Although vitamin A is one of the most versatile essential vitamins for our body, it is usually in the shadow of vitamin C or D.
Nevertheless, the nutrient is indispensable for the immune system, eyes, skin, and cell growth. Moreover, it is not absorbed equally well from every food.
Since vitamin A deficiency can have a drastic effect on our health, you can find out everything you need to know about the symptoms and how to prevent them here.
What Is Vitamin a Deficiency?
Vitamin A is necessary to ensure the proper functioning of the following processes of the human body:
- Maintenance of healthy vision
- Growth and development of children
- Ensuring organ and immune function
- Supporting muscle gain
- Protection against free radicals
For example, the following deficiency symptoms can occur if you don’t get enough vitamin A through your diet:
- Increased risk of infection
- Blemished skin
- Impaired vision
Accordingly, vitamin A deficiency, in particular, is known to be a significant cause of blindness.
Benefits of Vitamin A
Why vitamin A deficiency is a global problem that can lead not only to blindness but even death becomes clear when we survey the broad scope of the nutrient’s role (Wiseman et al. 20171).
1. Improves Fertility
Vitamin A is already instrumental in developing children in the womb (Hadi et al. 20002).
Furthermore, retinol deficiency can limit fertility. While it reduces sperm quality in men, it affects ovules quality in women (Clagett-Dame et al. 20113).
2. Boosts Immune System
The intestinal wall relies on vitamin A to form and maintain cells (Huang et al. 20184).
Therefore, sufficient consumption of bioavailable retinol helps stop the transport of pathogens from the intestine to vital organs.
Besides, even mild vitamin A deficiency is sufficient to increase the risk of respiratory diseases and diarrhea (Sommer et al. 19845).
3. Helps Renew Skin
Vitamin A is an all-around active substance when it comes to our skin. It can slow down its aging process, promote its renewal, smooth it (Park 20156).
For this reason, recent studies recommend natural foods rich in zinc, vitamin A and E to fight acne (Ozuguz et al. 20147).
Fortunately, these same nutrients are found together in natural vitamin A foods.
4. Maintains Vision
Damage to the retina from oxidative stress-so-called age-related macular degeneration is the most common cause of blindness in the elderly population (Beatty et al. 20008).
While natural foods containing vitamin A can counteract this disease, studies have shown beta-carotene supplements alone are not sufficient to halt its progression (Evans et al. 20179).
Additionally, vitamin A distribution programs in developing countries save the eyesight and lives of nearly half a million children annually (Sommer 201410).
5. Reduces Inflammation
In addition to infectious diseases, insufficient intake of vitamin A mainly promotes chronic inflammation in the body.
Therefore, the anti-inflammatory effect of retinol from the skin to the lungs is indispensable (Reifen 200211).
6. Strengthens Bones
While people with lower levels of vitamin A in their blood have a higher risk of bone fractures, those with the highest levels have a lower risk (Zhang et al. 201712).
To strengthen bones and minimize fractures’ risk, collaborative consumption with vitamin D is crucial, such as in the form of eggs (Joo et al. 201513).
7. Supports Muscle Gain
Scientists found a decrease in muscle protein synthesis in rats that consumed too little vitamin A through diet (Narbonne et al. 197814).
In addition to muscle development and maintenance, retinol also impairs weight gain, as vitamin A deficiency in the diet can lead to obesity (Zulet et al. 200815).
8. Regulates Genes
Most people are not aware that your diet can have a direct impact on your genetic makeup. Accordingly, the bioactive components of retinol can directly regulate genes (McGrane 200716).
Thus, your diet can have an impact on your offspring.
9. Fights Cancer
Animal studies suggest that retinoids can prevent and suppress cancer (Sankaranarayanan et al. 199617).
Similarly, other studies state that retinoids can inhibit tumor development and prevent second primary tumors’ development (Sun et al. 200218).
Unlike foods containing vitamin A, beta-carotene preparations do not show the same anticancer effects (Lee et al. 199919).
Foods to Prevent Vitamin A Deficiency Symptoms
Few people know that vitamin A1 – better known as retinol – is found only in animal foods.
For this reason, sources of true, pre-formed vitamin A1 are limited to the following food groups:
- Dairy products
For example, regular consumption of pasture-raised liver is the most effective way to obtain optimal vitamin A levels.
