The dietary needs of mammals

Like the rest of the animal kingdom, mammals need food for energy and the maintenance of bodily processes such as growth and reproduction. The chemical compounds used to supply the energy and building materials are obtained by eating plants or organic material. Both plant- and animal-based sources of food are made up of highly complex compounds that need to be digested and broken down into simpler forms.

Four of the most common naturally occurring elements— oxygen, carbon, hydrogen, and nitrogen—make up 96% of the total body weight of an animal. The remaining 4% is made up of the seven next most abundant elements—calcium, phosphorus, potassium, sulfur, sodium, chlorine, and magnesium, in that order. Necessary for many physiological processes, any change in their concentrations may be deleterious or fatal.

An animal's major dietary components are fat, water, protein, and minerals. The main digestion products of these compounds are amino acids (from proteins), various simple sugars that are present in the food or derived from starch digestion, short-chain fatty acids (from cellulose fermentation), and long-chain fatty acids (from fat digestion). The oxidation of these digestion products yields virtually all the chemical energy needed by animal organisms.

Despite carbohydrates' essential role in animal metabolism, their total concentration is always less than 1%. The two major animal carbohydrates are glucose and glycogen.

Body lipids act as energy reserves, as structural elements in cell and organelle membranes, and as sterol hormones. Because lipids can be stored as relatively non-hydrated adipose tissue containing 2-15% free water, eight times more calories per unit of weight can be stored as fat than as hydrated carbohydrates. This is the reason fat storage is essential for active animals, while carbohydrates are a major energy reserve for plants. Hibernating mammals deposit fat and may double their body weight at the end of the summer prior to hibernation; the white adipose tissue reserves allow them to survive the winter.

In addition, there are 15 elements making up less than 0.01% of the body of a mammal. These elements occur in such small amounts that they became known as trace elements. Still, they have vital physiological and biochemical roles. Iron, for instance, is a key constituent of hemoglobin in blood and several intercellular enzyme systems. While the amount of iron found in an adult human is only 0.14 oz (4 g)—70% in hemoglobin, 3.2% in myoglobin, 0.1% in cytochromes, 0.1% in catalase, and the remainder in storage compounds in the liver—growing animals need more iron, and adult females need to replace that which is lost in reproductive processes such as the growth of the fetus and menstruation. The dietary requirement for adult mammals is very small since iron (from the breakdown of hemoglobin) is stored in the liver and used again for hemoglobin synthesis. Other trace elements include copper, zinc, vanadium, chromium, manganese, molybdenum, silicon, tin, arsenic, selenium, fluorine, and iodine.

Animals can differ markedly in their vitamin requirements. Ascorbic acid (vitamin C), for example, can be synthesized by most mammals, but humans and a few other mammals such as non-human primates, bats, and guinea pigs, need to have it supplied in their diet.

Ruminants do not appear to need several vitamins in the B group since the microbial synthesis of vitamins in the ruminant stomach frees these animals from having to seek out additional dietary sources.

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