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How Foods Work

Food is our nourishment.  We appreciate food for its appetizing qualities like taste, appeal and aroma.  “That looks delicious!” Or we search for food when we’re hungry, “What’s in the fridge?”

Beyond appetite and hunger, this section is about understanding food as our source of necessary nutrients, “Is this food good for me?”  Knowing what makes up food and how food functions in our body, gives us confidence to choose food wisely.

Food nutrients fall into two basic categories:

1. Macronutrients

2. Micronutrients

 Additionally, in both categories there are foods called “functional” that have beneficial effects on health.  According to the American Dietetics Association, functional foods have physiologically active components from both plant (phytochemicals) and animal (zoochemical) sources, that provide a health benefit beyond the traditional nutrients they contain.


“Macro” means large quantity.  Macronutrients are nutrients we need in large amounts in our diet.  They are the carbohydrates, fats, and proteins in plant and animal structures that we need to obtain energy and the building materials for making our own unique body structures.


These energy packed nutrients provide fuel for the body and bulk for proper elimination. The main categories are sugars, starches, and fiber.  Sugar and starches supply energy to the body in the form of glucose [1], which is the only energy source for red blood cells and is the preferred energy source for the brain, central nervous system, placenta, and fetus.

Sugars important to us are monosaccharides and disaccharides.  Monosaccharides (“mono”, meaning one chemical bond) are simple sugars that are easily absorbed by the small intestinal lining. Disaccharides (“di”, meaning two chemical bonds) must first be split into monosaccharides by enzymes of the intestinal villi so that they can be absorbed into the bloodstream.

Of the many naturally occurring monosaccharides, those valuable to us as nutrients are glucose [1], fructose, and galactose. They are the only ones we can absorb.  The disaccharides we can digest and then absorb are sucrose (fructose + glucose [1]), lactose (glucose [1] + galactose), and maltose (glucose [1] + glucose [1]).

Starches are polysaccharides (“poly,” meaning many chemical bonds).  Starches are complex carbohydrates that provide lots of energy compacted in a small quantity of food. Starches are digested or broken down into the disaccharide, maltose, by these enzymes: ptyalin in saliva and amylase in pancreatic juice.  The resulting maltose is broken down into glucose [1] by the enzyme, maltase, that is produced in intestinal villi.  Thus, the end product of digestion of starch is the monosaccharide, glucose [1], for absorption into the bloodstream.

Fiber is composed of nondigestible carbohydrate and lignan found only in plants.  Fiber maintains good bowel health.  It provides bulk, important for moving the bowels, and acts as a prebiotic, stimulating the growth of friendly, necessary bacteria in our colon.  Fiber can be soluble and insoluble.   Soluble fiber develops a mucilagenous texture while insoluble fiber passes through the bowel unchanged.  Fiber is not absorbed into the bloodstream.

The recommended amount of carbohydrates is 45 to 65% of total calories. The recommended dietary intake of fiber is 14 grams per 1,000 calories consumed.

These nutrients supply energy and essential fatty acids and serve as a carrier for the absorption of the fat-soluble vitamins A,D,E, and K and carotenoids. Fats serve as building blocks of membranes and play a key role in numerous biological functions.

Dietary fat is found in foods derived from both plants and animals.  The desirable fats are polyunsaturated and monounsaturated fatty acids.

Polyunsaturated Fatty Acids
There are two categories of essential (must have) polyunsaturated fatty acids (PUFA) called omega-3 and omega-6.  These two types of PUFAs oppose and balance each other in vital body functions necessary to maintain health, especially of the eyes and brain.

  1. The three omega-3 fatty acids are alpha-linolenic acid [2] (ALA), eicosapentaenoic acid [3] (EPA [3]) and docosahexaenoic acid [4] (DHA [4]).  ALA is derived from plants while EPA [3] and DHA [4] are derived from fish and shellfish.
  2. The two omega-6 fatty acids are linoleic acid [5] (LA) and arachidonic acid [6] (AA [6]). LA is derived from plants while AA [6] is derived from meat. AA [6] can be made in the body from sufficeint LA.


Monounsaturated Fatty Acids
These fats are derived from plant sources such as olive oil.

Other dietary fats are saturated fatty acids (butter, animal fat), cholesterol, and trans fat (hydrogenated oil like Crisco).  Nationwide, prepared foods like cakes, cookies, pies, and French fries provide approximately 80% of trans fat from hydrogented plant oils compared to 20% that occur naturally in food from animal sources.  These fats are considered undesirable and should be limited as much as possible.

