Nutrients Must Be Soluble

In order to use the nutrients that we take in when we eat, we must first break the food down into its nutritive components. These components are then either absorbed by the body, or they pass through the intestinal tract and are removed from the body in the feces. The nutrients that are absorbed pass through the lining of the intestinal tract into the blood. The blood carries these nutrients to the sites where they will be reassembled and used by the body. If the nutrients are not used immediately, they will either be stored for later use, or excreted in the urine. Each of these fates of the absorbed nutrients (immediate use, storage, or excretion via urine) requires that the nutrients be soluble. To be transported from the stomach to other parts of the body, the nutrients must either be soluble in water (the main component of blood plasma), or be solubilized by some other particles (e.g., proteins) that are carried in the blood. Nutrients that are stored in the body are typically stored in fat cells, so they must be soluble in fat. And of course, to be excreted via the urine, a nutrient must be water-soluble. Hence, understanding the solubility of nutrients in the different substances of the body is very important for understanding how they can be used or processed in the body.

Scientists have developed several ways to discuss the important concepts of solubility. For salts (ionic solids) that dissociate into ions in water, such as the compounds containing the dietary minerals, a solubility product (K_sp) is typically given. The solubility product is the equilibrium constant for the dissociation reaction of the compound into ions in aqueous solution. This quantity is useful, for instance, in determining which compound containing a given mineral is more soluble, and hence would be better absorbed as a dietary supplement (e.g., calcium carbonate vs. calcium sulfate). The solubility of organic molecules, such as the vitamins, is quantified using a different scale known as Hidebrand solubility parameters (which will not be discussed in this tutorial). Organic molecules may be soluble in water or in lipids, depending on the functional groups on the molecule. A vitamin's solubility in water or in lipids determines where it can be used, and whether it will be stored in fat cells or excreted from the body if it is not needed for immediate use.