In this section, we discuss the findings regarding the factors influencing the consumption of each of the food groups—cereals, miscellaneous (such as condiments, coffee and tea), eggs, fish and seafood, fruits, meat and poultry, milk and milk products, oils and fats, pulses, roots and tubers, sweets (sugar and honey), and vegetables—used in assessing the household dietary diversity score (HDDS). Following that, we estimate the effect of crop diversification on dietary diversity.
3.1. Determinants of Food Groups Consumed by Households in Nigeria
A logistic regression was carried out to examine the socio-economic characteristics of households that determine their consumption of each of the 12 food groups. The results reported in
Table 2 revealed the following:
Cereals: It was found that, compared to the southern region, households from the northern region of Nigeria (North Central and North West) have the higher likelihood to consume cereals. This is rather expected because most cereal production in Nigeria emanates from the northern region relative to southwest. Additionally, asset ownership, food expenditure and land size all significantly increased the likelihood of consuming cereals (
Table 2). Moreover, the results implied that as households increase their food expenditure, the consumption of cereals increased. This is consistent with the findings of Musyoka et al. [
97] and Torres [
98]. The increase in land size could lead to having more land being used to produce cereals, which could make households increase their consumption of cereals. This finding is also in line with Yawson et al. [
99], that land size plays a significant role in expanding production cereals.
Spices: It was observed that food expenditure and asset ownership increase the likelihood to consume spices. In Nigeria, the southern region is known to be high consumer of spices compared to the northern region. In corroboration, we found that living in the northern Nigeria (North Central and North East) reduces the probability of consuming spices. On the other hand, increased food expenditure and ownership of assets increased the likelihood of consuming more spices. Increased income vis-à-vis assets strengthens the purchasing power of households to expend more on food.
Eggs: The households of the northern region are known as higher consumers of eggs compared to the southern region. Contrary to this belief, we found a negative association between egg consumption and the northern region. The probable reason for this might be a result of conflict and extreme terrorism that have afflicted the region in recent years. This is consistent with the findings of [
100], asserting that conflict, social crises and terrorism are responsible for increased food poverty in the region, among other things. Additionally, the age of the household’s head significantly reduced the likelihood of consuming eggs. This observation is in accordance with the belief that eggs are consumed by younger individuals and not for older individuals. Conversely, food expenditure and agricultural revenue significantly increased the consumption of eggs as a food group, implying that households with more proceeds from agricultural production are able to consume more eggs with the revenue accrued from the sales of their produce.
Fish and sea foods: Increased food expenditure significantly increased the consumption of fish and sea foods. This could be due to the fact that the higher the amount of money allocated for food consumption, the higher the probability of dietary diversification. For those in South East and South South zones, a significant increase was observed in the consumption of fish and sea foods relative to those in the South West. This could be due to the fact that the South South is a riverine area with abundant fish and other sea foods. Additionally, marital status, increased age of the household’s head, household non-food expenditure and household size increased the consumption of this food group. This supports the belief that older people should consume less meat and more fish and sea foods. In the same vein, being married increased the consumption of fish and sea foods. This is in line with the result on household size—this finding could be due to the fact that married people are expected to have large households, and as such, are expected to purchase more fish and sea foods as a good source of protein, especially for the children in the households.
Fruits: The sex of the household’s head (male), asset index, and food expenditure positively and significantly determined the consumption of the fruit food group in the study area. Being a male-headed household, increasing food expenditure of the household and asset index increased the consumption of the fruit food group. However, residing in the region of the country (North Central, North East and North West) where there is conflict negatively determined households consuming fruits. In addition, household size negatively determined the consumption of fruits as a food group. This implies that an increased number of household members decreased the consumption of fruits.
Meat: Meat is a very good source of protein and its consumption is very important for improving nutrition outcomes, especially among children. We found that being a male household head, asset index, household food expenditure, land size and non-food expenditure positively and significantly determined a households’ consumption of meat. This implies that male-headed households are more likely to consume meat than female-headed ones, as the male head allocates more expenditure on food, influencing the decision of a household caregiver to purchase more meat for the household’s consumption. This is supported by Abubakar [
101], who observed that female-headed households are less likely to report meat consumption than the males. On the other hand, being married, the age of the household’s head, residing in North Central, North West and South East relative to the South West showed a negative and significant consumption of meat. The finding that being married reduces the consumption of meat supports earlier findings that married households consume more fish and sea food. Moreover, even though households in the northern region practice animal husbandry, they eat less meat than those in South West. The reason is that the northern herdsmen rear meat-producing animals for generating income rather than for consumption.
