|Food and Nutrition Bulletin Volume 11, Number 3, 1989 (UNU, 1989, 73 pages)|
|Prevention and control of vitamin-A deficiency|
by the Subcommittee on Vitamin A Deficiency Prevention and Control
The Subcommittee on Vitamin A Deficiency Prevention and Control of the Food and Nutrition Board's Committee on International Programs conducted workshop on Strategies and Priorities for Research on the Influence of Vitamin A Supplementation on Morbidity on 28 April 1988. The purpose of the workshop was to consider the influences of vitamin A on the immune system, the role of vitamin A in the differentiation and maintenance of epithelial tissue, the current research and recent findings on vitamin A and morbidity, and the ethical dimensions of such research.
This report was written by the subcommittee subsequent to the workshop and represents the consensus of its members. It does not give equal weight to all hypotheses considered at the workshop but emphasizes those relating vitamin A to infections within the context of field studies. In addition, the report includes a detailed discussion of methods of assessment of vitamin-A status. It does not discuss the ethical issues in any great detail because these were discussed very thoroughly in its previous report and in Dr. Porter's paper below (pp. 36-40).
The following hypotheses were addressed at the workshop:
The word marginal is used to describe the range of vitamin-A deficiency of interest to the subcommittee; it refers to the subclinical stage preceding the appearance of night blindness and the conjunctival and corneal changes characteristic of xerophthalmia. Although it is difficult to define marginal vitamin-A deficiency adequately, it is nonetheless clear that in countries where xerophthalmia is a significant public health problem a large proportion of children are afflicted with marginal deficiency. The World Health Organization considers prevalences of corneal xerosis and keratomalacia (X2 + X3A + X3B)* in excess of 0.01% to indicate a significant public health problem affecting large numbers of children . In such areas, many more children will show less severe clinical signs (X1A, X1B) and night blindness (my), and even more will have low plasma vitamin A, low vitamin-A stores in the liver, and vitamin-A-deficient diets. Xerophthalmia is known to be associated with diminished host defences and increased risk of infection. Similarly, marginal vitamin-A deficiency may lower resistance to infection, a possibility that needs to be confirmed and quantified through adequately designed studies. Because so many children are likely to be affected by marginal vitamin-A deficiency, the study of this condition and its implications constitutes a significant research priority.
The subcommittee's first report considered effects of vitamin A on mortality and provided advice on the conduct of field studies designed to test the hypothesis that vitamin-A supplementation lowers child mortality rates . Mortality studies require very large samples in order to achieve satisfactory statistical power.
The study of morbidity related to vitamin A is more feasible since infections occur more frequently and fewer subjects need be studied. Mortality and morbidity studies are complementary. The demonstration of an effect of vitamin-A supplementation on the incidence and severity of gastrointestinal and respiratory infections would strengthen the persuasiveness of findings on mortality because such a demonstration would provide a plausible pathway leading to the mortality effects. Similarly, the demonstration of effects of marginal vitamin-A deficiency on epithelialtissue differentiation and maintenance and on immunocompetence would further validate both morbidity and mortality findings.
The subcommittee has assessed progress to date in regard to morbidity effects and has determined that further research is required. Salient research gaps are as follows.
Vitamin-A deficiency both in man and in laboratory animals impairs immunological responses . Although not all of the immune responses have been tested in man, the consensus of results in published studies indicates that the following parameters are altered. Reduced vitamin-A intake and low serum-rehnol concentrations are associated with decreased delayed hypersensitivity response, lower serum-antibody response to some antigens, decreased lymphocyte response to mitogens, decreased natural-killer-cell activity, delayed rejection of grafts, changes in complement level, and phagocyte dysfunction. Several published reviews are available [4-6]. In animals deficient in vitamin A, morbidity and mortality increased after infectious challenge 13]. The significance of these findings in humans remains to be established.
In general, changes in immunocompetence often precede obvious infection and can be viewed as a functional index of nutritional deficiency. Changes in immune responses may mediate heightened susceptibility to infection in populations deficient in vitamin A.
More information should be obtained through research in the following areas:
The literature suggests that marginal vitamin-A deficiency is associated with increased incidence or severity of infections (or both). The evidence seems to be much stronger for respiratory than for gastrointestinal infections. The interpretation of the results to date is difficult because studies have failed to fully document vitamin-A status or to control for factors associated with both vitamin-A deficiency and the risk of infection.
An example is the need to control for environmental sanitation. Vitamin-A-deficient children may be from lower-income families with poorer housing and sanitary facilities. Children with adequate vitamin-A status, even when living in the same communities, may be from better-off homes. Thus, failure to control for environmental sanitation would overestimate the importance of vitamin A for morbidity. Similarly, vitamin-A deficiency generally coexists with other nutritional problems that are thought to affect the risk of morbidity. Studies of marginal vitamin-A deficiency and morbidity must at least control for protein-energy malnutrition and other deficiencies such as of zinc and iron.
