Food Sources

If we drank only water and no other liquids, we could meet our body’s need for fluid. Most of us, however, consume fluids in addition to water throughout the day. Some fluids also contain other nutrients. Consider the wealth of nutrients found in milk (skim or whole), fruit juices, and soups. Some fruits and vegetables contain as much as 85% to 95% water. Watermelon, grapes, oranges, lettuce, tomatoes, and zucchini have high water content. Most foods contain water, but some are better sources of fluids than others. Generally, we depend on beverages as our main source of fluids.

The Adequate Intake (AI) recommendations for water are about 13 cups a day for men and 9 cups a day for women. This amount is in addition to fluids from foods consumed throughout the day, such as fruits and vegetables.¹ Although the minimum amount needed by healthy adults may be about 4 cups, higher amounts are optimum considering an individual’s physiologic status and energy output.

Our primary source of water should be the liquids we drink (Table 8-1). Notice that coffee, tea, alcohol, and soft drinks are not listed as primary sources. Although they do contain water, coffee, tea, and alcohol act as diuretics, which cause an increase in water loss via the kidneys as urine. Soft drinks add fluid to the body, but they contain solutes (sugar, salt, various chemicals) that must be diluted as they enter the bloodstream. Drinking a soda increases the concentration of these solutes in the blood. The body responds by pulling fluid from the cells into the bloodstream to dilute the sugar and salt. The body loses the increased fluid in the bloodstream when it is excreted as urine. In addition, the body responds to the increased solutes and decreased fluid content by once again triggering the thirst mechanism.

Bottled water has become a mainstay in U.S. beverage selections and an economic force. Sales of bottled water have reached to more than $11 billion.² Products range from imported sparkling mineral waters to spring waters to waters treated from nearby reservoirs. Although the price range is equally broad, the common denominator is that Americans enjoy the convenience of water as a beverage when available in portable containers and single portions.

Water Quality

The minerals found naturally in water vary. Hard water refers to water that contains high amounts of minerals such as calcium and magnesium. Drinking this water can provide a significant amount of these nutrients. Non-nutrition-related problems from hard water can develop; mineral deposits may damage appliances and other machinery that interacts with water, and soap suds are reduced. To reduce these problems, a filtration process can be installed to soften water by replacing some minerals with sodium chloride (salt). Soft water containing sodium, however, can be a problem for sodium-sensitive individuals, such as those at risk for hypertension. To prevent health problems, water softeners may be used on only the hot tap in kitchens, leaving the cold tap unsoftened for consumption.

Another aspect of water quality is contamination. For example, many older buildings have pipes with lead solder joints that can release lead into the water that sits in or runs through them. If the level of lead in water is more than 15 parts per billion (ppb), pregnant women, infants, and children are advised to drink bottled water because even low levels of lead can seriously impair normal development. The local health department can recommend a competent laboratory that tests household water quality.

To reduce the chance of lead leaking into drinking and cooking water, do the following:

Water treatment processes can remedy some contamination concerns. Others, such as industrial pollution, can be difficult to identify. Complications of bacterial contamination or inadvertent exposure of water to carcinogenic industrial substances can lead to health problems that range from simple gastroenteritis to cancer. Municipal and regional water processing plants take great care to ensure the safest water supply possible. Some water sources (private wells, surface water, springs, and cisterns), however, may exceed the maximum contaminant levels set by the Environmental Protection Agency (EPA).

The most severe water-related threats such as cholera and typhoid are no longer public health hazards in North America. Other potential industrial and environmental pollutants, however, can enter our water supply and endanger our health. Small suppliers may not have the financial means to improve technologic surveillance.

Poorer countries throughout the world continue to struggle with unsafe water supplies (see the Cultural Considerations box, Who Will Bring Water to the Bolivian Poor?). Without the financial and technologic knowledge and resources, many people become ill by consuming bacteria-contaminated water. Increased incidence of stomach cancer is associated with exposure to Helicobacter pylori, which is sometimes found in contaminated waters.