The recommended total fat intake is between 20 and 35% of calories for adults.  A fat intake of 30 to 35% of calories is recommended for children 2 to 3 years of age and 25 to 35% of calories for children and adolescents 4 to 18 years of age.  Few Americans consume less than 20% of calories from fat.  Fat intakes that exceed 35% are associated with both total increased saturated fat and calorie intakes.

These nutrients are vital to every cell in our body.  They form our basic structures such as muscle, skin, cartilage, ligaments and hair.  Proteins are part of antibodies, hormones, and enzymes.

Proteins are built from 20 different amino acids [7] joined by peptide bonds.  Certain amino acids [7] are called essential.  We must get them from our diet and from these we can make other amino acids [7] as needed.  The essential amino acids [7] are arginine, histidine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, and valine.

A fresh supply is needed every day because our body does not store protein.  However, excessive intake of protein is not good.  The excess  will be broken down to yield energy that is stored as fat and nitrogen waste that must be excreted through the kidneys.  The amount of protein we need is 0.8 g/ kg of body weight, depending on age, sex, level of activity and health.


“Micro” means small quantity.  Micronutrients are nutrients we need in small amounts in our diet.  They are the vitamins and minerals we must have to benefit from the energy and content of carbohydrates, proteins, and fats we eat.
These micronutrients fall into two categories: fat soluble (meaning they dissolve in fat) and water soluble (meaning they dissolve in water).

Fat soluble vitamins are A,D,E and K.  Absorption of these vitamins is greatly impaired by fat malabsorption (seen as foul, puffy, floating stool that is called steatorrhea).

Vitamin A [8] is a family of fat-soluble compounds that play an important role in vision, bone growth, reproduction, cell division, and cell differentiation (in which a cell becomes part of the brain, muscle, lungs, etc.)  Vitamin A [8] helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses . Vitamin A [8] also may help lymphocytes, a type of white blood cell, fight infections more effectively.

Vitamin A [8] promotes healthy surface linings of the eyes and the respiratory, urinary, and intestinal tracts.  When those linings break down, it becomes easier for bacteria to enter the body and cause infection.  Vitamin A [8] also helps maintain the integrity of skin and mucous membranes, which also function as a barrier to bacteria and viruses.

Retinol is one of the most active, or usable, forms of vitamin A [8], and is found in animal foods such as liver and whole milk and in some fortified food products.  Retinol is also called preformed vitamin A [8].  It can be converted to retinal and retinoic acid, other active forms of the vitamin A [8] family.

Provitamin A carotenoids are darkly colored pigments found in plant foods that can be converted to vitamin A [8]. Of the 563 identified carotenoids, fewer than 10% are precursors for vitamin A [8].  Among these, beta-carotene is most efficiently converted to retinol.  Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A [8], but only half as efficiently as beta-carotene.  Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A [8] activity but have other health promoting properties.

Vitamin D [9] is a fat soluble vitamin that is found in food and can also be made in our body after exposure to ultraviolet (UV) rays from the sun.  Sunshine is a significant source of vitamin D [9] because UV rays from sunlight trigger vitamin D [9] synthesis in the skin.

Vitamin D [9] exists in several forms, each with a different level of activity. Calciferol is the most active form of vitamin D [9].  Other forms are relatively inactive in the body. The liver and kidney help convert vitamin D [9] to its active hormone form.   Once vitamin D [9] is produced in the skin or consumed in food, it requires chemical conversion in the liver and kidney to form 1,25 dihydroxyvitamin D, the physiologically active form of vitamin D [9].  Active vitamin D [9] functions as a hormone because it sends a message to the intestines to increase the absorption of calcium [10] and phosphorus [11].

The major biologic function of vitamin D [9] is to maintain normal blood levels of calcium [10] and phosphorus [11].  By promoting calcium [10] absorption, vitamin D [9] helps to form and maintain strong bones.  Vitamin D [9] also works in concert with a number of other vitamins, minerals, and hormones to promote bone mineralization.  Without vitamin D [9], bones can become thin, brittle, or misshapen.  Vitamin D [9] sufficiency prevents rickets in children and osteomalacia in adults, two forms of skeletal diseases that weaken bones.

Research also suggests that vitamin D [9] may help maintain a healthy immune system and help regulate cell growth and differentiation, the process that determines what a cell is to become.

Vitamin E [12] is a fat-soluble vitamin that exists in eight different forms.  Each form has its own biological activity, which is the measure of potency or functional use in the body.  Alpha-tocopherol (α-tocopherol) is the name of the most active form of vitamin E [12] in humans.  It is also a powerful biological antioxidant.  Vitamin E [12] in supplements is usually sold as alpha-tocopheryl acetate, a form that protects its ability to function as an antioxidant. The synthetic form is labeled “L” while the natural form is labeled “D”. The synthetic form is only half as active as the natural form.