Further research is recommended in the following areas:
Indicators of vitamin-A status
In its first report , the subcommittee noted the importance of adequately characterizing the vitamin-A status of study populations. Such data will allow for stratification by vitamin-A status if required in the data analysis and will increase flexibility in extrapolating the results to other populations. The report also noted the problems in the measurement of vitamin-A status that require further methodological work:
Vitamin-A status is determined by total body stores and can be thought of in relative terms as deficient, depleted, adequate, excessive, or toxic. Deficient, excessive, and toxic concentrations are manifested by clinical signs and symptoms or by biochemical measures interpretable both for individuals and populations. Quantitative measurement of intermediate levels of vitamin A (depleted and adequate) is problematic and in need of research.
Vitamin-A status is also affected by absorption and other factors that increase metabolism, including infection and protein-energy malnutrition. Such factors should be considered in the design of studies to investigate the impact of vitamin A on morbidity.
Field-applicable indicators of marginal vitamin-A status currently recognized include serum concentrations, the relative dose-response test, and conjunctival-impression cytology. Each of these has limitations in feasibility and practicality in the field as well as in quantitative interpretation. Detailed discussions of methodologies for clinical histories and dietary histories are not specific to studies of vitamin A and morbidity and thus will not be discussed in detail here.
The interpretation of serum concentrations of vitamin A as reflectors of status on an individual and population basis have been discussed in a previous publication . In the past, the interpretation of population distribution curves was largely based on comparisons with non-representative population surveys from developing countries from which cutoff points for deficient, low, and acceptable levels were derived . Recently available are age- and sex-specific distribution curves for representative, relatively well nourished populations living in the United States [8; 9]. Although these reference curves appear to be independent of ethnic differences, they are influenced by environment [10; 11], and their usefulness as a universal reference needs to be validated. The most appropriate reference curve for serum levels for field studies of vitamin A and morbidity would be from a representative population known to have adequate body stores while living in an environment characteristic of an at-risk population. Studies are needed to establish appropriate reference curves for populations residing outside the United States and in high-risk areas. To obtain such references, distribution curves established before and repeated after large-dose supplements - for example, 30 days after a 200,000IU supplement - would be appropriate. These curves could then be compared with the US reference curves to determine whether there are differences.
Relative dose-response test
Whereas to demonstrate a rise in homeostatically controlled serum concentrations in response to increased intake or supplementation requires serially obtained samples with an intervening period for stabilization, the relative dose-response (RDR) test can detect depleted stores with only a five-hour interval between dosing and sampling [12; 13]. This test, modified to suit various conditions, has been successfully applied in both clinical and field-survey conditions. It has been validated by an intravenous procedure for dosing coupled with direct liver biopsy in a limited number of children with liver disease and adults [14;15]. The number of direct quantitative validation studies needs to be expanded to obtain greater assurance as to the range and confidence intervals of the association of a positive RDR test with vitamin-A concentrations in the liver. However, this can be accomplished only under specialized clinical circumstances, not in field studies in developing countries. Indirect validation with a before-and-after supplementation procedure also needs to be extended to population groups in which heavy intestinal parasitism, malabsorption, and protein-energy malnutrition are common. In addition, a proposed adaptation of the RDR method using didehydroretinol (DR, or vitamin A2), a natural derivative of retinal (vitamin Al, the common form of vitamin A in foods and formed in the gut from carotenoids) , needs to be tested in human populations. If the DR adaptation proves reliable in human populations, only a single blood sample after five hours might be required, making the test much more feasible for field morbidity studies . The RDR approach to assessment of depletion requires additional validity tests under field conditions to verify that mild to moderate protein-energy malnutrition, infections, or other conditions that may lower absolute serum concentration do not impede this short-term response and hence limit the applicability of the test for reliably indicating depletion of body stores.
The histologically based conjunctival-impression cytology (CIC) test requires fewer logistical and technological resources than a direct biochemical determination of vitamin A. In theory, this increases its practicality for field studies in developing countries [18|. Validation against liver biopsies and the RDR test has been reported in a clinical setting among a very limited number of children with liver disease . These studies suggest that the absence of goblet cells in the impressions corresponded to a liver concentration of <20 microg per gram, or a positive RDR, or both. Additional quantitative validation studies are needed but, as with the RDR, can be accomplished only under special clinical conditions. In a larger field study conducted in Guatemala, the CIC evaluated against the RDR was reported to lack sensitivity although it was specific [20; 21]. This field validation trial needs to be repeated. The effect of potential confounders such as concurrent eye infections, e.g. conjunctivitis and trachoma, should also be studied. The report of a strong linear correlation between abnormal CIC and relative plasma concentrations is not consistently reported by others under clinical or field conditions and needs confirmation . Interpretation of the histological picture relative to degrees of vitamin-A depletion needs to be standardized.
Research needs in relation to vitamin-A assessment are as follows:
Indicators of nutritional status
The following issues related to indicators of nutritional status should be considered in designing studies of vitamin A and morbidity:
Indicators of morbidity
Measures of immunocompetence
The following measures of immunocompetence may be considered in the evaluation of vitamin-A-deficient individuals and populations before and after any proposed intervention. The rationale for choosing these four tests is based on the consistency of abnormalities observed in published studies and will be discussed further in the subcommittee's next report. The ultimate choice will be dictated by the nature of the study and practical considerations such as the availability of laboratory facilities and costs.