Milk: We found that regarding asset ownership, households having access to extension services and food expenditure positively and significantly determined households’ consumption of milk in the study area (see
Table 2). An increased asset ownership implies that there is an increase in income; hence, households are rich enough to purchase milk. Households can acquire knowledge on the consumption of different food groups (in this case, milk) through nutrition information they received from extension workers. In the same vein, households who can afford to increase their non-food expenditure can also increase their consumption of milk. However, the age of the household’s head, household size and food expenditure negatively determined the consumption of milk as a food group. Larger household sizes consuming less milk could be due to the cost implication of its purchase. In addition, there is an increase in milk consumption as a result of households’ increased food expenditure. The consumption of milk as a food group was negatively and significantly determined by residing in the North Central and North East geopolitical zones relative to the South West. As previously observed, residents in the northern zones as compared with the southwest consume less animal-sourced protein-rich foods.
Oil and Fat: Being married and household food expenditure positively and significantly determined the consumption of oil and fat (
Table 2). Consistently, dwelling in the South East and South South positively and significantly determined the consumption of oil and fat. It is believed that these regions in Nigeria consume a lot of oil and fat compared to other regions, such as the northern parts. Additionally, residing in the North Central and North East zones relative to the South West negatively determined the consumption of oil and fat.
Pulses: Food and non-food expenditure positively and significantly determined the likelihood of households to consume pulses (
Table 2). In the same vein, residing in the South East relative to being in the South West positively and significantly determined the likelihood of consuming pulses. However, a negative significance was observed when North Central, North East and North West zones were considered relative to the South West. This implies that pulses are less consumed by households in the northern region, while they are more consumed by people in the South West. Land size and access to extension services were also negatively related to the consumption of pulses.
Root and tuber: From
Table 2, we can see that socio-economic characteristics negatively and significantly determine the consumption of roots and tubers, including: age of the household’s head, access to extension services, access to credit from cooperatives and food expenditure. The same was observed with households residing in the North Central, North East and North West zones relative to the South West. This is because roots and tubers are not a key component of diets in northern Nigeria. On the other hand, asset index and non-food expenditure of the household positively and significantly determined the consumption of roots and tubers.
Sweets: Furthermore, the results revealed that asset index, access to extension services, food and non-food expenditure all positively and significantly determine the consumption of sweets (
Table 2). The same was observed with households residing in the North Central, North East, North West, and geopolitical zones relative to the South West zone.
Vegetables: Agricultural revenue, household food and non-food expenditure positively and significantly determined households’ consumption of vegetables. Thus, increasing the agricultural revenue of households as well as expenditure on food and non-food increases their consumption of vegetables because the households have enough money to purchase these vegetables. On the contrary, residing in the North Central, North East and North West relative to South West negatively and significantly determined the consumption of vegetables. The results imply that households in northern Nigeria consume fewer vegetables as compared to those in the South West zone.
3.3. Crop Diversity and Nutrition Diversity
The three regression models (OLS, Poisson and IV-Poisson) used in the analysis showed the relationship between crop diversity and nutrition diversity. Two indicators were used to measure crop diversity: Firstly, crop diversity considered as crop count (that is, the number of crops grown by the households) was used. Secondly, we proxied crop diversification with agricultural revenue from the different crops grown in the households. The rationale of the link emerges from the assumption that even though smallholder farmers are involved in mixed cropping, they cannot produce all the food items that would be consumed in the households. Hence, income generated from crop diversification can be used to purchase non-cultivated food items to enhance their dietary diversity. The analysis was carried out for the entire country using pooled data (
Table 4); the disaggregated results by sector of residence (urban and rural) are presented in
Table 5 and
Table 6. Although three regression results (OLS, Poisson, and IV-Poisson) are presented, our main focus is on the IV-Poisson estimation strategy results. The OLS results were found to be biased as they failed to fulfill the assumption of count variables. According to Greene [
102], a common approach for count data models is to use a Poisson estimator. The Poisson estimator assumes equi-dispersion; that is, the mean and variance of the dependent variable are assumed to be equal. This assumption is often violated and can lead to incorrect standard errors. Overdispersion, where the conditional variance exceeds the conditional mean, is a common phenomenon. However, due to possible endogeneity between crop diversity and dietary diversity [
27,
61,
96], we used the IV-Poisson estimation technique to explore the relationship between crop diversification and dietary diversification.
Our main discussion of the reported findings below will focus on the results emerging from the instrumental variable approach. We found that in some cases, the age of the household’s head had a negative and significant relationship with nutrition diversity (HDDS). This implies that an increase in the age of the household’s head decreases the likelihood of having dietary diversification. This could be due to the fact that the older the household’s head becomes, the less likely she or he is to obtain additional income from off-farm activities which can be used to purchase diverse food groups in a bid to increase dietary diversity. These findings are also consistent with those of Babatunde et al. [
81].
