Feeding butter with elevated content of trans-10, cis-12 conjugated linoleic acid to lean rats does not impair glucose tolerance or muscle insulin response
BACKGROUND Conjugated linoleic acid (CLA) has been shown to be an effective supplement for reducing fat mass in animals, whereas results in humans have been inconsistent. OBJECTIVE This is a meta-analysis of human studies in which CLA was provided as a dietary supplement to test its efficacy in reducing fat mass. DESIGN We searched the PubMed database (National Library of Medicine, Bethesda, MD) and references from the resulting search to identify studies in which CLA was provided to humans in randomized, double-blinded, placebo-controlled trials and in which body composition was assessed by using a validated technique. RESULTS We identified 18 eligible studies. Of these, 3 were single-isomer studies, and results comparing CLA isomers were inconclusive. We compared the length of treatment by using studies in which a mixture of purified isomers were used and those in which purified trans-10,cis-12 isomers were used. This comparison indicated that the effect of CLA was linear for up to 6 mo and then slowly approached an asymptote at 2 y. An analysis of the dose effect indicated that fat loss compared with placebo was -0.024 kg x g CLA(-1) x wk(-1) (P=0.03). After adjustment to the median dose of 3.2 g CLA/d, CLA was effective and produced a reduction in fat mass for the CLA group alone (0.05 +/- 0.05 kg/wk; P<0.001) and for the CLA group compared with placebo (0.09 +/- 0.08 kg/wk; P<0.001) CONCLUSION Given at a dose of 3.2 g/d, CLA produces a modest loss in body fat in humans.