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1 From the Stanford Center for Research in Disease Prevention, Stanford University School of Medicine, Stanford University, Palo Alto, CA.
2 Supported by grant 5-R01 CA68082-03 from the National Cancer Institute and grant 99-00530V-10173 from the California Department of Health Services, Cancer Research Program. 3 Address reprint requests to DM Matheson, Stanford Center for Research in Disease Prevention, Stanford University School of Medicine, 1000 Welch Road, Palo Alto, CA 94304-1825. E-mail: donna.matheson{at}stanford.edu.
ABSTRACT
Background: Food insecurity is a critical variable for understanding the nutritional status of low-income populations. However, limited research is available on the relation between household food insecurity and children's nutritional status.
Objective: Our objective was to examine the relations among household food insecurity, household food supplies, and school-age children's dietary intakes and body mass indexes (BMIs).
Design: A sample of 124 predominantly Hispanic, fifth-grade children and their mothers were surveyed as part of a school-based obesity-prevention program. Data on the children's weights and heights were collected and three 24-h dietary recalls were conducted. The mothers provided reports of household food insecurity and household food supplies.
Results: Food insecurity was negatively associated with the children's BMIs and household food supplies but not with the children's food intakes. However, a secondary analysis showed that as payday approached, children from the most food-insecure households had significant decreases in energy intakes and meat consumption.
Conclusions: This is one of the first studies to report a significant association between food insecurity and children's nutritional status. The ages and sex-adjusted BMIs of the food-insecure children were lower than those of the food-secure children but were still within the normal range. The lower BMIs in the food-insecure children may have been due to short-term, yet periodic food restrictions that resulted as household food supplies diminished before payday. Future research is needed to assess the physiologic and psychological effects of periodic food restriction on children's health.
Key Words: Household food security • dietary intake • nutritional status • obesity • Hispanic children
INTRODUCTION
In the United States, 14.8% of households with children aged <18 y (12 million children) experience some level of food insecurity (1). Therefore, food insecurity may be a core variable for understanding the nutritional status of low-income populations (2). Food insecurity is defined as "limited or uncertain availability of nutritionally adequate and safe foods or limited or uncertain ability to acquire acceptable foods in socially acceptable ways" (3). Although household food insecurity is associated with socioeconomic status, 50% of food-insecure households have incomes above the poverty line (4). Therefore, the construct of food insecurity adds to our understanding of the relation between poverty and nutritional status beyond what is discerned by socioeconomic variables, such as household income and employment status. Furthermore, food insecurity may be a more sensitive measure of the food issues experienced by low-income families, especially the psychological and social ramifications of a lack of food, than is household income alone (4,5).
There has been growing interest in the hypothesized link between food insecurity and obesity (6,7). First, research with adults has indicated that food security is positively associated with obesity (7). However, this relation has not been explored in school-age children, despite evidence that the prevalence of obesity in children has increased dramatically over the past 2 decades (8). Second, food insecurity has been negatively associated with women's nutrient intakes (9). Again, this relation has not been examined in children, although there is ample data to suggest that children's food intakes do not meet current recommendations (10). Third, household food insecurity has been associated with decreased household food supplies (9), and household supplies of fruit and vegetables have been associated with children's intakes of these foods (11). However, the relations among household food security, food supplies, and children's food intakes are not known. Accordingly, the purpose of this research was to examine the associations among household food security, household food supplies, children's food intakes, and children's weight in a predominately Hispanic sample of fifth-grade children. The relations that we examined are illustrated in the conceptual model shown in Figure 1. Although temporal relations are implied in this model, they could be tested by this cross-sectional research design.
FIGURE 1. . Relation between household food security, household food supplies, children's at-home food consumption, and children's BMIs.
SUBJECTS AND METHODS
Subjects
Data were collected between September 1999 and June 2000 from families of fifth-grade students enrolled in 8 elementary schools in northern California. Mothers were recruited at school events and through telephone solicitation. The schools provided telephone numbers for all eligible children. Consent was obtained from the children and their mothers during the first face-to-face interview. The study was approved by the Stanford University Administrative Panel on Human Subjects in Medical Research. A total of 178 mothers were contacted and 131 participated. One child refused to complete the 24-h dietary recalls and 6 children refused to have their height and weight measured; therefore, the sample used in this research consisted of 124 mother-child dyads. Mothers were the respondents in 121 of the 124 interviews (97.5%). Two aunts and one grandmother, who were the primary female caregivers, were included in the analysis. Slightly more children were boys (n = 65; 52.4%) than girls and the average age of the children was 10.7 y (range: 9.9–12.5 y). Most of the mothers identified their ethnicity as Hispanic (87.1%; 108 of 124). Accordingly, 95 (76.6%) of the interviews were conducted in Spanish, but only 3 (2.4%) of the children's dietary recalls were conducted in Spanish. There were no statistical interactions among age, sex, and ethnicity.
