22 Energy for Growth

22.1  History

Only in recent times have sciences such as physics and chemistry become divorced from practial subjects such as raising animals for us to eat. People were far more sensible in the old days.  Many of the scientists who were interested in agriculture were also interested in chemistry or physics, and vice versa.  Thus, in the history of this topic, energy for growth, some of the great names of physics and chemistry were actively thinking about biological and agricultural phenomena.

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

• It is difficult to pinpoint the earliest origins of the concept of metabolism. Carbon was one of the chemical elements known in the ancient World and the discoveries of the other major elements in animal respiration were considerably later: hydrogen in 1766 by Henry Cavendish, nitrogen in 1772 by Daniel Rutherford, and oxygen by Joseph Priestly in 1774.
•  In the period from 1779 to 1784, Lavoisier and Laplace in France were undertaking experiments similar to those undertaken in a modern‑day undergraduate course on metabolism.
  
• A guinea pig was kept in an ice calorimeter for 10 hours and the metabolic heat generated by the animal was measured by the amount of ice that was melted.
  
• The amounts of heat and carbon dioxide generated were compared to the equivalent amounts produced by combustion of known amounts of charcoal.
  
• In relation to the amount of carbon dioxide produced, the amount of heat produced metabolically by the guinea pig was somewhat more than that produced by combustion of pure carbon. Despite this quantitative inbalance, Lavoisier and Laplace concluded correctly that carbonaceous material from the guinea pig had undergone some type of combustion involving air drawn into the animal's lungs.
• It was already known oxygen was picked up by the blood stream from the air in the lungs. Lavoisier and Laplace had now found  the oxygen was withdrawn from the blood stream and utilized by the body tissues.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 Cavendish demonstrated when he burnt hydrogen and obtained water vapour.
  
• This was confirmed metabolically by the observation of  air exhaled by the guinea pig having a diminished proportion of oxygen, but an increased proportion of carbon dioxide and water vapour.
  
• At about this time, similar experiments were undertaken by Crawford in Britain.

22.2 The respiratory quotient

The RESPIRATORY QUOTIENT (RQ) is the volume of carbion dioxide exhaled divided by the volume of oxygen used. The RQ was introduced in 1849 by Regnault and Reiset who devised a closed‑circuit apparatus for the measurement of oxygen consumption and carbon dioxide production. Small animals with a high surface to volume ratio were found to have a greater rate of respiration per unit of body weight than large animals. It was concluded this was due to the greater rate of heat loss in small animals.

                 Carbohydrates = 1.0

                 Protein                 =  0.8

                 Fat                        =  0.7

• Nitrogen in urine is be used to measure the amount of protein metabolized.
  
• A bomb calorimeter measures the heat of combustion of various foods.
• For starches and fats, both methods of calorimetry (live animal and bomb) give similar results, but in the animal calorimeter less heat is obtained from proteins than in the bomb calorimeter.
  
• Thus, as well as a major correction for urea production, the nitrogen to carbon ratios of other nitrogenous wastes such as uric acid and creatinine also hasto be taken into account.
  
• Creatinine is a metabolic end product of nonenzymatic dehydration of intracellular creatine, largely excreted unchanged by the kidneys.
• From values determined by calorimetry, it was found the different caloric values of  food substances are caused largely by differences in the amount of hydrogen that they contain, as shown by the weight of water vapor that they yield on combustion .
• The first calorimeter large enough to hold an adult steer was constructed at Pennsylvania State College. In modern closed circuit respiration chambers for small animals, oxygen is supplied internally from cylinders of compressed gas, and carbon dioxide and water are absorbed for the calculation of respiratory quotients. Studies on larger animals require an open circuit system in which oxygen and carbon dioxide levels of ingoing and outgoing air are determined on a continuing basis and integrated over time.
• Heat production and muscular work are essential for the animal to stay alive, and growing tissues compete with each other and with these essential services to obtain chemical energy from nutrients circulating in the vascular system.
• Lean meat with a water content of approximately 80% contains approximately 4.7 kJ/g, while the energy content of fat is approximately eight times greater .
• Mammalian basal metabolic rate is approximately 12.5 kJ per kg^0.75 per hour.

22.3 Feed intake and digestion

• Animals have considerable control over the amount and the nature of the things they eat. In many practical and experimental situations this behavior may be a primary factor in the regulation of growth.
  
