Vitamin Absorption

This topic generates a lot of rhetoric for supplement marketing stories, very little of it factual. Knowing the science for absorption pathways is like seeing a magician's tricks from the back side. The Lady doesn't really disappear. For vitamins, this adds clarity to dosage interpretations. Here are some of the back side glimpses into vitamin absorption:

• No nutrients are 100% absorbed
• Time release tablets can both increase or reduce (mostly) absorption percentages depending on the type of release mechanisms used, the digestive capabilities of each person, and the digestive method for each nutrient. The general rule is not to use time release for the majority of nutrients, except maybe Alpha Lipoic Acid. 
• Calcium averages about 33%. More or less depends upon body needs and amount ingested.
• Most calcium supplements are within a 5% point difference in comparative absorption studies
• Nutrient promotions that claim 75% are usually referring to a solubility percent indicating potential absorption rather than actual absorption.
• Calcium first has an active absorption process regulated by calcium blood levels, and then further down the intestinal tract, calcium and the other minerals compete for abosprtion through osmotic gradients.
• This free competition of minerals at this point highlights why excess calcium can hinder abosprtion of other minerals when un-balanced amounts are consumed.  
• There is little proof that liquid vitamins are better absorbed than powder or capsules, except when there are major digestive limitations and /or the tablets do not quickly breakdown. 
• There are some quick release tablets that dissolve faster than capsules.
• Vitamins in a liquid form are active and need more stabilizers to prevent breakdown or interaction with other nutrients.
• There are limits to how much of a vitamin can be absorbed at one time. Mega-dosages of many nutrients are largely wasted, like calcium and vitamin B1.
• Some nutrients like vitamin B1 ( and other B vitamins) have an active carrier system to aid absorption. ref  
• This system is reduced or down-regulated when vitamin levels are normal or in excess and increased when vitamin or nutrient levels are low.
• Concentrated nutrients can crowd out other nutrients for absorption sites. Vitamin E over K, zinc over copper, calcium over magnesium, Iron and vitamin E interaction, etc
• Vitamins and minerals often compete for the same carrier proteins.
• Intestinal wall docking stations can become saturated.
• Digestive processes breakdown most if not all supplements into isolated parts. 
• Natural vitamins often enjoy increased absorption advantages over synthetic vitamins, but there are some exceptions, like folic acid.
• High fiber diets can slow absorption down and / or reduce assimilation percentages, which can be either an advantage or a detriment.
• Fat soluble nutrients in powder form have a very low absorption percent. i.e. CoQ10 @ 5%.
• Emulsified oils in base increase fat soluble vitamin absorption by up to 300% or more.
• This research (<looking for unrestricted site for blocked reference) shows a 5 times greater amount of vitamin D absorbed in normal subjects compared to those with diminished fat metabolism 
• Some nutrient forms do have as much as 300% better absorption percentage over other forms of the same nutrient.
• "Food Form Vitamins" may have some absorption advantages. 
• Vitamin B1 is absorbed at about 30% up to 2.5 mg. ref The percentages rapidly decrease with higher dosages. From 5 to 20 mg, the extra absorption is only about 1.3%, most is eliminated thru colon. (NOTE: this is from a very old study, and may just reflect the amounts measured in plasma at a point after ingestion. Actual absorption might be far greater. A European study puts it at 95%, that is rather high.) 
• Vitamin B2 is absorbed on a linear increase up to about 20 mg. then rapid decrease. One study showed an increase in excretion of vitamin B2 of over 90% from extra over the base requirements. ref