Investigating and Quantifying Food Insecurity in Nigeria: A Systematic Review

: Given the recent increase in the number of Nigerians estimated to be at risk of food insecurity, it is crucial to explore the array of tools used to quantify food insecurity (FI). This exploration will help determine the prevalence and severity of FI in Nigeria. This review explored the scope of FI research carried out in Nigeria to examine how the design was quantiﬁed. A systematic review was performed to compile the accessible Nigerian studies. Seventy-nine studies were reviewed. Eighteen used the Household Food Insecurity Access Scale module (HFIAS) to investigate FI status; thirteen used the recommended daily calorie requirement approach; twelve employed the Household Food Security Survey Module (HFSSM); ten used the food insecurity index (through household per capita food expenditure); seven used the Food Insecurity Experienced Scale (FIES); two used the Food Consumption Score (FCS); and the others employed less standardized or thorough approaches. Different prevalence levels and gravities of FI in the Nigerian populations were documented. The prevalence of FI varied from 12% to 100%, based on the instrument and demography being studied. In accordance with the ﬁndings of this review, the authors propose standardization of the FI instrument and highlight the need for a measurement tool that would be appropriate for the Nigerian setting. This will enable researchers to attain a comprehensive knowledge of the occurrence rate of FI in Nigeria, leading to improved food-and nutrition-sensitive policy development.


