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In the 21st century, an every growing number of men and women are flocking to the Internet and World Wide Web to make purchases of products and to engage services from different providers. One type of product line that now is offered in abundance on the Internet is that of vitamins and nutritional supplements. At the present time there literally are thousands of different website venues that cater to people who are interested in purchasing and utilizing vitamins and other nutritional supplements.

Unfortunately, there is a small segment of the Internet market consisting of scams and flim-flam artists. There are people who have created less than reputable sites in the vitamin and nutritional supplement business. Naturally, if you are new at seeking these types of products online, you will want to avoid these types of disreputable sites. There are some tips and pointers that you should keep in mind when seeking reputable sites on the Internet that are involved in the vitamin and nutritional supplement trade.

First of all, if you are a novice at seeking and purchasing vitamins and nutritional supplements online, seek at those websites that have been established in business for a reasonable amount of time. Of course, there are websites that deal in these products that crop up all of the time -- and many of these sites are very good. However, until you get your feet wet and understand what to look for in regard to legitimate websites that market vitamins and nutritional supplements, you will be better served by relying on those sites that have been better established.

Second, as you go about looking for a reliable website from which to make the purchase of vitamins and nutritional supplements, get references from other people who have used one or another. In many ways, referrals and recommendations are the best way through which you can make reasonable and educated decisions about which websites to select when you are in the market for vitamins and nutritional supplements for your use and for use by your family.

By educating yourself and by taking your time in researching different websites, you will be able to find a reliable and reputable venue through which you can make the purchase of vitamins and nutritional supplements.

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Vitamins are chemical elements the body can't make itself and that must thus be supplied.

Besides fat, carbs, proteins ( amino acids ), minerals and trace elements, we must consume these with our foods or with additions.

Vitamin B-complex comprises a considerable number of vitamins which exist as a family. They shouldn't be taken individually. In this modern age, many millions of people suffer with a deficiency of vitamin B for 1 or 2 reasons, chief among which are : stress, packaged foods in the diet, poisons, processed sugar, drugs, cooking, starvation. Deficiency of vitamin B leads to anemia and neurological abnormalities ; inadequacy in kids could cause extreme damage, much being revertible. A standard level of serum vitamin B doesn't guarantee adequacy, methylmalonic acid concentrations ( either serum or pee ) are a way more trusty metabolic measure of vitamin B metabolism. Folks following a pure vegetarian ( vegan ) diet are at risk ( fifty percent ) for metabolicvitamin B deficiency. Vitamin B is in all animal products ( liver, muscle flesh, eggs, and dairy goods are sources, in order from wealthiest to poorest sources ).

Plant foods contain tiny if any active vitamin B ; produce grown in soil fertilized with cow dung may contain more B than commercially grown produce.Other good B vitamin sources are : baked potato, banana, spinach, soybeans, wheat germ, cantaloupe, tuna in water, navy beans, bok choy, avocado, sunflower seeds, chicken fillet, turnip greens for example. Probiotic additions aren't an adequate source of vitamin B ; some products work much better than others. A deficiency of vitamins B can boost your possibility of heart problems. They help in keeping a substance called homocysteine under control by breaking it down. If the homocysteine levels go too high, your blood may clot simpler which increases the possibility of a coronary or stroke. There are plenty of paths to getting plenty of B vitamins : eat a range of fresh fruits and vegetables, select dark green leafy types, not light green ones, eat whole grains like brown rice, wholeweat pasta, multi grain cereals, oatmeal and barley rather than those with refined bleached flour, eat beans ( or split peas ) each day.

