils. Together, these foods provide key nutrients, such as potassium, calcium, magnesium, fiber, and protein, which support healthy blood pressure regulation.

At the same time, DASH recommends limiting foods high in saturated fat, including fatty meats, full-fat dairy products, and tropical oils like coconut and palm oil, as well as sugar-sweetened beverages and sweets. When it comes to sodium, the DASH plan suggests keeping intake below 2,300 milligrams per day, noting that further reduction to 1,500 milligrams per day can lead to even greater blood pressure–lowering benefits.^[6]

Although the updated Dietary Reference Intakes (DRIs) for sodium no longer include a formal Tolerable Upper Intake Level (UL), the Adequate Intake (AI) values now take chronic disease risk into account. Strong evidence shows that lowering sodium intake helps reduce blood pressure, which in turn lowers the risk of cardiovascular disease—especially for individuals who are salt-sensitive.

Potassium: Your Heart’s Quiet Helper

Potassium (K) is everywhere in your body—and for good reason. It is the most abundant positively charged electrolyte inside your cells, where it helps keep normal body processes running smoothly. Potassium plays a key role in fluid balance, nerve signal transmission, and muscle contraction, from your biceps during a workout to your heart beating in your chest.

Food Sources of Potassium

Getting enough potassium is less about supplements and more about everyday food choices. Potassium is naturally found in many common foods, especially fresh, minimally processed options. Some of the best sources include fruits and vegetables, dairy products like milk and yogurt, beans and legumes (such as lentils and soybeans), and protein foods like salmon and beef. Building meals around these foods not only boosts potassium intake but also supports overall diet quality.

Potassium is a nutrient of concern, based on national dietary intake data showing that most people in the United States consume less potassium than recommended. Adequate potassium intake is important for blood pressure regulation and overall cardiovascular health.

Potassium needs are best met through food rather than supplements. Most potassium supplements available in the United States contain no more than 99 milligrams per serving, while adult potassium needs range from approximately 2,600 milligrams per day for women to 3,400 milligrams per day for men. This large gap highlights why potassium inadequacy must be addressed through potassium-rich foods, such as fruits, vegetables, legumes, dairy products, and whole grains, rather than through supplementation.

Potassium Deficiency and Toxicity

Low potassium intake is linked to several health concerns, including higher blood pressure, kidney stones, lower bone mineral density, and increased risk of type 2 diabetes. Diets low in potassium—especially when paired with high sodium intake—are associated with a greater risk of hypertension, while higher potassium intakes may help lower blood pressure, particularly in salt-sensitive individuals. Although research hasn’t fully established a cause-and-effect relationship, the evidence suggests potassium plays an important role in long-term health.

There is no Upper Intake Level (UL) for potassium from foods because healthy kidneys can excrete excess amounts. However, this does not mean potassium supplements are risk-free. High-dose supplements can be harmful, so potassium needs are best met through food rather than pills.

Chloride: Sodium’s Sidekick

Chloride helps with fluid balance, acid-base balance, and nerve cell transmission. It is also a component of hydrochloric acid, which aids digestion in the stomach.

Table salt is 60% chloride, so most chloride in the diet comes from salt. Each teaspoon of salt contains 3.4 grams of chloride. The chloride AI for adults is 2.3 grams. Therefore, the chloride requirement can be met with less than a teaspoon of salt each day. Other dietary sources of chloride include tomatoes, lettuce, olives, celery, rye, whole-grain foods, and seafood.

Chloride deficiency is rare since most foods containing sodium also provide chloride, and sodium intake in the American diet is high.

Attributions

This section is an adaptation of “Vitamins and Minerals Involved in Fluid and Electrolyte Balance” in Nutrition: Science and Everyday Application, v. 1.0 by Alice Callahan, PhD; Heather Leonard, MEd, RDN; and Tamberly Powell, MS, RDN licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.