Magnesium Hydroxide: This form is often used as an antacid and/or in laxatives. It can be found in over-the-counter products, such as milk of magnesia. Because It has poor bioavailability, it is considered one of the least optimal forms to use as a supplement. Magnesium Oxide: This form of magnesium is one of the most commonly used in supplements. It is desirable because it is inexpensive and the compound is very small, so large amounts of elemental magnesium can be delivered without taking up much space in a tablet or capsule. Magnesium oxide has long been considered a poor source of magnesium,
It is now clear that poor bioavailability limits curcumin’s use. With respect to the whole food approach in particular, large quantities of turmeric powder can be consumed and may not result in beneficial concentrations of curcumin in the blood.
Trials have shown that low levels of curcumin are present in the blood samples of participants administered high doses of curcumin (Anand et al., 2007). Some estimates suggest that only about 2% of curcumin taken orally is transported via the blood to the tissues. Furthermore, Pan and colleagues showed that 99% of curcumin in the bloodstream is conjugated curcumin (Pan et al., 1999). Studies have shown that conjugated curcumin has significantly lower bioactivity as compared to Free Form Curcumin and is readily excreted by the kidneys (Ireson et al., 2001; Pal et al., 2014; Shoji et al., 2014; Choudhary et al., 2015). Moreover, conjugated curcumin is also bulkier since it has a glucuronide or sulphate group attached, so less able to cross the lining of blood vessels and the blood brain barrier (BBB) (Krishnakumar et al., 2015)
Factors for Poor Curcumin Bioavailability Are:
Insolubility / Instability
Turmeric is highly insoluble and has a very hard time dissolving in water, much like sand. Its curcuminoids are hydrophobic and lipophilic (i.e. water hating, fat loving), and as a result, have a high tendency to clump and then crystallize as the pH gradient shifts through the digestive process. This significantly inhibits absorption into the gut and bloodstream.
Unfortunately, what does manage to escape is readily broken down by the liver through biotransformation.
Rapid Elimination / Short Half-Life
The rapid elimination of curcuminoids through the bowel further reduces curcumin’s bioavailability since its half-life is less than two hours, with little curcumin previously absorbed into the bloodstream (Sharma et al., 2001). Half-life refers to the time at which half the amount (50%) of an ingested substance will have been eliminated.