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GlobalFusion brings you its latest icon of broad-range action health supplement which gives you true measurable benefits. ReBorn lets you count years and years of full and vibrant living, as what it is suppose to be. Because we long to add not just years but value to a longer blissful life with people we love and to do what matters most. ReBorn is designed to give your system not just the boost and protection it needs but most importantly improves your body's natural repair ability. Best taken with ReBorn Milkshake, you fully maximize the nutrients you take and increase your body's ability to shield it from disease-causing toxins you get from the air, water and food you take. ReBorn also helps battle the toxins our body releases when you go under stress. With every capsule of ReBorn, you don't only get the promise of a longer healthier life, you are assured of it. Protection and Boost: improved Digestion + Healthy Liver + Manage Cholesterol level + Healthy Heart + Control Blood Sugar + Healthy Arteries + Prevents Gallstones + Anti-Ageing + Fat Reduction + Increased Nutrient Absorption + Anti Toxin + Gives Extra Energy + Fights Infections + Healthy Eyes + Strong Bones + Pancreatic Health + Improves Skin Elasticity + Fights Inflammations + Improves Sleep + Protects Lungs + Prostate + Brain Function Would you like to look and feel 10 years younger? Call or text 09178052572. Add me in Facebook ck.agbayani@gmail.com, Skype zerolimits.gfi@gmail.com, Yahoo Messenger ck_agbayani ...
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In January 2011, representatives from USANA Health Sciences, Inc., and the Women's Tennis Association (WTA) met in Sydney, Australia, to announce the expansion of their partnership. Beginning this year and continuing through 2013, USANA Health Sciences is now not only the official health supplement supplier, but also an official partner, and USANA's Essentials are the official multivitamin of the WTA. Hear from officials, including Alan Bergstrom, USANA's vice president of customer service, as well as from Samantha Stosur, one of the WTA's new USANA Brand Ambassadors.

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Vitamin D is a fat-soluble vitamin found in foods such as fatty fish, liver, and milk. It is also absorbed into the body from the sun’s ultraviolet rays. If you don’t eat Vitamin D containing foods often, you can get the recommended daily requirement from supplements.

Vitamin D works in the body by maintaining normal levels of calcium and phosphorus. By aiding in the absorption of calcium, vitamin D helps form and keeps bones strong. Without vitamin D the bones will become brittle.

A vitamin D deficiency occurs when a person’s body can’t absorb vitamin D from the gastrointestinal tract, they don’t get enough vitamin D in their diet, have limited sun exposure, and when the kidneys can’t properly convert vitamin D to its active form.

People who are deficient in vitamin D can develop diseases that weaken bones. Children develop rickets and adults get osteomalacia. Rickets causes skeletal deformities. Osteomalacia causes weak bones and muscles.

Women with osteoporosis should make sure to get adequate amounts of vitamin D. This will help prevent bone fractures by keeping bones stronger. It’s a good idea to talk with your doctor about adding vitamin D supplements to your day if you have osteoporosis.

Adequate vitamin D has been shown to provide protective abilities against colon cancer. To determine if vitamin D deficiency actually causes cancer, or if it’s just protective against certain types of cancer, more testing has to be done.

People with Alzheimer’s disease often get hip fractures. If they have vitamin D deficiency, this may be the reason. Often, someone with Alzheimer’s disease doesn’t get out much, so is not absorbing vitamin D from the sun’s rays. Vitamin D supplements should be discussed with the treating physician.

If you breast feed your child, be sure to supplement vitamin D. Failing to do so can result in an infant with rickets if the mother is vitamin D deficient herself.

Darker skinned people are more prone to vitamin D deficiency because they can’t absorb as much vitamin D from the sun’s rays.

Obese people are also at greater risk for vitamin D deficiency because more subcutaneous fat doesn’t let the vitamin D hormones circulate in the body properly.

People who don’t absorb dietary fat correctly are also at risk for vitamin D deficiency. Conditions that cause this problem include Crohn’s disease, cystic fibrosis, celiac disease, liver disease, pancreatic enzyme deficiency, and people who have surgery that removes part of the stomach.

Taking supplements with vitamin D, eating foods rich in vitamin D naturally, getting adequate sun exposure, and maintaining a healthy weight are all things we can do to reduce the risk of becoming vitamin D deficient. It’s very important to incorporate all these into your life.

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Article by Ray Torynin

A, B, C, D, E and K are the main different types of vitamins that are essential for normal metabolic rate, the method which helps sustain everyday living in us. A balanced diet is that diet which is made up of all sorts of necessary nutrients in accurate proportions. There exist a number of sub-types of vitamin B like B1, B2, B3, etc. Vitamin C and B vitamins are drinking water soluble vitamins whilst nutritional vitamins A, D, E and K are fat soluble nutritional vitamins. This implies your system can retail store nutritional vitamins A, D, E, and K in fat; but vitamin C and B nutritional vitamins are flushed off on a regular basis as a result of urine together with other excretory substance. Vitamin B deficiency can have an effect on your mood and brain functionality. Prior to talking about the relationship concerning vitamin B12 deficiency and depression, allow us to consider a seem on the leads to and signs of vitamin B12 deficiency.

