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Conjugated linoleic acids (CLA) are believed to influence body composition, blood lipids and certain endocrine parameters in animals and humans. The aim of this study was to investigate the effects of a six months dietary supplementation of 7 g CLA-oil (containing 54% CLA) daily in two groups of male and female resistance-trained athletes who were at a different training stage. The volunteers were matched according to their previous training: 7 beginners (3♀/4♂) and 7 advanced athletes (2♀/5♂). During the intervention period they performed a standardized training routine three times per week. Blood samples were taken and body mass index, body composition (bioelectrical impedance assessment) and nutrient intake (7-day food record) were recorded at baseline as well as during and following dietary supplementation Results: Serum lipid concentrations, serum leptin, soluble leptin receptor and IGF-I levels or body composition were similar in the two categories of athletes after CLA supplementation. However, despite a higher energy intake, a significant reduction of body fat (P <;0.05) was observed and both groups tended to increase their body cell mass (not significant). Total body water increased in the novice athletes (P <:0.05). Furthermore, total cholesterol (P = 0.049) increased over baseline levels in the novice athletes. These levels remained within the physiological range. In all athletes there was a significant correlation between percentage body fat and leptin (baseline: r2 = 0.46, P = 0.01, CLA: r2 = 0.49, P = 0.011), as well as between fat mass and serum leptin levels (baseline: r2 = 0.35, P = 0.033, CLA: r2 = 0.60, P = 0.002). Conclusions: Over a period of six months no differences were observed in the effects of a commercial CLA-triacylglyceride (54% CLA, 7g/d) on selected endocrine parameters, blood lipids, food intake and body composition between advanced and novice resistance-trained athletes who take part in a regular training program. [ABSTRACT FROM AUTHOR]