Research design
The study was conducted as part of a school-based obesity-prevention study funded by the National Cancer Institute. In this cross-sectional survey, we measured children's dietary intakes, height, and weight, and collected data on household food security and food inventories from the mothers. Height and weight were measured during school hours. Face-to-face interviews conducted after school hours were used to collect the children's first dietary recall and the mothers' reports of household food security. Follow-up telephone calls were used to obtain the remaining 2 children's dietary recalls and the 2 household food inventories from the mothers. The same bilingual research assistant conducted all of the interviews with the mothers, registered dietitians conducted the 24-h dietary recalls, and trained research assistants measured the children's heights and weights.
Assessment of children
Height and weight
Standing height was measured twice, to the nearest millimeter, with a portable direct reading stadiometer while the subjects were shoeless. If the 2 measures differed by >5 mm, a third measurement was made. Body weight was measured twice, to the nearest 0.1 kg, with a digital scale while the subjects were wearing light indoor clothing and no shoes. If the 2 measures differed by >0.2 kg, a third measure was obtained. The mean of the 2 measures, or the median of 3 measures, was used in the analysis. The body mass index (BMI) was calculated as weight (kg)/height2 (m).
Dietary intake
Three nonconsecutive 24-h dietary recalls were conducted with the children as the primary respondents. Ten validation studies have provided support for the use of 24-h dietary recalls in children as young as third grade (12–21) and as the most appropriate method for collecting dietary data from diverse cultural groups (22). Registered dietitians collected the dietary recalls by using standard protocols, including meal location, from the Nutrient Data System for Research (NDS-R) (23). Mothers were consulted in both the face-to-face and the telephone dietary recalls to clarify food details, food-preparation methods, or brand names that the children could not recall. Three dietary recalls, one on a weekend day and 2 on weekdays, were conducted to account for intraindividual variations in dietary intake (24,25). Dietary recalls were collected 1 wk apart to account for the potential monthly fluctuation in dietary intakes in food-insecure populations (26). To check for interinterviewer reliability with the dietary recalls, a subsample of 22 (5%) telephone recalls were tape-recorded and reentered by a second dietitian.
The NDS-R database (versions 4.01 and 4.02) is derived from the US Department of Agriculture Handbook No. 8, and includes many Hispanic food items (27). The number of servings from 9 food categories, energy intake, and the percentage of energy from fat were used to describe the children's diets. The 9 food categories were as follows: meat; breads, cereals, and grains; fruit; fruit scored by using 5-A-Day criteria (28); vegetables; vegetables scored by using 5-A-Day criteria; milk; sweets and snacks; and convenience foods. Foods used as ingredients in mixed dishes and standard servings of whole foods were counted in the meat, fruit, vegetables, and breads, cereals, and grains categories. Meat included beef, pork, poultry, fish, sausage, and cold cuts. The breads, cereals, and grains category included yeast breads, tortillas, breakfast cereals, pasta, rice, and noodles. Fruit or vegetables included all fruit or vegetables and fruit or vegetable juices, even those in mixed dishes such as beans in burritos or vegetable garnishes on sandwiches. We also counted servings of fruit or vegetables by using a second method, based on the 5-A-Day criteria. The 5-A-Day criteria for fruit or vegetables included standard serving sizes of 100% fruit or vegetable juices and all fresh, frozen, canned, or dried fruit or vegetables. Fried or pickled vegetables and fruit or vegetable dishes that contained >30% of energy from fat, including French fries and fruit desserts, were not counted with this method. Milk included all fluid milk, including chocolate milk. Sweets and snacks included cookies, cakes, pies, candy, chocolate, potato or tortilla chips, pretzels, and popcorn. Convenience foods included all frozen, canned, dehydrated, or prepackaged main dishes or soups. Precooked frozen or dehydrated vegetables (ie, frozen French fries and dehydrated mashed potatoes) were also classified as convenience foods. This category was not mutually exclusive from the other food categories.
Assessment of mothers
Demographics
Demographic variables, including household composition and the child's age and sex, were collected during the face-to-face interviews with the mothers.