Water soluble vitamins include thiamin [13] (B1), riboflavin [14] (B2), niacin [15] (B3), pantothenic acid (B5), pyridoxine [16] (B6), folic acid [17] (B9), vitamin B12 [18] and vitamin C [19].

Thiamin [13] (B1) is needed to convert carbohydrates into energy. It is essential for cardiac and nerve function.  Thiamin [13] must be supplied daily because it can not be stored.  Folate [17] deficiency impairs the absorption of thiamin [13]. The adult RDA for thiamin [13] is 1.5 mg.

Riboflavin [14] (B2) is needed for metabolism of carbohydrates, amino acids [7], and lipids.  Riboflavin [14] provides antioxidant protection.  Because flavoproteins are involved in the metabolism of several other vitamins (vitamin B6 [20]niacin [21], and folic acid [22]), severe riboflavin [14] deficiency may impact many enzyme systems. The adult RDA for riboflavin [14] is 1.2 mg.

Niacin [15] (B3) is essential as a coenzyme in the metabolism of carbohydrates, amino acids [7], and lipids.  Many enzymes require the niacin [15] coenzymes, NAD and NADP.  Niacin [15] is essential for the health of skin, mucous membranes, and the brain and nervous system.  The adult RDA is 15 mg.

Pantothenic acid (B5)
Pantothenic acid is a component of coenzyme A (CoA), an essential coenzyme in a variety of reactions that sustain life. CoA is required for chemical reactions that generate energy from food (fat, carbohydrates, and proteins). The synthesis of essential fats, cholesterol, and steroid hormones requires CoA, as does the synthesis of the neurotransmitters (chemical needed for nerve transmission), acetylcholine, and the hormone, melatonin.  Heme, a component of hemoglobin, requires a CoA-containing compound for its synthesis. Metabolism of a number of drugs and toxins by the liver requires CoA.

Pyridoxine [16] (B6) is a water-soluble vitamin that exists in three major chemical forms: pyridoxine [16], pyridoxal, and pyridoxamine. It performs a wide variety of functions in the body and is essential for good health.  For example, vitamin B6 [16] is needed for more than 100 enzymes involved in protein metabolism. It is also essential for red blood cell metabolism. The nervous and immune systems need vitamin B6 [16] to function efficiently, and it is also needed for the conversion of tryptophan (an amino acid) to niacin [15] (vitamin B3 [15]).

Hemoglobin within red blood cells carries oxygen to tissues.  Our body needs vitamin B6 [16] to make hemoglobin within red blood cells.  Vitamin B6 [16] also helps increase the amount of oxygen carried by hemoglobin.

An immune response is a broad term that describes a variety of biochemical changes that occur in an effort to fight off infections. Calories, protein, vitamins, and minerals are important to our immune defenses because they promote the growth of white blood cells that directly fight infections.  Vitamin B6 [16], through its involvement in protein metabolism and cellular growth, is important to the immune system.  It helps maintain the health of lymphoid organs (thymus, spleen, and lymph nodes) that make our white blood cells.  Animal studies show that a vitamin B6 [16] deficiency can decrease our antibody production and suppress our immune response.

Vitamin B6 [16] also helps maintain our blood glucose [1] (sugar) within a normal range. When caloric intake is low, our body needs vitamin B6 [16] to help convert stored carbohydrate or other nutrients to glucose [1] to maintain normal blood sugar levels.

Folate [17] is a water-soluble B vitamin that occurs naturally in food.  Folic acid [17] is the synthetic form of folate [17] that is found in supplements and added to fortified foods.

Folate [17] gets its name from the Latin word “folium” for leaf. A key observation of researcher Lucy Wills nearly 70 years ago led to the identification of folate [17] as the nutrient needed to prevent the anemia of pregnancy. Dr. Wills demonstrated that the anemia could be corrected by a yeast extract. Folate [17] was identified as the corrective substance in yeast extract in the late 1930s, and was extracted from spinach leaves in 1941.

Folate [17] helps produce and maintain new cells.  This is especially important during periods of rapid cell division and growth such as infancy and pregnancy.  Folate [17] is needed to make DNA and RNA, the building blocks of cells.  It also helps prevent changes to DNA that may lead to cancer. Both adults and children need folate [17] to make normal red blood cells and prevent anemia.  Folate [17] is also essential for the metabolism of homocysteine, and helps maintain normal levels of this amino acid.

