Applied Animal Nutrition-I

BASAL AND FASTING METABOLISM

Basal metabolism

• The term Basal Metabolism or Basal Metabolic rate (BMR) refer to the heat production of an animal resting in a thermally neutral environment (temperature range in which environmental temperature does not stimulate normal metabolism, approximately 25^oC) and in a post-absorptive state (that is after the digestion and absorption of the last food ingested has stopped).
• During this rest period although the animal will be doing no external or digestive work nor will it have any emotional excitement, still it will carry on a variety of internal processes, which are essential to life.
• These processes include respiration, circulation, maintenance of muscular tonus, production of internal secretions, etc.
• In the absence of feed, the nutrients required to support these activities must come from the breakdown of body tissues itself.
• The heat production can be determined by direct calorimetry, or by indirect calorimetry.

The conditions which are essential for measuring metabolic rate, are :

• Good nutritive condition: This implies that the previous diet of the animal has been adequate, especially as regards to energy and protein. Poor state of previous nutrition tends to decrease basal heat production.
• Environmental temperature: Temperature of about 25^oC  is specified  as it provides thermoneutral environment.
• Rest: by this way the minimum muscular activity can be achieved. This is very difficult for any kind of animal other than man.
• Post-absorptive state: state when the process or digestion or absorption disappears. It is reached by an overnight starvation in case of human, but for ruminants it may require about three or four days. Any ruminants can hardly fulfill this condition; hence it is measured after a starvation period of about 5 hours. Because of the fact that the last two conditions cannot be fulfilled and a modification is recommended for ruminant animals, hence the term resting metabolism is used in place of basal metabolism.
• To determine if an animal has reached a post absorptive state, measurement of heat production to the point of a constant minimum level can be made.
• Measurement of the respiratory quotient (RQ) to the point that the non protein RQ of fat (0.7) is reached also indicates that a post absorptive state has been acheived.
• In ruminants a decline in methane excretion to a minimum level indicates  a postabsorptive  state. By third day of fasting it declines to 0.5 litre from 30L in sheep and in cattle to 2 litres from 200L per day. So the measurement of basal metabolism in the ruminant cannot have the exact significance as it has in humans.

Fasting metabolism

• Fasting metabolism refers to the heat production at specified times after the last feeding. In ruminats the value determined is referred as fasting metabolism rather than as basal metabolism.This should not be confused with the term fasting catabolism,which also includes energy voided in the urine of fasting animals.
• To avoid some of the problems associated with a four day fast in ruminants, some workers have determined heat production over a specific time period after the last feeding and have referred this value as standard metabolism.
• The term resting metabolism has been used to denote the heat eliminated when an animal is lying at rest, though not strictly in a thermoneutral environment or in the postabsorptive state.