Vitamins from A-Zinc — Vitamin A

On January 2, 2010, in Fat Solubles, Vitamins, by Andrea

Vitamin A

Vitamin A — What is it?

Vitamin A is a generic term that refers to compounds with the biological activity of retinol.  These compounds include the precursor, or provitamin A carotenoids – principally beta-carotene, alpha-carotene, and beta-cryptoxanthin, which are provided in the diet by green, yellow, or orange vegetables and by some fruits, and preformed vitamin A, namely retinyl esthers (such as retinyl palmitate and retinyl acetate) and retinol itself which are present in foods of animal origin – mainly in organ meats such as liver, other meats, eggs, and dairy products.

Rarely in research, you may come across connotations of vitamins A1 and A2.  Vitamin A1 is retinol as mentioned above, while vitamin A2 is dehydroretinol, an oily yellow alcohol found in some fish that is 40% less active in mammals.  These differentiations are rare in common usage – retinol is typically called as such rather than vitamin A1 and the other vitamin A analogues such as beta-carotene do not have such distinctions as they are not technically vitamin A – just precursors to the vitamin.

What is it good for?

Vision

Regulation of gene expression

Immunity – commonly known as the anti-infective vitamin, it is required for normal function of the immune system; plays a central role in development and differentiation of white blood cells

Growth and Development – essential for embryonic development; during fetal development, retinonic acid (RA) functions in limb development and formation of heart, eyes and ears; RA has been found to regulate expression of the gene for growth hormone

Red Blood Cell Production – stem cells are dependant on retinoids for differentiation into red blood cells; vitamin A appears to facilitate mobilization of iron from storage to developing red blood cells for incorporation into hemoglobin

Where do I get it?

As a fat-soluble compound, beta-carotene’s absorption in the GI tract depends on the fat content of the meal with which it is eaten.  It is less easily absorbed than retinol and must be converted to retinol and retinal by the body.  For this reason, a conversion factor has been created to represent this.  The most recent international standard of measure for vitamin A is Retinol Activity Equivalents (RAE) which represent vitamin A activity as retinol.

2 micrograms (mcg) of supplemental beta-carotene can be converted by the body into 1mcg of retinol, giving it an RAE ratio of 2:1.

1 mcg of dietary vitamin A     =     1mcg retinol     =     1:1 RAE ratio
1 mcg supplemental vitamin A     =     1mcg retinol     =     1:1 RAE ratio
2mcg supplemental beta-carotene     =     1mcg retinol     =     2:1 RAE ratio
12mcg dietary beta-carotene     =     1mcg retinol     =     12:1 RAE ratio
24 mcg dietary alpha-carotene     =     1mcg retinol     =     24:1 RAE ratio
24mcg dietary beta-crytoxanthin     =     1mcg retinol     =     24:1 RAE ratio

1 IU = 0.3mcg of retinol


Food Sources

Food Vitamin A (IU)* %DV**
Liver, beef, cooked, 3 ounces 27,185 545
Liver, chicken, cooked, 3 ounces 12,325 245
Milk, fortified skim, 1 cup 500 10
Cheese, cheddar, 1 ounce 284 6
Milk, whole (3.25% fat), 1 cup 249 5
Egg substitute, ¼ cup 226 5
Food Vitamin A (IU)* %DV**
Carrot juice, canned, ½ cup 22,567 450
Carrots, boiled, ½ cup slices 13,418 270
Spinach, frozen, boiled, ½ cup 11,458 230
Kale, frozen, boiled, ½ cup 9,558 190
Carrots, 1 raw (7½ inches) 8,666 175
Vegetable soup, canned, chunky, ready-to-serve, 1 cup 5,820 115
Cantaloupe, 1 cup cubes 5,411 110
Spinach, raw, 1 cup 2,813 55
Apricots with skin, juice pack, ½ cup 2,063 40
Apricot nectar, canned, ½ cup 1,651 35
Papaya, 1 cup cubes 1,532 30
Mango, 1 cup sliced 1,262 25
Oatmeal, instant, fortified, plain, prepared with water, 1 cup 1,252 25
Peas, frozen, boiled, ½ cup 1,050 20
Tomato juice, canned, 6 ounces 819 15
Peaches, canned, juice pack, ½ cup halves or slices 473 10
Peach, 1 medium 319 6
Pepper, sweet, red, raw, 1 ring (3 inches diameter by ¼ inch thick) 313 6


Vitamin Sources

Most multivitamins contain vitamin A as a combination of a retinyl esther and beta-carotene.  There have been some studies showing a higher risk of osteoporsis in older adults taking more than 5,000 IU of retinol per day, which has led many companies to reduce the retinol content in their multivitamin supplements to 750mcg (2,500 IU).  Additionally, there have been some studies showing supplemental beta-carotene as a pro-oxident, meaning it helps grow free radicals in the body rather than reduce them.  However, there has not been a push to reduce or limit beta-carotene from multivitamin supplements as of yet.  In fact, there are some experts that still believe a separate RDA should be set specifically for beta-carotene should be set — one that is set approximately 6x the current RDA for the entire vitamin A RDA is today.