Vitamin B12 [18] is also called cobalamin because it contains the metal cobalt.  This vitamin helps maintain healthy nerve cells and red blood cells.  It is also need to help make DNA, the genetic material of all cells. Vitamin B12 [18] is bound to the protein in food.  Hydrochloric acid in the stomach releases B12 [18] from proteins in foods during digestion.  Once released, vitamin B12 [18] combines with a substance called gastric intrinsic factor.  This complex can then be absorbed by the intestinal tract.

Vitamin C [19] is also called ascorbic acid. Vitamin C [19] is required for the synthesis of collagen, an important structural component of blood vessels, tendons, ligaments, and bone. Vitamin C [19] also plays an important role in the synthesis of the neurotransmitter, norepinephrine.  Neurotransmitters are critical to brain function and are known to affect mood.

In addition, vitamin C [19] is required for the synthesis of carnitine, a small molecule that is essential for the transport of fat to cellular organelles called mitochondria, for conversion to energy.  Recent research also suggests that vitamin C [19] is involved in the metabolism of cholesterol to bile acids, which may have implications for blood cholesterol levels and the incidence of gallstones.

Vitamin C [19] is also a highly effective antioxidant. The adult RDA is 60 mg.


Minerals are grouped according to amounts we need each day.  Macrominerals are those we need 100 mg a day or more.  Microminerals (trace) are those we need less than 100 mg a day.  Ultratrace minerals are those found in small quantities in the body

Although there are other minerals important for health, the following are commonly malabsorbed or lost through diarrhea or vomiting in celiac disease and other malabsorption disorders. They include calcium [10], copper [23], iron [24], magnesium [25], potassium [26], phosphorus [11], selenium [27], and zinc [28].

Calcium [10] is a macromineral.  The amount of calcium [10] required daily by children 1-3 years is 500 mg. Children 4-8 years need 800 mg. Males and females 9-18 years need 1300 mg.  Men and women 19-50 years old need 1000mg.   Older males and females need 1200 mg.

Calcium [10] is the most abundant mineral in the human body, has several important functions.  More than 99% of total body calcium [10] is stored in the bones and teeth where it functions to support their structure.  The remaining 1% is found throughout the body in blood, muscle, and the fluid between cells.

Calcium [10] is needed for muscle contraction, blood vessel contraction and expansion, the secretion of hormones and enzymes, and sending messages through the nervous system. A constant level of calcium [10] is maintained in body fluid and tissues so that these vital body processes function efficiently.

Bone undergoes continuous remodeling, with constant resorption (breakdown of bone) and deposition of calcium [10] into newly deposited bone (bone formation).  The balance between bone resorption and deposition changes as people age.  During childhood there is a higher amount of bone formation and less breakdown. In early and middle adulthood, these processes are relatively equal.  In aging adults, particularly among postmenopausal women, bone breakdown exceeds its formation, resulting in bone loss, which increases the risk for osteoporosis (a disorder characterized by porous, weak bones).

Copper [23] is an ultratrace mineral.  RDA for copper [23] has not been established.  Estimated safe and adequate amounts in the diet for children are 0.7 to 2 mg. daily and 1.5 to 3 mg. daily for adolescents and adults. Copper [23] is found mostly concentrated in the liver and certain areas of the central nervous system, particularly the brain.  It is stored in the liver and excreted in bile salts.

Copper [23] is essential for making red blood cells and helps the body absorb iron [24].  As part of ceruloplasmin, it acts as a catalyst in the formation of hemoglobin, the oxygen-carrying blood component. Copper [23] improves use of oxygen in the cells.

Copper [23] is an essential component of many enzymes.  Both copper [23] and zinc [28] are involved in the enzyme called superoxide dismutase (SOD).  Copper [23] is needed for the functioning of the amino acid, tyrosine, and improves function of the brain chemicals, epinephrine, norepinephrine and dopamine.

Copper [23] plays a key role in certain hormones.  It is needed for balancing and secreting thyroid hormones and for making the adrenal hormone adrenaline.

Iron [24] is a micromineral.  The amount of iron [24] required daily by children 1 to 10 years is 10 mg.  Adolescent boys need 12 mg. and girls need 15 mg.  Men 19 – 50 years need 10 mg. and women 19 – 50 years need 15 mg. Older women need 10 mg.  Pregnant women need 30 mg.

Iron [24] is essential to most life forms and to normal human physiology.  Iron [24] is an integral part of many proteins and enzymes that maintain good health.  In humans, iron [24] is an essential component of proteins involved in oxygen transport.  It is also essential for the regulation of cell growth and differentiation.  A deficiency of iron [24] limits oxygen delivery to cells, resulting in fatigue, poor work performance, and decreased immunity. On the other hand, excess amounts of iron [24] can result in toxicity and even death.