Introduction
The global community is faced with one of the most challenging issues in recent times-food insecurity.Food insecurity (FI) is experienced in almost all regions of the world, especially in Asia and Africa, as more than 50% (425 million) of the people in the world affected by hunger in 2021 were in Asia and more than one third (278 million) lived in Africa [1].About 12% of the world's population was affected by severe FI, while more than 2 billion people were either moderately or severely food insecure in 2021 [1].
According to the 2023 Global Report on Food Crises, more than one quarter of a billion people are experiencing acute levels of hunger, while some are on the verge of extreme deprivation of food [2].Increases in conflict events, rising poverty, escalating inequalities, widespread underdevelopment, climate crises, and the COVID-19 pandemic have also contributed to the worsening of global food insecurity [2].It is worth noting that Africa (especially sub-Saharan Africa) is experiencing worsening FI, which is reflected in the inclusion of six African countries (Sierra Leone 40.5/100,Madagascar 40.6/100,Burundi 40.6/100,Nigeria 42/100, Sudan 42.8/100, and the Congo Dem.Rep. 43/100) among the top 10 bottom-ranking countries with the lowest Global Food Security Index (GFSI) score in the 2022 overall food security environment.No African countries were among the top 10 high-ranking countries [3].According to the Food and Agriculture Organization (FAO) and its partners' projections, the impact of the Russia-Ukraine war threatens food security globally.The ripple effect of the war will have numerous implications for global agricultural markets, and this can potentially worsen the state of food and nutrition security for many nations in the coming years [1].
Food insecurity and hunger are on the rise in Nigeria.In January 2023, the United Children Fund (UNICEF) projected that 25 million Nigerians are at risk of experiencing hunger by the third quarter of 2023.This revealed an increase of about 8 million in hunger from the 2022 estimates [4].Nigeria ranked 107th out of 113 countries and 25th out of 28 sub-Saharan Africa (SSA) countries with a GFSI score of 42/100 in the GFSI 2022 [3,5].Also, in the 2022 Global Hunger index (GHI), Nigeria ranked 103rd out of 121 countries, scoring 27.3/100.This indicates a serious level of hunger and suggests that Nigeria is not on track to achieve the Sustainable Development Goal 2 (SDG2) target by 2030 [5,6].
X-raying the food security environment in Nigeria, as reported in the GFSI 2022 under the four pillars for food security (Figure 1), indicated that Nigeria had the lowest score (25/100) globally on the affordability category.In the availability category, Nigeria ranked 108th globally and 26th in the region.The report showed that the very weak performance score (39.5) was a result of the "very weak" (0-39.9)scores obtained in certain indicators, such as agricultural research and development (30.7/100), supply-chain infrastructure (23.9/100), sufficiency of supply (25.5/100), political and social barriers to access (31.6/100), and food security and access policy commitments (0/100) [3].
Agriculture 2023, 13, x FOR PEER REVIEW2 of 33 high-ranking countries [3].According to the Food and Agriculture Organization (FAO) and its partners' projections, the impact of the Russia-Ukraine war threatens food security globally.The ripple effect of the war will have numerous implications for global agricultural markets, and this can potentially worsen the state of food and nutrition security for many nations in the coming years [1].
Food insecurity and hunger are on the rise in Nigeria.In January 2023, the United Children Fund (UNICEF) projected that 25 million Nigerians are at risk of experiencing hunger by the third quarter of 2023.This revealed an increase of about 8 million in hunger from the 2022 estimates [4].Nigeria ranked 107th out of 113 countries and 25th out of 28 sub-Saharan Africa (SSA) countries with a GFSI score of 42/100 in the GFSI 2022 [3,5].Also, in the 2022 Global Hunger index (GHI), Nigeria ranked 103rd out of 121 countries, scoring 27.3/100.This indicates a serious level of hunger and suggests that Nigeria is not on track to achieve the Sustainable Development Goal 2 (SDG2) target by 2030 [5,6].
X-raying the food security environment in Nigeria, as reported in the GFSI 2022 under the four pillars for food security (Figure 1), indicated that Nigeria had the lowest score (25/100) globally on the affordability category.In the availability category, Nigeria ranked 108th globally and 26th in the region.The report showed that the very weak performance score (39.5) was a result of the "very weak" (0-39.9)scores obtained in certain indicators, such as agricultural research and development (30.7/100), supply-chain infrastructure (23.9/100), sufficiency of supply (25.5/100), political and social barriers to access (31.6/100), and food security and access policy commitments (0/100) [3].Food security (according to the United Nations definition) is referred to as "People having at all times, physical, social and economic access to sufficient, safe and nutritious food which meets their dietary needs and food preferences for an active and healthy life" [7].The term "food insecurity" simply refers to the opposite of food security, where conditions of food security are not met, and it can be broadly conceptualized along a range from mild to severe [8].The definition contains four widely established pillars of food security, namely: availability, accessibility, utilization, and stability.The four pillars are important in understanding food security at any level, such as regional, household, and individual levels [9][10][11][12][13][14][15].
Generally, conditions that expose people to FI and hunger include extreme poverty, unemployment, corruption, unstable food access, ill health, non-existent social protection programmes, and terrorism [8,[14][15][16][17][18][19].FI is not only experienced in developing nations, as developed nations are also affected.In Australia, for instance, over 2 million households were affected by severe FI in 2022 [20], while in the United States of America (USA), an estimated 14.8% of children resided in food-insecure households [8,21].It is generally believed that FI risk is unevenly distributed.The most vulnerable groups, such as children, women, the elderly, and internally displaced people (IDPs), usually suffer higher levels of Food security (according to the United Nations definition) is referred to as "People having at all times, physical, social and economic access to sufficient, safe and nutritious food which meets their dietary needs and food preferences for an active and healthy life" [7].The term "food insecurity" simply refers to the opposite of food security, where conditions of food security are not met, and it can be broadly conceptualized along a range from mild to severe [8].The definition contains four widely established pillars of food security, namely: availability, accessibility, utilization, and stability.The four pillars are important in understanding food security at any level, such as regional, household, and individual levels [9][10][11][12][13][14][15].