Vitamin B12 Deficiency: Leads to

No animal or plant can make vitamin B12. They acquire this vitamin from your microorganisms in the soil. Microorganisms like bacteria, yeasts, molds, and algae are very excellent resources of vitamin B12. Animals are able of storing a lot more B12 than vegetation. Outstanding sources of vitamin B12 involve shrimp, venison, salmon, scallops and beef. Milk, cheese, eggs, caviar, fish, and so on. also have vitamin B12. Sea greens like kelp, blue-green algae, tempeh, miso, tofu, Brewer's yeast do include some volume of vitamin B12 nevertheless they are usually not referred to as quite excellent sources of vitamin B12. The principle cause of vitamin B12 deficiency is its limited resources. Vegans and vegetarians frequently experience from B12 deficiency as they don't take in animal meals. Getting older decreases the potential of intestines to take in vitamin B12 and other nutritional vitamins. So elderly often undergo from vitamin deficiencies. Individuals that are diagnosed with stomach difficulties or digestion complications may establish vitamin deficiencies. For example, individuals acquiring Crohn's sickness, blind loop syndrome, celiac sickness, atrophic gastritis, and so forth. may perhaps develop B12 deficiency. Overuse of particular medications can influence absorption of nutritional vitamins inside intestines.

Vitamin B12 Deficiency: Indicators

Indicators of B12 deficiency may possibly fluctuate from man or woman to individual. Females may possibly practical experience menstrual complications. Dandruff, problem in blood clotting and in swallowing is often noticed. Some may possibly endure from red and sore tongue, paleness, excessive weakness, reduced reflexes, weak pulse or heart palpitations. Memory complications, nervousness, depression and tingling sensation or numbness in feet are many of the normally noticed indicators of B12 deficiency. It's been observed that significant deficiency of vitamin B12 can result in irreversible nerve and brain harm leading to dementia. Vitamin B12 deficiency symptoms give us an concept with regards to the functions of vitamin B12.

Vitamin B12 Deficiency and Depression

Considered one of the primary features of vitamin B12 (along with other B nutritional vitamins like B6 and folate) will be to develop brain chemicals like 'norepinephrine' that management mood along with other brain capabilities. Norepinephrine plays an important function in mind-body relationship as well as in a variety of reactions on the person. Low ranges of norepinephrine can cause lessened power, reduced stamina, tiredness, lack of determination and issues in concentrating. Very low amounts of chemical compounds in brain like epinephrine and norepinephrine cause psychological challenges and cognitive complications. They have an impact on the mood on the individual significantly, primary to depression.

Vitamin B12 deficiency can cause many different types of nervous process problems. Scientific tests present that vitamin B12 shots can perform great for disturbed sleep-wake rhythms. It can be a reality that sleep-wake disturbances can lead to mood ailments. B12 promotes sound sleep while in night time and alertness through the day. Vitamin B12 can be involved in strengthening the myelin sheath that protects the nerves. B12 encourages correct cycling of proteins while in the body. While you know, protein is crucial for growth and restore of human body cells; so absence of protein (amino acids) due to deficiency of vitamin B12 can affect psychological well being of an individual. Additionally, vitamin B12 deficiency also can influence carbohydrate and fat metabolism, major to numerous wellbeing problems, which include depression.

Oral doses of vitamin B12 might not be absorbed appropriately within the intestines. So vitamin B12 injections in many cases are proposed just in case of vitamin deficiency. Vitamin B12 supplementation can assist decrease the signs and symptoms of depression but it won't be as efficient as antidepressants. Similarly, psychological counseling is usually critical to have rid of depression signs. Vitamin supplements shouldn't be taken without prior consultation using the physician. In case of vitamin B12 deficiency and depression also, you must consult a psychiatrist for prompt and right therapy.

