the water-soluble vitaminslatifa.pbworks.com/w/file/fetch/53004026/l… · ppt file · web...

The Water-Soluble Vitamins1. B-Vitamins:•Thiamin•Riboflavin•Niacin•Pantothenic Acid•Biotin•Vitamin B-6•Folate•Vitamin B-122. Vitamin C3- Choline

Dr. Latifah Al-Oboudi 2012

Water-Soluble Vitamin Overview

• Like fat-soluble vitamins, water-soluble vitamins are essential organic substances needed in small amounts for the normal function, growth, and maintenance of body tissues.


Vitamins and related compounds (e.g., choline) work together to maintain health.



• In contrast to the fat-soluble vitamins, only small amounts of water-soluble vitamins are stored in the body.

• The risk of water-soluble vitamins toxicity tends to be low because they are readily removed by kidneys and excreted in the urine.

• Tolerable Upper Intake levels have been set for only 4 of the water-soluble vitamins and choline.



• water-soluble vitamins are destroyed during cooking easily.

• A food’s vitamin content can be decreased by exposure to heat, light, air, and alkaline substances.

• water-soluble vitamins can leach into cooking water, whereas fat-soluble vitamins can leach into cooking fats and oils.



• Retention of the B-vitamins and vitamin C is greatest in foods that are prepared by steaming, stir-frying, and microwaving. These cooking methods limit exposure to heat and water.

• Fruits and vegetables are especially important sources of many vitamins.


Coenzymes: A common Role of B-Vitamins

• All B-vitamin form coenzymes, which are small, organic molecules that are a type of co-factor.

• Metals (e.g., zinc or magnesium) are another type of cofactor.

• Cofactors combine with inactive enzymes (called apoenzymes) to form active enzymes (called holoenzymes) that are able to catalyze specific reactions.


The enzyme-coenzyme interaction. The B-vitamins form coenzymes, which are compounds that enable specific enzymes to function.



• In foods, B-vitamins are present as vitamin or as coenzymes, both of which are sometimes bound to protein.

• Digestion frees B-vitamins from coenzymes or protein.

• Unbound (free) vitamins are the main form absorbed in the small intestine. Typically, about 50 to 90% of the B-vitamins in the diet are absorbed.



• Once inside cells, the coenzyme forms of the vitamins are resynthesized.

• Vitamin supplements sold in the coenzyme form have no specific benefits to consumers because vitamins must be released from the coenzyme before they can be absorbed.


Grains: One Important Source of B-Vitamins

• Grains are an important source of many B-vitamins. However, when grains are milled, the seeds are crushed and the germ, bran, and husk layers are removed. This refining process leaves just the starch-containing endosperm, which is the only portion of the grain used to make white flour, as well as many bread and cereal products.


When grains are milled, the bran and germ are removed and discarded, leaving only the starch-rich endosperm.


Grains: One Important Source of B-Vitamins

• Because the discarded parts are rich in nutrients, milling leads to a loss of vitamins and minerals.

• To counteract this nutrient loss, bread and cereal products made from milled grains are enriched with 4 B-vitamins—thiamin, riboflavin, niacin, and folic acid– and with the mineral iron.


• This practice, however, still leaves the products with less vitamin B-6, potassium, magnesium, zinc, fiber, and phytochemicals than in whole-grain products because whole grains contain the germ and the bran, as well as the endosperm.


• When compared with white rice, brown rice has 93% more vitamin B-6, 50% more potassium, 160% more zinc, 435% more magnesium, and 550%more fiber.

• Similarly, compared with

white bread, whole-wheat bread has 200% more vitamin B-6, 250% more potassium, 260% more zinc, 370% more magnesium, and 285% more fiber.


• Nutrition experts therefore recommend that whole-grain products, such as brown rice, oatmeal, and whole-wheat bread and pasta, be consumed daily.


Table 13_03a


Absorption , Transport, Storage, and Excretion of Thiamin

• Thiamin is absorbed mainly in the small intestine by a sodium-dependent active absorption process.

