Conjugated linoleic acid effects on circulating hormones, metabolites and lipoproteins, and its proportion in fasting serum and erythrocyte membranes of swine.

BACKGROUND: Conjugated linoleic acids (CLA)# refer to a group of linoleic acid (18:2)-derived isomers with conjugated double bonds mostly at carbon atoms 9 and 11 or 10 and 12, and with all possible cis and trans combinations. CLA is a newly recognized nutrient that functions to regulate energy retention and metabolism and that causes a serum lipoprotein profile considered to be less atherogenic. However, rodent models that have been frequently used for these studies are only of limited use because of distinct differences in physiology, compared with man. Additionally, possible differences in food intake between the experimental groups remained often unconsidered in those studies. Thus, it can not be excluded that the beneficial effects of CLA reported in a series of studies may be due, at least partially, to differences in nutrient and energy ingested.

AIMS OF THE STUDY: This prompted us to undertake an investigation on the action of CLA by using a pig model and a feeding regimen with controlled amounts of food and antioxidants. The parameters used to assess CLA-specific action were selected hormones and metabolites involved in energy metabolism, individual lipoproteins and the appearance of CLA in fasting serum and erythrocyte membranes. Blood as an easily available biological sample was used for investigation.

METHODS: For that purpose 16 adult female pigs were divided into two groups of 8 each, and were isoenergetically fed diets containing 0 (control diet) or 1% level of CLA (by weight) for 6 weeks. Plasma concentrations of total and free thyroxine (T4), total and free triiodothyronine (T3), and insulin were measured by radioimmunoassays. Plasma non-esterified fatty acids (NEFA), beta-hydroxybutyrate, circulating blood ATP and other clinical chemical variables were determined using enzymatic assays. The concentration of alpha-tocopherol was determined by high performance liquid chromatography. The lipoproteins VLDL (density <1.019 kg/L), LDL (density 1.019 - 1.063 kg/L), and HDL (density >1.063 kg/L) were isolated by step-wise ultracentrifugation. Fatty acids of the dietary oils, serum and blood cell membranes were separated and quantified by gas chromatography.

RESULTS: At week 6, body weights of the pigs fed the CLA-supplemented diet were not different from that of the controls. CLA-treated pigs exhibited a 37% higher concentration of fasting serum insulin than their controls receiving no CLA (P = 0.11). Circulating free and total T4 and T3 as well as serum levels of beta-hydroxybutyrate, alpha-tocopherol, protein, glucose, urea, creatinine and circulating blood ATP remained unaffected by CLA supplementation. Serum concentrations of non-esterified fatty acids were reduced by 38% in CLA-treated pigs relative to the controls, although this difference was not significant. CLA-treated pigs tended to have lower leukocyte counts in blood than their controls (P <0.1). Erythrocyte and platelet counts, the hematocrit and hemoglobin concentration were similar between the groups. Serum of CLA-treated pigs showed a trend toward increased levels of triacylglycerols, cholesterol and phosphatidylcholine in the very low density and low density lipoproteins (LDL), without distinct changes in the high density lipoprotein fraction (HDL). The LDL cholesterol to HDL cholesterol ratio was significantly increased by CLA. When pigs were fed CLA at a dietary level of 1%, limited proportions of CLA appeared in fasting serum (1.6%) and erythrocyte membranes (1.1%).

CONCLUSIONS: Under the present experimental conditions there appeared to be parallels between the effects of CLA and the reported effects of trans fatty acids in the mode of action on lipoproteins and insulin. The failure to demonstrate significant beneficial effects of CLA on the lipoprotein profile which have been observed in other studies requires further research.