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Exercise & Fitness Message Board


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[QUOTE=Evl316]I heard or read somewhere recently (don't ask where, I don't recall) that a person shouldn't take a multi vitamin that contains vitamins beyond 100% RDA as the receptors in the stomach can only absorb so much at one time. Although I think this statement was somewhat crap it does raise some interesting questions.

If there is a limited amount of "stuff" that can be absorbed by the receptors over a period of time what is absorbed first and why? Do certain vitamins, minerals, amino acids get absorbed faster than others.

I understand that even if all of the vitamins you are taking aren't all absorbed it's better than not taking them at all. In other words I'd rather have expensive piss than not be getting enough of what I need. [/QUOTE]

Thats not necessarily the issue, to me at least. It can be hard on the kidneys to take high doses of a lot of vitamins, and some can even become toxic but a multivitamin is an easy way to get what you need for the day and I don't think I have ever heard of anyone having adverse health as a result of daily multivitamin use. I don't know which vitamins get absorbed first and how much of what gets used, but part of the reason that the % of RDA is so high for several of the vitamins is because a lot of the usable vitamins can be destroyed in the stomach (like vitamin B12 especially, also like glutamine). So while the supplement may contain 1600% of the RDA of the vitamin, if 5% of that is all your body actually gets and uses, the rest is just padding. So it serves its purpose I guess. I also know that calcium can prevent the absorption of other vitamins, so it's better to take at night if you can. I haven't heard about any other "vitamin competitors", but they may exist.

[QUOTE]But what if a person is taking a large amount of a few certain vitamins or amino acids and taking them all at the same time and they are competing for uptake into the blood stream and not all can make it? Wouldn't it be better to take these things at a more spread out interval so everything gets into the blood stream?[/QUOTE]

This is actually exactly what I do for that very reason. I have never found any studies or recommendations to do this, but supplementing all at once seems kind of excessive, but I take a small handfull of stuff daily.... If Im gonna bother to buy and take vitamins, I want to be relatively sure that my body has the chance to use them. And eating 6x a day gives me ample opportunity. :)

[QUOTE]Honestly I don't really beleive that this is an issue. I mean how many medications does it say to take with food, for example? The idea of a person taking more vitamins than there body can handle at one time doesn't seem likely to me. On the other hand only so much protein can be absorbed by the body at one time so why wouldn't the same be true of other things? I would appreciate other people's opinions on this [COLOR=DarkOrange]undefined[/COLOR] subject.[/QUOTE]

Like I said, it's just my opinion, but I agree with you. I'm sure you can only break down/absorb/use so much at a time, so I take a low dose multi in the morning and a few other things throughout the day and rely on diet to fill in the bulk. Also remember, RDA doesn't really work for a healthy, fit, athletic individual. Your needs will be higher than your sedentary average Joe's.
To the best of my understanding of the questions...


[QUOTE=Evl316]I heard or read somewhere recently (don't ask where, I don't recall) that a person shouldn't take a multi vitamin that contains vitamins beyond 100% RDA as the receptors in the stomach can only absorb so much at one time. Although I think this statement was somewhat crap it does raise some interesting questions.[/QUOTE]
You can take more than 100% of the RDA, up to the Tolerable Upper Limit, but so many things affect how much is absorbed through the stomach/intestine -- the functional state of the digestive system, mucosal surface areas for absorption, certain health behaviours (eg. smoking, alcohol), excessive presence of a certain nutrient that decreases/increases absorbability of another nutrient, the quality of the supplement (eg. organic, synthetic), if the nutrient is from a food or supplement source, whether the correct enzymes needed for digestion/absorption are present, etc.

Excessive fat-soluble vitamins in the body is an example of why exceeding the 100% RDA is not advisable because these vitamins are stored in the liver (or body fat) and therefore, can reach toxic levels.


[QUOTE=Evl316]If there is a limited amount of "stuff" that can be absorbed by the receptors over a period of time what is absorbed first and why?[/QUOTE]
Food stuffs are all broken down and absorbed in different parts of the digestive system, due to the presence of enzymes that can only function in certain acidic levels.

For example, carbs begin their digestion in the mouth, the stomach only digests proteins, and fat in the upper part of the small intestine. Then, the emptying rate of food from the stomach into the duodenum also differ -- eg. solid, semi-solid, fat content; carbs are usually emptied first (easiest to break down), then proteins, then fats. This staggering method ensures that the most absorption can take place in the intestine since the intestine can only handle so much at a time.