Ownership of assets has a positive significant relationship with dietary diversity in the model (column 3) using the crop count, but not in the agricultural revenue models (column 6). This implies that the ownership of assets leads to the households consuming more diversified diets; this is in line with the findings of Pellegrini and Tasciotti [
27]. The findings from the study of Mannaf and Uddin [
103] also suggest that the ownership of assets (such as livestock) is expected to reduce food insecurity and increase the likelihood of diversifying diets.
Household size was found to have a negative association with dietary diversity (both in the number of crops grown and agricultural revenue models). By implication, the larger the household becomes, the lower the food groups available to members, especially in low-income households where there is weak purchasing power for many food groups. This finding confirms Babatunde et al. [
81] and Adebayo et al. [
100], who observed that an increase in household size increases the probability of food poverty. In addition, access to credit has a positive and significant relationship with dietary diversity. Access to financial services has been associated with the adoption of productivity-enhancing technologies [
69]. Therefore, if income increases through increased productivity, this could increase the purchasing power of households so they can purchase diverse food groups for consumption.
Food expenditure is positively influencing the likelihood to diversifying nutrients in Nigeria. Various studies have revealed that increased food expenditure increases the amount allocated to each food group, which will most likely increase the possibility of diversifying the food consumed by the household [
97,
98,
104]. Non-food expenditure has a positive significant relationship with HDDS for all the models estimated. This implies that when households spend an additional budget on non-food items, they also increase expenditure on food items (see models 2 and 3), and it is expected that their dietary diversity increases.
On the relationship between crop diversity and dietary diversity using a number of crops grown by the household, we found that crops grown positively and significantly influences the diversification of diet in the household. This is possible since as the number of food crops increases the household tends to have more option to upgrade and diversify from the consumption of unbalanced diets that possibly influence malnutrition, stunting and wasting especially among children. Thus, the higher the crop count, the higher the likelihood to consume divers’ kinds of food groups. This agrees with several findings in the literature, which revealed that an increase in farm production diversity is responsible for the number of food groups consumed in the household [
63,
97,
98,
104,
105,
106,
107,
108,
109].
The use of agricultural revenue as a proxy for crop diversity provided a consistent result with the number of crops grown by the households. This finding suggests that crop diversification is a key determinant of dietary diversity in Nigeria, as revealed also in other countries such as Malawi [
61], Ethiopia [
108] and India [
63]. Additionally, the plot of polynomial associations between crop diversity and dietary diversity suggests that the higher the crop diversification, the higher the dietary diversity is in Nigeria (
Figure 1).
Finally, the geopolitical zones in Nigeria were also modelled as a factor influencing dietary diversity. The estimation in all models revealed a negative and significant relationship between the region of residence and the nutritional diversity of households across Nigeria, especially for the northern region (North Central, North East and North West). Northern Nigeria have been plagued with security issues in recent years, which have contributed to a downward trend in development and growth indicators in the region, despite several interventions to reduce hunger, malnutrition and poverty.
In order to obtain a better idea regarding the situation in rural and urban households, the data were disaggregated to investigate the relationship of crop diversity with dietary diversity. Interestingly, some similar findings were found from the relationship between the cofounding factors used along with crop counts and agricultural revenue as a function of dietary diversity. For instance, variables such as the age of the household’s head, asset ownership, food and non-food expenditure as well as being resident in the northern region of the country, particularly the North East, all had a significant relationship.
However, the sex of the household’s head was found to be positively related to nutrition diversity in the urban areas, while the reverse was the case in the rural areas. This implies that a male-headed household reduces nutrition diversity in rural Nigeria, while it increases it in urban Nigeria. The probable reason for this might be the empowerment, social and human capital level of male-headed households in the urban areas compared to rural households. Household size, access to credit and use of firewood significantly determine dietary diversity in rural Nigeria. Specifically, an additional member of the household reduces the nutritional diversity of such households. This could be due to the fact that an additional household member will put a strain on the consumption of the household. Access to credit, on the other hand, increases the dietary diversity of households as they will have enough funds for their expenditures.
In relation to crop diversity and nutrition diversity, we also found that crop diversification and agricultural revenue have a positive and significant relationship with dietary diversity in rural households, while an inverse relationship was recorded in the urban households. The findings suggest that low crop diversification has the likelihood of reducing dietary diversification. Although farming is a rural phenomenon compared to urban, these findings do not necessarily suggest that the rural households were able to diversify diets more than the urban households. Specifically, linking crop diversification with dietary diversification, rural households seem to be in a better position than urban households in Nigeria. Our findings suggest that crop diversification is a key factor in reducing malnutrition in Nigeria. This is further supported by the non-parametric analysis expressed in
Figure 2 and
Figure 3. This finding, which is in line with the results reported by other studies, suggests that crop diversification increases the diversification of diets, especially in rural households [
63,
97,
98,
105,
106,
107,
108,
109].