Great Lakes Dairy Sheep Symposium - the Department of Animal ...

Introduction
MILK FAT SYNTHESIS AND ITS REGULATION IN DAIRY SHEEP
Adam L. Lock,* Liam A. Sinclair, † and Dale E. Bauman*
*Cornell University, New York, USA
† Harper Adams University College, Shropshire, UK
Milk and dairy products were recognized as important foods as early as 4000 B.C. as
evidenced by rock drawings from the Sahara. Today, the important contribution of milk and
dairy products in meeting our dietary requirements for energy, high quality protein and several
key minerals and vitamins are well documented. With the projected growth in world population
and the increased demand for animal-derived food products as living standards improve, dairy
products will undoubtedly continue to be an important dietary source of nutrients (Bauman et al.,
2005a). Fat is an important component of milk for a number of reasons. Nutritionally, fat is the
major energy component of milk and it accounts for many of the physical properties,
manufacturing characteristics, and sensory attributes of milk and dairy products. The fat content
of milk is highly variable and the environmental and physiological factors affecting milk fat have
been extensively characterized. Furthermore, nutritional quality has become increasingly
important in food choices because of consumer awareness of the link between diet and health;
importantly, milk fat has been shown to contain a number of bioactive components that may
provide benefits to human health which has led to an interest in “designing” milk fat to improve
its healthfulness and functional properties. Nutrition is the predominant factor affecting milk fat
and it represents a practical tool to alter the yield and fatty acid composition of milk fat. The
impact of nutrition on fat content and fatty acid composition of milk in the bovine has been
extensively reviewed (Palmquist et al., 1993; Lock and Shingfield, 2004; Lock and Bauman,
2004). Therefore, the purpose of this review is to provide an overview of the biology of milk fat
synthesis in ruminants, discuss an innovative method for altering the fat content of sheep milk
and its possible application, and consider the manipulation of the fatty acid composition of sheep
milk fat.
Milk Fat Synthesis
Fat is the most variable component in the milk of ruminants. The concentration of fat in milk
is influenced by animal and environmental factors such as breed, diet, stage of lactation, season
of year, ambient temperature, and body condition (Figure 1). Milk fat consists of droplets of
triglycerides that are coated with cell membrane (milk fat globular membrane; MFGM). Thus,
the majority of the fat content of milk is triglycerides (~95%) with phospholipids, cholesterol,
diacylglycerols, monoacylglycerols, and free fatty acids constituting the remainder (Lock and
Shingfield, 2004). Ruminants are estimated to have over 400 different fatty acids comprising
milk fat triglycerides, but the majority of the fatty acids have chain lengths between four and
eighteen carbons. In ruminants, fatty acids in milk fat arise from two sources that contribute
equally (molar basis), de novo synthesis within the mammary gland and uptake of preformed
fatty acids from circulation (McGuire and Bauman, 2002). An overview of the metabolic
pathways and some of the key enzymes involved in milk fat synthesis are shown in Figure 2.
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