In fact, due to its unparalleled nutrient density, recent studies recommend adults eat liver several times per week (Kicinska et al. 201920).
However, eating liver once a month may be sufficient to counteract vitamin A deficiency symptoms (van Stuijvenberg et al. 202021).
So why are plant sources not as effective against the symptoms of vitamin A deficiency?
Beta-Carotene May Not Relieve Vitamin A Deficiency Symptoms
Most Internet sources will suggest plant-based foods to you first and foremost as vitamin A sources. The following orange vegetables are always put in the spotlight:
- Sweet potatoes
However, although these plants are considered vitamin A foods, they only provide precursors called carotenoids.
However, it is essential to know the difference between carotenoids and retinoids to prevent the symptoms of vitamin A deficiency.
While animal sources provide pre-formed vitamin A (retinol), plant sources only offer provitamin A (beta-carotene).
Since true retinol has much higher bioavailability, the body can utilize it more efficiently. In contrast, our body must first convert provitamin A into retinol to benefit from it.
However, this conversion of beta-carotene to vitamin A is not self-evident, since according to studies, women absorb only about 3.3% and men about 2.25% of beta-carotene (Lin et al. 200022; Hickenbottom et al. 200223).
Subsequently, the body needs to convert these ingested carotenoids to retinol in a further step. As a result, the body can only convert small amounts of beta-carotene to vitamin A.
For example, in this study, on average, only 3% of absorbed beta-carotene was converted to retinol (Hickenbottom et al. 200224).
Because nutrient claims do not distinguish between provitamin A and vitamin A, they are usually confusing.
Although International Units (UI) compensate for conversion efficiency loss, they do not compensate sufficiently. In this context, 1 UI corresponds exactly to 0.3 mcg of retinol and 0.6 mcg of beta-carotene.
However, beta-carotene’s actual effect is probably far less than half the retinol equivalent. Hence, you better eat several pounds of carrots to meet the true daily vitamin A requirement.
How to Absorb Vitamin A to Prevent Deficiency Symptoms
Many people don’t realize that vitamin A is fat-soluble. While B and C vitamins are water-soluble, A, D, E, and K vitamins are fat-soluble.
Therefore, we cannot properly absorb vitamin A if we eat a low-fat diet. Besides, fat helps increase the conversion of carotenoids to true retinol.
Furthermore, vitamin D is significantly involved in the absorption of retinol. And such as vitamin A, vitamin D is also fat-soluble.
On the one hand, vitamin D helps the body utilize vitamin A, and on the other hand, it counteracts potential toxicity.
For example, vitamin D consumption is necessary to strengthen bone mineralization and prevent any fractures by absorbing vitamin A (Joo et al. 201527).
Fortunately, however, nature leaves nothing to chance.
Many natural foods that contain vitamin A are equally rich in fat and vitamin D. For example, these are:
- Cod liver
Moreover, using healthy animal fats like grass-fed butter or organic lard will help you absorb retinol more efficiently.
In addition to improved nutrient absorption, animal fats have the advantage that they contain few polyunsaturated fatty acids, so they are better suited for frying than all vegetable oils except virgin coconut oil.
You get healthy omega-3 polyunsaturated fatty acids from fatty fish anyway, as well as primarily vitamin A and D.
With this in mind, it’s easy to see why cod liver oil has the reputation of being a historic natural remedy.
Also, you may allow yourself 15 minutes of sunshine a day to boost your body’s vitamin D synthesis if the weather permits it.
Vitamin A Deficiency Symptoms
Vitamin A is involved in various body functions, most notably related to growth, vision, and immune defense.
However, the symptoms of vitamin A deficiency explained below can vary in severity in different people.
If you suspect any symptom of vitamin A deficiency, you should immediately consult your trusted physician.
Vitamin A deficiency can affect oocyte quality and thus inhibit fertility.
Similarly, scientists suggest that too little intake of retinol can affect sperm quality and reduce fertility in men (Clagett-Dame et al. 201128).
Furthermore, vitamin A deficiency in the mother during pregnancy is associated with higher infant mortality rates (Hadi et al. 200029).
Nevertheless, be cautious with supplements as they may cause malformations (Guillonneau et al. 199730).
