Farm Animal Metabolism and Nutrition

Chapter 6

Glucose Availability and

Associated Metabolism

R.W. Russell and S.A. Gahr

Division of Animal and Veterinary Sciences, West Virginia University,

Morgantown, West Virginia, USA

Introduction

Glucose is the sugar that is central to carbo-
hydrate metabolism in all vertebrates. It is
the carbohydrate that circulates throughout
the body via blood plasma and/or cells, the
concentration of which is tightly con-
trolled. Failure of control in either direc-
tion has dire physiological consequences,
e.g. coma, energy ‘spillage’, dehydration
and vascular damage. Although the scheme
of metabolism is constant throughout
nature, the strategies of procuring and
metabolizing carbohydrates vary markedly,
even among farm animals.
Classification of animals by feeding
types yields three categories: (i) herbivores,
consumers of plants; (ii) carnivores, con-
sumers of animals; and (iii) omnivores,
consumers of both plants and animals. The
majority of farm animals are herbivores,
e.g. cattle, sheep, goats, rabbits, horses and
catfish; or omnivores, e.g. swine and
poultry. Carnivores have not been
prominent farm animals, although fur-
bearing animals are predominantly carni-
vores. The increase in aquaculture has also
increased the number of farm animal carni-
vores, e.g. trout, salmonoids and alligators.

Aquaculture has also challenged traditional

animal scientists to expand their under-

standing of metabolism to include that of

carnivores and poikilotherms, cold-

blooded animals.

There are three major routes by which

animals procure glucose.

1.Direct enzymatic digestion and absorp-

tion of dietary carbohydrates. Non-glucose

dietary carbohydrates that are absorbed are

processed to glucose before passage from

the liver such that extrahepatic metabolism

of carbohydrate is restricted to glucose

metabolism.

2.Fermentation of dietary carbohydrates

by enteric microbial ecosystems. Glucose is

synthesized by the host, primarily from

end-products of the microbial ecosystem.

Typically, fermentation chambers are

located near the beginning or end of the

alimentary canal, or both.

3.Direct enzymatic digestion of dietary or

enteric microbial protein with absorption

of peptides and amino acids. Selected

amino acids from the absorbed peptides

and amino acids then become substrate for

gluconeogenesis, the synthesis of glucose

from non-carbohydrate precursors.

© CAB International2000. Farm Animal Metabolism and Nutrition

(ed. J.P.F. D’Mello) 121