The B Vitamins
The current reasoning is that since B vitamins are soluble in water, any excess taken would just be quickly flushed out of the body so you can safely take any amount, even mega-dosages. Researchers have now discovered that this is not entirely true, but many health advisors apparently are not yet aware of these facts. And Vitamin companies that are increasing the vitamin dosages in formulas also appear to be unaware of this new information. Folic acid and vitamin D are two nutrients dramatically increased in supplements by 100% to 500% recently. Without knowing the true facts, these 
changes might put certain subsets of the consuming public in jeopardy.
Nutrient absorption, utilization, and effectiveness have now become paramount for health maintenance and proper dosage recommendations.
You might think this is a well studied area, but in reality very little research existed prior to 1995. The last ten years finally brought some insightful information, but for some reason, it hasn't filtered down to the public in a meaningful presentation. Media News and product marketing have repeated clichés which do little justice to reveal these truths. Strange, especially since these aspects are some of the most telling statistics for supplement validity. Maybe it is because if you knew this information, you would not buy into the mega dose mania without supervision. Some products have scientific support while some stretch a glimmer of truth beyond recognition. Here is a chart that shows the absorption pathways for each vitamin, but not the absorption percentages. This chart shows nutrient absorption percentages from different foods.
Liquid multiple vitamin companies try to gain an advantage by claiming greater absorption over tablets or capsules. While it sounds logical, there is very little actual proof, unless of course, the tablet does not breakdown according to USP 30 minute requirements, or the digestive system is compromised, often a factor for Seniors. No vitamins are 100% absorbed from any source as claimed by liquid vitamin marketing, especially for the fat soluble vitamins A, D, E & K which would be jeopardized by a liquid water rather than a fat emulsified delivery system. Yes, some liquid vitamins might get absorbed faster, but properly prepared capsule or tablet vitamins catch up soon enough. The actual differences for some nutrients between liquid and capsules may be less than 5% in healthy people, with the liquids not even always on top. Some Vitamins in a liquid medium are very unstable. Nasal sprayers and sublingual dissolving tablets have slight advantages for some nutrients, but not as much as promoted when scientific research tests absorption.
How Absorption Rates are Determined?
There are about 4 factors Scientists consider:
- Blood cell and blood plasma content and saturation points,
- Percentage that spills into urine and the first point spillage starts.
- Elimination rate from blood plasma and amount of metabolites formed. Metabolites are formed as the vitamin begins to get processed. Metabolites can be co-enzymes which do most of the work attributed to vitamins or the forms created when these co-enzymes breakdown.
- Tissue content and saturation point. Very difficult to measure and determine for some nutrients.
Generally, blood plasma reaches a saturation point first, then spillage into urine, next blood cell saturation, and last tissue saturation. All the while fecal elimination is progressively increasing, and at mega-dosages can reach very high percentages. A test measuring just plasma levels would not necessarily give blood cell levels or tissues levels. Measuring the metabolites formed is often influenced by other nutrients needed for their generation. The time it takes to reach these different saturation points is governed by time, body size, and dosage amount factors. It is possible that lower intake levels could actually reach and maintain saturation points before mega-dosages since the body develops defensive processes to quickly eliminate higher amounts, often by drastically turning off absorption mechanisms.
Vitamin B1 Thiamine
Absorption of thiamine at dietary levels is by an active transport protein carrier, while at higher mega dosages, absorption is be passive assimilation.
UPDATE: The next study is from a now very old work. It appears the Scientists used plasma levels to measure increase in absorption. It is now known that plasma levels remain low while absorption levels can be very high. Thiamine is rapidly eliminated if already sufficient. Current absorption levels now are at 95%. Rather high, but even in these new studies, the plasma levels did not increase very much.
One of the best studies on Vitamin B1 assimilation reports that intake of 2.5 mg is the amount for the top percentage absorption rate at about 30%. Over 5 mg, percentages rapidly decline until at about 20 mg there is little if any further increases found in blood plasma or urine metabolites. This is because vitamin B1 has an active absorption process and needs a carrier element, usually a protein, to transport it across the intestinal wall barrier. At higher intake dosages, there could be a small amount of passive absorption taking place as well. ref
Update: new research on Vitamin B1 shows that at mega amounts, 100 mg - 1500 mg, the passive transport overrides the controlled active process and follows nonlinear increasing blood levels. This is far beyond any nutritional application since foods do not naturally have such large amounts. Plus, there could of course be increased absorption interference of other nutrients which is why these mega-doses should be monitored by a health professional. This new study did not access the biological aspects of these higher dosages. ref
Vitamin B2 Riboflavin
Vitamin B2 is absorbed by a passive osmotic gradient method and exhibits a generally straight line linear increase up to at least 20 mg. Measuring the amount excreted reveals at least 60% of the ingested dose is absorbed. Over 2.5 mg it begins to be eliminated in urine, thus the yellow color.
