Invited review: Palmitic and stearic acid metabolism in lactating dairy cows

ABSTRACT Energy is the most limiting nutritional component in diets for high-producing dairy cows. Palmitic (C16:0) and stearic (C18:0) acids have unique and specific functions in lactating dairy cows beyond a ubiquitous energy source. This review delineates their metabolism and usage in lactating dairy cows from diet to milk production. Palmitic acid is the fatty acid (FA) found in the greatest quantity in milk fat. Dietary sources of C16:0 generally increase milk fat yield and are used as an energy source for milk production and replenishing body weight loss during periods of negative energy balance. Stearic acid is the most abundant FA available to the dairy cow and is used to a greater extent for milk production and energy balance than C16:0. However, C18:0 is also intimately involved in milk fat production. Quantifying the transfer of each FA from diet into milk fat is complicated by de novo synthesis of C16:0 and desaturation of C18:0 to oleic acid in the mammary gland. In addition, incorporation of both FA into milk fat appears to be limited by the cow's requirement to maintain fluidity of milk, which requires a balance between saturated and unsaturated FA. Oleic acid is the second most abundant FA in milk fat and likely the main unsaturated FA involved in regulating fluidity of milk. Because the mammary gland can desaturate C18:0 to oleic acid, C18:0 appears to have a more prominent role in milk production than C16:0. To understand metabolism and utilization of these FA in lactating dairy cows, we reviewed production and milk fat synthesis studies. Additional and longer lactation studies on feeding both FA to lactating dairy cows are required to better delineate their roles in optimizing milk production and milk FA composition and yield. Key words: stearic acid , palmitic acid , milk fat , milk production , body condition INTRODUCTION Energy is and will continue to be the major nutritional challenge to the ever-increasing lactation productivity of dairy cows. Because of this, dairy producers and nutritionists have increased the use of high-energy feed ingredients, such as fat, in lactating dairy cow diets. Dry, ruminally inert fat supplements have become common feed ingredients in diets because of their energy content and versatility on farms, where they can be added to grain mixes, mineral mixes, TMR, or top dressed. Dry, rumen-inert fats usually contain high concentrations of long-chain FA (LCFA), with the most common being palmitic (C16:0), stearic (C18:0), oleic (C18:1), and linoleic (C18:2). Research over the last several years has shown FA are not just a ubiquitous source of energy, but have metabolically different functions in the cow and contribute to the productive function of cows in different ways. This paper discusses the roles of C16:0 and C18:0, the 2 most common SFA found in rumeninert fats, in the metabolism and productivity of lactating dairy cows. CHEMICAL PROPERTIES OF PALMITIC AND STEARIC ACID Palmitic acid is a 16-carbon SFA denoted as n-hexadecanoic acid with a chemical formula of CH 3 (CH 2 ) 14 CO 2 H. It has a melting point of 62.8°C and an acid dissociation constant (pK a ) of 4.78 Stearic acid is an 18-carbon SFA denoted as n-octadecanoic acid with a chemical formula of CH 3 (CH 2 ) 16 CO 2 H. Michel Eugene Chevreul first described C18:0 in the early 1800s DIGESTION AND ABSORPTION Most naturally occurring fats consumed by the dairy cow are in the form of triacylglycerols (TAG), phospholipids, and glycolipids. Bacteria and protozoa in the rumen hydrolyze these complex lipids into LCFA, glycerol, sugars, choline, serine, phosphates, and other organic compounds FA Flow into the Duodenum The flow of C18:0 from the rumen is several times greater than the amount consumed because of biohydrogenation of PUFA in the forages and grains fed to dairy cows. The effects of feeding multiple ingredients and supplemental fats on microbial metabolism of FA in the rumen and FA flow into the duodenum are reported in the meta-analysis of On exiting the rumen, most FA are present as salts of sodium, potassium, or calcium combined in an insoluble particulate phase of feed particles and microbial cells. These salts are dissociated and protonated to a great extent in the abomasum due to low pH, and thus enter the duodenum mostly as nonionized FFA. If these FFA are not absorbed, they may reform as salts as pH Rumen-inert and AV blend intake and outflow values are means of feeding supplements at 3 and 6% of the diet DM. 3 The FA composition of the rumen inert fat was 50.8% C16:0, 4.2% C18:0, and 35.5% cis C18:1. 4 The FA composition of the AV blend was 17.0% C16:0, 17.2% C18:0, and 34.5% cis C18:1