The structure of water—two hydrogen atoms bonded to one oxygen atom—allows it to provide a base for biochemical reactions in the body and to easily move through the various compartments of cells and body systems. As the basis of body fluids, water can host other substances of different electrical charges and characteristics. Intracellular fluids (within the cell) are composed of water plus concentrations of potassium and phosphates. Interstitial fluids (between the cells) contain concentrations of sodium and chloride. Extracellular fluids include interstitial fluid and encompass all fluids outside cells, including plasma and the watery components of body organs and substances (Box 8-1).

Regulation of Fluid and Water in the Body

Our bodies have delicate but efficient mechanisms for maintaining appropriate fluid levels. The intake of fluids is balanced with the output through urine, sweat, feces, and insensible perspiration (Figure 8-1). Regulation of fluid in the body is of physiologic importance because water makes up 50% to 60% of the weight of an average adult; the percentages are even higher for infants, whose body weight is 75% to 80% water (Figure 8-2). Fortunately, all we need to do is take in enough fluids and our bodies’ natural systems take care of the rest.

Homeostasis (physiological equilibrium) is maintained by electrolytes that include minerals and blood proteins. Two of the most important minerals are sodium and potassium. The extracellular distribution of fluid depends on sodium, and potassium influences intracellular water. Water moves within and between the cells in interstitial fluids in response to the levels of these minerals. An imbalance is corrected by mechanisms that cause thirst and regulate the ability of the kidneys to release or retain fluids.

Thirst, a dryness in the mouth, stimulates the desire to drink liquids. We often ignore our thirst until mealtimes. The thirst mechanism is controlled by the hypothalamus and involves several steps. The sodium and solute levels in blood increase as the water level in the body gets low. This causes water to be drawn from the salivary glands to provide more fluid for the blood. The mouth then feels dry because less saliva, which keeps the mouth moist, is produced. This sensation, thirst, stimulates the drinking process. If the thirst mechanism is faulty, as it may be during illness, physical exertion, or aging, hormonal mechanisms also help conserve water by reducing urine output.

The mechanisms of the kidneys regulate the amounts of water excreted. Obligatory water excretion of at least 500 mL (1 pint) must be excreted daily, regardless of the amount ingested, to clear the body of waste products. The mechanism relies on the combined actions of the brain, kidneys, pituitary gland, and adrenal gland. When fluid in the body becomes low, the hypothalamus stimulates the pituitary gland to release antidiuretic hormone (ADH). ADH is secreted in response to high sodium levels in the body or too low blood pressure or blood volume. The target organ of the hormone is the kidney. The kidneys then conserve water by decreasing excretion of water, and the retained fluid is recycled for use throughout the body.

When the sodium concentration in the kidneys gets high (too much fluid excreted), another process kicks in to counteract the lowered blood volume and pressure. The kidneys release renin, an enzyme that activates the blood protein angiotensin. Angiotensin raises blood pressure by narrowing blood vessels; it is a vasoconstrictor. Angiotensin also prompts the adrenal gland to release the hormone aldosterone. The target organ of aldosterone is the kidney. The effect is to decrease excretion of sodium, causing the kidneys to respond by retaining fluid in the body.

Fluid and Electrolytes

Dissolved in body fluids are minerals and other organic molecules required for the regulation of both intracellular and extracellular fluid distribution. Fluids follow salt concentrations; this means that cells can control fluid balance by directing the movement of mineral salts.

Electrolytes are minerals that carry electrical charges or ions (particles) when dissolved in water. These minerals separate into positively charged ions (cations) or negatively charged ions (anions). The primary extracellular electrolytes in body fluids are sodium (Na⁺/cation) and chloride (Cl⁺/anion), and the primary intracellular electrolyte is potassium (K⁺/cation). To maintain fluid balance, cells control the movement of electrolytes. Water will follow sodium concentration. Moving electrolytes in and out of the cell membrane requires transport proteins. The sodium/potassium pump is a transport protein that works to exchange sodium from within the cells for potassium. Other ions are also exchanged.

In addition to water regulation, the kidneys also regulate electrolyte levels. If body levels of sodium are low, aldosterone directs the kidneys to reabsorb or retain more sodium. This in turn results in potassium being excreted so the balance of electrolytes is maintained.

