General Physiology of Nutrient Absorption

Nutrients enter the blood by passing through the intestinal mucosa. Intestinal nutrient transport can occur via two distinct pathways. One is termed the paracellular pathway and represents the movement of a nutrient between the absorptive enterocytes on the intestinal villi. This transport pathway is an energy-independent diffusional process and depends on the electrochemical gradient across the mucosa and its permeability characteristics to the nutrient in question. The characteristics of the diffusional pathway are not regulated in response to nutrient deficiency or excess. A second transport pathway represents the transcellular movement of a nutrient across the intestine. The transcellular transport rate of the nutrient is composed of both diffusional and carrier-mediated transport pathways. Often in response to changes in nutrient status the number of nutrient carriers will be changed to facilitate appropriate increases or decreases in intestinal absorption to help maintain nutrient homeostasis. Within a class of nutrients there can be substantial differences in absorption rates. For example, in the case of minerals, monovalent minerals (such as sodium, potassium, and iodine) are absorbed at very high efficiency approximating complete absorption, while multivalent elements (such as chromium, heavy metals, and iron) are relatively poorly absorbed (1-20%). In addition, because some nutrient transporters, e.g., the divalent metal transporter DMT-1 that is responsible for intestinal iron transport, can also transport more than one type of mineral, unintended alterations in the absorption rate of one mineral may occur by a process of co-adaptation in response to changes in the status or physiologic need for another mineral. For example, in iron deficiency, iron absorption is increased, but the absorption of cadmium and lead are also inadvertently increased. However, these effects (homeostatic regulation and co-adaptation) are responses to a change in physiological state and should not be confused with alterations in nutrient 'bioavailability' per se, a characteristic of an individual food, a complex mixed meal, or a longer term characteristic of a particular dietary pattern.

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