Nutrition 330 Introductory Nutrition

Study Guide: Unit 10

Water and the Major Electrolytes

Often overlooked as an essential nutrient, water is the most indispensable and abundant component of living cells. A person can survive without food for several weeks but can live without water for no more than a few days. Severe malfunctions can result from loss of body fluids of 10%, and death from loss of 20%. The reason is that water is involved in many biochemical reactions and serves as an important solvent and medium for vital materials.

Maintenance of fluid and electrolyte balance in the body requires proper amounts of sodium, chloride, and potassium, which—along with calcium, phosphorus, sulphur, and magnesium—constitute the major minerals present in the body. The major minerals in humans are defined as those making up 0.01% or more of body weight. One one-hundredth of a percent is not a large amount, yet many minerals—called trace minerals—are present in the body in much smaller concentrations. Calcium is discussed in Unit 11, while iron and zinc (trace minerals) are looked at in Unit 12.

Like vitamins, water and minerals do not yield energy; unlike vitamins, they are inorganic in their chemical composition. Minerals are indestructible, even after being burned to ashes. Minerals are lost in cooking only when leached out into the cooking water.

In this unit, we look at the functions of water in the body and at how the body maintains water balance. We discuss the importance of electrolytes in regulating the distribution, composition, and acidity of body fluids. We provide an overview of the functions of water, deficiency and toxicity symptoms, and major food sources of sodium and potassium.

This unit consists of two sections:

10.1—Water and Electrolytes
10.2—Sodium and Potassium

Objectives

After completing this unit, you should be able to

1. identify the functions of water in the body.
2. describe the causes of fluid and electrolyte imbalance, and describe how acid‑base balance is maintained.
3. discuss the following about sodium and potassium:
   1. Identify their functions in the body.
   2. Describe the symptoms of deficiency and excess intake.
   3. Discuss their relationship to blood pressure.
   4. List some of their major food sources.
   5. Discuss dietary strategies to reduce salt and sodium intake.

10.1 Water and Electrolytes

Introduction

Water comprises about 60% of an adult’s total body weight. The percentage is generally lower in the elderly, in women, and in obese persons; and higher in children and athletes. Among body tissues, muscle cells have the highest water content, while skeletal cells have the lowest. Of the total body water, approximately two‑thirds is in the intracellular fluid and one‑third is in the extracellular fluid, which includes the plasma, lymph, spinal fluid, secretions, and interstitial fluid around the cells.

Objectives

After completing this section, you should be able to

• identify the functions of water in the body.
• describe the causes of fluid and electrolyte imbalance, and describe how acid‑base balance is maintained.

Key Terms

After completing section 10.1, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 11: Introduction and “Water and the Body Fluids,” pages 363–368

Water in the Body

The many functions performed by water in the body are summarized in the textbook. The authors indicate that one of these functions is to help form macromolecules—particularly glycogen—and some of the proteins in muscle. Muscle has a much higher water content than does body fat. Therefore, in fasting, water loss is substantial as there is loss of glycogen and lean body mass. Fat storage, however, requires little water, which is why obese individuals have less total body water in proportion to their weight than do muscular athletes, and have more difficulty in cooling themselves during hot weather or when they have a fever.

Water helps to soften fecal matter when it is absorbed by dietary fibre; it thereby increases fecal bulk and contributes to easy elimination.

Under normal conditions, the loss of water through the kidneys, lungs, feces, and skin is balanced by the intake of water from liquids and food and by metabolic water produced during energy metabolism (water balance). Since there is no water storage per se in the human body, water losses must be replaced daily. The regulation of water intake and excretion is related directly to electrolyte balance in body fluids. Fluid balance refers to the balance between water inside and outside body cells, but water balance and fluid balance are often used interchangeably.

Reading Assignment

• Chapter 11: “Fluid and Electrolyte Balance,” “Fluid and Electrolyte Imbalance,” and “Acid‑Base Balance,” pages 368–373

Electrolyte Balance

Electrolytes are charged ions produced when a salt, acid, or base dissociates in water. These charged particles are capable of conducting an electric current. Cations are positively charged ions; anions are negatively charged ions. In any electrolyte solution, the number of cations is balanced by the number of anions, so that electroneutrality is maintained. Sodium is the major cation and chloride the major anion in the extracellular fluid. In the intracellular fluid, the major cations are potassium and magnesium, and the major anions are phosphate and proteins.

