Nutritargeting.

The term "nutritargeting" in analogy to the term "drug targeting" means targeting nutrients to specific "target" tissues. What is the rationale for this idea? Some tissues obviously are able to accumulate micronutrients selectively and to use them predominantly for specific functions. It has, for instance, been known for a long time that the accumulation of beta-carotene in the skin does not only provide a "golden-yellow" color but considerable antioxidative protection as well. Yet beta-carotene is only one of many antioxidants, which can be detected in the skin. Other carotenoids, for example, lutein and zeaxanthine, are preferentially found in the macula lutea, the so-called yellow spot in the eye. Here, carotenoids are subject to a metabolism typical for that tissue, which cannot be found in other tissues (e.g., formation of meso-zeaxanthine). In addition, they can specifically be absorbed into the macula. In the macula, they protect the retinal pigment epithelial cells against oxidative damage from UV light. Indeed, these two carotenoids can be protective against age-dependent macula degeneration. Another example is the tissues that are particularly rich in vitamin C, for example, the cortex of the suprarenal gland or the lens: here, vitamin C fulfills both antioxidative functions and metabolic ones as it helps in the formation of collagen structures. Approximately 40% of the body's ascorbate is stored in skeletal muscle because this tissue is relatively abundant and its cellular concentration is tenfold higher than the plasma level. Similarly, the intracellular ascorbate concentration in the brain (3 mM) greatly exceeds the level in the extracellular fluid (200-400 muM). The majority of ascorbate is stored in the astroglial cells that are capable of reducing great quantities of DHAA to ascorbate, which then becomes available for release back into the extracellular fluid. Thus, the accumulation of vitamins respectively micronutrients in single tissues is not limited to a pure storage process like the storage of vitamin A in the liver, but is often connected with important and tissue-specific metabolic functions. When single micronutrients are applied for prevention or even intervention in diseases of organs or tissues, they are usually administered in higher doses for a longer period of time. The hope is to accumulate it this way sufficiently in the tissue and to thus be able to ensure the therapeutic success. This procedure, however, leads to a "flooding" of the whole organism with micronutrients and their potential enrichment in tissues which would usually not accumulate the respective micronutrient. Thus, unexpected side effects may occur. An attractive solution to these problems in the future could be to wrap up or apply micronutrients in such a way that they can selectively reach the targeted tissue. For this approach, called "drug targeting" by pharmacologists, one could introduce the analogous expression "nutritargeting" with respect to micronutrients. For such a nutritargeting there are already a lot of examples and developments which show that it is possible to accumulate micronutrients in target tissues while simultaneously circumventing or protecting other tissues. A substantial requirement for the development of "carriers" for nutritargeting is the availability of procedures or specific carriers, which allow the selected nutrients to bypass the main barriers that are encountered when, for example, circumventing the enteral route in the targeting process. The entrance areas for such a targeting are the nasal mucosa, the oral mucosa, the cornea, the skin, or the lung. In the case of enteral application of proteins, the packaging has to resist gastric digestion and the body must be able to absorb the particles through the intestinal mucosa without hydrolyzing the proteins in order for them to reach the systemic circulation. Another field in which nutritargeting may play an important role is the diseases where either systemic absorption is not possible (e.g., malabsorption/maldigestion) or where local deficits occur, which may not or only inadequately be supplied by systemic application.