This study is aimed at determining the content of water soluble vitamins in amateur sportsmen before and after the use of reduction diet. Twenty adult male amateur sportsmen aged between 20 and 43 qualified for this study. The participants adhered to individually adjusted reduction diets for 3 months. Diet supplements were excluded from the diet during that time. Vitamins were analyzed before and after the reduction diet using HPLC. The following vitamins were analyzed: B1, B2, B3, PP, B5, B6, B7, B9, B12 and vitamin C. Statistically significant increase of riboflavin in blood plasma (p=0.0254) and statistically significant (p=0.048) decrease in the concentration of niacin was found when compared to the period before the diet. Appropriate level of riboflavin can be obtained by the consumption of proper amount of diary in the diet. Supplementation with PP is advised and, in case of improper diet, the supplementation of B1, B7 and B12 may also be considered.