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History

The discovery of vitamin A may have stemmed from research dating back to 1906, indicating that factors other than carbohydrates, proteins, and fats were necessary to keep cattle healthy. By 1917 one of these substances was independently discovered by Elmer McCollum at the University of Wisconsinadison, and Lafayette Mendel and Thomas Burr Osborne at Yale University. Since "water-soluble factor B" (Vitamin B) had recently been discovered, the researchers chose the name "fat-soluble factor A" (vitamin A). Vitamin A was first synthesized in 1947 by two Dutch chemists, David Adriaan van Dorp and Jozef Ferdinand Arens.

Equivalencies of retinoids and carotenoids (IU)

As some carotenoids can be converted into vitamin A, attempts have been made to determine how much of them in the diet is equivalent to a particular amount of retinol, so that comparisons can be made of the benefit of different foods. Unfortunately the situation is confusing because the accepted equivalences have changed. For many years, a system of equivalencies was used in which an international unit (IU) was equal to 0.3 g of retinol, 0.6 g of -carotene, or 1.2 g of other provitamin-A carotenoids. Later, a unit called retinol equivalent (RE) was introduced. 1 RE corresponded to 1 g retinol, 2 g -carotene dissolved in oil (it is only partly dissolved in most supplement pills, due to very poor solubility in any medium), 6 g -carotene in normal food (because it is not absorbed as well as when in oils), and 12 g of either -carotene, -carotene, or -cryptoxanthin in food (these molecules only provide 50% of the retinol as -carotene, due to only half the molecule being convertible to usable vitamin).

Newer research has shown that the absorption of provitamin-A carotenoids is only half as much as previously thought, so in 2001 the US Institute of Medicine recommended a new unit, the retinol activity equivalent (RAE). 1 g RAE corresponds to 1 g retinol, 2 g of -carotene in oil, 12 g of "dietary" beta-carotene, or 24 g of the three other dietary provitamin-A carotenoids.

Substance and its chemical environment

Micrograms of retinol equivalent per microgram of the substance

retinol

1

beta-carotene, dissolved in oil

1/2

beta-carotene, common dietary

1/12

alpha-carotene, common dietary

1/24

gamma-carotene, common dietary

1/24

beta-cryptoxanthin, common dietary

1/24

Because the production of retinol from provitamins by the human body is regulated by the amount of retinol available to the body, the conversions apply strictly only for vitamin A deficient humans. The absorption of provitamins also depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin.

The conclusion that can be drawn from the newer research is that fruits and vegetables are not as useful for obtaining vitamin A as was thought; in other words, the IU's that these foods were reported to contain were worth much less than the same number of IU's of fat-dissolved oils and (to some extent) supplements. This is important for vegetarians. (Night blindness is prevalent in countries where little meat or vitamin A-fortified foods are available.)

A sample vegan diet for one day that provides sufficient vitamin A has been published by the Food and Nutrition Board (page 120). On the other hand, reference values for retinol or its equivalents, provided by the National Academy of Sciences, have decreased. The RDA (for men) of 1968 was 5000 IU (1500 g retinol). In 1974, the RDA was set to 1000 RE (1000 g retinol), whereas now the Dietary Reference Intake is 900 RAE (900 g or 3000 IU retinol). This is equivalent to 1800 g of -carotene supplement (3000 IU) or 10800 g of -carotene in food (18000 IU).

Recommended daily intake

Vitamin A

Dietary Reference Intake:

Life Stage Group

Recommended Dietary Allowances (RDA)

Adequate Intakes (AI*)

g/day

Upper Limit

g/day

Infants

06 months

712 months

400*

500*

600

600

Children

13 years

48 years

300

400

600

900

Males

913 years

1418 years

19 - >70 years

600

900

900

1700

2800

3000

Females

913 years

1418 years

19 - >70 years

600

700

700

1700

2800

3000

Pregnancy

19 - >50 years

750

770

2800

3000

Lactation

19 - >50 years

1200

1300

2800

3000

(Note that the limit refers to synthetic and natural retinoid forms of vitamin A. Carotene forms from dietary sources are not toxic.)

