Nutrient intake, loss, metabolism, and requirements are subject to considerable changes over time and between individuals. Even intake can never be constant. This is in spite of the fact that most foods are always available nowadays, due to extensive world trade. Other factors like age, gender, or a person's state of health, lead to varying nutrient needs, different metaboliza-tion, and storage capacities.
The fact that none of the measurable parameters have a "normal" value is a result of the important impact of genetic variability: Instead, there is just a more or less narrow normal range. One of the causes is a certain variability of the amino acid sequence of proteins. For instance, there are several forms of hemoglobin, which differ in their oxygen-binding capacity. Under normal conditions this does not necessarily affect their physiological function. But in some cases (sickle-cell anemia, thalassemia) it does. A similar situation can be assumed to exist with regard to enzymes and transport proteins involved in nutrient metabolism. These variables, based on mostly intracellular conditions, need to be factored into the evaluation of individual nutrient requirements.
The amount of a particular nutrient in the blood plasma is usually not a good parameter for determining nutrient availability. Nevertheless, the body frequently uses plasma content of nutrients as internal reference value (A). Hormonal and nonhormonal mechanisms regulate uptake, excretion, and/ or release from storage in such a way that the registered value in the plasma is equal to the internal reference value. The function of this homeostasis is to ensure adequate nutrient supply to those tissues that need them most urgently at a given time.
The example of vitamin A shows that these homeostatic mechanisms often preclude a simple assessment of nutrient availability from easily accessible compartments like blood (B). With a sufficient vitamin A supply, the vitamin A content of the liver—its main storage organ—is 300-1000 |ug vitamin A/g. Serum content ranges between 50 and 90 ug/dl (with individual variations). Even if no more vitamin A is consumed, the blood level is maintained for 12-15 months during which the liver contents continue to decrease. A marginal deficiency in the serum is detectable only during the last stage, just before complete exhaustion of liver storage.
Additionally, the wide range of normal values makes it hard to interpret serum values. Consequently, a serum value within normal range is of no diagnostic value and cannot be used to infer the vitamin A status of the entire organism.
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