Vitamin D Solubility
By Jens Allmer
Vitamin D, in fact a hormone, is said to be soluble in fat. Therefore, it has been cautioned, that too large an intake or production of D3 or more precisely Cholecalciferol can be dangerous since it could be stored in fat. Solubility is not that easy, especially when considering biological systems. First of, anything is soluble in anything to a varying degree. When saying D3 is fat soluble, we also need to consider that there are three different molecules that we mean when we say D3 (see vitamin D for a disambiguation).
When discussing solubility, in chemistry, there are two components, the solvent and the solute. The solvent is the substance that dissolves the solute, while the solute is the substance being dissolved. Too much sol..? Let’s consider sugar and water. Water is the solvent and sugar is the solute. You cannot disolve (make disapear from sight) an unlimited amount of sugar in water. The solution (solvent and solute together) will become more and more viscous (thick) the more sugar you add to a given amount of water. At one point, the sugar crystals will not go into solution anymore and then you determined the solubility of sugar in water. Let’s consider sucrose (table sugar) as a particular sugar. You would be able to disolve the unbelievable but true amount of around 2kg of sucrose in one liter of water. Crazy! Changing the solvent or solute will lead to different outcomes. Increasing the temperature will also make sugar more soluble in water. In biology, another component comes into play. Let’s call it a facilitator or solubilizer for fun to stay with the sol.. nomenclature. These facilitators are well soluble in water and can bind to molecules that are not well soluble in water to solubilize them. This facilitates their transport through the blood which is not pure water, of course. Such solubilizers are typically binding or carrier proteins. For Cholecalciferol, this would be the D-binding protein (DBP).
Let’s have a look at the three molecules associated with the name D3. Cholecalciferol is produced from 7-dehydrocholesterol. Cholecalciferol is hydroxylated in the liver and to some degree in muscle. The introduction of an OH group to any molecule makes is more soluble or more hydrophil (water-loving). Calcifediol (the hydroxylated Cholecalciferol) is further hydroxylated in the kidney thus making it even more soluble (in water) and forming Calcitriol. Calcitriol is inactivated by further hydroxylation thus forming Calcitronic acid. The latter is well soluble in water and is excreted via the kidney. So in terms of solubility in water we get (where Cholecalciferol is least and Calcitronic acid is most soluble in water):
Cholecalciferol < Calcifediol < Calcitriol < Calcitronic acid.
This does not directly speak to the solubility of these molecules in the blood, however. We need to consider DBP which binds these molecules. In fact, the same rules apply to some degree. We can therefore ask how soluble is Cholecalciferol in DBP. We typically call this the binding affinity, though, and not the solubility. Considering this, we have a different outcome (where calcitronic acid has the weakest and calcifediol the strongest propensity to bind DBP):
Calcitronic acid < Cholecalciferol < Calcitriol < Calcifediol.
Cholecalciferol is considered insoluble in water. Calcifediol is also considered insoluble in water. The same is true for Calcitriol which is also considered insoluble in water. For all practical reasons, these molecules are insoluable when compared to the sucrose example above. However, as stated above, anything can be disolved in anything to a degree. For Cholecalciferol that means that 0.000013 mg can be disolved in one liter at 25 °C (reference). That may sound ridiculously little, but it amounts to 1014 molecules of Cholecalciferol in five liters of blood. Now that sounds a lot, but compared to the amount of cells in the human body it is not a lot. For the other molecules, I couldn’t find actual figures, but the solubility in water as stated above holds true due to the chemical properties (each additional OH group increases hydrophility of the molecule and increases solubility in water).
For the binding affinity to DBP, I also couldn’t find much numbers for some of the molecules (7-dehydrocholesterol, Calcitronic acid). These two are however estimated to have such a low binding affinity that they cannot compete with Cholecalciferol. Cholecalciferol also has an extremely low binding affinity (7 * 10-7M-1). Calcitriol has a high binding affinity (4 * 107M-1) and Calcifediol has the highest affinity among these molecules (5 * 107M-1) but only slightly more than Calcitriol.
What is the practical implication of all this? All these molecules are not well soluble in water and need solubilizers. DBP solubilizes all these molecules but works best for Calcifediol. This molecule is also an intermediary storage for D3 and due to its highest binding affinity can stay in the blood for weeks (bound to DBP, of course). Calcitriol stays only for a day or so when bound to DBP. Now, Cholecalciferol when ingested does not readily bind to DBP and does not significantly dissolve in water but is solubilized by a different molecule: chylomicrons. Interestingly, the chylomicron bound Cholecalciferol is readily channeld to fat cells and muscle cells. Perhaps a story for some other time.**