Generally, conditions that expose people to FI and hunger include extreme poverty, unemployment, corruption, unstable food access, ill health, non-existent social protection programmes, and terrorism [8,[14][15][16][17][18][19].FI is not only experienced in developing nations, as developed nations are also affected.In Australia, for instance, over 2 million households were affected by severe FI in 2022 [20], while in the United States of America (USA), an estimated 14.8% of children resided in food-insecure households [8,21].It is generally believed that FI risk is unevenly distributed.The most vulnerable groups, such as children, women, the elderly, and internally displaced people (IDPs), usually suffer higher levels of FI [8,19,[22][23][24][25].FI may be momentary in nature, given the possibility of households slipping into and coming out of FI in response to evolving situations, supporting the idea that FI is best conceptualized as a dynamic phenomenon rather than a fixed construct [15,[26][27][28][29].In consideration of the weighty possible implications of FI, comprehending the methods employed by researchers to study FI, and the results obtained, are instrumental in designing food-and nutrition-sensitive policies and their implementation.About two and a half decades ago, many researchers carried out FI research in Nigeria, utilizing various measuring tools like their counterparts in other parts of the world.Several approaches have been established to study FI across global, national, and household/individual domains [11].About three decades ago, FI measurement tools were used to assess economic access to food, using, for instance, household income or the Household Consumption and Expenditure Survey (HCES) as proxies for food security, although the measures did not capture the experiences of households accessing food or the strategies employed in coping with insufficient food access.To overcome this limitation, the Radimer/Cornell scale (experience-based) was created [30].In most cases, other experience-based approaches that were developed from [30] hinged on three central themes: "anxiety/uncertainty over running out of food, concerns over and adjustments to the quality, and quantity of food in the diet" [8,30].The tools used different recall periods (30-day, 12 months) where the gravity of FI is in relation to the quantity of agreeable answers, with a higher number revealing extreme conditions [8,30].The United States Department of Agriculture (USDA) Household Food Security Survey Module (HFSSM) is another approach commonly used in US-based studies for measuring FI, and it has been extensively adopted in many other countries.The module contains a lineup of questions (18 items) developed from the collective insights of FI at the household level, and it is executed through a survey-based measurement, usually within the timeframe of the prior 12 months.The findings may be presented as an unbroken continuum of severity, or by applying cutoff levels to assign households to four specific categories.In the absence of children, the module utilizes 10-item questions.It is crucial to highlight that HFFSSM adopts a flexible scoring system instead of a strict numerical threshold for classifying households [8,15,31,32].
The Household Food Insecurity Access Scale (HFIAS), created by the Food and Nutrition Technical Assistance Project (FANTA), is also another FI measuring tool that has been tested and approved in various countries and widely used across the globe [33].The module utilizes nine-item questions, which centre on the household food access experience within a 30-day recall.The households are categorized into four categories depending on the severity experienced, from food secure to varying levels of FI [33].The Food Insecurity Experience Scale (FIES), developed by the FAO, is the most recent tool used in measuring FI.The FIES utilizes eight-item questions, which relate to occurrences during the preceding 12 months.This approach prioritizes the subjective experiences shared by the interviewed individuals and downplays the role of money in food access [8,34,35].In contrast to the HFIAS and HFSSM, the FIES measures individual FI and has been extensively utilized in the assessment of FI [8,15,[34][35][36][37][38][39][40][41].The food insecurity index is another construct for estimating household FI through the mean of per capita household food expenditure (MPCHFE).In this method, the line of household food security is established at two thirds of the MPCHFE; while households are food secure if their MPCHFE surpasses the food security line, they are classified as food insecure if they fall below it.The approach is commonly utilized in African studies; for instance, in Nigeria and Ethiopia [42][43][44][45][46]. Another approach to estimating household FI involves investigating the food security status of each household by comparing their food consumption against the established food security threshold using the recommended daily calorie requirement.The households satisfying or surpassing the recommended daily calorie intake (1800-2710 Kcal in most studies) are categorized as food-secure households while those households that fall below the threshold are grouped as food insecure [47][48][49][50].The use of the Food Consumption Score (FCS) is another key performance measure created by the World Food Programme (WFP) for assessing the occurrence of FI in a country or region (WFP 2007;Jones et al. 2013).
It is significantly guided by the knowledge of how dietary diversity and household food access are interconnected.This approach uses 7-day recall, while the study participants report on the consistency of the household consumption of eight food groups.The food consumption recurrence of each food group is then multiplied by an assigned weight for each group, while the values derived are added together to obtain the FCS.This score is then transcribed to a discrete variable using standard cutoff values [11,51].This approach is not widely utilized by researchers in various countries or regions, unlike HFFIAS, HFSSM, and FIES.Taking into account the extensive body of academic literature and research that has explored FI over some decades, it is noteworthy that there are systematic reviews (SRs) on food (in)security in different countries and regions, but those that focus on assessing and quantifying FI in Nigeria are quite scarce.For instance, there exist SRs of the literature on the understanding and measurement of FI in Australia [15], the associated factors of FI in older adults in the USA [52], the FI studies in South Africa, which point out opportunities for enriching policy insights [53], and the effects of agricultural interventions on food security in northern Ghana [54].At the regional level, [55] explored a descriptive SR on FI and health outcomes in Southern Africa, while [56] reviewed the interconnectedness of household FI, dietary diversity, and stunting in SSA.This study is the first SR that is devoted to investigating food security research in Nigeria.The approaches utilized by researchers are similar to those in other parts of the world, employing various methods, such as HFIAS, HFSSM, and FIES (the most recent), to estimate FI [19,41,57].While some studies utilized researchers' constructs, others modified or adapted approaches from standardized measures, especially the HFSSM, leading to one-item and three-item questions [58][59][60][61].However, inconsistencies in measuring FI are likely to impact the reported occurrence, which affects food-and nutrition-sensitive policy development.The purpose of this review is to systematically assess the scientific research papers that claim to examine FI in Nigeria; explore the diversity of scholarly investigations, with their measurement tools in Nigeria; and present a broad picture of the gravity of FI in Nigeria as highlighted in these scholarly articles.

