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, as listed in  Table 9.3.

Water Content of Foods: Sources of Water in the Diet

Water comes from both beverages and foods. Approximately 20–30% of daily water intake is supplied by foods, particularly fruits and vegetables. Many fruits and vegetables contain 80–95% water by weight, making them important contributors to hydration. In fact, some fruits, such as watermelon, contain a higher percentage of water than beverages such as milk.  Refer to Table 9.4 for water content ranges.

Foods with higher water content can contribute substantially to hydration while also providing vitamins, minerals, and fiber.

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 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—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 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, underscoring why hydration is about more than just drinking water and why electrolyte balance matters for health and performance across the lifespan.

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 maintain fluid balance in your body and is essential for sending nerve signals and allowing muscles, including your heart, to contract. The issue isn’t sodium itself—it’s getting far more than the body needs.

Food Sources of Sodium

Although sodium occurs naturally in many foods, most dietary sodium comes from processed, packaged, and restaurant foods rather than from salt added at the table.

Manufacturers use sodium to enhance flavor, improve texture, and extend shelf life. As a result, foods such as breads, cereals, sauces, soups, and condiments can contribute substantial amounts of sodium even when they do not taste salty.

Current dietary guidelines recommend limiting sodium intake to less than 2,300 mg per day. However, the average American consumes approximately 3,400–3,500 mg of sodium daily, largely due to the widespread use of sodium in processed foods.^[5]

Health Effects of Too Much or Too Little Sodium

Sodium deficiency is uncommon because sodium is widely available in the food supply. Excess sodium intake is a much greater concern. Consuming too much sodium can increase the risk of hypertension (high blood pressure), although individuals vary in their response. People who are salt-sensitive experience larger increases in blood pressure when consuming a high-sodium diet. Salt sensitivity is more common among older adults, African Americans, women, and individuals with overweight or obesity. ^[6]

One of the most effective dietary strategies for lowering blood pressure is the Dietary Approaches to Stop Hypertension (DASH) eating pattern. DASH emphasizes fruits, vegetables, whole grains, low-fat dairy products, beans, nuts, fish, and poultry while limiting foods high in saturated fat, added sugars, and sodium. Research shows that DASH can lower blood pressure and reduce cardiovascular disease risk. The plan recommends limiting sodium intake to less than 2,300 mg per day, with additional benefits often seen at 1,500 mg per day. ^[7]

Although sodium is essential for life, most Americans consume more than recommended. Choosing more whole foods, limiting heavily processed foods, and paying attention to Nutrition Facts labels can help reduce sodium intake and support long-term cardiovascular health.

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, helping 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.

Use the table below as a quick reference to review the major functions, food sources, and problems associated with inadequate or excessive amounts of water, sodium, potassium, or chloride.

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.