Preformed vitamin A is found almost exclusively in animal products, such as human milk, glandular meats, liver and fish liver oils (especially), egg yolk, and whole milk and dairy products. Preformed vitamin A is also used to fortify processed foods, that may include sugar, cereals, condiments, fats, and oils.
Pro-vitamin A carotenoids are found in green leafy vegetables (e.g., spinach, amaranth, and young leaves from various sources), yellow vegetables (e.g., pumpkins, squash, and carrots), and yellow and orange noncitrus fruits (e.g., mangoes, apricots, and papaya).
Booth, S.L., Johns, T. & Kuhnlein, H.V. 1992. Natural food sources of vitamin A and pro-vitamin A. UNU Food and Nutrition Bulletin, 14: 6–19, added in his/her UNU Food and Nutrition Bulletin:
Red palm oil produced in several countries worldwide is especially rich in pro-vitamin A.Burití Palm. 1975. In: Report, Ad Hoc Panel of the Advisory Committee on Technology Innovations, Board on Science and Technology for International Development, Commission on International Relations. Underexploited tropical plants with promising economic value. P. 133-137. Washington, DC, National Academy Sciences. Another possible rich source of vitamin A, according to this writer:
Some other indigenous plants also may be unusually rich sources of pro-vitamin A. Such examples are the palm fruit known in Brazil as burití, that is found in areas along the Amazon (as well as elsewhere in Latin America),
Vuong, L.T. 1997. An indigenous fruit of North Vietnam with an exceptionally high ß-carotene content. p. 2. Sight and Life Newsletter. Another author from Vietnam about a fruit in Vietnam that they call "gac":
and the fruit known as gac in Vietnam, that is used to colour rice, particularly on ceremonial occasions.
FAO/WHO
Foods containing pro-vitamin A carotenoids tend to be less biologically available but more affordable than animal products. It is mainly for this reason that carotenoids provide most of the vitamin A activity in the diets of economically deprived populations.
Dietary intake and patterns
Rodriguez-Amaya, DB. 1997. Carotenoids and food preparation: the retention of provitamin A carotenoids in prepared, processed, and stored foods. Arlington, VA, John Snow, Inc./OMNI Project, added statement:
Vitamin A status cannot be assessed from dietary intake alone, but dietary intake assessment can provide evidence of risk of an inadequate status. Quantitative collection of dietary information is fraught with measurement problems. These problems arise both from obtaining representative quantitative dietary histories from individuals, communities, or both and from interpreting these data while accounting for differences in bio-availability, preparation losses, and variations in food composition data among population groups.Parker, R.S. et al. 1999. Bio-availability of carotenoids in Human subjects. Proc. Nutr. Soc., 58 :1–8 and IVACG. 1989. Report of the International Vitamin A Consultative Group. Guidelines for the development of a simplified dietary assessment to identify groups at risk for inadequate intake of vitamin A. Washington, DC, International Life Sciences Institute- Nutrition Foundation.
This is especially difficult in populations consuming most of their dietary vitamin A from pro-vitamin carotenoid sources. Simplified guidelines have been developed recently in an effort to improve the obtaining of reliable dietary intake information from individuals and communities.
World and regional supply and patterns
Périssé, J. & Polacchi, W. 1980. Geographical distribution and recent changes in world supply of vitamin A. Food and Nutrition, 6: 21–27 and ACC/SCN. Second report on the world nutrition situation. Vol.1. Global and regional results, October 1992. Vol. 2, March 1993. (see Table 16 above)
In theory the world’s food supply is sufficient to meet global requirements. Great differences exist, however, in the available sources (animal and vegetable) and in per capita consumption of the vitamin among different countries, age categories, and socio-economic groups. VAD as a global public health problem, therefore, is largely due to inequitable food distribution among and within countries and households in relation to need for ample bioavailable vitamin A sources.
FAO/WHO. 1988. Requirements of vitamin A, iron, folate and vitamin B12. Report of a Joint FAO/WHO Expert Consultation. Rome, Food and Agriculture Organization.
Earlier FAO global estimates in 1984 indicated that preformed vitamin A constituted about one-third of total dietary vitamin A activity.Rodriguez-Amaya, DB. 1997. Carotenoids and food preparation: the retention of provitamin A carotenoids in prepared, processed, and stored foods. Arlington, VA, John Snow, Inc./OMNI Project.
World availability of vitamin A for human consumption at that time was approximately 220 μg of preformed retinol per capita daily and 560 μg RE from pro-vitamin carotenoids (about 3400 μg carotenoids for a 6:1 conversion factor) per person per day, for a total of about 790 μg RE. These values are based on supply estimates and not consumption estimates. Losses commonly occur during food storage and processing, both industrially and in the home.
ACC/SCN. Second report on the world nutrition situation. Vol.1. Global and regional results, October 1992. Vol. 2, March 1993.
The estimated available regional supply of vitamin A from a more recent global evaluation shown in Table 16 (above) illustrates the variability in amounts and sources of vitamin A.
The variability is further complicated by access to the available supply, that varies with household income, poverty being a yardstick for risk of VAD. VAD is most prevalent in Southeast Asia, Africa, and the Western Pacific, where vegetable sources contribute nearly 80 percent or more of the available supply of retinol equivalents.
US Department of Agriculture, Agricultural Research Service. 1998. Food and Nutrient Intakes by Individuals in the United States, by Sex and Age, 1994-96, pp. 197. Nationwide Food Surveys Report No. 96-2.
