dryness of the conjunctival epithelium and cornea. the conjunctiva becomes dry, thick, and wrinkled. indicative is the appearance of bitot's spots, which are clumps of keratin debris that build up inside the conjunctiva. if untreated, xerophthalmia can lead to dry eye syndrome, corneal ulceration and ultimately to blindness as a result of cornea and retina damage. although xerophthalmia is an eye-related issue, prevention (and reversal) are functions of retinoic acid having been synthesized from retinal rather than the 11-cis-retinal to rhodopsin cycle. throughout southeast asia, estimates are that more than half of children under the age of six years have subclinical vitamin a deficiency and night blindness, with progression to xerophthalmia being the leading cause of preventable childhood blindness. estimates are that each year there are 350,000 cases of childhood blindness due to vitamin a deficiency. the causes are vitamin a deficiency during pregnancy, followed by low transfer of vitamin a during lactation and infant/child diets low in vitamin a or beta-carotene. the prevalence of pre-school age children who are blind due to vitamin a deficiency is lower than expected from incidence of new cases only because childhood vitamin a deficiency significantly increases all-cause mortality. according to a 2017 cochrane review, vitamin a deficiency, using serum retinol less than 0.70 µmol/l as a criterion, is a major public health problem affecting an estimated 190 million children under five years of age in low- and middle-income countries, primarily in sub-saharan africa and southeast asia. in lieu of or in combination with food fortification programs, many countries have implemented public health programs in which children are periodically given very large oral doses of synthetic vitamin a, usually retinyl palmitate, as a means of preventing and treating vad. doses were 50,000 to 100,000 iu (international units) for children aged 6 to 11 months and 100,000 to 200,000 iu for children aged 12 months to five years, the latter typically every four to six months. in addition to a 24% reduction in all-cause mortality, eye-related results were reported. prevalence of bitot's spots at follow-up were reduced by 58%, night blindness by 68%, xerophthalmia by 69%. gene regulation ra regulates gene transcription by binding to nuclear receptors known as retinoic acid receptors (rars; rarα, rarβ, rarγ) which are bound to dna as heterodimers with retinoid "x" receptors (rxrs; rxrα, rxrβ, rxrγ). rars and rxrs must dimerize before they can bind to the dna. expression of more than 500 genes is responsive to retinoic acid. the process is that rar-rxr heterodimers recognize retinoic acid response elements on dna. 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. this response upregulates or downregulates the expression of target genes, including the genes that encode for the receptors themselves. to prevent excess accumulation of ra it must be metabolized and eliminated. three cytochromes (cyp26a1, cyp26b1 cyp26c1) catalyze the oxidation of ra. the genes for these proteins are induced by high concentrations of ra, thus providing a regulatory feedback mechanism. embryology in vertebrates and invertebrate chordates, ra has a pivotal role during development. altering levels of endogenous ra signaling during early embryology, both too low and too high, leads to birth defects, including congenital vascular and cardiovascular defects. of note, fetal alcohol spectrum disorder encompasses congenital anomalies, including craniofacial, auditory, and ocular defects, neurobehavioral anomalies and mental disabilities caused by maternal consumption of alcohol during pregnancy. it is proposed that in the embryo there is competition between acetaldehyde, an ethanol metabolite, and retinaldehyde (retinal) for aldehyde dehydrogenase activity, resulting in a retenoic acid deficiency, and attributing the congenital birth defects to the loss of ra activated gene activation. in support of this theory, ethanol-induced developmental defects can be ameliorated by increasing the levels of retinol or retinal. as for the risks of too much ra, the prescription drugs tretinoin (all-trans-retinoic acid) and isotretinoin (13-cis-retinoic acid), used orally or topically for acne treatment, come with warnings to not be used by pregnant women or women who are anticipating becoming pregnant, as they are known human teratogens. immune functions vitamin a deficiency (vad) has been linked to compromised resistance to infectious diseases. in countries where early childhood vad is common, vitamin a supplementation public health programs initiated in the 1980s were shown to reduce the incidence of diarrhea and measles, and all-cause mortality. vad also increases the risk of immune system over-reaction, leading to chronic inflammation in the intestinal system, stronger allergic reactions and autoimmune diseases. lymphocytes and monocytes are types of white blood cells of the immune system. lymphocytes include natural killer cells, which function in innate immunity, t cells for adaptive cellular immunity and b cells for antibody-driven adaptive humoral immunity. monocytes differentiate into macrophages and dendritic cells. some lymphocytes migrate to the thymus where they differentiate into several types of t cells, in some instances referred to as "killer" or "helper" t cells and further differentiate after leaving the thymus. each subtype has functions driven by the types of cytokines secreted and organs to which the cells preferentially migrate, also described as trafficking or homing. reviews based on in vitro and animal research describe the role that retinoic acid (ra) has in the immune system. ra triggers receptors in bone marrow, resulting in generation of new white blood cells. ra regulates proliferation and differentiation of white blood cells, the directed movement of t cells to the intestinal system, and to the up- and down-regulation of lymphocyte function. if ra is adequate, t helper cell subtype th1 is suppressed and subtypes th2, th17 and itreg (for regulatory) are induced. dendritic cells located in intestinal tissue have enzymes that convert retinal to all-trans-retinoic acid, to be taken up by retinoic acid receptors on lymphocytes. the process triggers gene expression that leads to t cell types th2, th17 and itreg moving to and taking up residence in mesenteric lymph nodes and peyer's patches, respectively outside