In the treatment of lifestyle diseases, including metabolic syndrome and type 2 diabetes, it is important to lower body mass and fat tissue, and consequently, to increase insulin-sensitivity. Unfortunately, it often happens that low- energy diet which would lower overweight is not observed and, thus, it does not bring the expected effects. This paper discusses the influence of three diets — control, high-fructose, and high-fatty diet — on absorption of energy from food in order to transform it into body mass. The kJ/g ratio which describes this process has been calculated. In the tested diets, the addition of fructose (79.13±2.47 kJ/g) or fat (82.48± 2.28 kJ/g) results in higher transformation of energy into body mass than in the case of control diet (89.60±1.86 kJ/g). The addition of Na[VO(O 2 ) 2 (4,4 ′ -Me 2 -2,2 ′ -bpy)] • 8H 2 O (where 4,4 ′ -Me 2 -2,2 ′ -bpy=4,4 ′ -dimethyl-2,2 ′ -bipyridine) results in sta- tistical increase of that ratio: fruct ose diet (86.88±0.44 kJ/g), fat diet (104.68±3.01 kJ/g), and control diet (115.98±0.56 kJ/g), respectively. Fat diet statistically influences the decrease of kidney mass in comparison to th e other diets. The application of the tested vanadium compound results also in the statistical decrease of the fatty liver caused by fructose and fat diet.