Household food insecurity
Food insecurity was measured by using the 18-item US Department of Agriculture's Core Food Security Module. The food-insecurity construct consists of 4 components: 1) quantity of food, 2) quality of food, 3) food anxiety, and 4) coping mechanisms taken by the household to augment food supplies (29,30). Households are categorized into 4 levels of food insecurity: 1) food secure, 2) food insecure without hunger, 3) food insecure with moderate hunger, and 4) food insecure with severe hunger (ie, the children's food intakes are restricted). This instrument has been used in national surveys, and the reliability and validity for use with low-income women are well established (5,31).
Household food supplies
A 40-item household inventory of food supplies was collected from the mothers over the telephone. Using a checklist of food categories adapted from the Block food-frequency questionnaire (32) and a previously validated self-reported shelf inventory (9,33), we asked the mothers to indicate whether they had "none," "a little," or "a lot" of each food category. Food-insecure households may have depleted food supplies at some point during a month, so 2 measurements—made 1 wk apart—were collected (26). For analysis, food items were classified into categories similar to those used in the dietary recall. The household food-supply categories included meat (6 items); breads, cereals, and grains (5 items); fruit (9 items); vegetables (10 items); milk (1 item); sweets and snacks (6 items); and convenience foods (3 items). Fruit and vegetables included in mixed dishes (eg, vegetables on pizza) were not included in the household inventory.
Statistics
Descriptive statistics for the children's BMIs and dietary intakes and the mothers' reports of household food security and household food supplies were calculated. To adjust for age and sex differences in the children's BMIs, we compared the children's BMIs with those in the 1999 age- and sex-specific growth charts of the Centers for Disease Control and Prevention (CDC; 34). To determine whether children in the sample were stunted, we compared the children's median heights with the median heights-for-age from the 1999 age- and sex-specific CDC growth charts (34). Variables that described dietary intake included the number of servings from each food category, energy intake, and the percent of energy from fat. Only foods that the children consumed at home were included in the tests of association between the children's food and nutrient intakes, household food security, and household food supplies. Hearn et al (11) reported an association between school-age children's fruit and vegetable intakes and household availability of fruit and vegetables. Likewise, diminished household food supplies are associated with food insecurity (9). Therefore, comparing all foods consumed by children, including foods consumed outside the home, with their household food supplies could limit our ability to specifically examine the relation between household food security, household food supplies, and children's food intakes.
The primary analyses in this research examined the associations among children's dietary intakes, children's BMIs, household food supplies, and household food security. An unequal variance t test was used to assess the difference in BMIs between children living in food-secure and food-insecure households. Jonckheere's tests for ordered alternatives were used to assess the associations between the level of household food security and household food supplies, children's dietary intakes, or children's height. This test is based on the sum of the Wilcoxon statistics for each pair that obey an a priori order restriction (35). The test has a greater power than the omnibus one-way analysis of variance or the Kruskal-Wallis test when the data obey an a priori ordering, such as food-insecurity levels, and it also protects against false-positive results. Because the data were not normally distributed, Spearman's rank-order correlation coefficients were used to assess the associations between the children's food intakes and BMIs.
Secondary analyses were conducted to test the hypothesis that food-insecure families are more likely to have ample food after payday but begin to run out of food or restrict food intake as they reach the end of a pay period. Changes in the children's at-home food intakes relative to payday were examined in families who were food secure, food insecure, or food insecure with hunger. On the basis of biweekly periods, with paydays on Fridays, we calculated the number of days from payday for each child's dietary recalls. The children's at-home energy intakes, percentages of energy from fat, and food intakes were regressed against the number of days from the putative paydays to the day of recall. A negative slope indicated that consumption decreased as the number of days from payday increased. The slopes of the resulting least-squares regression lines for each food category and fat or energy intake were compared across the level of food security by using Jonckheere's test for ordered alternatives. The results are expressed as medians and means ± SDs unless stated otherwise.
RESULTS
Level of food security
About 65.0% of the mothers (n = 80) reported that their households were food secure, 24.4% (n = 30) reported that their households were food insecure without hunger, 8.9% (n = 11) that their households were food insecure with hunger, and 1.6% (n = 2) that their households were food insecure with severe hunger. One mother refused to complete the food-security scale. Mothers who reported food insecurity with hunger and food insecurity with severe hunger were grouped together in subsequent analyses because of the low number of respondents in each of these cells.
Children's weight status
The average BMI of the sample was 20.70 ± 4.17 (range: 13.10–36.81). Adjustment for age and sex with the use of the CDC growth curves showed that, on average, the children's BMIs were at the 73rd percentile. The boys' mean BMI was 20.93 ± 4.50 (75th percentile), whereas the girls' mean BMI was 20.44 ± 3.80 (71st percentile). The difference between the boys' and girls' BMIs was not significant (P = 0.51).
