Despite the perception that grass-fed beef contains healthier fats, nutritional differences between different grass-fed systems and grain-fed systems are not well explored. Therefore, the present study compares the fatty acid profiles of four different Western United States beef systems including (1) steers finished in a feedyard (120 days; CON), (2) steers grass-fed for 20 months (GF20), (3) steers grass-fed for 20 months with a 45-day grain finish (45GR), and (4) steers grass-fed for 25 months (GF25). Longissimus thoracis serving of n-3 PUFA was lowest for CON compared to all other treatments (P < 0.05). No difference in eicosapentaenoic acid or docosapentaenoic acid was observed between 45GR, GF20, and GF25 (P > 0.05). Grass-fed treatments resulted in t10-18:1:t11-18-1 ratios 15 to 24 times lower compared to CON (P < 0.05). Grass-and grain-finishing resulted in nutritional trade-offs rather than nutritional absolutes with GF25 and 45GR resulting in a more favorable fatty acid profile than GF20.

Vaccenic acid (VA, trans11-18:1) and cis-9, trans11-conjugated linoleic acid (also known as rumenic acid; RA) have received widespread attention as “beneficial” trans fatty acids (TFA) due to their putative health benefits including anti-diabetic properties. Feeding oilseeds (e.g. sunflower seeds or flaxseeds) to cattle given forage-based diets have been shown to enrich their meat and milk fat with VA and RA. However, there is shortage of studies evaluating the health effects of the resulting enriched fat. The objective of the present experiment was to determine the effects of beef fat enriched with VA and RA on parameters related to glucose and insulin tolerance in in high-fat-fed C57BL/6 Mice. Thirty-six 8-week-old male c57/6J mice (25±1.4 g body weight) were randomly assigned for 18 weeks to either a control low fat diet (CLF; 10% total calories from fat), a control high fat diet based on lard (CHF; 45% total calories from fat), or a VA&RA-enriched high fat diet based on flaxseed-fed beef fat (EHF, 45% total calories from fat). Compared with CLF, feeding either CHF or EHF resulted in negative metabolic outcomes associated with high fat diets including adiposity, impaired insulin and glucose tolerances, and hepatic steatosis. However, the EHF diet induced a higher (P &lt; 0.05) liver weight and liver triglyceride content compared with the CHF diet. In addition, the area under the curve for glucose tolerance test (GTT) in EHF mice was higher (P &lt; 0.05) than in CHF group. Collectively, the findings from this study, suggest that feeding beef fat enriched with VA and RA (i.e. EHF) worsens glucose tolerance and liver steatosis in high-fat-fed mice. Additional studies are needed to determine if the adverse effects of EHF on glucose tolerance and liver fat accumulation are related to VA and RA or due to other TFA present in EHF.