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3 International Institute of Tropical Agriculture, PMB 5320, Ibadan, Oyo State, Nigeria; 4 University of Ibadan, College of Medicine, Department of Human Nutrition, Ibadan, Oyo State, Nigeria; 5 University of Agriculture-Abeokuta, Department of Nutrition, PMB 2240, Abeoukuta, Ogun State, Nigeria; 6 University of Development Studies, Navrongo, Ghana; and 7 Beltsville Human Nutrition Research Center, U.S. Department of Agriculture, Beltsville, MD
* To whom correspondence should be addressed. Email:-bdixon{at}cgiar.org.
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ABSTRACT |
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 TOP ABSTRACT IntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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Introduction |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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20–24% of child mortality from measles, diarrhea, and malaria and 3% of mortality associated with other infectious causes can be attributed to VAD (7). Improving the vitamin A status of young children in developing countries can reduce child death rates by 20–50% (8), suggesting that a substantial portion of their mortality is attributable to VAD. In women, in addition to ocular lesions, VAD may be an important factor contributing to maternal mortality and poor pregnancy and lactation outcomes. In a double-blind, cluster randomized trial the effects of of low-dose supplementation with vitamin A or ß-carotene on mortality related to pregnancy in Nepal, there was a reduction of 40 and 49% among those who received vitamin A and ß-carotene, respectively (9). VAD is also likely to increase vulnerability to other disorders, such as iron-deficiency anemia, for both women and children, and to worsen growth deficits in children (10). Limited information is available about the current vitamin A status of children <5 y old in Nigeria. Studies conducted have been regional and not nationally representative (11–14). The number of studies conducted is limited as is the number of subjects evaluated (11,15). Further, the studies dealt with specific aspects of VAD, which focused on a particular population and in different geographical locations in Nigeria (16). Other studies related to clinical signs of VAD (17,18). Complications of conditions such as malnutrition, measles, and diarrhea, which are commonly associated with VAD, rank among the leading causes of morbidity in Nigerian children (19).
Although some progress has been made in combating VAD, urgent action is needed to accelerate the efforts to bring it under control. It is therefore critical to undertake a holistic national survey to generate current data on VAD and health practices and care, for both planning and programming in Nigeria. The information should also be useful to all agencies with primary and secondary mandates with a focus on nutrition. The objective of the survey was to assess the vitamin A status of children <5 y old in Nigeria.
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Subjects and Methods |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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The sampling strategy was designed to obtain a nationally representative sample of the population. After examination of the obvious and documented relations among 1) the agroecology and type of farming systems; 2) crops grown and food consumed; and 3) type of food consumed (intake) and micronutrient deficiencies, the country was initially stratified according to major agroecological zones and predominant food crops within the zones. Due to lack of information on the principal variables of interest in the study, we postulated that a large sample size, spread across agroecological zones and food type, would be fairly representative of the whole country. The 3 major agroecological zones in Nigeria are the dry savanna, moist savanna, and humid forest. The agroecologies are characterized by the length of the growing period and annual rainfall. The humid forest has a growing period of >270 d, whereas the moist and dry savannas have growing periods of 181–270 and 90–180 d, respectively (Table 1). We used a stratified multistage procedure, with stratification at 2 levels [agroecological zone and predominant food crops]. The predominant food crops range from mainly cereal food crops to root crops and combinations of the two. The predominant food crops and their areas included maize (corn), sorghum, millet, rice, groundnuts (peanuts), and cowpeas in the dry savanna zone; maize, sorghum, millet, rice, groundnuts, cowpeas, cassava, yams, and sweet potatoes in the moist savanna zone; and cassava, yams, sweet potatoes, and oil palm in the humid forest zone.
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Further stratified sampling was adopted with probabilities proportional to the number of LGAs in each category, subject to the additional conditions that a minimum of 1 LGA from each group within the state and a maximum of 18 for all of the states would make up a particular geopolitical zone. This was achieved on a practical level by the random selection of 1 state from each of the 36 categories (12 states x 3 LGA groups), and the allocation of the remaining 36 samples proportionately to the total population of LGA groups. A total of 72 LGAs were selected.
Enumeration areas (EA). To facilitate the actual location of therural/urban LGA from selected LGAs, the EAs that fell entirely within a selected LGA were further sampled; 3 areas were randomly selected from the list of EAs within a selected LGA. From these selected EA, a further breakdown of EA was made for the selection of households. A comprehensive list of all of the EAs in selected LGAs was obtained from the National Population Commission. An EA is the smallest geographical cluster of households as delineated by the National Population Commission for the purpose of enumeration. The population of each EA was obtained from the LGA headquarters. Finally, 216 EAs were selected.
