Adolescents often fail to meet the intake requirements for micronutrients with their food. On the other hand, increasing number of individuals are supplementing their diet with dietary supplements (DS). The aim of the present study was to determine whether health risk due to inadequate diet and/or use of DS exists among Slovenian adolescents. Data on DS use, physical activity (SHAPES questionnaire), micronutrients intake (two times 24-h recall), eating habits (KIGGS questionnaire), as well as socio-demographic and anthropometric characteristics (body height and body mass) were collected within the ACDSi (Analysis of Children's Development in Slovenia) cross-sectional study, performed in 2014 on a representative sample of Slovenian adolescents aged between 14 and 19 (N=1463), enrolled into 15 secondary schools in different Slovenian regions. The results of our study demonstrated that DS use is widespread (69 %) among adolescents, who use DS mostly by their own decision (41 %) or follow the advice of their parents/other relatives (30 %). DS users are more likely to be males, physically more active individuals, members of sports clubs, more often involved in team sports, and daily DS users also spend more time per week for training than other adolescent athletes. The use of DS in adolescents contributes significantly to their absolute intake of most vitamins and certain minerals, which in DS users increases the percentage of individuals, who meet the recommendations for daily intake of certain vitamins and minerals set by the German Food Association (DGE), but not to the extent that DS users would exceed the upper tolerable level for daily micronutrient intake. Results also demonstrated that with their diet alone (without DS) adolescents consumed less vitamins and minerals than recommended by the DGE. On the other hand, adolescents consumed 2-3 times more sodium than recommended minimum value by DGE. The results of our study indicate that intake of some vitamins and minerals with diet alone is significantly higher in vigorously active adolescents, as compared to other adolescents. Nevertheless, the percentage of individuals, who met DGE intake recommendations for certain vitamins and minerals, was not significantly higher in vigorously active adolescents, as compared to others (with the exception of vitamin K in males). The low intake of most vitamins and minerals in the vast majority of adolescents is due to the fact that compared to the Optimized mixed diet (OMD) recommendations, around three quarters of adolescents of both genders consume significantly less fruits (36 % of OMD recommendations) and vegetables (30 % of OMD recommendations), as well as less milk/dairy products (40 % of OMD recommendations), cereals/cereal products (54 % of OMD recommendations), and fish (33 % of OMD recommendations). On the other hand, almost three quarters of adolescents exceeded the recommended amount of meat/meat products (320 % OMD) and sweet/savory snacks (453 % OMD). Based on the results of this doctoral thesis we emphasize the necessity to significantly improve the adolescents' nutritional habits, so that they will be able to meet the intake requirements for all micronutrients, which applies to all adolescents, regardless of the extent of their phisical activity. Adolescents need to be better educated about the importance of healthy nutrition and possible negative effects of unnecessary DS use.