The current RDA for vitamin A is:

Age
(years)
Children
(mcg RAE)
Males
(mcg RAE)
Females
(mcg RAE)
Pregnancy
(mcg RAE)
Lactation
(mcg RAE)
1-3 300
(1,000 IU)
4-8 400
(1,320 IU)
9-13 600
(2,000 IU)
14-18 900
(3,000 IU)
700
(2,310 IU)
750
(2,500 IU)
1,200
(4,000 IU)
19+ 900
(3,000 IU)
700
(2,310 IU)
770
(2,565 IU)
1,300
(4,300 IU)

Keep in mind that the RDA is the amount needed to avoid deficiency symptoms – not to achieve an optimum level.  Unfortunately, there have been no studies conducted to determine what the optimum vitamin A level is as of now.

Deficiency

Severe zinc deficiency often accompanies vitamin A deficiency.  Zinc is required to make retinol binding protein (RBP), which transports vitamin A, so a zinc deficiency limits the body’s ability to move vitamin A stores from the liver to body tissues where they are needed.  Zinc also helps to protect against potential toxicity of retinol.  Additionally, a zinc deficiency results in decreased activity of the enzyme that releases retinol from it’s storage form, retinyl palmitate, in the liver.  Zinc is also required for the enzyme that converts retinol into retinal.  Despite these well-documented associations between zinc and vitamin A, the health consequences of zinc deficiency on vitamin A nutrition status in humans is still unclear as of now.

Vitamin A deficiency may exacerbate iron deficiency anemia.  Vitamin A supplementation has beneficial effects on iron deficiency anemia and has been show to improve iron nutritional stores status among children and pregnant women.  A combination of supplemental vitamin A and iron seems to reduce anemia more effectively than supplemental iron or supplemental vitamin A alone.  Additionally, there have been some studies in which iron deficient anemia has been helped by supplemental vitamin A, even in the absence of a vitamin A deficiency.

Night blindness is one of the first symptoms of vitamin A deficiency.  Vitamin A deficiency contributes to blindness by making the cornea very dry, which damages it and the retina.

Deficiency in A also diminishes the body’s ability to fight infections.  In vitamin A deficient individuals, the cells lining the lungs lose the ability to remove disease-causing microorganisms.  This may contribute to a larger amount of pneumonia cases seen in vitamin A deficient individuals.

Keep in mind that a subclinical deficiency does not exhibit the signs and symptoms of a deficiency, but still may have the adverse effects of the deficiency.  A mild form of deficiency may increase the risk of developing respitory or diarrheal infections, decrease growth rate and slow bone development in children, and decrease the liklihood to survive a serious illness.

A vitamin A deficiency early in life could have adverse effects on neurologic and behavioral development and function later in life.  Some researchers even believe that schizophrenia may result from vitamin A deficiency.

Toxicity

Hypervitaminosis A refers to a high storage level of vitamin A in the body that can lead to toxic symptoms.  Severe cases may result in liver damage, hemmorrhage, and coma, and may generally only occur in individuals with long-term consumption in excess of 8,000-10,000 mcg/day or 25,000-33,000 IU/day.

Acute toxicity is relatively rare.  Symptoms include nausea, vomiting, headache, fatigue, loss of appetite, dizziness, dry skin, desquamation, cerebral edema, bone and joint pain, blurred vision, lack of muscular function, and abnormal liver function.

It is important to note a few things about vitamin A toxicity.  First, the Upper Limit (UL) does not apply to malnourished individuals receiving vitamin A either periodically or through fortification programs as a measure of preventing a vitamin A deficiency.  Additionally, beta-carotene and other carotenoids in foods, even when consumed in high levels have not produced toxicity; therefore, the UL does not include the carotenoids.

Age
(years)
Children
(mcg)
Males
(mcg)
Females
(mcg)
Pregnancy
(mcg)
Lactation
(mcg)
0-1 600
(2,000 IU)
1-3 600
(2,000 IU)
4-8 900
(3,000 IU)
9-13 1,700 (5610 IU)
14-18 2,800 (9,240 IU) 2,800 (9,240 IU) 2,800 (9,240 IU) 2,800 (9,240 IU)
19+ 3,000 (10,000 IU) 3,000 (10,000 IU) 3,000 (10,000 IU) 3,000 (10,000 IU)

There are some troubling concerns with the two major forms of vitamin A found in supplements today.