Selection of households. From each LGA/EA, 30 households were randomly selected according to size, i.e., the number of households in the selected LGA/EA from the target population of households with children <5 y old. A total of 6480 households were selected.
Subjects. For the selection of subjects for biochemical and health measurements, a pair comprised of a mother and a child <5 y old was selected from the identified households. However, if there was more than 1 child <5 y old, the youngest child was selected. In addition, when there was more than one wife in the household, the youngest wife was selected with her youngest child. If there was no suitable mother-child pair in the selected household, another household was selected. This also applied if the mother refused to participate in the survey. An ethical clearance for the conduct of the survey, including the collection of biological samples, was obtained after due process from the Federal Ministry of Health through the Nutrition Division of the Ministry. Data collection took place from August to October 2001.
The research group in each state included a state supervisor, three 2-person interview teams, and a team of laboratory technologists, who were responsible for biological sample collection and processing.
Collection of blood samples and laboratory analysis
Blood samples were drawn from the children by venipuncture at a health facility close to the survey site. The blood samples were allowed to clot, centrifuged (10 x g; 1000 min) to obtain serum, labeled legibly, and stored at –20°C. If there was no health facility, arrangements were made with the village head for the use of his compound. After processing, the samples were placed in a cold box. The temperature in the cold box was maintained at –4°C with frozen ice packs. At the end of each day, the serum samples collected were transferred to the Primary Health Care clinic, which served as a temporary collection/storage center using the cold box. The process of sample collection was conducted with consideration for the health and safety of both collector and subject. The Federal Ministry of Health through the National Primary Health Care Development Agency obtained permission from the Primary Health Care centers in the state and LGAs to use their cold chain facilities for the temporary storage of samples. The samples were stored at the designated centers for a maximum of 14 d after which they were transported frozen to the International Institute of Tropical Agriculture, Ibadan, Nigeria where they were kept at –20°C for a maximum of 30 d. The samples were air-freighted in dry ice to South Africa for analysis.
Serum retinol was measured using a slightly modified version of the reversed-phase HPLC method described by Catignani and Bieri (22). All samples were analyzed at the Medical Research Council, Tygerberg, Capetown, South Africa.
A prevalence of serum retinol <0.70 µmol/L (<20 µg/dL) >20% was suggested as defining public health problems related to VAD (23). The following recommended cut-off values for determining vitamin A status were used in the present study: serum retinol 
0.70 µmol/Lwas considered adequate; serum retinol <0.70 µmol/L was indicative of low or marginal VAD; serum retinol <0.35 µmol/L was indicative of severe VAD.
In addition to assessing the vitamin A status of children <5 y old in Nigeria, their iron status was assessed using serum ferritin concentration kits [Ferritin Mab Solid Phase Component system, Cal 06B–160850, ICN Pharmaceuticals. For control, the Chiron Diagnostic Ligant 123 Cal 97600 was used (24)]. All samples were analyzed at the Medical Research Council laboratory, Tygerberg, Cape Town, South Africa. The iron status results will be presented elsewhere.
We observed that 20% of the samples had serum ferritin concentration >100 µg/L. This concentration in children <5 y old may indicate the presence of inflammation or infection (24–26). Therefore, we used elevated serum ferritin as an indicator of potential infection. Serum C-reactive protein (CRP) is present only during episodes of acute inflammation (27). Analysis of CRP, which indicates the presence of infection and/or acute inflammation, was carried out on a subsample (n = 100) of subjects with serum ferritin levels >100 µg/L [Technicon RA and OpeRa Systems product no. TO1-1940-01 and no. T23-3073-01 and T23-3074-01 Bayer Testpoint Protein Controls, Technicon Instruments] (28). All samples were analyzed at the Medical Research Council laboratory, Tygerberg, Cape Town, South Africa. Sixty percent of the children sampled had elevated levels of CRP, indicating the presence of inflammation and potential infection. It was show that infection can induce VAD and that serum retinol may be depressed after infection (29).
