The Partitioning Of Fat In Ruminantscan

The partitioning of fat in ruminantscan

nutrition be used as a tool to

regulate marbling?

D.W. Pethick1, L. Mclntyre2, G. Tudor3 and J.B. Rowe4

�School of Veterinary Studies, Murdoch University, WA, 6150

*Agriculture Western Australia, South Perth, WA, 6151

3AgricuIture Western Australia, Bunbuty, WA, 6231

4Department An imal Science, University of New England, Armidale, NSW, 2351

Summary

In this paper we propose that regulation of the glucose/

insulin axis by diet will result in increased fattening at

the marbling (intramuscular) depot. The pattern of fat

accretion in the whole carcass is first described to

emphasise that the marbling depot is relatively late

maturing. The pathways of fat metabolism are then

discussed with the aim of examining the possibility of

regulating fat partitioning between depots in ruminants.

The importance of acetate versus glucose + lactate as

substrates for lipogenesis differs between depots in

the ruminant and it is proposed that this might allow for

differential control of lipogenesis. The role of diet in the

manipulation of pathways for lipogenesis in ruminants

is then explored. Both glucose infusion and diets

promoting starch digestion in the small intestine

increase the activity of enzymes (ATP citrate lyase)

associated with lipogenesis from glucose indicating a

greater relative importance of glucose as a lipogenic

substrate and this may have an impact on the relative

growth of different fat depots via the glucose/insulin

axis. Chromium supplementation to ruminants

consuming a diet low in chromium also increases the

relative importance of glucose as a precursor for

lipogenesis and causes redistribution of fat away from

the subcutaneous site. Finally the effects of feeding

different cereal grains and chromium supplementation

to 150 day fed steers on visual marbling score are

discussed. It is concluded that starch digestion in the

small intestine is associated with increased visual

marbling score.

Introduction

Consumer preferences and the need to maximise

profitability in the rural sector have resulted in an

enormous research effort to maximise muscle and

minimise fat in meat producing animals such as pigs,

sheep and cattle. However, leanness at all sites is not

always desirable and some markets will pay a premium

for beef meat containing extra levels of fat (marbling). In

this review we first discuss the development of fat tissue

during growth and how this relates to consumer

preferences. We then examine the biochemical pathways

for lipogenesis in ruminants and discuss dietary

mechanisms for regulating these pathways. Finally, the

possibility for altering the distribution of fat between

depots in ruminants is explored with particular emphasis

on marbling.

Growth of adipose tissuepractical

aspects

Adipose tissue is deposited in specific depots which

are similar for all mammals. The primary depots are within

the abdominal cavity (perirenal, mesenteric and

omental), intermuscular, subcutaneous and

intramuscular. However, the proportions differ between

the species and are influenced by age. Thus the pig has

more subcutaneous fat (70% of total body fat) and less

abdominal fat than sheep and beef cattle (Wood, 1984).

Adipose tissue depots are thought to develop in

the order of abdominal, intermuscular, subcutaneous

and finally intramuscular (Vernon, 198 1). The

progressive development of some fat depots in cattle is

shown in Figure 1. The data suggest that abdominal fat

changes largely in line with carcass (or body) weight

while all other depots increase relative to carcass weight.

Thompson et al. (1987) measured chemical fat content

at all sites in mature Merino sheep and found

intramuscular fat was 7% of total body fat. This

compared with subcutaneous at 24%, intermuscular

20%, kidney fat 1 1%, omental fat 16% and mesenteric at

6%. Therefore the marbling depot is of moderate to small

size.

Beef produced for the Australian domestic market

originates from cattle yielding a carcass weight of about