A controlled dietary intervention study will be carried out based on the stable isotope method the investigators have developed to quantify both the absorption of beta-carotene and its bioconversion to retinol in oil and in a mixed diet. For 4 weeks the participants will consume 2 capsules/day, 7 days/week with each capsule containing 100-mg [13C10] beta-carotene and 100-mg [13C10] retinyl palmitate. For two weeks they will consume a diet with high levels of beta-carotene in vegetables and fruits and for the other 2 weeks they will consume a diet with low levels of beta-carotene in vegetables and fruits supplemented with an extra amount of beta-carotene in oil. Samples of blood and faeces will be taken. The study hypothesis is that the absorption of beta-carotene in oil in comparison with the absorption of beta-carotene in a mixed diet differs by a factor of 3 to 6. The investigators want to measure the influence of the food matrix of vegetables and fruits on the absorption of beta-carotene in ileostomy patients on a western diet.