Besides fluid balance, electrolytes serve as buffers in the maintenance of acid‑base balance in body fluids. The pH of the body fluids is closely regulated within a narrow range, as shown in Figure 11‑8 on page 372. This regulation is particularly evident in the case of blood: blood pH values below 7.35 indicate acidosis; those above 7.45, alkalosis. Life can be maintained only within blood pH values between 7.0 and 7.8. The pH of body secretions and excretions are more variable. Gastric juices can be as low as pH 2.0, and pancreatic juices as high as pH 8.2. Proteins, bicarbonate, and phosphate are the body’s major buffering systems.

As we saw in Unit 6, amino acids in proteins have a unique structure, containing both acidic and basic groups. Proteins are plentiful and can serve as an important buffer both in the cells and in the plasma. In addition to proteins, bicarbonate, and phosphate—the three major buffers—the lungs, skin, GI tract, and kidneys all help to regulate the body’s acid‑base balance. Although slow, the kidneys are the most important organs for maintaining the acid‑base balance. They act by selectively altering the rate of excretion of various ions: hydrogen, sodium or potassium, and bicarbonate.

Study Questions

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10.2 Sodium and Potassium

Introduction

When we talk about electrolytes in body fluids, the three major minerals involved are sodium, chloride, and potassium. They are provided in the diet and are readily absorbed by the small intestine into the blood. They are filtered by the kidneys, and then either reabsorbed or excreted in the urine. Approximately 98% of ingested sodium and chloride and about 85% of ingested potassium appear in the urine. The rest is lost in the feces, and a small amount of sodium and chloride is lost in sweat.

The relationship of sodium to high blood pressure has been investigated for many years. An inverse relationship exists between potassium and high blood pressure.

Objectives

After completing this section, you should be able to

• identify the functions of sodium and potassium in the body.
• Describe the symptoms of deficiency and excess intake of sodium and potassium.
• Discuss the relationship of sodium and potassium to blood pressure.
• List some of their major food sources of sodium and potassium.
• Discuss dietary strategies to reduce salt and sodium intake.

Reading Assignment

• Chapter 11: “Sodium” and “Chloride,” pages 373–376

Sodium

Functions

Sodium is by far the most abundant cation in the extracellular fluid where it is involved in maintaining fluid balance and acid‑base balance. It also functions in conducting nerve impulses and in control of muscle contraction.

Deficiency

Since sodium is abundant in the food supply and humans tend to consume more than is required, dietary deficiency of sodium is unlikely. As discussed on pages 375–376 of the text, a low blood‑sodium level or low blood volume can result from excessive sweating in a hot climate (heatstroke) or as a result of heavy exercise, vomiting, diarrhea, burns, untreated diabetes, hemorrhages, overuse of diuretics for heart failure or kidney disease, use of very low sodium diets, or adrenocortical insufficiency—for example, in Addison’s disease.

Symptoms of sodium deficiency are loss of body weight, muscle cramps, weakness, anorexia, mental apathy, and coma.

Toxicity

Very high intakes of sodium can be toxic, causing internal hemorrhages, vomiting, peripheral circulatory failure, respiratory depression, and death. Toxicity does not occur in the normal ranges of dietary intake, but only in accidental situations; for example, when a high concentration of salt is given in infant formula instead of sugar.

Sodium and Blood Pressure

Hypertension, one of the most common chronic diseases in industrialized countries, is a major factor in coronary heart disease, stroke, and renal failure. Evidence indicates that excess sodium intake, in the form of sodium chloride, is a major cause. It is thought that salt‑sensitive or sodium‑sensitive people with a high salt intake are particularly prone to develop hypertension.