2. Dry Eyes
Eye problems are among the first symptoms that come to mind for people associated with vitamin A deficiency.
Dry eyes, or the impaired ability to produce tears, are the best known initial vitamin A deficiency symptoms.
Also, children in developing countries whose diets are deficient in vitamin A are most at risk for developing dry eyes (West 200231).
Furthermore, inadequate vitamin A is the most common cause of blindness in children that could have been prevented by proper nutrition (Wiseman et al. 201732).
3. Night Blindness
Another known symptom of vitamin A deficiency equally affects vision. Severe cases of vitamin A lack can lead to night blindness (Al Binali 201433).
In one study, women who suffered from night blindness were given vitamin A in the form of both foods, such as liver and supplements.
In this study, both forms of vitamin A improved their condition. Over several weeks of treatment, the women increased their vision in the dark by 50% (Haskell et al. 200534).
4. Dry Skin
Skin problems are among the best-known symptoms of vitamin A deficiency. For example, some people may notice dryness, dandruff, or itching when their vitamin A intake is too low.
In individual cases, this dryness may also be associated with the scalp and hair.
People who suffer from eczema show precisely these symptoms.
For this reason, a low retinol level is considered a promising parameter for the early detection and treatment of potential eczema (Biswas et al. 201835).
Studies indicate that nutrition plays a significant role in developing and controlling acne (Ozuguz et al. 201436).
Moreover, the association between insufficient vitamin A levels and acne development has been repeatedly established (El-Akawi et al. 200637).
Vitamin A is an antioxidant that can protect skin cells from oxidative stress such as UV rays. Accordingly, studies suggest that it can slow down their aging process, promote skin renewal and counteract acne (Park 201538).
6. Impaired Wound Healing
Vitamin A promotes collagen formation, an essential building block for skin, hair, muscles, and bones.
Researchers, therefore, conclude that this effect on collagen production could improve wound healing in the context of clinical treatments (Wicke et al. 200039).
Accordingly, a study on older men demonstrated that retinoids administered in the form of cream could dramatically improve wound healing.
7. Respiratory Infections
Scientists first discovered Vitamin A as the anti-infection vitamin for a good reason (Green et al. 192840).
Accordingly, even mild vitamin A deficiency is sufficient to promote the risk of respiratory infections (Sommer et al. 198441).
In addition to pneumonia, foods with vitamin A also reduce infection rates and mortality associated with tuberculosis or malaria (Huang et al. 201842).
Retinol is essential to maintain the intestinal wall, preventing pathogens from entering the bloodstream (Huang et al. 201843).
Besides, it offers anti-inflammatory properties (Reifen 200244).
Therefore, it is not surprising that a recent study suggests a link between vitamin A deficiency and Crohn’s disease.
It found that people who had too little vitamin A in their blood developed chronic gut inflammation more often (Soares-Mota et al. 201545).
9. Poor Growth
Vitamin A plays an essential role in the formation of healthy cells.
Accordingly, vitamin A is essential for developing vital major organs and growth in children (Hadi et al. 200046).
For this reason, an insufficient supply of the critical vitamin can delay a child’s growth and, in particular, inhibit bone formation.
Retinol plays a significant role in blood formation.
Accordingly, scientists at the German Cancer Research Center suggest, vitamin A deficiency can cause a loss of important blood stem cells, damaging the hematopoietic system in the bone marrow (Cabezas-Wallscheid et al. 201747).
Furthermore, it is well-documented that vitamin A deficiency can lead to anemia (Jafari et al. 201348).
How Much Vitamin a per Day to Prevent Deficiency Symptoms?
Since vitamin A deficiency can be dangerous, if you suspect any of the symptoms, consult your trusted doctor immediately.
Health authorities recommend that men consume 900 mcg, women 700 mcg (770 mcg during pregnancy), and children and adolescents 300 to 600 mcg of vitamin A per day (ODS 202049).
Accordingly, these amounts should be ideal for preventing deficiency symptoms.
Although consuming too much vitamin A is harmful, it is unlikely if you consume only natural foods.
In contrast, vitamin A toxicity can occur precisely when people consume supplements in excessive amounts for weeks or months.
Moreover, excessive vitamin A supplementation can even lead to hair loss (Almohanna et al. 201950)
With this in mind, natural foods with retinol are the safer and healthier choice.