Furthermore, in Southeast Asia the total available supply is about half of that of most other regions and is particularly low in animal sources. In contrast, the Americas, Europe, and Eastern Mediterranean regions have a supply ranging from 800 to 1000 μg RE/day, one-third of which comes from animal sources. Recent national data from the USA Continuing Survey of Food Consumption
Centers for Disease Control and Prevention. 1998. National Health and Nutrition Examination Survey III, 1988-1994. CD-ROM Series 11, No. 2A, April 1998. Hyatsville, MD.
and the National Health and Nutrition Examination Survey
Gregory, J., Foster, K., Tyler, H. & Wiseman, M. 1990. The Dietary and Nutritionl Survey of British Adults. London, HMSO.
included mean dietary intakes of children 0–6 years of age of 864 ± 497 and 921 ± 444 μg RE daily. In the Dietary and Nutritional Survey of British Adults,
Tyler, H.A., Day, M.J.L. & Rose, H.J. 1991. Vitamin A and pregnancy [letter]. Lancet, 337:48–49.
the median intake of men and women 35-49 years old was 1118 μg RE and 926 μg RE, respectively, which corresponded to serum retinol concentrations of 2.3 μmol/l and 1.8 μmol/l, respectively. In another selected survey in the United Kingdom, median intakes for nonpregnant women who did not consume liver or liver products during the survey week were reported to be 686 μg RE daily.
Bloem, M.W., de Pee, S. & Darnton-Hill, I. 1997. Vitamin A deficiency in India, Bangladesh and Nepal. In: Gillespie S, ed. Malnutrition in South Asia. A regional profile. p.125–144. UNICEF Regional Office for South Asia.
The available world supply figures in Table 16 were recently reassessed based on a bio-availability ratio of 1:30 for retinol to other pro-vitamin A carotenoids.
Yin, S. 1998. Green and yellow vegetables rich in pro-vitamin A carotenoids can sustain vitamin A status in children. FASEB J., 12: A351 and Jalal, F. 1998. Serum retinol concentrations in children are affected by food sources of β- carotene, fat intake, and anthelmintic drug treatment. Am. J. Clin. Nutr., 68: 623-9.
This conversion factor was justified on the basis of one published controlled intervention study conducted in Indonesia and a limited number of other studies not yet published in full. Applying the unconfirmed conversion factor to the values in Table 16 would lead to the conclusion that regional and country needs for vitamin A could not be met from predominantly vegetarian diets. This is inconsistent with the preponderance of epidemiologic evidence. Most studies report a positive response when vegetable sources of pro-vitamin A are given under controlled conditions to deficient subjects freed of confounding parasite loads and provided with sufficient dietary fat.
Parker, R.S. et al. 1999. Bio-availability of carotenoids in Human subjects. Proc. Nutr. Soc., 58 :1–8 and FAO/WHO. 1988. Requirements of vitamin A, iron, folate and vitamin B12. Report of a Joint FAO/WHO Expert Consultation. Rome, Food and Agriculture Organization.
Emerging data are likely to justify a lower biologic activity for pro-vitamin A carotenoids because of the mix of total carotenoids found in food sources in a usual meal.
FAO/WHO
This Consultation concluded that the 1:6 bioconversion factor originally derived on the basis of balance studies should be retained until there is firm confirmation from ongoing studies that use more precise methodologies.
Evidence for making recommendations
Indicators of vitamin A deficiency
WHO/UNICEF. Indicators of VAD and their use in monitoring intervention programmes. WHO/NUT/96.10. pp. 66. World Health Organization, Geneva.
Ocular signs of VAD are measured by clinical examination and history and are quite specific in preschool-age children. However, these are rare occurrences that require examination of large populations to obtain incidence and prevalence data. Sub-clinical VAD is more prevalent, requiring smaller sample sizes to obtain valid prevalence estimates.
Sommer, A. 1994. VAD and its consequences: A field guide to their detection and control. 3rd ed. Geneva, World Health Organization, 1994.
A full description of clinical indicators with coloured illustrations for each is found in the WHO Field Guide.
Christian, P. 1998. Working after the sun goes down. Exploring how night blindness impairs women’s work activities in rural Nepal. Eur. J. Clin. Nutr., 52: 519–524.
The most frequently occurring is night blindness, which is the earliest manifestation of xerophthalmia. In mild form it is generally noticeable after stress from a bright light that bleaches the rhodopsin (visual purple) found in the retina. VAD prolongs the time to regenerate rhodopsin, thus delays adaptation time in dark environments. Night-blind young children tend to stumble when going from bright to dimly lighted areas and they, as well as night-blind mothers, tend to remain inactive at dusk and at night.Underwood, B.A. & Olson, J.A., eds. 1993. A brief guide to current methods of assessing vitamin A status. A report of the International Vitamin A Consultative Group (IVACG). Washington, DC, Nutrition Foundation.
No field-applicable objective tool is currently available for measuring night blindness in children under about 3 years of age. It can be measured by history in certain cultures.
Sommer, A. History of nightblindness: a simple tool for xerophthalmia screening. Am. J. Clin. Nutr.,1980, 33:887–891.
When night blindness is prevalent, many cultures coin a word descriptive of the characteristic symptom that they can reliably recall on questioning, making this a useful tool for assessing the prevalence of VAD.
Questioning for night blindness is not consistently a reliable assessment measure where a local term is absent. In addition, there is no clearly defined blood retinol level that is directly associated with occurrence of the symptom. Vitamin A – related night blindness, however, responds rapidly, usually within 1–2 days, to administration of vitamin A.
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