Methods
A systematic exploration was embarked upon to find all of the research works carried out in Nigeria on food (in)security.The key search terms were "food insecurity" OR "food security" OR "food availability" OR "food utilisation" OR "food access" AND "Nigeria."The searched databases included PubMed, the Lens database (https://www.lens.org/(accessed on 25 July 2023)), and SCOPUS.In order to gain a full collection of scholarly papers that reported on FI in Nigeria, no restrictions were placed on the date of publication.However, only scholarly papers published in English were considered, while research documents, such as unpublished articles, books, book chapters, conference abstracts, encyclopedias, theses, dissertations, editorials, and reviews, were debarred.All identified scholarly papers were downloaded into EndNote X7.
The literature search identified 1783 studies, out of which 853 were duplicates.The titles and abstracts of the remaining 930 studies (original research papers) were read consciously, while 708 studies were found not referring directly or indirectly to FI research in Nigeria, resulting in 222 studies left for another stage of assessment.However, only studies with full-text access were reviewed.Publications that failed to meet the inclusion criteria were removed at this stage, while others that may meet the criteria for another level of investigation were kept for further examination.The full texts of 222 studies were obtained for another level of assessment.The authors individually read all of the articles at this stage and determined if each publication would be retained based on the inclusion criteria.Out of the 222 studies, 124 were removed for not meeting the inclusion criteria upon closer assessment.Meanwhile, 19 additional articles (to give a total of 143 studies) were removed as a result of no identifiable method of measuring FI and no record of FI prevalence rate in the studies (for instance, [62][63][64][65]).Furthermore, the reference lists of articles were equally checked in case there might be studies that met the inclusion criteria.However, this search did not yield any additional articles.Only 79 studies have been included in this review, as presented in Figure 2. In this stage, information drawn from these studies includes the location of the study; the population group; the study focus; the findings; the primary method; the measured FI; the method for assessing FI; FI prevalence; and the number of respondents.
Agriculture 2023, 13, x FOR PEER REVIEW5 of 33 group; the study focus; the findings; the primary method; the measured FI; the method for assessing FI; FI prevalence; and the number of respondents.