Statistical analysis
All of the data sets were analyzed centrally at the International Institute of Tropical Agriculture. All statistical analyses were done using the Statistical Software, SAS version 8 (30). Data input was accomplished using MS-Access and MS-Excel. Descriptive statistics were generated. The significance of differences in proportions was tested by a 
2 test. Duncan's multiple range test was used to test significant differences among mean serum retinol concentrations. The significance of differences between the percentage frequencies was assessed using the 2-tailed Student's t test; differences with P < 0.05 were considered significant. Data are presented as the median and mean for serum retinol concentration and the percentage frequencies for VAD.
During analysis, the data set was stratified as follows: all children <5 y old (n = 3099); children <5 y old after the removal of those with potential infection (n = 2404); and those with potential infection (n = 695). Consequently, the results are presented in a stratified manner, grouped as described above. It should be noted that the number of blood samples analyzed from children does not equal the number of households surveyed; this was due mainly to poor response rate and unclear sample labeling during collection.
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Results |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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The mean retinol concentrations for children living in the rural and urban sectors were similar and higher (P < 0.05) than those of children living in the medium sector for all children <5 y old surveyed (Table 3). After removal of samples with potential infection, the results did not differ. In children with potential infections, the mean serum retinol concentration was higher in those children living in urban areas (P < 0.01) than in those living in rural and medium areas (Table 3).
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When the data were disaggregated by agroecological zone, marginal VAD was higher (P < 0.05) in children living in the dry savanna than in those living in the moist savanna or humid forest zones for all children surveyed. The moist savanna zone did not differ from the humid forest zone (P < 0.05). The percentage frequency of severely deficient children was greater (P < 0.05) in those living in the humid forest than in those living in the dry or moist savanna (Table 4).
When we removed samples from children with potential infection, a greater proportion of children (P < 0.05) living in the dry savanna suffered from marginal VAD than children living in the moist savanna or humid forest. The incidence of severe deficiencies was intermediate in the dry savanna, tended to be lower in the moist savanna (P < 0.06), and was greater (P < 0.05) in the humid forest (Table 4).
In children with potential infections, the proportion of marginal VAD was higher in children living in the dry savanna (P < 0.009) than in those living in the moist savanna or humid forest. The incidence of severe deficiencies was higher in children living in the dry savanna, intermediate in those living in the humid forest (P < 0.05), and lower in those living in the moist savanna (P < 0.05). When we combined those who were marginally deficient with those who were severely deficient, 1 of 2 children living in the dry savanna suffered from VAD (Table 4).
When we disaggregated the data by urbanization (sector), the proportion of marginally deficient children <5 y old was different (P < 0.05) for all children surveyed. Marginal VAD was higher in children living in the medium and rural sectors, and lower in those living in the urban sector (P < 0.05). The incidence of severe deficiencies was higher in children living in the medium sector, intermediate in the rural sector, and lower in the urban sector (P < 0.05). When we combined those who were marginally deficient with those who were severely deficient, 1 of 3 children <5 y old living in the medium sector suffered from VAD (Table 5).
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We disaggregated the data for children with potential infection by sector; few children living in the urban sector suffered marginal VAD (P < 0.0076). Severe VAD was higher in children living in the medium sector, intermediate in those living in the rural sector, and lower in those living in the urban sector (P < 0.05). When we combined those who were marginally deficient with those who were severely deficient, 2 of 5 children <5 y old living in the medium sector suffered from VAD and 1 of 3 children <5 y old living in the rural sector suffered from VAD (Table 5).
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Discussion |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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However, compared with recent (1997–2000) data from some other sub-Saharan African countries with similar mortality rates in children <5 y old, the Nigerian prevalence rate appears similar to that of Uganda (27.9%) and higher than that of Tanzania (24%); however, in terms of number of children affected, Nigeria has the greatest number at 5,520,800 followed by Uganda at 1,227,600 and Tanzania at 1,272,000 (31).
Information on the location of children <5 y old in Nigeria suffering from VAD was also provided in the present survey. Among the 3 agroecological zones, the dry savanna zone had the highest VAD prevalence (>30%), which is also higher than the national average. This zone consists of states in the far northern, drier part of the country. It is characterized by a crop-growing period that could be as short as 30 d/y. The principal food crops in the zone are cereals and legumes, and a low intake of vitamin A (and their precursors)-rich foods might be one reason for such a high prevalence of VAD. It is significant to point out that oil palm is not cultivated or consumed regularly in this zone. It is important to note that even after the removal of children with potential infection from the data set, VAD prevalence in the dry savanna zone was still the highest among the zones, and 5% above the national average. This result is in agreement with that of Ajaiyeoba who reported a high prevalence of VAD in the northeast and northwest geographical regions (16). The dry savanna agroecological zone is the major agroecological zone in these geographical regions. The PIC survey data (11) indicated that VAD [serum retinol concentration <0.70 µmol/L or (<20 µg/dL)] was heavily concentrated in the northern part of the country, with rates of 17% in the northwest and 12% in the northeast in children <6 y old. Also relevant are the percentage frequencies of the different types of illness recorded during the survey, which indicated that the dry savanna zone had the fewest children who never had measles, diarrhea, and whooping cough (data not presented). This suggests a higher illness rate for children in this zone.