According to the Institute of Medicine of the National Academies, "RDAs are set to meet the needs of almost all (97 to 98 percent) individuals in a group. For healthy breastfed infants, the AI is the mean intake. The AI for other life stage and gender groups is believed to cover the needs of all individuals in the group, but lack of data prevent being able to specify with confidence the percentage of individuals covered by this intake."

Sources

Egg.

Vitamin A is found naturally in many foods:

liver (beef, pork, chicken, turkey, fish) (6500 g 722%)

carrot (835 g 93%)

broccoli leaf (800 g 89%) - According to USDA database broccoli florets have much less.

sweet potato (709 g 79%)

butter (684 g 76%)

kale (681 g 76%)

spinach (469 g 52%)

pumpkin (400 g 41%)

collard greens (333 g 37%)

Cheddar cheese (265 g 29%)

cantaloupe melon (169 g 19%)

egg (140 g 16%)

apricot (96 g 11%)

papaya (55 g 6%)

mango (38 g 4%)

pea (38 g 4%)

broccoli (31 g 3%)

milk (28 g 3%)

Note: data taken from USDA database bracketed values are retinol activity equivalences (RAEs) and percentage of the adult male RDA per 100g.

Conversion of carotene to retinol varies from person to person and bioavailability of carotene in food varies.

Metabolic functions

Vitamin A plays a role in a variety of functions throughout the body, such as:

Vision

Gene transcription

Immune function

Embryonic development and reproduction

Bone metabolism

Haematopoiesis

Skin health

Antioxidant Activity

Vision

The role of vitamin A in the vision cycle is specifically related to the retinal form. Within the eye, 11-cis-retinal is bound to rhodopsin (rods) and iodopsin (cones) at conserved lysine residues. As light enters the eye the 11-cis-retinal is isomerized to the all-"trans" form. The all-"trans" retinal dissociates from the opsin in a series of steps called bleaching. This isomerization induces a nervous signal along the optic nerve to the visual center of the brain. Upon completion of this cycle, the all-"trans"-retinal can be recycled and converted back to the 11-"cis"-retinal form via a series of enzymatic reactions. Additionally, some of the all-"trans" retinal may be converted to all-"trans" retinol form and then transported with an interphotoreceptor retinol-binding protein (IRBP) to the pigment epithelial cells. Further esterification into all-"trans" retinyl esters allow this final form to be stored within the pigment epithelial cells to be reused when needed. The final conversion of 11-cis-retinal will rebind to opsin to reform rhodopsin in the retina. Rhodopsin is needed to see black and white as well as see at night. It is for this reason that a deficiency in vitamin A will inhibit the reformation of rhodopsin and lead to night blindness.

Gene transcription

Vitamin A, in the retinoic acid form, plays an important role in gene transcription. Once retinol has been taken up by a cell, it can be oxidized to retinal (by retinol dehydrogenases) and then retinal can be oxidized to retinoic acid (by retinal oxidase). The conversion of retinal to retinoic acid is an irreversible step, meaning that the production of retinoic acid is tightly regulated, due to its activity as a ligand for nuclear receptors. Retinoic acid can bind to two different nuclear receptors to initiate (or inhibit) gene transcription: the retinoic acid receptors (RARs) or the retinoid "X" receptors (RXRs). RAR and RXR must dimerize before they can bind to the DNA. RAR will form a heterodimer with RXR (RAR-RXR), but it does not readily form a homodimer (RAR-RAR). RXR, on the other hand, readily forms a homodimer (RXR-RXR) and will form heterodimers with many other nuclear receptors as well, including the thyroid hormone receptor (RXR-TR), the Vitamin D3 receptor (RXR-VDR), the peroxisome proliferator-activated receptor (RXR-PPAR) and the liver "X" receptor (RXR-LXR). The RAR-RXR heterodimer recognizes retinoid acid response elements (RAREs) on the DNA whereas the RXR-RXR homodimer recognizes retinoid "X" response elements (RXREs) on the DNA. The other RXR heterodimers will bind to various other response elements on the DNA. Once the retinoic acid binds to the receptors and dimerization has occurred, the receptors undergo a conformational change that causes co-repressors to dissociate from the receptors. Coactivators can then bind to the receptor complex, which may help to loosen the chromatin structure from the histones or may interact with the transcriptional machinery. The receptors can then bind to the response elements on the DNA and upregulate (or downregulate) the expression of target genes, such as cellular retinol-binding protein (CRBP) as well as the genes that encode for the receptors themselves.

