<p>We reported earlier that adult Fabpl<i>Cre</i>;<i>Apc</i>^15lox/+ mice with either daily UV exposure or vitamin D supplementation developed not significantly fewer intestinal tumours but less tumour bulk (reduced outgrowth) than control mice that remained vitamin D deficient. Here, we report on a prior exploratory study in which parents (Fabpl<i>Cre</i> and <i>Apc</i>^15lox/15lox) and pups were fed either the vitamin D deficient (&lt; 5 IU/kg) or supplemented (1500 IU/kg) diet before selecting the proper genotype and continuing on in adulthood with the respective diets. Then, a lifelong vitamin D rich diet did significantly reduce the number of intestinal tumours from 10.9 to 7.5 tumours/mouse (<i>p</i> = 0.02) at an age of 200 days. For this reduction, the vitamin D apparently needed to be already present in the earliest stages of tumour initiation shortly after intestinal truncation of floxed <i>Apc</i> alleles in utero. If our experimental results apply to other vitamin D susceptible cancer types, it may explain differences between observational studies on (long term) vitamin D statuses and randomized trials on (4–5 years) vitamin D supplementation in relation to cancer risk.</p> Graphical abstract <p></p>