In contrast, even a single consumption of 66 grams of liver per month could help preschoolers in South Africa avoid the need for vitamin A supplementation (van Stuijvenberg et al. 202053).
Don’t Ignore Vitamin A Deficiency Symptoms
Vitamin A deficiency is one of the most substantial health problems in developing countries, contributing to childhood blindness and increased mortality rates.
Nevertheless, one should not underestimate the symptoms of vitamin A deficiency in Western society. What plagues developing countries can also have an impact here.
There, it is primarily the increased risk of infection from vitamin A deficiency that drives mortality rates.
Against the Corona pandemic’s backdrop, it is therefore just as essential for our immune system to absorb sufficient vitamin A through food.
Since the conversion of beta-carotene to retinol is greatly overestimated according to studies, reaching for foods with pre-formed vitamin A is advisable. For example, these are fish, eggs, liver, and dairy products.
However, anyone who suspects vitamin A deficiency symptoms, ranging from dry eyes to acne and infertility to respiratory infections and poor wound healing, should see a doctor immediately.
Vitamin A Deficiency Symptoms FAQ
How do you fix vitamin A deficiency?
If you witness any signs of vitamin A deficiency, consult a health professional immediately. Your doctor will be able to design an individual plan to fix your issue. Besides, foods with pre-formed vitamin A, such as liver, fatty fish, eggs, and dairy products, may help.
What blocks vitamin A absorption?
Above all, you have to distinguish between retinol and beta-carotene. While retinol is active vitamin A, your body has to convert beta-carotene into retinol, which results in minimal absorption rates. Vitamin D and healthy fats can improve the absorption of the fat-soluble vitamin A.
Which disease is caused due to lack of vitamin A?
Vitamin A deficiency can cause anemia, acne, infertility, and various eye issues, ranging from dry eyes to complete blindness.
Which foods are high in Vitamin A?
The most potent food with vitamin A is liver – whether from beef, chicken, pork, or cod. Other options with bioavailable retinol for vitamin A deficiency include eggs, dairy products, and fatty fish.
1Beatty S, Koh H, Phil M, Henson D, Boulton M. The role of oxidative stress in the pathogenesis of age-related macular degeneration. Surv Ophthalmol. 2000 Sep-Oct;45(2):115-34. doi: 10.1016/s0039-6257(00)00140-5. Review. PubMed PMID: 11033038.
2Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability of the conversion of beta-carotene to vitamin A in women measured by using a double-tracer study design. Am J Clin Nutr. 2000 Jun;71(6):1545-54. doi: 10.1093/ajcn/71.6.1545. PubMed PMID: 10837297.
3Hickenbottom SJ, Follett JR, Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design. Am J Clin Nutr. 2002 May;75(5):900-7. doi: 10.1093/ajcn/75.5.900. PubMed PMID: 11976165.
4Hickenbottom SJ, Follett JR, Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design. Am J Clin Nutr. 2002 May;75(5):900-7. doi: 10.1093/ajcn/75.5.900. PubMed PMID: 11976165.
5Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability of the conversion of beta-carotene to vitamin A in women measured by using a double-tracer study design. Am J Clin Nutr. 2000 Jun;71(6):1545-54. doi: 10.1093/ajcn/71.6.1545. PubMed PMID: 10837297.
6Hickenbottom SJ, Follett JR, Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design. Am J Clin Nutr. 2002 May;75(5):900-7. doi: 10.1093/ajcn/75.5.900. PubMed PMID: 11976165.
7Wiseman EM, Bar-El Dadon S, Reifen R. The vicious cycle of vitamin a deficiency: A review. Crit Rev Food Sci Nutr. 2017 Nov 22;57(17):3703-3714. doi: 10.1080/10408398.2016.1160362. Review. PubMed PMID: 27128154.
8Hadi H, Stoltzfus RJ, Dibley MJ, Moulton LH, West KP Jr, Kjolhede CL, Sadjimin T. Vitamin A supplementation selectively improves the linear growth of indonesian preschool children: results from a randomized controlled trial. Am J Clin Nutr. 2000 Feb;71(2):507-13. doi: 10.1093/ajcn/71.2.507. PubMed PMID: 10648265.
