To the best of our knowledge, this is the first study in Bangladesh that used the updated version of the CIAF (i.e., the CISAF) to estimate the prevalence of severe under-5 undernutrition and its associated socioeconomic factors. Though the three most conventionally used stunting, wasting, and underweight indicators represent different physiological manifestations of undernutrition, they all share similar causal factors . Individual assessments of stunting, wasting, and underweight underestimate the total burden of undernutrition. Since children may suffer from more than one type of undernutrition, only the data on prevalence may not provide a convincing estimate of the proportion of undernourished children in the population. Here the application of CIASF identifies all malnourished children and provides a single estimate of the burden of severe undernutrition among the children under age 5 .
In this study, we used the aggregated CISAF measurement to investigate severe under-5 child undernutrition in Bangladesh. More than one out of 10 children under-5 (11%) has been found to suffer from severe undernutrition. In a press release of UNICEF (2020), the prevalence of moderate to severe underweight and stunting was found to be about 23 and 28% in 2019, respectively . Similarly, other organizations in Bangladesh (such as icddr,b, and Save the Children) working on child undernutrition have recently reported a similar prevalence rate of either severe stunting or underweight. But none of them used such a composite method to measure severe underweight, i.e., CISAF. So, the actual composite scenario of severe undernutrition among the under-5 children in Bangladesh has been overlooked. Severe stunting, underweight or child undernutrition in Bangladesh was investigated in different studies. Our research results are in line with these previous studies that rural under-5 children experience severe undernutrition than the urban ones [25,26,27,28]. But the composite index for measuring severe undernutrition, that is CISAF was not applied in those studies.
On the contrary, according to a few studies, severe under-5 undernutrition was more prevalent in the urban areas of Bangladesh [29, 30]. It is noteworthy that these studies [29, 30] used the data of the last two decades. There has been a significant change in the urban areas of Bangladesh for the last few years, such as universal access to primary education, improving the household socioeconomic status, adequate maternal healthcare utilization, improvement of transportation facilities, and awareness about nutrition, which ultimately improved the overall nutritional status in the urban areas [27,28,29]. Compared to this, rural areas did not get the touch of such extensive development in Bangladesh. It might be one of the plausible explanations behind such a higher prevalence of severe under-5 undernutrition among the rural children of Bangladesh.
The prevalence of severe under-5 child undernutrition was more dominant among the children of uneducated parents. One in four children of the uneducated parents experienced severe undernutrition regardless of being born in rural or urban areas. Further, low birth weight children had a greater odd of being severely malnourished irrespective of the rural or urban context. For example, in our study, children with low birth weight were more likely to experience severe undernutrition (3.99 times the odds) living in the urban areas than rural children with low birth weight (2.84 times the odds). Also, one-fifth of the under-5 children born in rural and urban areas with a small birth weight experienced severe undernutrition. These findings were matched with the previous studies of Pakistan, Nepal, Malawi, Mexico, and Iran, which reported that children born with small birth weight were more likely to experience different types of undernutrition [19, 31,32,33,34]. Generally, children born with a low birth weight gain inadequate amounts of height and weight . Thus, they may remain shorter and lighter, and might suffer from severe undernutrition without adequate nutritional support. Additionally, children with small birth weights are often born to households with low socioeconomic status and mothers with poor health conditions . Due to the irregular distribution of food for children in poverty-stricken households, and the knowledge gap of parents/caregivers for adequate micronutrient supplementation, children can be deprived of nutritious food intake and eventually suffer from acute undernutrition [12, 14]. Again, parental illiteracy is frequently connected with low birth weight and other variables, such as inadequate maternal healthcare access and child caregiving. All these interlinked factors are substantially responsible for the poor nutritional outcomes of mothers and the low birth weight among the children [37, 38].
Poor household status has been found as one of the strongly associated factors for severe undernutrition among under-5 children in several studies [5, 39, 40], which is matched with our findings. Nearly one-fifth of the children who lived in the least wealthy households experienced severe under-5 undernutrition, regardless of the urban-rural context of this study. The odds of severe under-5 undernutrition were 2.44 times higher for children living in rural areas and 2.40 folds higher for urban children from the poorest wealth index. Poor parents often cannot afford a minimum diet and proper postnatal care for their children . Other plausible explanations, such as increased clustering of urban poor in slums with limited access to public health and nutrition services and amenities, high population density, poor quality drinking water, inadequate sanitation facilities, are also blameworthy for the higher prevalence of severeundernutrition among the under-5 children of the poor households .