• It is transported mainly by red blood cells in its coenzyme form(thiamin pyrophosphate).

• Little thiamin is stored; only a small reserve is found in muscles and the liver.

• Any excess intake is rapidly filtered out by the kidneys and excreted in the urine.


Absorption , Transport, Storage, and Excretion of Riboflavin

• In the stomach, hydrochloric acid(HCL) release riboflavin from its bound forms.

• Free riboflavin is absorbed primarily via active transport or facilitated diffusion in the small intestine.

• In the blood, riboflavin is transported by protein carriers. • Riboflavin is converted to its coenzyme forms in most tissues,

but this occurs mainly in the small intestine, liver, heart, and kidneys.

• A small amount of riboflavin is stored in the liver, heart, and kidneys.

• Any excess intake is excreted in the urine.


Absorption , Transport, Storage, and Excretion of Niacin

• Nicotinic acid and nicotinamide are readily absorbed from the stomach and the small intestine by active transport and passive diffusion.

• After being absorbed, niacin is transported via the portal vein to the liver, where it is stored or delivered to the body’s cells.

• Niacin is converted to its coenzyme forms in all tissues .

• Any excess niacin is excreted in the urine.


Absorption , Transport, Storage, and Excretion of Pantothenic Acid

• The pantothenic acid portion of any coenzyme A in the diet is released during digestion in the small intestine. It is then absorbed and transported throughout the body bound to red blood cells.

• Storage is minimal and is in the coenzyme form.

• Excretion of pantothenic acid is via the urine.


Absorption, Transport, Storage and Excretion of Biotin

• In the small intestine the enzyme biotinidase releases biotin from biocytin and other biotin-dependent enzymes found in foods, free biotin is absorbed in the small intestine via a sodium-dependent carrier, biotin is stored in small amounts in the muscles liver and brain, and its excretion is mostly via the urine, although some excreted in bile.


Absorption, Transport, Storage, and Excretion of Vitamin B-6

• The coenzyme form is normally converted to the free vitamin form for absorption.

• Vitamin B-6 is transported via the portal vein to the liver, where most of it is phosphorylated.

• From the liver, the phosphorylated forms(mainly plp) are released for transport in the blood bound to the transport protein albumin.

• Muscle tissue is the main storage site for vitamin B-6.• Excess vitamin B-6 is generally excreted in the urine.


Absorption, Transport, Storage, and Excretion of Folate

• To be absorbed, folate polyglutamates must be broken down (hydrolyzed) in the GI tract to the monoglutamate form.

• Consumed with food, as in fortified cereal grains, absorption is slightly reduced.

• The portal vein delivers the monoglutamate form of folate from the small intestine to the liver, where it is converted to the polyglutamate form once inside a cell. ( This change allows folate to be trapped in a cell.)

• then, folate is either stored in the liver or released into the blood or bile.

• Folate is excreted in both the urine and feces


Absorption, Transport, Storage, and Excretion of Vitamin B-12

• The absorption of vitamin B-12 is quite complex. • In food, vitamin B-12 is bound to protein. HCL and

pepsin in gastric juice release vitamin B-12 from these proteins.

• In the stomach, the free vitamin B-12 binds to R-protein, which originates in the salivary glands.

• In the small intestine, pancreatic protease enzymes release vitamin B-12


•then combines with intrinsic factor, a protein like compound, produced by parietal cells in the stomach, that enhances vitamin B-12 absorption.• The vitamin B-12 intrinsic factor complex travels to the ileum, where vitamin B-12 is absorbed and transferred to the blood transport protein transcobalamin.• Although vitamin B-12 is continually secreted into the bile, most of it is reabsorbed by enter hepatic circulation, thereby efficiently “recycling” this vitamin. • Little vitamin B-12 is excreted in the urine.• Normally, healthy adults absorb about 50% of the vitamin B-12 in foods.


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