Results
The brief snapshot of the key insights obtained from the 79 included studies is presented in Table 1 below.

Results
The brief snapshot of the key insights obtained from the 79 included studies is presented in Table 1 below.

Common Features of the Reviewed Studies
The first two included scholarly papers in this study were conducted in 2006 [57,62].Most of the studies in subsequent years (2007,(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019) had research articles ranging from 1-5.According to this review, the trend changed in 2020; this was occasioned by the emergence of the COVID-19 pandemic, which resulted in an increase in the number of scholarly papers published in subsequent years.Many of these papers include review articles, editorials, conference proceedings, books, and chapters in books.In 2020-2022, 11, 13, and 15 articles were included in this review, respectively.The number of respondents ranged in size from the smallest study (with 72 respondents) [101] to the highest-population study (using secondary data) of 5198 respondents [98].
About 17% (13) of the studies utilized secondary data sources [29,40,60,61,85,98,104,105,110,113,117,120,128] within five years (2018,(2020)(2021)(2022)(2023).The first research article that employed a secondary data source was published in 2018 [85].About 84% (66) of the studies used primary data in investigating and measuring FI in Nigeria.Three notable states in Nigeria with the highest number of FI studies (using primary data) were conducted in Oyo (15), Ogun (10), and Enugu (9), respectively (see Figure 3).According to this review, it is worth noting that no research works (employing primary data sources) were conducted in three states in northwest Nigeria; namely, Jigawa, Katsina, and Zamfara states.According to recent Cadre Harmonise data surveys (2018/2019 household survey), the northwest and the northeast were the two regions having the most acute levels of FI in Nigeria [130].The southwest had the highest number of FI research studies (45) conducted in the region, while the south-south had the least (9) FI research studies conducted in the region, and only three were reported in Federal Capital Territory (FCT), Abuja (see Figure 3).Furthermore, about 9% (7 articles) of studies in this review employed qualitative methodology [68,76,87,101,103,113,118], while the remaining 91% (72) studies were quantitative in nature.Apart from the survey nature of the studies, some (five studies) incorporated focus group discussion (FGDs) and key informant interviews (KIIs) [59,72,82,83,97].

