BACKGROUND 

Time on feed and grain-based diets work in concert to increase the amount of marbling as well as the concentration of oleic acid in marbling and subcutaneous adipose tissues. The increase in oleic acid is caused by a stimulation of stearoyl-CoA desaturase (SCD) gene expression by some factor in grain-based diets. The increase in oleic acid improves beef palatability, and previous research funded by The Beef Checkoff has indicated that greater amounts of oleic acid increases the healthfulness of beef. With subsequent funds from The Beef Checkoff, we have demonstrated that saturated fatty acids that are relatively abundant in finishing cattle plasma (i.e., palmitic and stearic acid) strongly influence gene expression in intramuscular adipose tissue, which should lead to increased marbling development. This proposal has as its primary goal to establish a practical application of our previous biochemical and molecular biology studies. Thus the objectives of this research were to establish that dietary palm oil, unlike soybean oil,will increase marbling scores without depressing animal production performance; and to demonstrate the effects of palm oil and soybean oil on the expression of SCD, PPARγ, and other regulatory genes during adipose tissue development.

METHODOLOGY

Cattle: Twenty-eight Angus or Angus x Brahman crossbred steers from the Texas A&M University Research Center at McGregor, TX were assigned to three groups of 8 or 9 steers and fed a basal diet without additional fat, with 3% palm oil (rich in palmitic acid), or with 3% soybean oil (rich in polyunsaturated fatty acids), added as top dressings. The steers were fed the diets for 10 wk. At that time, the steers were harvested at the Texas A&M University Rosenthal Meat Science and Technology Center. Samples were taken from 18 of the steers (n = 6 per treatment group, balanced across breed type) for in vitro measurements and analysis of plasma fatty acids. Subcutaneous adipose tissue samples were obtained from the 8th to 11th rib section immediately after the hide was removed.

Adipose tissue dissection: Intramuscular and subcutaneous adipose tissues were dissected from the longissimus muscle by established procedures. Samples were snap-frozen in liquid nitrogen for later extraction of RNA for gene analysis, or were stored at -20°C for subsequent analysis of adipocyte size and fatty acid composition.

Adipose tissue biopsies and plasma samples: At 14 and 16 months of age, subcutaneous adipose tissue samples were obtained by tailhead biopsy and blood samples were obtained by venipuncture. Biopsy samples were stored at -20°C for measurement of adipose tissue cellularity and fatty acid composition or stored at -80°C for extraction of RNA for measurement of specific gene expression. Blood samples were analyzed for fatty acid composition.

Fatty acid synthesis, enzyme activities, and cellularity: At sample collection, lipogenesis wasmeasured in s.c. adipose tissue as described previously (Martin et al., 1999). Additionally, samples of subcutaneous adipose tissue were immediately homogenized and centrifuged and the supernate was used to measure enzyme activities of glucose-6-phosphate dehydrogenase, 6- phosphogluconate dehydrogenase, NADP+-malic dehydrogenase, and fatty acid synthetase by established procedures. Subcutaneous adipose tissue was collected immediately from the steers and frozen at -80°C for determination of cellularity by osmium fixation, counting, and sizing. The fixed adipocytes were used for determination of cell size, volume, and cells/100 mg tissue, using a bright-field microscope (Olympus Vanox ABHS3, Olympus, Tokyo, Japan) and CCD Color Video Camera (DXC-960MD, Sony, Japan).

Gene expression: Real-time quantitative PCR (RT-qPCR) was used to measure the quantity of AMPKα, PPARγ, C/EBPβ, SCD, GPR41 and GPR43 mRNA relative to the quantity of ribosomal protein S9 (RPS9) mRNA in total RNA isolated from subcutaneous and intramuscular adipose tissues. The endogenous RPS9 control was used to normalize gene expression.

FINDINGS

As expected, plasma palmitic acid was increased by dietary palm oil (Figure 1a), whereas supplemental soybean oil increased plasma linoleic acid (the most abundant plasma fatty acid) and doubled the plasma concentration of a -linolenic acid (Figure 1b). Palm oil caused a small increase in the oleic acid content of subcutaneous adipose tissue (from 40.2 to 41.3%), whereas soybean oil decreased subcutaneous adipose tissue oleic acid content (to 38.9%). Carcass weight was unaffected by supplemental dietary oils (P = 0.45) (Table 1). Supplementary palm oil increased marbling scores (P < 0.09), from Small79 to Modest09, whereas soybean oil depressed marbling scores to Small55. Palm oil caused a small increase in adjusted fat thickness, whereas soybean oil decreased fat thickness (P = 0.15). Kidney, pelvic, and heart fat also tended to be greater in steers fed the palm oil supplement (P = 0.15). Overall maturity was lower by over 25% in steers supplemented with palm oil than in steers receiving the basal diet (P = 0.002). Palm oil supplementation increased lipid synthesis in vitro from glucose and acetate (both P = 0.03) compared to the control group. Soybean oil had no effect on lipogenesis or lipogenic enzyme activities, but glucose-6-phosphate dehydrogenase activity was greater (P = 0.10) in steers fed the palm oil supplement than in control steers, and a similar trend was observed for NADP+-malic dehydrogenase. Adipocyte volume in s.c. adipose tissue was not increased significantly by palm oil but was decreased by soybean oil supplementation (P = 0.004). The soybean supplement tended to cause a greater relative abundance of AMPKα at 10 wk of age (P= 0.10) and GPR43 (P = 0.12) than in control samples. No significant differences were detected for PPARγ, C/EBPβ, SCD, or GPR41. Increased AMPKα and GPR43 gene expression should have led to increased lipid filling in adipose tissue of soybean oil-fed steers, but this was not the case. Instead, soybean oil had a negative impact on marbling scores, whereas palm oil increased marbling scores and tended to increase overall carcass fatness.

IMPLICATIONS

Overall, this study demonstrated by providing saturated fatty acids to the diet of feedlot steers has the potential to increase carcass quality without negatively affecting beef fatty acid composition. Conversely, the unsaturated fatty acids in soybean oil depressed carcass quality. The next goal will be to identify a natural, cost-effective source of saturated fat that can be supplemented to feedlot cattle.