The proportion of children affected by VAD differed significantly among the agroecological zones, with 23.9% in the moist savanna zone and 29.9% in the humid forest zone. The moist savanna is characterized as the food basket of Nigeria, whereas the humid forest zone is characterized by the high production and regular consumption of oil palm among other food crops. In the present study, it was observed that most of the sauces, potages, and soups consumed were prepared using palm oil, a practice more common in the humid forest than in the other zones; nevertheless, the prevalence of VAD was still high and a public health problem. A possible explanation may be that the ß-carotene in palm oil is destroyed during processing, rendering it less bioavailable (32,33), or it is not consumed in suficient quantities to have an effect on the vitamin A status of children <5 y old (34). Previously, it was assumed that 6 µg of ß-carotene in all plant foods provided the same vitamin A activity as 1 µg of preformed retinol. However, recent data reveal that the amount of ß-carotene from plant foods, which provides the vitamin A activity equivalent to that of 1 µg of preformed retinol, is much greater than 6 µg and is also highly variable among foods, ranging from 8 µg to as much as 40–45 µg (35,36). This may further explain why VAD is common among children who consume diets previously thought to meet or nearly meet their vitamin A requirements (37).
Apart from the zones, VAD prevalence was found to be consistently higher in the rural sector than in the urban sector. Higher urban incomes and access to better social infrastructure could have contributed to this difference. The medium sector on the other hand had consistently higher VAD prevalence for children <5 y old. For each of 3 categories of children <5 y old surveyed (infection status), the prevalence of severe VAD in the medium sector was at least double that in either the urban or the rural sector. The reason for this is not clear.
Nationwide vitamin A supplementation has been promoted for a long time in Nigeria through primary health care and also the National Immunization Days, which target the population of children <5 y old and offer an optimal structure for delivering vitamin A supplements (38). Since 2000, supplementation has been linked directly with the national campaign against polio. The objective was to reach 39 million children aged 6–60 mo with high-potency doses of vitamin A along with polio vaccination. It was hoped that by the end of the year 2000, coverage of vitamin A would be close to 90% (39). Data from the Multiple Cluster Indicators Survey (40), however, showed that <25% of children between the ages of 06 and 59 mo had received vitamin A supplement in the preceding 24 mo. Furthermore, the data indicate that the regions with the most serious prevalence of VAD had received the least supplementation; although the proportion of children receiving a vitamin A supplement was as high as 47% in the southeast, it was only 16% in the northeast (dry savanna zone) and 10% in the northwest (dry savanna zone). Vitamin A supplementation is much more available in urban areas than in rural areas (40). Recent coverage data suggest that this objective was not achieved because coverage was estimated at 70% in 2004 (41).
At the time of the survey, 29.5% of the children <5 y old nationwide were vitamin A deficient. Among the agroecological zones and sectors, VAD was highest in the dry savanna at 31.3% with 32.6% in the medium sector. These levels are above the WHO biochemical cut-off value of 0.70 µmol/L in >20% of the population. Therefore, it can be concluded that VAD in children <5 y old is a severe public health problem in Nigeria and that although the proportion of children with low serum vitamin A levels varies agroecologically and across sectors, it is an important public health problem in all zones and sectors.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Supported by the U.S. Agency for International Development under terms of award no. LGA-G-00-93-00042, the United Nations Children's Fund, and Helen Keller International. The opinions expressed herein are those of the author(s) and do not necessarily reflect views of the USAID.
8 Abbreviations used: CRP, C-reactive protein; EA, enumerated areas; LGA, local government areas; PIC, participatory information collection; VAD, vitamin A deficiency.
Manuscript received 7 March 2006. Initial review completed 1 April 2006. Revision accepted 18 May 2006.
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LITERATURE CITED |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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