and on the inner wall of the small intestine. the net effect is a down-regulation of immune activity, seen as tolerance of food allergens, and tolerance of resident bacteria and other organisms in the microbiome of the large intestine. in a vitamin a deficient state, innate immunity is compromised and pro-inflammatory th1 cells predominate. skin deficiencies in vitamin a have been linked to an increased susceptibility to skin infection and inflammation. vitamin a appears to modulate the innate immune response and maintains homeostasis of epithelial tissues and mucosa through its metabolite, retinoic acid (ra). as part of the innate immune system, toll-like receptors in skin cells respond to pathogens and cell damage by inducing a pro-inflammatory immune response which includes increased ra production. the epithelium of the skin encounters bacteria, fungi and viruses. keratinocytes of the epidermal layer of the skin produce and secrete antimicrobial peptides (amps). production of amps resistin and cathelicidin, are promoted by ra. units of measurement 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. the situation can be confusing because the accepted equivalences have changed over time for many years, a system of equivalencies in which an international unit (iu) was equal to 0.3 μg of retinol (~1 nmol), 0.6 μg of β-carotene, or 1.2 μg of other provitamin-a carotenoids was used. this relationship was alternatively expressed by the retinol equivalent (re): one re corresponded to 1 μg retinol, to 2 μg β-carotene dissolved in oil, to 6 μg β-carotene in foods, and to 12 μg of either α-carotene, γ-carotene, or β-cryptoxanthin in food. newer research has shown that the absorption of provitamin-a carotenoids is only half as much as previously thought. as a result, in 2001 the us institute of medicine recommended a new unit, the retinol activity equivalent (rae). each μg rae corresponds to 1 μg retinol, 2 μg of β-carotene in oil, 12 μg of "dietary" beta-carotene, or 24 μg of the three other dietary provitamin-a carotenoids. substance and its chemical environment (per 1 μg) iu (1989) μg re (1989) μg rae (2001) retinol 3.33 1 1 beta-carotene, dissolved in oil 1.67 1/2 1/2 beta-carotene, common dietary 1.67 1/6 1/12 alpha-carotene, common dietary gamma-carotene, common dietary beta-cryptoxanthin, common dietary 0.83 1/12 1/24 animal models have shown that at the enterocyte cell wall, β-carotene is taken up by the membrane transporter protein scavenger receptor class b, type 1 (scarb1). absorbed β-carotene is converted to retinal and then retinol. the first step of the conversion process consists of one molecule of β-carotene cleaved by the enzyme β-carotene-15, 15'-monooxygenase, which in humans and other mammalian species is encoded by the bcm01 gene, into two molecules of retinal. when plasma retinol is in the normal range, gene expression for scarb1 and bc01 are suppressed, creating a feedback loop that suppresses β-carotene absorption and conversion. absorption suppression is not complete, as receptor 36 is not downregulated. dietary recommendations the us national academy of medicine updated dietary reference intakes (dris) in 2001 for vitamin a, which included recommended dietary allowances (rdas). for infants up to 12 months there was not sufficient information to establish a rda, so adequate intake (ai) is shown instead. as for safety, tolerable upper intake levels (uls) were also established. for uls, carotenoids are not added when calculating total vitamin a intake for safety assessments. life stage group us rdas or ais (μg rae/day) us upper limits (μg/day) infants 0–6 months400 (ai)600 7–12 months500 (ai)600 children 1–3 years300600 4–8 years400900 males 9–13 years6001700 14–18 years9002800 >19 years9003000 females 9–13 years6001700 14–18 years7002800 >19 years7003000 pregnancy <19 years7502800 >19 years7703000 lactation <19 years12002800 >19 years13003000 the european food safety authority (efsa) refers to the collective set of information as dietary reference values, with population reference intake (pri) instead of rda, and average requirement instead of ear. ai and ul defined the same as in united states. for women and men of ages 15 and older, the pris are set respectively at 650 and 750 μg re/day. pri for pregnancy is 700 μg re/day, for lactation 1300/day. for children of ages 1–14 years, the pris increase with age from 250 to 600 μg re/day. these pris are similar to the us rdas. the efsa reviewed the same safety question as the united states, and set uls at 800 for ages 1–3, 1100 for ages 4–6, 1500 for ages 7–10, 2000 for ages 11–14, 2600 for ages 15–17 and 3000 μg/day for ages 18 and older for preformed vitamin a, i.e., not including dietary contributions from carotenoids. safety vitamin a toxicity (hypervitaminosis a) occurs when too much vitamin a accumulates in the body. it comes from consumption of preformed vitamin a but not of carotenoids, as conversion of the latter to retinol is suppressed by the presence of adequate retinol. retinol safety main article: hypervitaminosis a there are historical reports of acute hypervitaminosis from arctic explorers consuming bearded seal or polar bear liver, both very rich sources of stored retinol, and there are also case reports of acute hypervitaminosis from consuming fish liver, but otherwise there is no risk from consuming too much via commonly consumed foods. only consumption of retinol-containing dietary supplements can result in acute or chronic toxicity. acute toxicity occurs after a single or short-term doses of greater than 150,000 μg. symptoms include blurred vision, nausea, vomiting, dizziness and headache within 8 to 24 hours. for infants ages 0–6 months given an oral dose to prevent development of vitamin a deficiency, bulging skull fontanel was evident after 24 hours, usually resolved by 72 hours. chronic toxicity may occur with long-term consumption of vitamin a at doses of 25,000–33,000 iu/day for several months. excessive consumption of alcohol can lead to chronic toxicity at lower intakes. symptoms may include nervous system effects, liver abnormalities, fatigue, muscle weakness, bone and skin changes and others. the adverse effects of both acute and chronic toxicity are reversed after consumption is stopped. in 2001, for the purpose of determining uls for adults, the us institute of medicine considered three primary adverse effects and settled on two: teratogenicity, i.e., causing birth defects,