Mineral Categories

Based on the amount of each mineral in the composition of our bodies, the 16 essential minerals are divided into two categories: major and trace minerals. To maintain body levels of major minerals, these minerals are needed daily from dietary sources in amounts of 100 mg or higher. In contrast, trace minerals are required daily in amounts less than or equal to 20 mg (Box 8-4). Although the required amounts differ greatly between the major and trace minerals, each is absolutely necessary for good health.

The Dietary Reference Intakes (DRIs) listed in this chapter for minerals are those for young adults ages 19 to 24.¹ Levels for other groups are noted when special mention is needed. Keep in mind that because nutrition is a relatively young science, new functions of minerals as nutrients in the human body are still being discovered.

Food Sources

The prime sources of minerals include both plant and animal foods. Valuable sources of plant foods include most fruits, vegetables, legumes, and whole grains. Animal sources consist of beef, chicken, eggs, fish, and milk products. The discussions of individual minerals highlight the best food choices (Box 8-5).

BOX 8-5   MyPlate

Vegetables

As noted in Chapter 7 (Vitamins), the health benefits of eating vegetables overlap with those of eating fruits. Both fruit and vegetable MyPlate categories provide rich sources of minerals and are valuable components of an overall healthy diet providing nutrients essential for the health and maintenance of our bodies (also see Box 7-2). Health benefits of eating vegetables and fruits as part of an overall health diet include reduced risk for stroke, coronary artery disease, and type 2 diabetes mellitus; protection against some cancers (mouth, stomach, colorectal); and, as an excellent source of fiber, may decrease risk of several chronic diet-related disorders. The recommendation is to eat at least cups of vegetables every day.

What Counts as a Cup?*

The focus of this MyPlate box is on portions of the vegetables group.

In general, 1 cup of raw or cooked vegetables or vegetable juice, or 2 cups of raw leafy greens can be considered as 1 cup from the vegetable group. The following lists specific amounts count as 1 cup of vegetables (in some cases equivalents for cup are also shown) toward your recommended intake.

VEGETABLE	AMOUNT THAT COUNTS AS 1 CUP OF VEGETABLES	AMOUNT THAT COUNTS AS CUP OF VEGETABLES
Dark Green Vegetables
Broccoli	1 cup, chopped or florets
	3 spears, 5 inches long, raw or cooked
Greens (collards, mustard greens, turnip greens, kale)	1 cup cooked
Spinach	1 cup cooked
	2 cups raw = 1 cup	1 cup raw = cup
Raw, leafy greens: spinach, romaine, watercress, dark green leafy lettuce, endive, escarole	2 cups raw = 1 cup	1 cup raw = cup
Orange Vegetables
Carrots	1 cup; strips, slices, or chopped; raw or cooked	1 medium Approximately 6 baby carrots
	2 medium
	1 cup baby carrots (approximately 12)
Sweet potato	1 large, baked ( inches or more in diameter)
	1 cup sliced or mashed, cooked
Winter squash (acorn, butternut, hubbard)	1 cup cubed, cooked	acorn squash, baked = cup
Dry Beans and Peas
Dry beans and peas (such as black, garbanzo, kidney, pinto, soybeans, black-eyed, split peas)	1 cup whole or mashed, cooked
Tofu	1 cup -inch cubes (approximately 8 oz)	1 piece, inches × inches × 1 inch (approximately 4 oz)
Starchy Vegetables
Corn, yellow or white	1 cup	1 small ear (approximately 6 inches long)
	1 large ear (8-9 inches long)
Green peas	1 cup
White potatoes	1 cup diced, mashed
	1 medium boiled or baked potato ( to 3 inches in diameter)
	French-fried: 20 medium to long strips ( to 4 inches long) (contains discretionary calories)
Other Vegetables
Bean sprouts	1 cup cooked
Cabbage, green	1 cup, chopped or shredded, raw or cooked
Cauliflower	1 cup, pieces or florets, raw or cooked
Celery	1 cup, diced or sliced, raw or cooked
	2 large stalks (11-12 inches long)	1 large stalk (11-12 inches long)
Cucumbers	1 cup raw, sliced or chopped
Green or wax beans	1 cup cooked
Green or red peppers	1 cup chopped, raw, or cooked	1 small pepper
	1 large pepper (3 inches in diameter, inches long)
Lettuce, iceberg or head	2 cups raw, shredded or chopped = 1 cup	1 cup raw, shredded or chopped = cup
Mushrooms	1 cup raw or cooked
Onions	1 cup chopped, raw or cooked
Tomatoes	1 large raw, whole (3 inches)	1 small raw, whole ( inches)
	1 cup chopped or sliced; raw, canned, or cooked	1 medium, canned
Tomato or mixed vegetable juice	1 cup	cup
Summer squash or zucchini	1 cup cooked, sliced or diced

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^*Accessed June 14, 2012, from www.choosemyplate.gov/food-groups/vegetables-counts.html.