In this study, uneducated parents were one of the strongly associated factors of severe child undernutrition, especially in rural areas. Educated parents were found to be significantly associated with better nutritional conditions during pregnancy and after birth and had been an indirect predictor of better child health throughout life [43, 44]. According to the studies from Bangladesh  and Pakistan , the likelihood of child undernutrition increased when their parents were uneducated, which is consistent with our study findings in rural areas. Again, maternal education could be an important determinant of child survival in rural areas since the child mortality and morbidity rates are usually high . Like our study finding, maternal lower educational status in rural areas increases the odds of severe child undernutrition. Hence, a greater focus on parental education facilities is required irrespective of geographical regions to accelerate the nationwide improvements in child nutrition status.
Children’s birth order ≥4 were 1.75 times more likely to experience severely malnourished living in urban areas. Studies from Bangladesh and India showed that children in higher birth order were more likely to be severely malnourished in Bangladesh’s urban areas  but in rural areas of India . Previous studies from Bangladesh , Congo , and Ethiopia  also reported that children with higher birth order were more likely to experience undernutrition regardless of urban-rural context. When the number of children increases, household members get competitive for foods, and the equality of providing necessary foods could not be maintained for all children . The risk of severe undernutrition is usually high in older children (i.e., age 4 to 5 years) in Nepal, Pakistan, Ethiopia, and Congo [17, 48, 50, 51].
In comparison, severe undernutrition is high in younger (age 1 to 2 years) children in India , indicating this problem’s complex nature. We found that toddlers (age 2 to 3 years) living in rural areas had higher odds of severe under-5 undernutrition than toddlers living in urban areas. A similar level of provision of health and nutritional care available for urban children might be the reason for the insignificant association between children’s age and severe undernutrition. At 6–36 months, inappropriate feeding behaviors and other factors (e.g., infection and food shortage) may be responsible for one-third of undernutrition cases, depending on population, place, time, and season . In addition, lack of attention in rural areas (urban-rural disparities) in receiving complementary feeding, access to health services, preventive and curative interventions influence nutrition outcomes .
Severeundernutrition is a multifaceted, complex phenomenon, involving many immediate causes (e.g., poor diet habits, diarrhea, ages of breastfeeding children) and underlying causes (e.g., income inequality, food insecurity, access to safe water, environmental hygiene) [55, 56]. The risk difference between most affluent and poorest was higher among children in rural areas than in urban areas, indicating a greater rich-poor gap. Such socioeconomic inequality can be reduced by increasing income-generating activities driven by public and private entities and introducing different microfinance programs. These endeavors need to be aimed at deprived and vulnerable individuals and ensure their participation with a standard wage structure under the national nutritional security system. In Ethiopia, improved per capita household income increased available funds for food expenditure and basic health care needs and developed children’s nutritional status . Moreover, universal education and standard health care should be available and accessible to all women, especially in rural and remote areas. Improving access to community-based education and standard health care to mother include conferring many benefits from improved caregiver practices; enhancing health and environmental knowledge; increasing educated and skilled workforce; living in better neighborhoods, reducing gender-based violence, child marriage, and early childbearing; and reducing maternal death rates in terms of improved maternal and child nutritional status .
The BDHS 2017–18 data used for this study was one of Bangladesh’s largest nationally representative samples. The stability of the data set allows changes over time to be monitored with some confidence. However, there were some limitations too for this study. First, we could not use the infant and young children feeding data to the models because the data available in the BDHS were only for the children aged 6 to 23 months. But our study considered children under age 5. So, there would be too many missing variables, and this study would be limited only to children under age 2. Further, data on potential confounders like diet, food insecurity, and parents smoking behavior were unavailable. Secondly, the cross-sectional data was insufficient to establish a causal relationship, consequently limiting the findings’ applicability. Thirdly, the BDHS data were collected retrospectively and self-reported, thus subject to underreporting, information bias, and recall bias. However, data were collected using validated tools and standard procedures. Using seven nutritional status measurements, CISAF provided a credible estimate of the overall proportion of severe under-5 child undernutrition and the complex interplay between individual, community, public policy, and environment level associated factors.