It would be important to role out the confounding effect of concurrent or recent infection on the basis of clinical findings and estimation of complement-reactive protein and endotoxins in the blood.
Strategies for research
The subcommittee recognizes that no single research design and set of procedures can or should be followed in a variety of settings in different countries with differing social contexts and resource limitations that affect feasibility. Nonetheless, the subcommittee wishes to present some guidelines that may prove useful in designing future studies.
Choice of population
It is desirable to test the major hypotheses in populations with various levels of vitamin-A nutriture.
Choice of deign
The subcommittee strongly recommends that double-blind randomized studies with placebo controls be used where feasible (as always, with due consideration for scientific and ethical issues). A controlled study is likely to yield information of greater scientific validity than a non-experimental study (e.g. a case-control study). Thus, experimental studies are preferable for unequivocally testing the hypotheses of interest.
In its previous report , the subcommittee reviewed theoretical and practical aspects of controlled trials. It noted, for example, that baseline comparability is ensured on average by random assignment of treatment and control interventions. The choice of the level of randomization - by individual, household, or community - is a matter of balancing considerations of study size with feasibility. If cluster sampling is used, appropriate statistical methods should be followed.
The likelihood of strict baseline comparability can be improved by randomization within strata on factors that relate to the outcome variables and that vary substantially among units to be randomized. Adequate stratification increases the precision of estimates, but must be weighed against the added field work that may result.
If strict comparability at baseline is not achieved in unstratified studies, adjustments are still possible during data analysis either by stratification or by regression analyses. This is possible, of course, only if the appropriate data have been collected at baseline.
The subcommittee recognizes the value of non-experimental studies and believes that they should be conducted where experiments are not feasible. Case-control studies, in particular, seem appropriate for studies of marginal vitamin-A status and risk of morbidity.
Case definition for such studies may present problems, however. If a case is defined as a child with diarrhoea at a given time and a control as a child without, the definition of a case will be weak as a result of the ephemeral nature of diarrhoea. Case definition could be improved by collecting histories of past infections; recall biases should be dealt with on the basis of the type of data. Another possible approach would be to select cases on the basis of marginal vitamin-A status, identify matched controls, and measure outcome through longitudinal follow-up. Such a design raises ethical issues related to the failure to treat those with identified clinical conditions. The subcommittee concurs with Feachem , who holds that such studies are unethical.
A major problem in case-control studies is the difficulty of satisfactorily matching for characteristics known to affect both the outcomes and vitamin-A status in the same direction. Case-control studies carried out to date have been equivocal because of poor matching. Variables to be matched should include age, sex, anthropometric and socio-economic status, nutritional status (anthropometric indicators of protein-energy malnutrition, indicators of iron and zinc deficiency, and other indicators where appropriate), environmental sanitation, and access to health care.
Content of the proposal
Below are listed issues which should be addressed in the design of proposals for future studies of vitamin-A supplementation and its relation to morbidity.
Specify the hypothesis
Research proposals should clearly state whether the hypothesis is to test the effect of marginal vitamin-A status on morbidity or to test the effect of vitamin-A supplementation on morbidity outcomes. If the hypothesis concerns testing the effect of marginal vitamin-A status, quantitative measures of depletion of vitamin A will be needed. The stated hypothesis should also indicate whether there is any intent to test the mechanisms by which vitamin-A deficiency or supplementation may lead to changes in morbidity - for example, by changes in immunocompetence. It should also state whether the intent is to study only morbidity per se or also severity of illness.
Specify sampling considerations and population description
Specify expected effects
Describe quality-control and data-management procedures
Specify the analysis plan
This should detail the specific methods to be used to test the study hypotheses and should, in addition, address the following issues:
This report is intended to provide guidance for developing programmes to assess the impact of vitamin A on child morbidity. It also gives an overview of the research priorities in this area, focusing on field studies of the impact of vitamin A on infectious diseases.
Subcommittee on Vitamin A Deficiency
Prevention and Control
Reynaldo Martorell (Chairman), Food Research Institute, Stanford University, Stanford, California
Abdelmonem A. Afifi, School of Public Health, University of California, Los Angeles, California
Guillermo Arroyave, School of Family and Consumer Sciences, San Diego State University, San Diego, California
Ranjit Kumar Chandra, Directory of Immunology, Memorial University of Newfoundland and Janeway Child Health Centre, St. John's, Newfoundland, Canada
Frank Chytil, Department of Biochemistry, School of Medicine, Vanderbilt University, Nashville, Tennessee
Samuel Preston, Population Studies Center, University of Pennsylvania, Philadelphia, Pennsylvania
Mervyn W. Susser, Columbia University, New York, New York
Frederick Trowbridge, Division of Nutrition, Center for Health Promotion and Education, Centers for Disease Control, Atlanta, Georgia
Barbara A. Underwood, National Eye Institute, National Institutes of Health, Bethesda, Maryland
Virginia H. Laukaran, Staff Officer, Food and Nutrition Board
Susan Berkow, Staff Officer, Food and Nutrition Board
Frances Peter, Deputy Director, Food and Nutrition Board
Jean Shirhall, Editor