Energy is the capacity of doing work, while metabolism is the overall balance of constructive and destructive changes that occurs within living organisms.


In the period from 1779 to 1784, Lavoisier and Laplace in France undertook on animal metabolism.

It was already known by about this time that oxygen was picked up by the blood stream from the air in the lungs. Lavoisier and Laplace had now found that the oxygen was withdrawn from the blood stream and utilized by the body tissues. The extra heat produced by the guinea pig was, of course, obtained from the formation of water by oxidation of hydrogen - a fact that Cavendish demonstrated when he burnt hydrogen and obtained water vapour. This was confirmed metabolically by the observation that the air exhaled by the guinea pig had a diminished proportion of oxygen but an increased proportion of carbon dioxide and water vapour.

Respiratory quotient

The RESPIRATORY QUOTIENT is the volume of carbon dioxide exhaled divided by the volume of oxygen used.

Carbohydrates = 1.0
Protein = 0.8
Fat = 0.7

Small animals with a high surface to volume ratio have a greater rate of respiration per unit of body weight than large animals, caused by the greater rate of heat loss in small animals.

Specific oxygen consumption

Metabolic rate also may expressed as the rate of oxygen consumption.

Growth and energy

From these points we may conclude that it is metabolically more expensive to deposit fat than to deposit lean meat, and that we do not want our meat animals racing around like wild horses burning off their feed energy.

At the cellular level as well

When applied at the other end of the scale, to individual animal cells, energy requirements for biosynthesis and maintenance may be expressed in terms of chemical energy as moles ATP. For example, in cultured mammalian cells, 1.6 x 10 E-11 moles ATP per cell are required for biosynthesis, while a greater amount, 2.9 x 10 E-11 moles ATP per cell, is required for maintenance.

Lost feed

Feed energy that is digested and absorbed, and which ends up circulating in the vascular system, is only a fraction of that which passes along the gut. From the total feed intake or consumption (C), a certain portion or rejecta (FU) is lost to the animal. The rejecta may be composed of feces or regurgitated feed (egesta, F) and substances that are excreted by the kidneys or lost through the skin (excreta, U). Metabolizable energy, or assimilation (A), is the portion of the feed that may be utilized by an animal or by a population of animals for respiration (R),

A = R + P = C - FU

Heat production

The term respiration is used here in a broad sense that is equivalent to oxidative metabolism, or to the liberation of energy for all aspects of body maintenance such as heat production and muscular work. Quite large differences (15%) in heat production may exists between breeds of cattle, and cattle with a low skin temperature under normal conditions may tend to exhibit a high growth rate. However, in practice, it is difficult to use the relationship between heat loss and rate of growth to assess productivity. Bulls, for example, exhibit a high heat loss coupled with a rapid growth rate. In this case, the energy lost as heat is more than offset by the greater efficiency of lean meat deposition relative to fat deposition. The bottom line is: that energy lost to keep the animal warm cannot be used for animal growth.

Feed intake

If you don't eat, you don't grow, as many generations of parents have yelled at their kids across the dinner table. Farm animals have considerable control over the amount and the nature of the things that they eat. In many practical and experimental situations this behavior may be a primary factor in the regulation of growth. Control may be passive, as in the increased energy cost of locomotion as an animal grows heavier. Active controlling elements may involve hypothalamic feed-back circuits. The medium of communication may be represented by the levels of circulating factors such as glucose, amino acids, fatty acids , and steroids (which are partitioned between aqueous and lipid compartments of the body). Within the central nervous system, opioid peptides such as beta-endorphin, methionine- and leucine-enkephalin, and dynorphin are involved in modulating feed intake.


Ruminants consume a considerable volume of roughage, and the rumen may be filled before energy requirements are satisfied. Normally, however, feeding behavior changes to accomodate differences in the energy content of the feed. Control signals may be transmitted by acetate and propionate sensors in, or near the rumen. But there is also a complex interaction between diet and the growth of the gut itself, such that diets with high levels of complex carbohydrates and inadequate protein may cause extra growth of the gastrointenstinal tract.


Behavioural factors also are involved in the animal's growth responses to long-term stress. Stress diminishes live weight gains, mainly because of a decrease in fat deposition.

Competition between tissues

Body tissues may compete for circulating nutrients. In ruminants, for example, the energy flow to skeletal muscle is increased by STH while the flow to adipose tissue is increased by insulin. In other words, circulating factors such as hormones may govern the energy distribution system by switching on and off the energy assimilation systems of each type of tissue.

Insulin is particularly important in controlling the distribution of nutrients from the blood stream since it turns on the whole energy catching system of a cell. At the cell membrane it increases glucose and amino acid transport, in the cytoplasm it activates ribosomes and mitochondria while inhibiting lysosomes, and in the nucleus it modulates the synthesis of DNA and RNA. Receptors on the cell membrane recognize insulin molecules and bind them, although some insulin does enter the cell, perhaps to exercise its long-term control functions. Insulin receptors may be heterogeneous in nature with differences occurring in their affinity and capacity for binding insulin molecules. The concentration of receptor sites on the cell membrane and the affinity of receptor sites for insulin are both variable, and are interrelated with the overall levels of insulin in the blood stream.

Physiological age

Certain brain cells may be able to keep track of animal age. These cells may operate a time tally mechanism that matches animal age against the body mass anticipated at any particular time. Evidence from experimental animals suggests that animals have a "set-point" for body weight, and that this may be decreased by experimental lesions in the lateral hypothalamus.

The rates of many reactions within eukaryotic cells, including the synthesis of DNA for cell division, are regulated by calcium ions bound to calmodulin to form a biocatalyst. Reactions are accelerated when the cell membrane alters its permeability and allows a brief influx of calcium ions from outside the cell. The calcium-calmodulin complex remains active until calcium ions are ejected from the cell by a vigorous calcium ion pump in the cell membrane.