• There are a number of possible control systems for the regulation of energy balance by consumption.
• 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. 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.

 22.4 Energy distribution models

Although the growth equation of von Bertalanffy may be translated into energy terms, the other growth equations in which deceleration is a function of body weight or of age present more of a problem. One approach to the problem is given by Sir John Hammond's  idea of body tissues competing for circulating nutrients.

There are a number of astute points about this model;

•  (1) it concerns itself only with assimilation,
  
• (2) it is based on a priority system where vital organs make the greatest claim, and
  
• (3) it encompasses the division of energy between animal growth and animal reproduction.
  

• 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, insulin increases glucose and amino acid transport.<>
• In the cytoplasm, insulin activates ribosomes and mitochondria, while inhibiting lysosomes .
• In the nucleus, insulin 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.

Hammond's model was also supposed to explain the effect of reduced energy intake on muscle development. But this did not survive experimental testing.  For example, with pigs at live weights of 150 kg on a submaintenance diet, the tissues of the carcass are affected in reverse order to their order of anatomical development. Fat is reduced most, bone is reduced least, and muscle is intermediate in reduction.  For another example, with Angus steers on a resricted diet reducing live weight, bone and connective tissue may be relatively unaffected, muscle mass may be reduced, but there may be only relatively slight reductions in carcass fat content. Chemical analyses shows the loss of muscle weight is from protein, and not to dehydration or to mobilization of intramuscular fat.

• MUSCLE : BONE RATIOS (IMPORTANT WHEN WE LOOK AT WEIGHTS OF BONE-IN MEAT CUTS). 
• Within a breed, muscle to bone ratios may not be a fixed function of carcass weight, if animals have been placed on a low plane of nutrition  causing weight loss or inhibition of further growth.
  
• Muscle to bone ratios also may remain constant despite intensive genetic selection for leaness and growth rate.
• Dairy and beef breeds may differ in their muscle to bone ratios as a consequence of selection for milk yield versus meat yield.
• Differences in muscle to bone ratios between breeds of beef cattle appear to be a relic of an earlier period of breed improvement when certain breeds were selected for their muscular strength as draft animals.

  Berg and Butterfield  in 1976 proposed an alternative model of nutrient distribution during growth. Most researchers still agree with this modification.



Energy distribution between tissues is now envisaged as a combination of nutrient availability and tissue capacity. Hammond's model resembles growth equations in which specific growth rate is a function of age, essentially of physiological age.  As animals become older they become capable of reproduction and extensive fat deposition, and their priorities change accordingly. Conversely, the model of Berg and Butterfield resembles growth equations in which specific growth rate is a function of body size, because of the implied limits to tissue volume in all except adipose tissue. Thus, according to these two models, the animal "knows" either its physiological age or what size it should be.

• TEMPLATE THEORY. One idea is overall body dimensions might be dependent upon each tissue of the body regulating its own growth.  Each tissue type produces  both templates, with a catalytic action on growth, and antitemplates. Templates are thought to be confined to the cell producing them, while antitemplates diffuse out of the cell to become uniformly diluted through the remainder of the body. The concentration of antitemplates in the body at any particular time may be a balance between their continuous formation and degradation. Thus, definitive size is reached when the concentration of antitemplates in the whole body finally reaches a level where it blocks all intracellular templates.
• TIME TALLY THEORY.  Certain brain cells may be able to keep track of animal age. These cells might operate a time tally mechanism matching animal age against the body mass anticipated at any particular time.  Information on animal mass may be brought to the time tally by substances resembling antitemplates produced by one, or a small number of body tissues or organs. Evidence from experimental animals suggests  animals do have a "set‑point" for body weight, and that this may be decreased by experimental lesions in the lateral hypothalamus.

   22.5 Conclusion

  This lecture is more about what we don't know than what we do know! It might be possible to combine all these proposed growth‑controlling mechanisms into a single system with each tissue compartment being given

•  (1) a priority for obtaining energy,
  
• (2) an essential respiratory requirement, and
  
• (3) a preferred size calculated from an interaction between age and body weight.
  

• (4) a biological clock,
  
• (5) an intrinsic system for monitoring body mass, and logical questions concerning
  
• (6) pregnancy,
  
• (7) required muscular work,
  
• (8) environmental heat loss,
  
• (9) nutrient intake and
  
• (10) stress.

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