Purpose The purpose of this study is to evaluate the nutritional effects in Wistar rats of supplementation with stand-alone saturated fatty acid (SFA) or monounsaturated fatty acid (MUFA), the replacement of SFA by MUFA and the combination of both (SFA + MUFA) over a long period of time (13 weeks). Design/methodology/approach In total, 30 Wistar rats were used and randomly assigned to receive (n = 6): control – lab chow; lard (L20%) – lab chow with added lard (20%); olive oil (O20%) – lab chow with added olive oil (20%); lard replacement with olive oil (L20% –O20%) – during six weeks lab chow with added lard (20%) replaced by lab chow with added olive oil (20%) given during the past seven weeks of the trial; lard combination with olive oil (L10% + O10%) – lab chow with added lard (10%) and olive oil (10%). Food and caloric intake, weight gain, food and energy efficiency, body mass index, bone mineral composition and blood biochemistry were evaluated. Findings All diets with added fatty acids showed higher energy intake (p < 0.001), weight gain (p = 0.01), accumulation of adipose tissue (p = 0.02) and food and energy efficiency (p = 0.01) compared to the control group. All groups exhibited higher levels of blood triglycerides compared to the control group (p = 0.02). In addition, the L10% + O10% group developed hyperglycemia (p < 0.001); the L group showed higher amounts of non- high density lipoprotein (HDL-c) (p = 0.04); and the L20%−O20% group exhibited high levels of the triglyceride/HDL-c ratio (p = 0.04) in relation to the control. Originality/value These results indicate that regardless of the fatty acid type, consumption in large quantities of fatty acids for long periods of time can cause obesity and dyslipidemia.