Common Features of the Reviewed Studies
The first two included scholarly papers in this study were conducted in 2006 [57,62].Most of the studies in subsequent years (2007,(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019) had research articles ranging from 1-5.According to this review, the trend changed in 2020; this was occasioned by the emergence of the COVID-19 pandemic, which resulted in an increase in the number of scholarly papers published in subsequent years.Many of these papers include review articles, editorials, conference proceedings, books, and chapters in books.In 2020-2022, 11, 13, and 15 articles were included in this review, respectively.The number of respondents ranged in size from the smallest study (with 72 respondents) [101] to the highest-population study (using secondary data) of 5198 respondents [98].
About 17% (13) of the studies utilized secondary data sources [29,40,60,61,85,98,104,105,110,113,117,120,128] within five years (2018,(2020)(2021)(2022)(2023).The first research article that employed a secondary data source was published in 2018 [85].About 84% (66) of the studies used primary data in investigating and measuring FI in Nigeria.Three notable states in Nigeria with the highest number of FI studies (using primary data) were conducted in Oyo (15), Ogun (10), and Enugu (9), respectively (see Figure 3).According to this review, it is worth noting that no research works (employing primary data sources) were conducted in three states in northwest Nigeria; namely, Jigawa, Katsina, and Zamfara states.According to recent Cadre Harmonise data surveys (2018/2019 household survey), the northwest and the northeast were the two regions having the most acute levels of FI in Nigeria [130].The southwest had the highest number of FI research studies (45) conducted in the region, while the south-south had the least (9) FI research studies conducted in the region, and only three were reported in Federal Capital Territory (FCT), Abuja (see Figure 3).Furthermore, about 9% (7 articles) of studies in this review employed qualitative methodology [68,76,87,101,103,113,118], while the remaining 91% (72) studies were quantitative in nature.Apart from the survey nature of the studies, some (five studies) incorporated focus group discussion (FGDs) and key informant interviews (KIIs) [59,72,82,83,97].
Additionally, it was observed that studies that employed the "mean per capita food expenditure" approach used varying food security lines (FSL) for which households were categorized as either food secure (if the households met the threshold of a two third mean per capita food expenditure) and food insecure, if otherwise.For instance, refs.[46,66,73,79,86,92] had food security lines of NGN 7967, NGN 1948, NGN 4219, NGN 2643, NGN 10,704, and NGN 20,132 respectively.Furthermore, ref. [114] specified an FSL of NGN 2900 for their study that focused on the effects of climate-smart agricultural practices on food security among farming households in Kwara state, while [99] did not specify the FSL used in categorizing the households into food secure/food insecure households in their study.Furthermore, seven studies employed the FIES approach.Out of these five were COVID-19-related studies in Nigeria [40,41,106,127,128], while two studies were not related to 123].All of the COVID-19-related studies reported that household food access was hampered by the COVID-19 lockdowns in many parts of Nigeria.The study by Samuel et al. [40] reported that lockdown restrictions increased food expenditure and experiences of FI among the respondents.Meanwhile, ref. [106] utilized the COVID-19 National Longitudinal Phone Survey (COVID-19 NLPS) and found that 58.5% of Nigerian households experienced severe FI.Using the standard eight-item FIES in their study, ref. [41] found that only 6.9% of the Nigerian households were food secure, while emphasizing the inadequacies or non-existence of safety net programs, which further plunged the households into severe food insecurity during the pandemic.The study of Bwala et al. [127], which captured the psychosocial stress factors amid COVID-19, reported an increasing level of aggression and irritation among all categories of households, but this was greater among the low-income earners of the selected households in southwest and north-central Nigeria.Out of the 1674 households used in the study by [128] using the COVID-19 NPLS, about 4% were severely FI in the pre-pandemic, but they recorded 43% severe FI during the COVID-19 pandemic.
The method of using the food consumption score (FCS) in measuring FI was not quite popular among the studies included in this review.Only two studies used FCS [85,116].The investigation by [116] using the FCS and coping strategy index (CSI) found that 21.4% of the households were quality-food insecure, while 34.8% were economically vulnerable to FI.The study by [85] did not report the FI prevalence rate among the polygamous family structure in Nigeria but found polygamous households having better food security outcomes than monogamous households using nationally representative data collected in three waves (2011, 2013, and 2015).In Figure 3, the category "Others" referred to FI measurements that were characterized by FI proxies, adapted versions of some other methods, and less popular methods of measuring FI in the Nigerian studies included in this review.Some studies (for instance, [61,78,98]), employed two-item, three-item, and four-item questions to assess FI among the participants, respectively.The three-item version of the HFSSM was adapted by [60,82].Also, ref. [102] utilized a nine-item food security questionnaire for older children to evaluate the effectiveness of health education intervention on food security and other factors among adolescent girls in Maiduguri, Borno state, while [68,107] adapted the "Freedom from Hunger scale" (FFH) in assessing households' FI status in Enugu state [131].The study by [59] adapted the "Household Hunger Scale" (HHS) by [132] and DDS for assessing household food and nutrition security in Ohafia matrilineal society in Nigeria and found a 66% FI level among the households.The Dietary Diversity Score (DDS) was also used by [117] among households using nationally representative data; in addition, ref. [103] employed income level, education, and government policy in investigating farming households' food security in northern Nigeria.The study by [101] employed perception of the food security situation in the prevailing year (2019) to assess the FHHs in Enugu East Senatorial zone of the state, while [113] used changes in food intake as influenced by COVID-19 in exploring FI among residents in Nigeria during the first wave of COVID-19.The use of FI risk perception and management strategies was utilized by [87] in assessing FI among households in Oyo and Osun states and reported 87.6% FI prevalence.The work of [76] employed food habits and coping strategies among the elderly in Ogun state, while [118] adopted a self-reported one-item question "Since the COVID-19 crisis began, do you eat less or skip meals because there was no enough money or food" [133] to investigate the effects of COVID-19 on FI and other factors among women and girls living with or at risk of HIV in Nigeria and reported 76.1% FI prevalence.The food availability pillar of food security measurement was the focus of the study of [120], which also adopted a one-item self-reported question [134], "do you reduce the size of meal eaten in the household because of insufficient food," among female-headed households and reported 43.1% prevalence of FI among them.

