What is Sub-clinical vitamin A deficiency

Below are various authors' explanation about Sub-clinical vitamin A deficiency and the conclusion of FAO/WHO:

WHO/UNICEF. Indicators of VAD and their use in monitoring intervention programmes. WHO/NUT/96.10. pp. 66. World Health Organization, Geneva.

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. 

Underwood, B.A. 1990. Biochemical and histological methodologies for assessing vitamin A status in Human populations. In: Packer L, ed. Methods in Enzymology: Retinoids, Part B. pp. 242–250.New York, Academic Press.

Olson, J.A. 1992. Measurement of vitamin A status. Voeding, 53: 163–167.

Direct measurement of concentrations of vitamin A in the liver where it is stored or in the total body pool relative to known specific vitamin A–related functions (e.g., night blindness) would be the indicator of choice for determining requirements. This cannot be done with the methodology now available for population use. There are several practical biochemical methods for estimating sub-clinical vitamin A status but all have limitations.

WHO/UNICEF. Indicators of VAD and their use in monitoring intervention programmes. WHO/NUT/96.10. pp. 66. World Health Organization, Geneva.

Each method is useful to identify deficient populations, but no one of these indicators is definitive or directly related quantitatively to disease occurrence. The indicators of choice are listed in Table 17. These indicators are less specific to VAD than clinical eye signs and less sensitive for measuring sub-clinical vitamin A status. WHO recommends that where feasible at least two sub-clinical biochemical indicators, or one biochemical and a composite of non-biochemical risk factors, should be measured and that both types of indicators should point to deficiency in order to identify populations at high risk of VAD.

FAO/WHO 

Cut-off points given in Table 17 represent the consensus gained from practical experience comparing populations with some evidence of VAD with those without VAD. There are no field studies that quantitatively relate the prevalence of adverse health symptoms (e.g., incidence or prevalence of severe diarrheal disease) and relative levels of biologic indicator cut-off values. Furthermore, each of the biochemical indicators listed is subject to confounding factors, which may be unrelated to vitamin A status (e.g., infections).

Sommer, A. & Muhilal. 1982. Nutritional factors in corneal xerophthalmia and keratomalacia. Arch. Ophthalmol., 100: 399–403.

Although all biochemical indicators currently available have limitations, the biochemical indicator of choice for population assessment is the distribution of serum levels of vitamin A (serum retinol). Only at very low blood levels (<0.35 μmol/l) is there an association with corneal disease prevalence. 

Wachtmeister, L. 1988. Attempts to define the minimal serum level of vitamin A required for normal visual function in a patient with severe fat malabsorption. Acta Ophthalmol., 66: 341–348.

Blood levels between 0.35 and 0.70 μmol/l are likely to characterise sub-clinical deficiency, 

Flores, H. 1984. Assessment of marginal vitamin A deficiency in Brazilian children using the relative dose response procedure. Am. J. Clin. Nutr., 40: 1281–1289.

but sub-clinical deficiency may still be present at levels between 0.70 and 1.05 μmol/l and occasionally above 1.05 μmol/l. 

WHO/UNICEF. Indicators of VAD and their use in monitoring intervention programmes. WHO/NUT/96.10. pp. 66. World Health Organization, Geneva.

The prevalence of values below 0.70 μmol/l is a generally accepted population cut-off for preschool-age children to indicate risk of inadequate vitamin A status

Flores, H. 1991. Serum vitamin A distribution curve for children aged 2–6 y known to have adequate vitamin A status: a reference population. Am. J. Clin. Nutr., 54: 707–711. 

Pilch, S.M., ed. 1987. Analysis of vitamin A data from the health and nutrition examination surveys. J. Nutr., 117: 636–640.

and above 1.05 μ mol/l to indicate an adequate status. 

Pilch, S.M., ed. 1987. Analysis of vitamin A data from the health and nutrition examination surveys. J. Nutr., 117: 636–640.

Filteau, S.M. 1993. Influence of morbidity on serum retinol of children in a communitybased study in northern Ghana. Am. J. Clin. Nutr., 58: 192–197

As noted elsewhere, clinical and sub-clinical infections can lower serum levels of vitamin A on average as much as 25 percent independently of vitamin A intake (95,96). 

FAO/WHO 

Therefore, at levels between about 0.5 and 1.05 μ mol/l, the relative dose response or modified relative dose response test on a subsample of the population can be useful for identifying the prevalence of critically depleted body stores when interpreting the left portion of serum retinol distribution curves.