Nutrition of patients in Intensive Care Units (ICU) is a critical treatment that needs to be optimized according to the specific needs of each patient. This study concerns the problem of optimally mixing commercially available formulas for enteral nutrition to achieve any desired mix of nutrients. The mathematical model has been discussed and improved according to feedback received from physicians in ICUs; it is solved in negligible computing time by a free integer linear programming solver. A software tool has been developed with a user-friendly interface, to allow extending the testing phase to several hospitals; it allows studying the trade-off between different objective functions.