This study aims to investigate if the proportion of neuroendocrine tumor (NET) patients with normal vitamin values can be increased, with vitamin suppletion and a personalized diet, Effects of the intervention will be evaluated by quantitative analysis of blood and urine and questionnaires. The measurements, will be performed at baseline (t=0), after 4 weeks (t=4) and after 18 weeks at end of study (t=18). Furthermore at t=18 a semi-qualitative interview will be performed.