Currently used dietary recommendations and requirements are generalized. It applies to both healthy and ailing individuals. These recommendations are meant to avert leading chronic illnesses such as: type 2 diabetes mellitus, obesity, hyperlipidemia, cardiovascular diseases and hypertension. In the future it might be possible to give dietary advice tailored to every - sick and healthy - individual. Nutrigenomics and nutrigenetics are two fields derived from nutrition science and genetics. Their main goal is to elucidate the influence of interactions between genes and diet on individuals' health. This paper shows the examples of metabolic response changes according to diet and chosen gene polymorphisms. It will enable an effective prevention or management of chronic diseases by accurate diet and lifestyle matched to an individual's genetic makeup. It could be useful especially to define predisposition for type 2 diabetes mellitus in young children. It will be possible to change their diet and lifestyle so that they could avoid this chronic disease. There will also be a possibility to detect early the beginning of the illness and choose/select proper treatment. It is important because type 2 diabetes mellitus frequency is up to 90% of all cases of diabetes mellitus. It's often diagnosed too late and a lot of patients have already developed complications caused by this disease. Successful dietotherapy will also be available in such disease entities as dyslipidemias, hypertension and micro- , macronutrients and vitamins defficiences.