Relation between food security and weight status
Children from food-secure households were significantly heavier for their height than were children from food-insecure households. The mean BMI for children from food-secure households was 21.19 ± 4.69 and the mean BMI for children from food-insecure households was 19.80 ± 2.88. An unequal variance t test showed that these groups were significantly different (P = 0.04). When sex was added to the model, there was no interaction effect between food-security level and the children's sex on BMI (P = 0.73).
Overall, the children's median height was 144.0 cm and the CDC median for boys or girls the same age was 143.0 cm, so children were slightly taller than would be expected. Jonckheere's test for ordered alternatives showed no significant association between household food-security level and children's height (P = 0.49). Therefore, there was no evidence of stunting in our sample.
Household food security, children's dietary intakes, and food supplies
The children's mean and median energy intakes, percentages of energy from fat, and food intakes—as all foods consumed or as only those foods consumed at home—are reported in Table 1. Spearman's correlations showed no significant associations between children's energy intakes, percentages of energy from fat, and food intakes and BMI, except for an inverse relation between the amount of breads, cereals, and grains eaten at home and BMI (r = -0.21, P = 0.02). The association between total breads, cereals, and grains consumed and BMI was nearly significant (r = -0.17, P = 0.06). The intraclass correlations between the dietary recalls coded by 2 independent dietitians were 0.91 (P < 0.001) for energy intake and 0.80 (P < 0.001) for percentage of energy from fat. These results indicate that the dietitians' coding of the 24-h dietary recalls was reliable (36).
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TABLE 1 . The number of servings of each food group, energy intakes, and the percentage of energy from fat consumed by boys and girls1
The children's at-home intakes of most foods, intakes of energy, and percentages of energy from fat were not significantly associated with the household level of food security. Only meat consumption was significantly lower in children from food-insecure households than in children living in food-secure households and food-insecure households with hunger (Table 2). This trend in decreased meat consumption with decreased household food security was significant, despite the U-shaped distribution in the data, because Jonckheere's test of ordered alternatives weights the means of each category by the number of families. There were so few families classified as food insecure with hunger that the trend in decreased meat intake with increased food insecurity was significant. In addition, as shown in Table 3, changes in at-home meat and energy intakes, relative to payday, were significantly associated with the food security level. Negative slopes indicate a decrease in food or nutrient intakes before payday. As payday approached, there were significant decreases in at-home energy and meat intakes in children from food-insecure households. These results indicate that the children living in food-insecure households had a more precipitous drop in at-home food intake within a pay period than did the children living in food-secure households.
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TABLE 2 . Jonckheere's test for ordered alternatives between household food-security status and the children's mean number of at-home servings from each food group, energy intakes, and the percentage of energy from fat1
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TABLE 3 . Jonckheere's test for ordered alternatives between household food-security status and the change in children's at-home food and nutrient intakes as payday approaches1
As hypothesized, the association between household food supplies and level of household food security was highly significant for most food categories (Table 4). Likewise, household food supplies were significantly associated with the children's at-home intake of fruit, meat, and sweets and snacks. (Table 5).
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TABLE 4 . Jonckheere's test for ordered alternatives of household food supplies and household food-security status1
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TABLE 5 . Spearman's rank-order correlation coefficients between household food supplies and children's at-home dietary intakes1
DISCUSSION
This is one of the first studies to examine the relation between food security and Hispanic children's nutritional status. Children from food-insecure households had lower body weights than did their peers from food-secure households. However, on average, these children were not underweight. Rather, the food-insecure children's body weights were within the normal range, whereas their peers from food-secure households were actually at risk of becoming overweight. Therefore, children living in food-insecure households do not need more food to sustain adequate growth. However, adequate growth is not the only indicator of nutritional well-being in children living in food-insecure households (3). Nutritional status is also influenced by the quality of foods consumed (2). In this research, the types of foods children consumed were significantly associated with their household food supplies. Household food supplies were significantly associated with household food-security status.
Our results support the model of food-management strategies for low-income families proposed by Campbell and Desjardin (37). These researchers suggest that mothers focus on feeding their children first, and in doing so may sacrifice their own nutrient needs. In our research, the status of household food security was clearly associated with household food supplies, but the status of household food security was only associated with the children's at-home intake of meat. It is likely that the mothers protected their children from the effects of food insufficiency in their homes as much as possible and only when household food shortages became acute (eg, just before payday) were the children's intakes affected. In our research, children's energy and meat intakes were significantly lower toward the end of a pay period in food-insecure households than in food-secure households. These periodic restrictions in food intake, if they occur repeatedly over a long time, may explain the lower BMIs observed in the food-insecure children. Furthermore, the long-term effects of periodic food restrictions on children's nutrient stores are unknown. However, meat—a primary source of dietary iron—was the food category most restricted in the children's diets, and there is evidence of an increased prevalence of iron deficiency in low-income children (38,39).