Vitamin B6 Pyridoxine
Vitamin B6 absorption was extensively reported in 2008 from the over 6000 participant National Health and Nutrition Examination Survey of 2003-2004. One interesting fact that emerged was that blood levels of the co-enzyme form of B6, pyridoxal 5' phosphate (PLP) decreased as men aged unless they took supplements. Young Women have lower levels due to such situations as child-birth and oral contraceptive use. The recommended amount for vitamin B6 established by the Government Nutrition Council is 2 mg. This amount falls about 50% short of protecting the last 10% of the population. At 3 to 4.9 mg, most people would be covered and only a few would still need higher intakes. Vitamin B6 at 3 to 4.9 milligrams was enough to keep Seniors protected from hyperhomocysteinemia, a possible factor in artery disease. This is why the Vitamin Workshop's new vitamin criteria set it's standards for Vitamin B6 between 5 to 20 mg. About 4 mg twice a day would be ample for blood cell saturation and ample tissue content. B6 also has a role to play in water balance within the body.
Vitamin B5 Pantothenic Acid, Pantethine,
Vitamin B5, pantothenic acid, can be utilized at slightly higher amounts, maybe up to 10-15 mg. The need is established at 7 mg. for adults. Since B5 is widely distributed in foods, it is rare that a deficiency would be found. Stress increases need.
Niacin, and Niacinamide
The recommended amount for Niacin is 30 mg. 50 milligrams will give a flushing sensation to the skin surfaces since Niacin opens up blood capillaries. The skin becomes red. As soon as the liver processes or buffers it, the flushing stops. Niacinamide is the buffered form of Niacin and does not cause a flush. Niacin is not a true vitamin in the strict definition for a vitamin. The body can build niacinamide from the amino acid tryptophan in 9 steps. Thus, the FDA adjusted the requirement for niacin or niacinamide or nicotinic acid, or nicotinamide, which are all the small thing with the amides representing a buffered form, to reflect this conversion from tryptophan to list as 1 niacin NE. The NE stands for Niacin Equivalant. Both 60 mg of tryptophan and 1 mg of niacin = 1 NE niacin.
CAUTION: Mega-dosages of NIACIN are being used to lower cholesterol. It is very effective when added with statins. BUT, you have to be monitored by a health Professional since the action of niacin OVER TIME can damage eye capillaries and liver functions. Do NOT take on your own.
This is the B vitamins reality. They are vitally critical to health, but only needed and utilized in very small amounts. More is not always better. Tissue levels would be the only area where extra might initially help, but offer no further benefit once tissue saturation is achieved and maintained. Once blood cells are saturated, or blood plasma, most extra spills rapidly out in urine. With B1, excess consumed over 5 mg is most likely not absorbed by the turning off active transport and is eliminated out the colon. At very large dosages, over 100 mg, passive absorption appears to have a linear increase through 1500 mg. This is not a normal nutritional application since foods rarely have over 2 mg.
Other B vitamins like Biotin and B12 don't seem to have problems at higher amounts, Biotin is set at 30 mg. With B12, large amounts can be utilized at one time and stored for later use, unlike the other water soluble B vitamins. As people age, B12 absorption is usually reduced and intakes have to increase to compensate. A similar situation exists for vitamin D. Fat soluble vitamins need a separate discussion.
OTHER FACTORS
The above information regards isolated vitamins. Vitamins from foods seldom if ever get to mega-dose milligram amounts so it is not much of a concern. But there are other concerns for vitamin supplements that influence absorption. A few nutrients have many different formats and each one could vary absorption by a few percentage points. How the tablets are made, coatings, fillers, and binders can all affect breakdown rates and thus absorption. Capsules can breakdown at different rates as well. Digestive conditions of course can dramatically affect rates and genetics also plays a role. So, science has to deal with these many specifics by using generalities to cover as many people and conditions as possible when establishing recommended guideline dosages.
CATCH 22
At the mega vitamin dosages some nutritionists recommend, such as for B vitamins at over 100 mg, saturation is achieved at much lower dosages and the extra over the saturation point often quickly generates greater elimination and even absorption interference? Surely they must have knowledge of these facts, especially since the phrase, "expensive urine" is mentioned often enough. Is there a therapeutic effect at monster potencies that overrides normal digestive principles? Can tissues supersaturate? Just because companies put out such high dosages of vitamins does not prove greater utilization or effectiveness. Research is lacking to answer all these questions concerning mega dosages. Some known facts need repeating here.
PRECAUTIONS FOR HIGHER DOSE B VITAMINS
Vitamin B6 and Folic acid have cautions for higher dosages. B6 over 100mg begins to negatively affect the nervous system in a few individuals and folic acid over about 650 mcg per day suppresses the immune system's killer cell activity. Plus it might be a factor in cancer growth rates and dementia progression. New research points out that high dose B1 might also influence cancer growth rates. While Neurologists have positively used very large dosages of some B vitamins in mental therapy, which might reflect serious genetic challenges to vitamin processing or metabolite conversions, this is beyond the nutritional scope of this website. A very real possibility exists for single vitamin mega-dosages to crowd out other vitamin absorption allowing it to achieve higher plasma levels and force greater uptake into the brain.
Satisfying nutritional needs is priority one, prime consideration for this discussion and new vitamin criteria.