9Guillonneau M, Jacqz-Aigrain E. [Teratogenic effects of vitamin A and its derivates]. Arch Pediatr. 1997 Sep;4(9):867-74. doi: 10.1016/s0929-693x(97)88158-4. Review. PubMed PMID: 9345570.
10Clagett-Dame M, Knutson D. Vitamin A in reproduction and development. Nutrients. 2011 Apr;3(4):385-428. doi: 10.3390/nu3040385. Epub 2011 Mar 29. Review. PubMed PMID: 22254103; PubMed Central PMCID: PMC3257687.
11Huang Z, Liu Y, Qi G, Brand D, Zheng SG. Role of Vitamin A in the Immune System. J Clin Med. 2018 Sep 6;7(9). doi: 10.3390/jcm7090258. Review. PubMed PMID: 30200565; PubMed Central PMCID: PMC6162863.
12Sommer A. Preventing blindness and saving lives: the centenary of vitamin A. JAMA Ophthalmol. 2014 Jan;132(1):115-7. doi: 10.1001/jamaophthalmol.2013.5309. PubMed PMID: 24407830.
13Park K. Role of micronutrients in skin health and function. Biomol Ther (Seoul). 2015 May;23(3):207-17. doi: 10.4062/biomolther.2015.003. Epub 2015 May 1. Review. PubMed PMID: 25995818; PubMed Central PMCID: PMC4428712.
14Ozuguz P, Dogruk Kacar S, Ekiz O, Takci Z, Balta I, Kalkan G. Evaluation of serum vitamins A and E and zinc levels according to the severity of acne vulgaris. Cutan Ocul Toxicol. 2014 Jun;33(2):99-102. doi: 10.3109/15569527.2013.808656. Epub 2013 Jul 5. PubMed PMID: 23826827.
15Beatty S, Koh H, Phil M, Henson D, Boulton M. The role of oxidative stress in the pathogenesis of age-related macular degeneration. Surv Ophthalmol. 2000 Sep-Oct;45(2):115-34. doi: 10.1016/s0039-6257(00)00140-5. Review. PubMed PMID: 11033038.
16Evans JR, Lawrenson JG. Antioxidant vitamin and mineral supplements for preventing age-related macular degeneration. Cochrane Database Syst Rev. 2017 Jul 30;7:CD000253. doi: 10.1002/14651858.CD000253.pub4. Review. PubMed PMID: 28756617; PubMed Central PMCID: PMC6483250.
17Sommer A. Preventing blindness and saving lives: the centenary of vitamin A. JAMA Ophthalmol. 2014 Jan;132(1):115-7. doi: 10.1001/jamaophthalmol.2013.5309. PubMed PMID: 24407830.
18Reifen R. Vitamin A as an anti-inflammatory agent. Proc Nutr Soc. 2002 Aug;61(3):397-400. doi: 10.1079/PNS2002172. Review. PubMed PMID: 12230799.
19Zhang X, Zhang R, Moore JB, Wang Y, Yan H, Wu Y, Tan A, Fu J, Shen Z, Qin G, Li R, Chen G. The Effect of Vitamin A on Fracture Risk: A Meta-Analysis of Cohort Studies. Int J Environ Res Public Health. 2017 Sep 10;14(9). doi: 10.3390/ijerph14091043. PubMed PMID: 28891953; PubMed Central PMCID: PMC5615580.
20Joo NS, Yang SW, Song BC, Yeum KJ. Vitamin A intake, serum vitamin D and bone mineral density: analysis of the Korea National Health and Nutrition Examination Survey (KNHANES, 2008-2011). Nutrients. 2015 Mar 10;7(3):1716-27. doi: 10.3390/nu7031716. PubMed PMID: 25763530; PubMed Central PMCID: PMC4377877.
21Narbonne JF, Daubeze M, Bonmort F. [Protein metabolism in vitamin A deficient rats. II. Protein synthesis in striated muscle]. Ann Nutr Aliment. 1978;32(1):59-75. PubMed PMID: 677623.
22Zulet MA, Puchau B, Hermsdorff HH, Navarro C, Martínez JA. Vitamin A intake is inversely related with adiposity in healthy young adults. J Nutr Sci Vitaminol (Tokyo). 2008 Oct;54(5):347-52. doi: 10.3177/jnsv.54.347. PubMed PMID: 19001764.