Exploring Food Insecurity in Different Settings
The studies reviewed were carried out in diverse populations in Nigeria.Thirteen studies investigated FI using the Nigerian public [29,40,60,61,85,98,104,106,110,113,117,120,128].Among these studies, three utilized online surveys with a state-wide population.Three studies also employed COVID-19 NLPS, while seven studies used secondary datasets from the General Household Survey (GHS) made available through NBS.Twenty-seven studies were carried out among farming households in different population groups in different states.The work of [47] measured FI among farming households in Kwara state and reported 38% FI prevalence.The study of [67] used farming households in Ekiti state and reported about 88% FI, while [69] was carried out among urban farm households in Abuja with 30% FI recorded.Other studies were carried out among farming households in southwestern Nigeria; for instance, [99] (Ogun and Oyo), [46] (Ogun), [81] (Ondo), [122] (Oyo, Ekiti and Ondo), [19] (Oyo and Ogun), [79] (Ondo and Ogun), and [124] (Oyo).Also, some studies were carried out among farming households (crop and livestock) in northern Nigeria.For instance, refs.[88,89,100,103,109] reported 37%, 73.9%, 54.5%, 73.5%, and 81.5% FI.Studies involving farming households in the southeast include [96,101].Also, ref. [123] carried out their study among farming households in the Niger Delta region (Bayelsa and Rivers states) employing the FIES method and reported 72% FI.Two studies reported on people (women and adolescent girls) living with or at risk of the human immunodeficiency virus (HIV) [84,118] and found 72% and 76% FI, respectively.Further studies reported on primary and secondary school children [57], preschool children [125], and households with under-five children [112] and found a higher percent of FI-76.3%, 98.8%, and 63%, respectively.Ukegbu et al. [94] reported research on university undergraduate students, while [103] was about school-aged adolescents, and they found 93% and 81% FI, respectively.Bwala et al. [127] reported on households with varying income groups during COVID-19 and found about 96% FI.

Discussion
This review has investigated the depth of FI research publications in Nigeria over the last seventeen years, cutting across the six regions of the country with diverse populations, including under-five children, people living with or at risk of HIV, rural and urban farm households, and preschool to university undergraduate studies.Nigerian researchers have employed diverse tools to investigate the prevalence of FI.These tools included tested and proven HFIAS, HFSSM, and FIES and unpopular and less dependable one-item and twoitem tools.It is of great concern that a sizable number of the studies employed FI proxies, unproven measures, and studies that presented unclear FI methodologies coupled with no information about the actual prevalence rate of FI at different population levels.With different tools identified in the studies included in this review, some of the common FI tools include HFIAS, HFSSM, recommended daily calorie requirement, and the food security index.The studies that utilized the HFIAS module had an FI ranging from 45.8% [90] to 98.8% [125]; studies employing HFSSM reported a range of FI from 37% [100] to 100% [124], and studies that measured FI through the recommended daily calorie requirement ranged from 23.5% [48] to 78% [122].
Many studies were conducted and published in almost every region of the country, but it was quite surprising that no food-(in)security-related research that met the inclusion/exclusion criteria for this review was included (found) from three states in Nigeria: Jigawa, Katsina, and Zamfara states (northwest region) (see Figure 3).Meanwhile, the northwest region is one of the two regions (northwest and northeast) reported in the 2018/2019 household survey and analyzed to have the highest acute (crisis level) levels of FI in Nigeria [130].Many FI and hunger-related studies are solicited from two regions that are characterized by high levels of hunger, food insecurity, poverty, armed banditry, Boko Haram insurgency, kidnapping, and cattle rustling [14,17,133,134].
Furthermore, the South-South region, with a total of nine studies (see Figure 3), highlights a significant research gap in understanding and measuring food insecurity among the populations in this region.The region is known for incessant incidents of crude oil spillage, with attendant deleterious impacts on the environment that exposed the communities to the problem of household FI, especially among the vulnerable groups (women and children).For instance, ref. [134] reported that 97% of the households in the oil-spill-affected communities in Bayelsa were food insecure when compared to 67% in the control communities.This emphasizes the need for more quality research on food (in)security in the region.Studies that employed the use of a minimum recommended daily calorie requirement per adult equivalent were not consistent with the use of a unified recommended calorie required per adult equivalent.The minimum recommended calories of the thirteen studies ranged from 1800 Kcal [119] to 2700 Kcal [49].There is a need for a single minimum recommended calorie to be used in determining FI among different settings.Also, the Nigerian government, through its agencies and other relevant nongovernment agencies, may propose a unified minimum recommended calorie to be used in any food-security-related studies in the country.
Some studies utilized single-item to three-item tools to measure FI prevalence at household and national levels.However, as indicated in this review, the quantification of FI using one-item to three-item studies, with questions including "if anyone reduces the size of meal in the household because of insufficient food" and "cut the size/skip a meal," found that FI ranged from 25% among people living with and without HIV in Nigeria during COVID-19 [62] to 43.1% among FHHs in Nigeria [120].Comparing the prevalence rate of FI using one-item to three-item tools with other broad and extensive measures [19,80,124] revealed that the use of one item might have underrated the prevalence of FI in Nigerian studies included in this review.This finding was corroborated by the studies from Australia, which opined that the use of a one-item measure underestimated the prevalence of FI by about 5% when compared with more extensive tools [135][136][137][138][139]. Furthermore, some studies in this review used small sample sizes where the results of the studies may not be generalized; for instance, refs.[47,101,114].It is worth noting that two studies from this review failed to report the prevalence rate of FI after data description and analyses [71,85].The FI prevalence rate (usually reported in percent) is an important indicator of the food insecurity level or status in a population and, as a matter of compulsion, it should be reported in the published studies.Among seven studies included in this review that employed qualitative methods of analyzing FI, [76] reported the lowest FI prevalence rate of 12%, assessing the food habits of the elderly in Ogun state.Also, through a qualitative method, ref. [113] reported the highest level of FI (90%) among adult Nigerians during the first wave of COVID-19.This review indicated that 71% of the studies that measured FI through qualitative methods used FI proxies, such as income level, education attainment, food insecurity risks, food habits, and government policy [76,87].Meanwhile, studies that quantified FI through quantitative methods used well-recognized and standardized FI tools, such as HFIAS, HFSSM, and FIES [12,39,109].Surprisingly, the work of [124] was the only study that reported 100% FI in this review, while the study was carried out among gari processing households in Oyo state.