In contrast to vitamins, minerals are stable when foods containing them are cooked. As inorganic substances, they are indestructible. Minerals may leach into cooking fluids but are still able to be absorbed if the fluid is consumed.

Although plants may contain an abundance of various minerals, some minerals in plants are not easily available to the human body. Bioavailability refers to the level of absorption of a consumed nutrient and is of nutritional concern. Binders such as phytic and oxalic acids may bind some minerals to the plant fiber structures. Binders are substances in plant foods that combine with minerals to form indigestible compounds, making them unavailable for our use. The amount of plant minerals available for absorption may depend on minerals in soils in which the plants are grown.

Minerals from animal foods do not have the same bioavailability issues. In fact, minerals from animal foods can be absorbed more easily than those from plants. However, fat content may be an issue for some animal foods. Lower fat sources of dairy and meat products are usually available and provide the same levels of minerals at a higher nutrient density. Liver is often cited as a good source of minerals, such as iron and zinc. But liver is also high in cholesterol and saturated fats and may contain toxins to which the animal may have been exposed. These factors, combined with liver’s somewhat unusual taste, often leaves the impression that good nutrient intake depends on eating healthy food that tastes bad. Other sources of each nutrient may be more appealing and equally as nutritious.

Food processing may reduce the amount of minerals available for absorption. Processing oranges into orange juice does not affect potassium levels naturally contained in oranges. However, processing whole-wheat flour into white flour does cause significant loss of minerals because the whole grain is not used. Iron is the only mineral returned to white flour through enrichment; zinc, selenium, copper, and other minerals are permanently lost.

Because we have difficulty obtaining high enough levels of some minerals naturally, fortification of manufactured foods has become commonplace. It is in this manner that food processing can serve the nutrient needs of consumers while still addressing the issues of convenience and taste appeal. Salt fortified with iodine is available; dry cereals have added minerals such as iron and an assortment of vitamins and other minerals.

Minerals are inorganic substances. As elements, they are found in the rocks of the earth. Their tendency to gain or lose electrons makes them electrically charged. Thus they have special affinities for water, which itself carries positive and negative charges. As we consume plant and animal foods containing minerals, we can incorporate them into our body structures (bones), organs, and fluids.

Regulation

Our dietary intake of calcium influences the deposition of calcium in our bones. Blood calcium levels, however, do not depend on a daily dietary calcium intake. Instead the skeletal supply of calcium provides the source of calcium to be distributed throughout the body through the circulatory system. If calcium blood levels get too low, three actions can occur to reestablish calcium homeostasis: bones release calcium, intestines absorb more calcium, and kidneys retain more calcium.

Hormones that regulate the level of calcium in body fluids control the release of calcium from bones. Hormones affecting blood levels include parathormone (parathyroid hormone), calcitriol (active vitamin D hormone), and calcitonin. Parathormone is secreted by the parathyroid gland in response to low blood calcium levels. It raises blood calcium levels by stimulating all three ways of providing calcium to body fluids. Vitamin D has a hormone-like effect as calcitriol and increases blood calcium levels by acting on all three systems. The third hormone involved, calcitonin, is released by the Special C cells of the thyroid gland. Calcitonin reacts in response to high blood levels of calcium by lowering both calcium and phosphate in the blood.

Reactions of very low or extremely high blood levels can occur if regulatory mechanisms are hindered by a lack of vitamin D or hormone malfunction. If calcium blood levels get too high, calcium rigor (with symptoms of hardness or stiffness of muscles) may occur. Conversely, if levels are too low, a person may experience calcium tetany, with spasms caused by muscle and nerve excitability.