Nutrient bioavailability, estimated as absorbability alone, can be measured by various in vitro methods. In vitro methods have obvious distinct advantages in that they are less expensive, rapid, and amenable to high throughput analyses. Often, experimental in vitro methods involve an initial 'digestion phase' where the food is treated with acid and digestive enzymes to simulate the initial steps of food breakdown. The digestion phase is then followed by a second phase wherein the goal is to estimate the potential relative availability of a nutrient. This usually involves the measurement of the concentration of the soluble nutrient of interest in a supernatant of the digested food following centrifugation or after dialysis of the digested food products across a semi-permeable membrane designed to select only low-molecular-weight complexes. Variations on this theme include the addition of radioactive isotopes following the digestion phase and the in vitro measurement of cellular uptake of the nutrient in a cell culture preparation or some appropriate index of nutrient uptake. In the case of iron, for example, cellular synthesis of ferritin, an iron storage protein, has been used. Similar applications of this in vitro technique are now appearing in the scientific literature for the measurement of phytochemical bio-availability, such as beta-carotene, lycopene, lutein, etc. However, although promising at the moment, there is little confidence that these methods can adequately replace in vivo methods of measuring nutrient bioavailability.
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