The vitamin A activity of cis isomers of β - carotene was not included in the determination of portion sizes because of uncertainty about the bioavailability of cis - β - carotene relative to all - trans - β - carotene. (academic.oup.com)
Dugan et al (2007) reported that the major trans isomer in beef produced from a 73 % barley grain diet is 10t - 18:1 (2.13 % of total lipid) rather than 11t - 18:1 (TVA)(0.77 % of total lipid), a finding that is not particularly favorable considering the data that would support a negative impact of 10t - 18:1 on LDL cholesterol and CVD» (nutritionfacts.org)
In addition to their antioxidant actions within LDL and HDL cholesterol, the vitamin E isomers reside in lipid bilayers where they not only act as antioxidants, but affect cell signaling, membrane - resident enzymes and other membrane - dependent processes. (lmreview.com)