Tissue fatty acid profiles can be used to quantify endogenous rumenic acid synthesis in lambs.

Proportions of vaccenic (trans-11 18:1) and rumenic (cis-9, trans-11 18:2) acids in mesenteric adipose, subcutaneous adipose, and longissimus muscle tissue lipids from lambs fed varying proportions of forages and concentrates were used to develop a mathematical model to predict exogenous and endogenous contributions to rumenic acid (RA) in the several tissues. From the model, we were able to estimate the proportion of absorbed RA, the proportion of vaccenic acid (VA) desaturated, the original proportion of VA in the tissue (before desaturation), and finally the proportion of RA synthesized endogenously. Estimates of endogenous RA were in the range of published data estimated by independent procedures. An independent data set of VA and RA in milk fat was used to challenge the model. Predictions were concordant with observations, although estimates of endogenous RA synthesis were lower than previous reports. Changing the amount of exogenous RA through manipulation of the diet influenced desaturation of VA inversely, so that endogenous RA synthesis was decreased when exogenous supply was increased (r = -0.80). The model should be challenged with data from human and nonruminant, as well as ruminant studies.