In our contemporary adipogenic environment even modest improvements in body fat mass could be of relevance. In the last years animal and human studies have investigated the potential benefit of CLA on body composition. However, inconclusive results are often derived from short-term studies. Long-term intervention trials with supplemental CLA on body composition have not been reviewed exclusively up to now. Therefore, the objective of this study was to review the evidence of prolonged CLA supplementation as well as its influence on body composition in humans, and to summarize results from safety assessments of CLA intake. A literature search was performed to find intervention trials with CLA supplementation and its effects on body composition, as well as on insulin sensitivity. Only prolonged (≥12 wk) studies on body composition were included. The investigated studies indicate a modest reduction and/or prevention of regain of body fat in overweight/obese subjects. Results on the influence of CLA on insulin sensitivity are inconsistent, with newer data rather adding to the safety of CLA. Impaired insulin sensitivity by CLA remains a safety concern, yet is seemingly restricted to diabetic subjects and single-isomer application. A meta-a2 lysis of extended studies only is warranted to quantitatively evaluate the effects of CLA on body composition. Future research may elucidate if CLA should be considered as a marginal missing, semi-essential nutrient in our present diet.