The weekly changes that we observed in the children's food intakes have important implications for dietary assessment in low-income populations. Our results suggest that within a 30-d period, dietary intakes of low-income children may be highly variable because of fluctuations in their family's food supplies. This dietary variability may influence the children's ability to accurately estimate their intakes, especially if using food-frequency questionnaires. Accordingly, in a validation study of a food-frequency questionnaire, inner-city school-age children had difficulty reporting average intakes (40), perhaps because their food portions varied depending on their family's food supplies. Standard food-frequency questionnaires, based on intakes over the past year, do not detect weekly changes in the food intakes of low-income children and may be difficult for children to answer. Instead, we recommend the use of multiple 24-h dietary recalls, collected over 1 mo, to determine the dietary intakes of low-income populations.
Our sample was similar to nationally representative samples of Hispanic children. First, the level of food security in our sample was similar to that in the sample for the Current Population Survey. In the Current Population Survey, collected between 1995 and 1999, 61.3% of Hispanic families with household incomes <130% of the poverty line were classified as food secure, whereas 64.5% of the families in our sample were classified as food secure. Second, similar to data from the Hispanic Health and Nutrition Examination Survey, the BMIs of children in our sample were high (41). The average BMIs of 10-y-old Hispanic boys and Hispanic girls in the Hispanic Health and Nutrition Examination Survey collected in 1982–1984 were 18.35 and 18.90, respectively. In comparison, in our sample, the average BMIs of the boys and girls were 20.93 and 20.44, respectively. Third, even though the BMIs of the children were normal to high, their food intakes were low. The Hispanic children who participated in the Continuing Survey of Food Intakes of Individuals in 1989–1991 had an average energy intake of 6.9 MJ/d, whereas the children in our sample had an average energy intake of 6.0 MJ/d (10). Furthermore, similar to other samples of low-income children, there was no association between energy intake and BMI, although we observed an inverse relation between the amount of breads, cereals, and grains consumed at home and BMI. Evers and Hooper (42) reported low muscle mass in their sample of 7- to 9-y-old children and suggested that the children were overweight because of a lack of physical activity. However, Johnson-Down et al (43) reported systematic underreporting of energy intake in overweight children aged 9–12 y and no association between physical activity and BMI. They concluded that the relation between energy expenditure and energy intake in overweight children is difficult to discern given the underreporting of nutrient intakes. Other researchers also showed that children's abilities to accurately complete 24-h dietary recalls are variable (13,17,44). Therefore, a limitation of our research was the use of 24-h dietary recalls as a measure of dietary intake. Finally, additional research is needed to examine the inverse association between breads, cereals, and grains consumed at home and BMI. The correlation between the breads, cereals, and grains category and the sweets and snacks category was not significant (r = -0.05, P = 0.57). Therefore, it does not appear that breads, cereals, and grains displaced more energy-dense desserts or snack foods in the diets of these children.
In summary, this research links the status of household food security to children's food intakes. Because this research was cross-sectional, causal relations between food insecurity and the nutritional health of children cannot be discerned. Furthermore, prospective studies investigating the longitudinal effects of food insecurity on children's well-being cannot be conducted given the ethical issues involved. However, our results do not indicate growth failure in food-insecure children, and it is unlikely that food-insecure children are chronically not meeting their energy needs. Nonetheless, food-insecure children may experience periodic food restrictions that may cause nutrient deficiencies and psychological distress. Children whose mothers report inadequate food supplies are more anxious and aggressive, more likely to steal (45), and more likely to be absent from school (46) than are children with adequate food. Moreover, household food insecurity has been associated with mothers' reports of anger, feelings of being an inadequate parent, and fear of losing custody of their children (47). Children's responses to and understanding of these disrupted family dynamics have not been systematically examined. Additional research aimed at understanding how low-income Hispanic families manage limited food resources is needed to develop programs that help alleviate short-term, acute food shortages. Moreover, research that compares food-assistance programs with or without community-based interventions designed to provide support and education for food-insecure families is needed. These data may guide the development of nutrition interventions that augment the food-assistance programs already available to low-income families.
ACKNOWLEDGMENTS
We thank Rosa Gill, Jennifer Styles, and Kara Hanson for their invaluable assistance in recruiting the sample and collecting and analyzing the data.
REFERENCES