Dermatology

Vitamin A appears to function in maintaining normal skin health. The mechanisms behind retinoid's therapeutic agents in the treatment of dermatological diseases are being researched. For the treatment of acne, the most effective drug is 13-cis retinoic acid (isotretinoin). Although its mechanism of action remains unknown, it dramatically reduces the size and secretion of the sebaceous glands. Isotretinoin reduces bacterial numbers in both the ducts and skin surface. This is thought to be a result of the reduction in sebum, a nutrient source for the bacteria. Isotretinoin reduces inflammation via inhibition of chemotatic responses of monocytes and neutrophils. Isotretinoin also has been shown to initiate remodeling of the sebaceous glands; triggering changes in gene expression that selectively induces apoptosis. Isotretinoin is a teratogen and its use is confined to medical supervision.

Retinal/retinol versus retinoic acid

Vitamin A deprived rats can be kept in good general health with supplementation of retinoic acid. This reverses the growth-stunting effects of vitamin A deficiency, as well as xerophthalmia. However, such rats show infertility (in both male and females) and continued degeneration of the retina, showing that these functions require retinal or retinol, which are intraconvertable but which cannot be recovered from the oxidized retinoic acid.

Deficiency

Main article: Vitamin A deficiency

Vitamin A deficiency is estimated to affect millions of children around the world. Approximately 250,000-500,000 children in developing countries become blind each year owing to vitamin A deficiency, with the highest prevalence in Southeast Asia and Africa. According to the World Health Organization (WHO), vitamin A deficiency is under control in the United States, but in developing countries vitamin A deficiency is a significant concern. With the high prevalence of vitamin A deficiency, the WHO has implemented several initiatives for supplementation of vitamin A in developing countries. Some of these strategies include intake of vitamin A through a combination of breast feeding, dietary intake, food fortification, and supplementation. Through the efforts of WHO and its partners, an estimated 1.25 million deaths since 1998 in 40 countries due to vitamin A deficiency have been averted.

Vitamin A deficiency can occur as either a primary or secondary deficiency. A primary vitamin A deficiency occurs among children and adults who do not consume an adequate intake of yellow and green vegetables, fruits and liver. Early weaning can also increase the risk of vitamin A deficiency. Secondary vitamin A deficiency is associated with chronic malabsorption of lipids, impaired bile production and release, low fat diets, and chronic exposure to oxidants, such as cigarette smoke. Vitamin A is a fat soluble vitamin and depends on micellar solubilization for dispersion into the small intestine, which results in poor utilization of vitamin A from low-fat diets. Zinc deficiency can also impair absorption, transport, and metabolism of vitamin A because it is essential for the synthesis of the vitamin A transport proteins and the oxidation of retinol to retinal. In malnourished populations, common low intakes of vitamin A and zinc increase the risk of vitamin A deficiency and lead to several physiological events. A study in Burkina Faso showed major reduction of malaria morbidity with combined vitamin A and zinc supplementation in young children.

Since the unique function of retinyl group is the light absorption in retinylidene protein, one of the earliest and specific manifestations of vitamin A deficiency is impaired vision, particularly in reduced light - night blindness. Persistent deficiency gives rise to a series of changes, the most devastating of which occur in the eyes. Some other ocular changes are referred to as xerophthalmia. First there is dryness of the conjunctiva (xerosis) as the normal lacrimal and mucus secreting epithelium is replaced by a keratinized epithelium. This is followed by the build-up of keratin debris in small opaque plaques (Bitot's spots) and, eventually, erosion of the roughened corneal surface with softening and destruction of the cornea (keratomalacia) and total blindness. Other changes include impaired immunity, hypokeratosis (white lumps at hair follicles), keratosis pilaris and squamous metaplasia of the epithelium lining the upper respiratory passages and urinary bladder to a keratinized epithelium. With relations to dentistry, a deficiency in Vitamin A leads to enamel hypoplasia.

Adequate supply of Vitamin A is especially important for pregnant and breastfeeding women, since deficiencies cannot be compensated by postnatal supplementation.. However, excess Vitamin A, especially through vitamin supplementation, can cause birth defects and should not exceed recommended daily values.