Almost two-thirds of iron [24] in the body is found in hemoglobin, the protein in red blood cells that carries oxygen to tissues.  Smaller amounts of iron [24] are found in myoglobin, a protein that helps supply oxygen to muscle, and in enzymes that assist biochemical reactions.  Iron [24] is also found in proteins that store iron [24] for future needs and that transport iron [24] in blood.  Iron [24] stores are regulated by intestinal iron [24] absorption.

Magnesium [25] is a macromineral.  The amount of magnesium [25] required daily by children 1 to 3 years is 80 mg. and  130 mg. for children age 4 to 8 years.  Boys and girls 9 to 13 years need 240 mg.  Adolescent boys 14 to 18 years need 410 mg. and girls need 360 mg.  Men 19 to 30 years need 400 mg. and women 310 mg.  Older men need 420 mg. and women need 320 mg.  Pregnant women need 400 mg.

The fourth most abundant mineral in the body, magnesium [25] is essential to good health. Approximately 50% of total body magnesium [25] is found in bone. The other half is found predominantly inside cells of body tissues and organs. Only 1% of magnesium [25] is found in blood, but the body works very hard to keep blood levels of magnesium [25] constant.

Magnesium [25] is needed for more than 300 biochemical reactions in the body. It helps maintain normal muscle and nerve function, keeps heart rhythm steady, supports a healthy immune system, and keeps bones strong. Magnesium [25] also helps regulate blood sugar levels, promotes normal blood pressure, and is known to be involved in energy metabolism and protein synthesis. There is an increased interest in the role of magnesium [25] in preventing and managing disorders such as hypertension, cardiovascular disease, and diabetes. Dietary magnesium [25] is absorbed in the small intestines.  Magnesium [25] is excreted through the kidneys.

Potassium [26] is a macromineral essential to life.  50 to 150 mEq is needed daily depending on level of exercise, sweating, and other factors. Along with sodium and chloride, potassium [26] is an “electrolyte.” Electrolytes exist as ions in body fluids and are found in all body fluids. Potassium [26] is mainly a component of fluids inside cells whereas sodium and chloride are mainly part of fluids surrounding cells. It is excreted primarily in the urine but also in stool and sweat.

Potassium [26] manages the balance of the body acids.  It opposes sodium to regulate the body’s water balance and distribution and maintain osmotic balance.  It works to lower blood pressure.  Potassium [26] controls muscle and nerve functions, being necessary for the conduction of nerve impulses.

Potassium [26] is essential for the movement of sugars, amino acids [7], and other  molecules across membranes. It helps dissolve and use calcium [10].

Phosphorus [11] is a macromineral second to calcium [10] in abundance in the body. The amount of phosphorus [11] required daily by children 1 to 3 years is 460 mg. and  500 mg. for children age 4 to 8 years.  Boys and girls 9 to 19 years need 1250 mg.  Men and women 19 years and older need 700 mg. Pregnant women 19 years and older need 700 mg.

Phosphorus [11] is in every cell.  While most of the phosphorus [11] content in the body is in the bones, 15% is in cells and body fluids.  Phosphorus [11] opposes calcium [10] to balance and maintain blood and urine acidity.  It combines with calcium [10] in building the skeleton.

Phosphorus [11] is necessary for carbohydrate, fat, and protein metabolism and to absorb nutrients into cells.  It is an essential component of phospholipids which contain glycerol, fatty acids and phosphorus [11].  It acts as a component for DNA, RNA, ADP and ATP.

Phosphorus [11] is essential to reproduction. It is important to muscle and nerve function and the processes of the ductless glands.

Selenium [27] is a micromineral.   The adult RDA is 70 mg for males and 55 mg for females.

Selenium [27] is essential to good health but required only in small amounts. Selenium [27] is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins help prevent cellular damage from free radicals.  Free radicals are natural by-products of oxygen metabolism that may contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play a role in the immune system.

Zinc [28] is a micromineral.  The amount of zinc [28] required daily by children 1 to 10 years is 10 mg.  Boys and men need 15 mg. Girls and women need 12 mg.  Pregnant women need 15 mg.

Zinc [28] is an essential mineral that is found in almost every cell.  It stimulates the activity of approximately 100 enzymes, which are substances that promote biochemical reactions in the body.  Zinc [28] supports a healthy immune system, is needed for wound healing, helps maintain our sense of taste and smell, and is needed for DNA synthesis.  Zinc [28] also supports normal growth and development during pregnancy, childhood, and adolescence.


USDA, Food and Nutrition Information Center. www.nal.usda.gov/fnic [29].