Absorption rates in the intestine for all digestion by-products also differ -- for example, vitamin B12 is absorbed in the lowest part of the small intestine (the ileum), while AAs and iron are absorbed in the upper two parts (duodenum and jejunum). As well, digested carbs, proteins, and fats don't all use the same brush border enzymes, receptors, or transporters for absorption (therefore, different means of functioning).

All the above are some conditions that affect rates of digestion and absorption into the bloodstream.


[QUOTE=Evl316]Do certain vitamins, minerals, amino acids get absorbed faster than others.[/QUOTE]
For proteins, they first need to be broken down into peptides and amino acids before being absorbed and this process can take up to 4 hours in the stomach.

For transport across the epithelial layer of the intestine, there are at least 2 types of peptidase brush border transporters situated on the membrane -- one for peptides and one for free amino acids. But which are taken up by the enterocyte first depends on who can find a co-transport partner -- eg. dependent on sodium ion availability. Once inside, the peptides are further broken down into individual AAs before they're absorbed into the bloodstream, though some AA are absorbed faster when in peptide form (eg. Glycine).

AAs are absorbed into the bloodstream by the active transport of one of 4 transport systems (while some AAs are just actively transported across). All 4 transport systems have different absorption rates -- eg. neutral AAs are absorbed the fastest; acidic AAs the slowest.

On the other hand, vitamins and minerals don't need to be digested since they aren't composed of subunits, so I would imagine that they would reach the small intestine earlier than the time needed for a peptide or AA to get there. I'm not sure of the exact absorption mechanism, but the uptake of fat-soluble vitamins (A, D, E, K) is definitely not as fast as that of the water-soluble vitamins (B & C vitamins).


[QUOTE=Evl316]But what if a person is taking a large amount of a few certain vitamins or amino acids and taking them all at the same time and they are competing for uptake into the blood stream ...[/QUOTE]

AAs (as well as fats, glycerol, and glucose) are all absorbed through different areas of the MILLIONS of villi on the mucosa of the small intestine (ie. the absorption area is huge, so competition would likely only occur if there is an excess of one substance over another needing the same carrier/transporter, eg. taking in too much vitamin E reduces the absorption of vitamins A and K).

Also, AAs have those 4 different transport mechanisms, each specialized for a certain type of AA. And they are not all lining up in front of the enterocyte's MANY receptors -- most must be transported across the membrane with an ion. Some people may lack a certain transport system so those associated AAs are out of the picture. Finally, when the AAs are absorbed, some require only simple diffusion, others active transport (different resources are being taxed).


[QUOTE=Evl316]... and not all can make it?[/QUOTE]
Excess amino acids are deaminated to urea in the liver, which is then removed by the kidneys. Unused water-soluble vitamins are also passed out in urine.


[QUOTE=Evl316]Wouldn't it be better to take these things at a more spread out interval so everything gets into the blood stream?[/QUOTE]
It's more like getting to the small intestine first. The supplements would probably have a better chance of reaching the small intestine and being absorbed if they were taken with a meal, as this would protect certain vitamins from being destroyed by the stomach acid. The breakdown of foods can also aid in digestion and absorption, eg. many nutrients can only be absorbed in the presence of other solutes/ions/nutrients/etc. If the body can only use X amount of a certain vitamin at a time, spacing out the supplements might be a good idea.

In an adequate/"normal" diet, I think the majority of digested foods that enter the small intestine get absorbed into the bloodstream. The most important thing about getting nutrients into the blood stream across the mucosa probably would be establishing the proper electrochemical gradient.


[QUOTE=Evl316]Honestly I don't really beleive that this is an issue. I mean how many medications does it say to take with food, for example? The idea of a person taking more vitamins than there body can handle at one time doesn't seem likely to me. On the other hand only so much protein can be absorbed by the body at one time so why wouldn't the same be true of other things? I would appreciate other people's opinions on this[COLOR=DarkOrange]undefined[/COLOR] subject.[/QUOTE]
It may be unlikely, but it can happen with over-indulgence, which is why there are upper limits for vitamins and minerals.

Generally, I think the digestive system is pretty efficient at absorbing most nutrients from food sources. For example, absorbed nutrients floating around in your bloodstream and not needed by your body are stored as fat.





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