Limitations of This Study
This review contained some limitations that should be mentioned.It is important to acknowledge the effort made to ensure that this review was extensive, but it is possible that some studies might have been omitted, especially those that might have been published after the database search was concluded.It is worth noting that this review is the first systematic review of FI investigation and quantification in Nigeria.With different kinds of FI measurement tools and diverse approaches employed by different studies, it was quite challenging juxtaposing the outcomes of various studies.This review is also limited to studies (articles) extracted from the stated research databases, while some other studies not indexed in these databases were not included.Nevertheless, the authors do not feel that the lack of a meta-analysis should undermine the validity of the findings in this study.

Areas for Further Research
The issue of food insecurity is a global phenomenon, although the African region is experiencing the highest burden in recent times.A systematic review of this nature may be extended as an African study to have a comprehensive understanding of food insecurity and quantification of food (in)security research endeavours in the region.

Conclusions
According to the authors' search, this is the first systematic review that assesses the comprehensive nature of food security studies conducted in Nigeria.This study revealed different approaches employed by researchers in gathering important details about food insecurity in Nigeria.These approaches included standardized FI measurement tools, adapted measures (one-item to three-item questions), and FI proxies.The one-item to three-item questions were found to present an underrated or underestimated prevalence of FI among different populations in the country.With different FI tools used by researchers in this review, it is quite difficult to comprehend the correct or accurate prevalence and gravity of FI in Nigeria.While some studies found FI prevalence as low as 12 to 21.4% [76,115], a study reported a 100% FI prevalence among gari processors in Oyo state [124].As revealed by the investigation of this review, the authors highlight the need for a measurement tool that would be appropriate for Nigerian settings and that would enable researchers to attain comprehensive knowledge of the FI prevalence and severity in the country, paving the way for improved food-and nutrition-sensitive policy development.

Figure 1 .
Figure 1.Authors' graph using data from the Global Food Security Index, 2022.

Figure 1 .
Figure 1.Authors' graph using data from the Global Food Security Index, 2022.

Figure 2 .
Figure 2. Flowchart of studies that met the search criteria, and the total number of studies included for review.

Figure 2 .
Figure 2. Flowchart of studies that met the search criteria, and the total number of studies included for review.

Figure 3 .
Figure 3.The Nigerian map showing the geographical distribution of the 66 research studies.

Figure 3 .
Figure 3.The Nigerian map showing the geographical distribution of the 66 research studies.

Figure 4 .
Figure 4. FI measurement tools employed in the studies included in this systematic review.

Table 1 .
Snapshot of the key insights obtained from the 79 scholarly articles included in this review.