Background: Previous studies have demonstrated that DHA n-3 FA can reduce obesity by inhibiting adipocyte differentiation. Objective: To investigate the potential of a DHA rich supplement (fish oil; FO) over non DHA (sunflower oil; SF) to reduce weight gain in dogs, when the animals were fed increased dietary energy intakes above maintenance levels (iso-caloric in both groups). The transcriptional, metabolic and phenotypic responses in two dog breeds were assessed. Design: Six beagles and greyhounds were divided equally into two treatment groups (FO and SF). During four weeks, energy intake was increased 20-80% above maintenance level achieved by feeding basal diet (kibble), coconut fat and supplement (FO or SF). Changes to body weight gain (BWG), apparent nutrient digestibility (Dry Matter, Fat, and Gross Energy), red blood cell (RBC) FA levels, White Blood Cell (WBC) inflammatory gene expression levels (HSP90, HSP70 and IL1β) were measured twice: at days 0 and 28. A paired t-test was used to determine significance between time periods, treatments or breeds. Outcomes: BWG in beagles fed FO were significantly higher (P<0.05) than those fed SF. No significant differences in BWG in greyhounds or between breeds were shown. Differences were not seen in apparent nutrient digestibility. HSP90 gene expression was up-regulated in the beagles fed FO (P<0.05), while HSP70 gene down-regulated for both breeds fed SF (P<0.05). A significant breed difference was observed for HSP70 fed SF (P<0.05). RBC EPA levels significantly increased in both breeds fed FO (P<0.05) with no significant difference in DHA levels. Only beagles fed SF showed significant increase in LA, AA (P<0.05). Both breeds fed SF showed significant decrease in GLA (P<0.05). Significant breed differences (P<0.05) were found in RBC FA levels of EPA (FO), LA, GLA and AA (SF). Conclusion: Results from the four weeks trial do not support the potential of DHA n-3 FA to reduce BWG. However, within the same time frame, WBC inflammatory gene expression and RBC FA levels showed differences between treatments and breeds. Therefore breed differences warrant further investigation and future studies should be designed for extended periods with larger datasets.