Toxicity

Main article: Hypervitaminosis A

Since vitamin A is fat-soluble, disposing of any excesses taken in through diet is much harder than with water-soluble vitamins B and C, thus vitamin A toxicity may result. This can lead to nausea, jaundice, irritability, anorexia (not to be confused with anorexia nervosa, the eating disorder), vomiting, blurry vision, headaches, hairloss, muscle and abdominal pain and weakness, drowsiness and altered mental status.

Acute toxicity generally occurs at doses of 25,000 IU/kg of body weight, with chronic toxicity occurring at 4,000 IU/kg of body weight daily for 615 months. However, liver toxicities can occur at levels as low as 15,000 IU per day to 1.4 million IU per day, with an average daily toxic dose of 120,000 IU per day. In people with renal failure 4000 IU can cause substantial damage. Additionally, excessive alcohol intake can increase toxicity. Children can reach toxic levels at 1,500 IU/kg of body weight.

In chronic cases, hair loss, dry skin, drying of the mucous membranes, fever, insomnia, fatigue, weight loss, bone fractures, anemia, and diarrhea can all be evident on top of the symptoms associated with less serious toxicity.

It has been estimated that 75% of people may be ingesting more than the RDA for vitamin A on a regular basis in developed nations. Intake of twice the RDA of preformed vitamin A chronically may be associated with osteoporosis and hip fractures. This may be due to the fact that an excess of vitamin A can block the expression of certain proteins that are dependent on vitamin K. This could hypothetically reduce the efficacy of vitamin D, which has a proven role in the prevention of osteoporosis and also depends on vitamin K for proper utilization.

High vitamin A intake has been associated with spontaneous bone fractures in animals. Cell culture studies have linked increased bone resorption and decreased bone formation with high vitamin A intakes. This interaction may occur because vitamins A and D may compete for the same receptor and then interact with parathyroid hormone which regulates calcium. Indeed, a study by Forsmo et al. shows a correlation between low bone mineral density and too high intake of vitamin A.

Toxic effects of vitamin A have been shown to significantly affect developing fetuses. Therapeutic doses used for acne treatment have been shown to disrupt cephalic neural cell activity. The fetus is particularly sensitive to vitamin A toxicity during the period of organogenesis.

These toxicities only occur with preformed (retinoid) vitamin A (such as from liver). The carotenoid forms (such as beta-carotene as found in carrots), give no such symptoms, but excessive dietary intake of beta-carotene can lead to carotenodermia, which causes orange-yellow discoloration of the skin.

Researchers have succeeded in creating water-soluble forms of vitamin A, which they believed could reduce the potential for toxicity. However, a 2003 study found that water-soluble vitamin A was approximately 10 times as toxic as fat-soluble vitamin. A 2006 study found that children given water-soluble vitamin A and D, which are typically fat-soluble, suffer from asthma twice as much as a control group supplemented with the fat-soluble vitamins.

Chronically high doses of Vitamin A can produce the syndrome of "pseudotumor cerebri". This syndrome includes headache, blurring of vision and confusion. It is associated with increased intracerebral pressure.

Vitamin A and derivatives in medical use

Retinyl palmitate has been used in skin cremes, where it is broken down to retinoic acid, which has potent biological activity, as described above.

The retinoids, a class of chemical compounds that are related chemically to retinoic acid, are used in medicine to modulate gene functions in place of this counpound. In general, like retinoic acid itself, these compounds do not have full vitamin A activity.

References

^ Carolyn Berdanier. 1997. Advanced Nutrition Micronutrients. pp 22-39

^ a b Wolf, George (2001-04-19). "Discovery of Vitamin A". Encyclopedia of Life Sciences. doi:10.1038/npg.els.0003419. http://www.mrw.interscience.wiley.com/emrw/9780470015902/els/article/a0003419/current/html. Retrieved 2007-07-21. 

^ Composition of Foods Raw, Processed, Prepared USDA National Nutrient Database for Standard Reference, Release 20 USDA, Feb. 2008

^ a b Chapter 4, Vitamin A of Dietary Reference Intakes for

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