Humans and other animals get the chemical energy and carboncontaining building blocks they need from carbohydrates, proteins, and fats. All these substances are carbon compounds with high potential energy.
A carbohydrate, protein, or fat is an example of a nutrient: a substance that an organism needs to remain alive. Food is any material that contains nutrients.
The amount of energy provided by foods is measured in kilocalories (on food labels, kilocalories are referred to as Calories). Because fats are rich in C-H bonds, they provide more energy than other nutrients: about 9 kcal/g versus about 4 kcal/g for carbohydrates and proteins.
Understanding which nutrients an individual needs, and in what amounts, are basic issues in research on nutrition in humans and other animals. Let’s consider what humans need to maintain good health.
In 1943 the Food and Nutrition Board of the U.S. National Academy of Sciences1 published the first Recommended Dietary Allowances (RDAs). The goal of the RDAs was to specify the amount of each essential nutrient that an individual must ingest to meet the needs of most healthy people. Although all nutrients are necessary for growth and survival of animals, essential nutrients are those that cannot be synthesized and must be obtained from the diet.
Humans require four classes of essential nutrients:
• Essential amino acids are amino acids that an animal cannot synthesize from simpler building blocks. The human diet requires nine essential amino acids, which must be obtained from food. (20 amino acids are required to manufacture most proteins; humans can synthesize 11 of them.)
• Essential fatty acids are fatty acids that an animal must obtain in its diet. Humans can synthesize all fatty acids except two, which must be obtained from eating certain plants or fish.
• Vitamins are organic, or carbon-containing, compounds that are vital for health but are required in only minute amounts. They have a variety of roles; several function as coenzymes in critical reactions. Table 1 lists a few of the vitamins for which RDAs have been established, notes their functions, and indicates the problems that develop if they aremissing in the diet.
Table 1: Some Important Vitamins Required by Humans
|Vitamin B1 (thiamine)||Legumes, whole grains, potatoes, peanuts||Formation of coenzyme in citric acid cycle|
|Vitamin B12||Red meat, eggs, dairy products; also synthesized by bacteria in intestine||Coenzyme in synthesis of proteins and nucleic acids and in formation of red blood cells|
|Niacin||Meat, whole grains||Component of coenzymes NAD+ and NADp+|
|Folate||Green vegetables, oranges, nuts, legumes, whole grains; also|
synthesized by bacteria in intestine
|Coenzyme in nucleic acid and amino acid metabolism|
|Citrus fruits, tomatoes, broccoli, cabbage, green peppers||Used in collagen synthesis, prevents oxidation of cell components, improves absorption of iron|
|Vitamin D||Fortified milk, egg yolk; also synthesized in skin exposed to sunlight||Aids absorption of calcium and phosphorus in small intestine|
• Minerals are inorganic substances used as components of enzyme cofactors or structural materials (see Table 2). Some, such as calcium and phosphorus, are needed in relatively large quantities. Others, such as iron and copper, are required in small or trace amounts. Minerals include ions of electrolytes, which influence osmotic balance and are required for normal membrane function. Sodium (Na+), potassium (K+), and chloride (Cl-) are the major ions of electrolytes in the human body.
Table 2: Major Minerals Required by Humans
|Minerals||Source in Diet||Function|
|Calcium (Ca)||dairy products, green vegetables, legumes||bone and tooth formation, nerve signaling, muscle response|
|Chlorine (Cl)||table salt or sea salt, vegetables, seafood||fluid balance in cells, protein digestion in stomach (HCl), acid–base balance|
|Fluorine (F)||fluoridated water, seafood||maintenance of tooth structure|
|Iodine (I)||iodized salt, algae, seafood||component of the thyroid hormones thyroxine and t3|
|Iron (Fe)||meat, eggs, whole grains, green leafy vegetables, legumes||enzyme cofactor; synthesis of hemoglobin and electron carriers|
|Magnesium (Mg)||whole grains, green leafy vegetables||enzyme cofactor|
|Phosphorus (P)||dairy products, meat, grains||bone and tooth formation; synthesis of nucleotides and Atp|
|Potassium (K)||dairy products; meat; nuts; fruits; potatoes, legumes, and other vegetables||nerve signaling, muscle response, acid–base balance|
|Sodium (Na)||table salt or sea salt, seafood||nerve signaling, muscle response, blood pressure regulation|
|Sulfur (S)||any source of protein||amino acid synthesis|
To obtain nutrients, animals must ingest them, usually via a mouth. The structure of animal mouthparts is therefore often highly specialized for the capture of specific types of food.