23McGrane MM. Vitamin A regulation of gene expression: molecular mechanism of a prototype gene. J Nutr Biochem. 2007 Aug;18(8):497-508. doi: 10.1016/j.jnutbio.2006.10.006. Epub 2007 Feb 22. Review. PubMed PMID: 17320364.
24Sankaranarayanan R, Mathew B. Retinoids as cancer-preventive agents. IARC Sci Publ. 1996;(139):47-59. Review. PubMed PMID: 8923019.
25Sun SY, Lotan R. Retinoids and their receptors in cancer development and chemoprevention. Crit Rev Oncol Hematol. 2002 Jan;41(1):41-55. doi: 10.1016/s1040-8428(01)00144-5. Review. PubMed PMID: 11796231.
26Lee IM, Cook NR, Manson JE, Buring JE, Hennekens CH. Beta-carotene supplementation and incidence of cancer and cardiovascular disease: the Women’s Health Study. J Natl Cancer Inst. 1999 Dec 15;91(24):2102-6. doi: 10.1093/jnci/91.24.2102. PubMed PMID: 10601381.
27Khillan JS. Vitamin A/retinol and maintenance of pluripotency of stem cells. Nutrients. 2014 Mar 21;6(3):1209-22. doi: 10.3390/nu6031209. Review. PubMed PMID: 24662164; PubMed Central PMCID: PMC3967188.
28Purton LE. Roles of retinoids and retinoic Acid receptors in the regulation of hematopoietic stem cell self-renewal and differentiation. PPAR Res. 2007;2007:87934. doi: 10.1155/2007/87934. PubMed PMID: 17846663; PubMed Central PMCID: PMC1950592.
29Jafari SM, Heidari G, Nabipour I, Amirinejad R, Assadi M, Bargahi A, Akbarzadeh S, Tahmasebi R, Sanjdideh Z. Serum retinol levels are positively correlated with hemoglobin concentrations, independent of iron homeostasis: a population-based study. Nutr Res. 2013 Apr;33(4):279-85. doi: 10.1016/j.nutres.2013.02.004. Epub 2013 Mar 11. PubMed PMID: 23602245.
30ODS. Vitamin A Fact Sheet for Health Professionals. Bethesda, MD: Office of Dietary Supplements, National Institutes of Health, 2020. Retrieved 2021 March 19, from https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/.
31Dibley MJ, Sadjimin T, Kjolhede CL, Moulton LH. Vitamin A supplementation fails to reduce incidence of acute respiratory illness and diarrhea in preschool-age Indonesian children. J Nutr. 1996 Feb;126(2):434-42. doi: 10.1093/jn/126.2.434. PubMed PMID: 8632216.
32Murphy S, West KP Jr, Greenough WB 3rd, Cherot E, Katz J, Clement L. Impact of vitamin A supplementation on the incidence of infection in elderly nursing-home residents: a randomized controlled trial. Age Ageing. 1992 Nov;21(6):435-9. doi: 10.1093/ageing/21.6.435. PubMed PMID: 1471582.
33van Stuijvenberg ME, Schoeman SE, Nel J, le Roux M, Dhansay MA. Liver is widely eaten by preschool children in the Northern Cape province of South Africa: Implications for routine vitamin A supplementation. Matern Child Nutr. 2020 Jul;16(3):e12931. doi: 10.1111/mcn.12931. Epub 2019 Dec 17. PubMed PMID: 31845541; PubMed Central PMCID: PMC7296811.
34Mozaffarian D, Rimm EB, Herrington DM. Dietary fats, carbohydrate, and progression of coronary atherosclerosis in postmenopausal women. Am J Clin Nutr. 2004 Nov;80(5):1175-84. doi: 10.1093/ajcn/80.5.1175. PubMed PMID: 15531663; PubMed Central PMCID: PMC1270002.
35Joo NS, Yang SW, Song BC, Yeum KJ. Vitamin A intake, serum vitamin D and bone mineral density: analysis of the Korea National Health and Nutrition Examination Survey (KNHANES, 2008-2011). Nutrients. 2015 Mar 10;7(3):1716-27. doi: 10.3390/nu7031716. PubMed PMID: 25763530; PubMed Central PMCID: PMC4377877.