Sources

The main dietary source of sodium (95%) is common salt (sodium chloride). Other forms are monosodium glutamate (MSG) and sodium bicarbonate (baking soda). Common salt is 40% sodium and 60% chloride. The average intake of salt in Western societies is about 6–10 grams (2.4–4 grams of sodium) daily. As much as 75% of this amount is added during processing. Foods of animal origin, such as meat and milk, have moderate amounts of sodium. Unprocessed fruit, vegetables, and cereals are low in sodium.

Although the requirement for sodium and other electrolytes has not been well established, estimates of minimum sodium requirements for humans have been as low as 115 mg per day for a sedentary adult living in a temperate climate. As you can see, our usual consumption of sodium exceeds the minimum requirement more than forty‑fold. In view of the potential benefits of a decreased salt intake (and no evidence of possible risks of deficiency), dieticians and other health professionals consistently recommend that people try to limit their salt intake. Canada’s Food Guide recommends choosing vegetables, fruit, grain products, meats, and alternatives that are prepared with little or no salt. Figure 11‑10 on page 376 clearly illustrates the effect of food processing on the sodium and potassium content of foods. The “how to” on page 375 discusses strategies to reduce salt and sodium intake.

Recommended Intake

The recommended intake for sodium is stated as an AI (acceptable intake) of 1500 mg per day. The UL (upper limit) is 2300 mg per day which is contained in approximately 1 teaspoon of table salt.

Reading Assignment

• Chapter 11: “Potassium,” pages 377–379

Potassium

Functions

Potassium is the major cation in the intracellular fluid. As an electrolyte, it helps to maintain fluid and acid‑base balance. Like sodium, it is important for neuromuscular activity, including that of the heart muscle. It also promotes cellular growth in protein synthesis and acts as a biological catalyst in energy metabolism.

Deficiency

Dietary deficiency of potassium is uncommon in normal diets, because potassium is widely distributed in food. However, deficiency may occur if one relies heavily on highly processed and convenience foods without fresh fruit and vegetables, or if one is on a very‑low‑calorie diet with limited food choices. Deficiency usually results from abnormal conditions that cause excessive loss of body fluids, such as vomiting, diarrhea, diuresis (production of abnormally large amounts of urine, such as in diabetic acidosis), and prolonged malnutrition. Symptoms of deficiency are muscular weakness, mental apathy, and cardiac arrhythmia or failure.

Toxicity

If injected into blood, potassium can stop the heart from beating. Special care must be exercised when using a salt substitute containing potassium chloride, especially by people with kidney failure. However, a high intake of potassium from foods does not cause toxicity because of its slow absorption through the intestine. The kidneys can normally control blood levels of potassium by increasing urinary excretion. If necessary, the body can also trigger vomiting to protect itself from rapid accumulation. Symptoms of potassium toxicity include mental confusion, numbness of extremities, poor respiration, weakening of heart action, and vomiting.

Sources and Recommended Intake

The average daily intake of potassium in the Canadian diet is about 2.8 grams. A minimum requirement is about 500–600 mg for a sedentary adult living in a temperate climate. However, the DRI recommends an intake of 4700 mg per day, which can be quite easily obtained from a variety of foods selected according to Canada’s Food Guide. Highly processed foods, such as cured meats, frozen dinners, ready‑to‑eat cereals, crackers, and baked pies and pastries have far more sodium and less potassium than do fresh fruits and vegetables and legumes. See Figure 11‑11 on page 378 for some major food sources.

Sodium, Potassium, and Hypertension

Sodium and potassium are closely associated with each other but are also opposites. The sodium intake of the great majority of the population is excessive and is closely associated with above normal blood pressure and hypertension. In stark contrast, the potassium intake of most people is low, and this is also associated with above normal blood pressure and hypertension. In both cases, the root of the problem is clear: the typical diet eaten across the Western world has an excessive intake of salt (which is mostly due to salt added to processed food), while an inadequate intake of unprocessed whole foods, such as fruit, vegetables, and whole grains, results in a poor intake of potassium. Another common feature of sodium and potassium is as follows: the high intake of sodium cannot be called “toxicity” in the usual meaning of the word, while, similarly, the low intake of potassium cannot be called “deficiency” in the usual meaning of the word.

Study Questions

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