Intakes of retinol not far above the RDA, but well under the UL may reduce bone mineral density and increase the risk of osteoporatic disease and breaks.  It is speculated that perhaps retinol increases the bone resorption response or perhaps interferes with vitamin D in the maintenance of calcium within the bones.  This problem has only been found with the retinol form of vitamin A and not with beta-carotene, which has caused some multivitamin manufacturers to limit the amount of retinol in their product to 750mcg (2,500 IU).  In fact, my bottle of Centrum has a warning about taking too much vitamin A in the form of retinol on the label.

My bottle of Centrum...

The picture for beta-carotene isn’t a bed of roses, either.  Believing beta-carotene to be a powerful anti-oxidant, researchers gave patients supplements of both retinol and beta-carotene supplements to ascertain exactly how good it really was.  The findings of the Beta-Carotene and Retinol Efficacy Trial (CARET) shocked everyone.  Results suggest that high-dose supplementation of vitamin A and beta-carotene should be avoided in people at high risk of lung cancer.  One hypothesis had to do with the high amount of oxidation in the lung tissue – but there were no definative conclusions reached except the spike of lung cancers in high risk individuals who received the extra supplementation.  Despite this higher risk, there has been no push to remove or reduce beta-carotene from multivitamins.  However, a quick Google search will find a few tailor-made vitamins without beta-carotene for smokers, at a premium price of course.

Miscellaneous

Under conditions of vitamin A adequacy, most mammals, including humans, store more than 90% of their total vitamin A in their liver.

The safety of vitamin A is frequently questioned during pregnancy.  With vitamin A, there is such a thing as “too little” as well as “too much.”  Vitamin A is important in fetal development during cellular and tissue differentiation.  Excess retinol during pregnancy is known to cause birth defects.  There have been no birth defects observed at doses of preformed A from supplements below 3,000mcg (10,000 IU/day), but since foods are commonly supplemented, it is advised to stay under 5,000 IU/day.  It should be noted that vitamin A from beta-carotene is not known to increase the risk of birth defects.

WLS Concerns

Beta-carotene is a poor supplement of vitamin A for those who have had RNY, DS, and VSG.

To begin with, beta-carotene requires an acidic environment to absorb properly.  A study was conducted to measure the absorption of retinol, retinyl palmitate, and beta-carotene in an achlorhydriac, or low gastric acidic environment.  To test the supplements, subjects were given a proton pump inhibitor (PPI) to neutralize gastric acidity over a period of time, then tested with a gastric probe to ensure the pH level.  Once a low gastric level was reached, individuals were given supplements, then serum levels were checked.  Serum concentrations of beta-carotene were significantly greater at a low gastric pH (ie normal) than those at a high gastric pH (ie achlorhydria, or low gastric acid environment).  Serum concentrations of retinol and retinyl palmitate were not significantly different between the low gastric pH and high gastric pH.  Those with a lower acid content will not absorb beta-carotene fully, if at all.  It is also important to note that as we age, even those of normal stomachs will begin to neutralize their gastric acidity.  This was confirmed by a study published in the New England Journal of Medicine in reference to the breakdown of calcium citrate versus carbonate — but the problem of acidity remains the same.

Additionally, beta-carotene is absorbed in the duodenum, which is bypassed in the RNY and DS procedures.  Since this absorption site is bypassed completely, beta-carotene does little-to-no good to patients with either surgery – especially coupled with the lower gastric-acid component present in both surgeries as well.

Those with Adjustable Gastric Bands (AGB’s) should take note as well – beta-carotene absorbs better when eaten with some fat.  Those who have been extreme in removing fat from their diets should note this key interaction.

For post-WLS patients who become deficient in vitamin A, one can find dry-form, or water-miscible forms of the vitamin.  It should be noted that labs should be watched carefully, as always, when supplementing.  There is some evidence that water-miscible forms of retinol vitamins can be more toxic than their oil-based counterparts – but the study conducted was on non-bypassed individuals who had no level of malabsorption.  There are many rules in the nutrition world that simply do not apply to us – but I’m not going to be the one to say which ones are which.  Labs are really your only guide to know where you stand nutritionally and you should follow them accordingly.

Information compiled from:

Handbook of Vitamins, 4th Edition
Linus Pauling Institute
Advanced Health and Life

Office of Dietary Supplements
Gastric acidity influences the blood response to a beta-carotene dose in humans
Water-miscible, emulsified, and solid forms of retinol supplements are more toxic than oil-based preparations
NEJM — Calcium absorption and achlorhydria

http://www.amazon.com/Handbook-Vitamins-CLINICAL-NUTRITION-DISEASE/dp/0849340225/ref=sr_1_1?ie=UTF8&s=books&qid=1262205681&sr=8-1
Related Posts with Thumbnails

1 Response » to “Vitamins from A-Zinc — Vitamin A”

  1. Alexandria says:

    Some studies also find that vitamin A may help hearing.

Leave a Reply