BIOLOGY IS STILL FULL OF SURPRISES
In the process of dissolution of an ionic solute in water, there are two forces taking part in the process, the intra-atomic attraction within a solute which holds the positive and negative ions together as a molecule – this is called lattice energy – and the inter-molecular attraction forces between water and each compound of the solute to pull the ions out of the molecule – called hydration energy.
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A solute dissolve in water when hydration energy > lattice energy. This is an exothermic process.
Certain substances in the digestive tract like phytic acid can interfere as well with the absorption pattern of some nutrients even if the nutrients are solubilized; other substances, such as vitamin D, can enhance nutrient absorption. All of these processes are governed by fundamental chemical properties and principles, such as polarity, physical structure as surface area, inter-molecular interactions, thermodynamics, and equilibrium.
When assessing an additive, it is important to integrate these elements. Even if two products have a similar chemical composition, their dissolution kinetics and their absorption could be significantly different.
If we take the example of Zinc oxide, the dissolution kinetics will vary depending on the chemical structure (zinc sulfate is more soluble than zinc oxide) but as well on their physicochemical properties. Sources with low specific surface area will not dissolve, being inert in the gut, whereas sources with high specific surface area will optimized their dissolution kinetics.
These differences of patterns will highly affect these ingredients absorption, especially in the presence of phytic acid. Zinc Sulfate will quickly dissociate into Zn2+ and these free cations will be captured by the phytic acid to form phytate complex.
If the objective behind the usage of Zinc Sulfate is simply nutritional, there will always be sufficient free Zn2+ circulating to be absorbed and cover the minimum dietary requirement of 80ppm. But if the rationale behind the supplementation of Zinc Sulfate is to leverage the antibacterial properties of Zn2+, there will not be sufficient circulating Zn2+ to be effective along all the digestive tract.
In that latter scenario, a product with slower solubilization pattern will turn out to be more effective to control bacterial population. The ingredient needs to be sufficiently soluble to provide sufficient free ions to act on the bacterial population but not too fast to avoid releasing all the free Zn2+ at once and avoiding the complexation by phytic acid. Despite all the ingredients providing Zn2+ ion, it is critical to select the source with the proper dissolution pattern in order to fulfill its role.
Another illustration comes with the different salts of formic acid. Formic acid is used in animal nutrition as a strategy to limit proliferation of intestinal bacteria around weaning. Formate is indeed transformed into the stomach into formic acid that then penetrates the bacteria and kill it by acidification. But, to be transformed, formate needs to be dissociated between the cation (Na+, Ca2+) and the anion (COOH-).
Different sources of formate are available on the market either as sodium formate or calcium formate. Despite both having a similar chemical composition, their solubility is very different. Sodium formate is 6 times more soluble than Calcium formate (970g/liter of water for Sodium formate against 170g/L for Calcium formate). Therefore, it will require a dosage and cost 6 times higher when using Calcium formate to provide the same quantity of formate and thus a similar effect on bacteria.
In an acidic solution, the dissolution of tends to be increased. Indeed, the high concentration of H+ of acidic solution will combine with the anions released and accelerate further the dissolution reactions. Therefore, if we want to measure the solubility of a compound in the stomach, it is critical to reproduce the similar acidity.
Temperatures play as well a critical role on compound dissolution kinetics. For many solids dissolved in liquid, the solubility increases with temperature. The increase in kinetic energy that comes with higher temperatures allows the solvent molecules to more effectively break apart the solute molecules that are held together by intermolecular attractions. But the sensitivity of compound solubility to temperatures varies a lot depending on the energy binding the two components. If the links between the two components is weak, a slight increase of energy will be sufficient to quickly reach (cf NaCl in table below). Otherwise, an important amount of energy will be required to accelerate the dissolution of the components.
We tend to believe that all molecules with similar chemical structure will have similar properties. Actually, chemistry and biology are more complex than that and many other factors need to be taken into consideration before concluding.