Nutritional factors can dramatically affect development of young animals during the early stage of life. The objective of the present study was to examine the effects of a neuroactive nutritional supplement (NNS) containing DHA, taurine, carotenoids and vitamins on the body weight and body composition of growing puppies. A total of twenty-four 2-month-old Beagles were fed a nutritionally complete and balanced base diet and a control supplement daily during an initial 1-month baseline assessment, after which they were divided into control and treatment groups. They were fed daily either control or treatment supplements in addition to the base diet from 3 to 12 months of age. Lean body mass and fat mass were assessed using quantitative magnetic resonance scans at 0 (baseline), 3, 6 and 9 months of treatment. Total body weight and lean body mass did not differ between groups over time. The puppies in the treatment group showed a trend of reduced fat gain compared with those in the control group, and with a marginally significant difference at 6 months (P = 0·05). At 3 months, insulin-like growth factor 1 was higher (P = 0·02) in the treatment group compared with the control group. At 9 months, fasting lipid levels were lower (P < 0·05) and fat-oxidation metabolite 3-hydroxybutyrate was higher (P < 0·05) in the treatment group compared with the control group. These results may indicate that NNS has an impact on puppy growth and development, possibly by promoting fat metabolism; further investigation would be necessary to determine the full impact of this supplement on growth and development.