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Original Research |
Nutrition Department (B.R.C., J.D.S., K.S.H.), University of Tennessee, Knoxville, Tennessee
Child and Family Studies Department (J.D.M.), University of Tennessee, Knoxville, Tennessee
College of Human Ecology, University of Tennessee, Knoxville, Tennessee
Address reprint requests to: Betty Ruth Carruth, PhD, RD, Nutrition Department, University of Tennessee, Knoxville, TN 37996-1900.
| ABSTRACT |
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Methods: Length (cm/month) and weight (kg/month) of white infants (n = 94) were measured five to nine times from 2 to 24 months of age. Mothers reported birth weights, infants ages at first introduction of supplementary food, illnesses and information sources about infant feeding. Simple linear regression equations were used to compute slopes for each child (unit changes in length and in weight by age). Stepwise linear regression was used to determine the effect on weight and length slopes by the introduction of supplementary foods (e.g., an infants age when cereal, fruit, juice, vegetables and a meat cluster were first added) to the diet. Breast feeding (months duration or ever fed), illness scores and gender were covariates in the regression models.
Results: A significant model (F = 10.09, p = .002) for weight gain (2 to 8 months) showed that gender explained 10% of the variance; for length slope, the model was non-significant and gender explained 3% of the variance. Females had a slower weight gain compared to that of males. None of the covariates or supplementary foods were retained in the models. Weight prior to 12 months was the best predictor (p = .0001, 54% of the variance) of weight gain 12 to 24 months.
Conclusions: Unit changes in weight or length for an infants age were not statistically associated with the timing of when supplementary foods were first added to the diet 2 to 8 or 12 to 24 months. Weight prior to 12 months was a significant predictor of weight gain 12 to 24 months.
Key words: infant weight, infant length, infant growth, infants diets, supplementary foods
| INTRODUCTION |
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Current infant feeding recommendations by the American Academy of Pediatrics include exclusive breast feeding for approximately the first six months after birth, and, in general, that water, juice and other supplemental foods are unnecessary in the first six months [2]. However, infant feeding studies have shown that the timing for introducing supplementary foods reflects cultural practices [35], characteristics of the mother [68], advice of physicians [911], a dietary enhancement to promote the infants sleeping through the night [1214] and factors influencing growth patterns during the first year of life [15,16]. Findings of several investigators indicate that some mothers add supplementary foods earlier than the recommended four to six months [3,10,17,18].
Heinig et al. examined mode of feeding, growth indices and addition of complementary foods after infants were four months of age and found no relationship between age of adding foods and growth [15]. In a study of infant age at time of introducing supplementary foods and age of the mother, adolescent mothers added cereal significantly earlier (p
.0001), but there was no significant difference by age of mother for the timing of adding other food groups [10]. Mehta et al. [19] conducted a prospective study about introducing solid foods early (3 to 4 months) and later (6 months) and the use of commercial foods versus parents choices of solid foods for their infant. There were no significant differences in body composition or anthropometric measurements associated with earlier or later introduction of foods or with commercial foods versus parents choice of solid foods. In a study by de Bruin et al., a comparison of energy utilization and growth in exclusively breast-fed and formula-fed infants (
4 months) showed no significant difference by mode of feeding with respect to length, weight and head circumference of infants in the first year of life [20].
In developing countries, retarded infant growth in the first year of life has been associated with decreased breast milk production and the lack of hygienic complementary foods that have adequate energy and nutrient densities to promote normal growth [21,22]. In two intervention studies of Honduran and Ghanaian infants, respectively, where nutritionally adequate and hygienic complementary foods were added to the diet (6 to 12 months of age), infants had improved growth [23] or grew similarly to exclusively breast fed infants in the first year of life [24]. In developed countries, the early addition of appropriate supplementary foods has resulted in normal growth patterns [11,14]. The literature also indicates that most mothers from disadvantaged populations add supplementary foods earlier than four months [10,22,25].
The addition of supplementary foods, irrespective of breast and formula feeding modes [20,26], may or may not have an effect on infant growth when other influencing factors are controlled, such as the familys socioeconomic status, perinatal events, low birth weight, energy and nutrient intake and desirable health status of the infant prior to and following the addition of supplementary foods.
The purpose of this study was to determine the effect of adding categories of supplementary foods on the growth of full-term, healthy infants for the period two to eight months of age, (i.e., the age range within which supplementary foods were first added to infants diets) and from 12 to 24 months of age. In addition, the relationships were determined among growth indices, illness scores, breast feeding and age of the infants when supplementary foods were first added to the diet.
| METHODS |
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Interview schedules were established using an incomplete block design (IBD) with randomized assignment of mothers at each interview period (2, 3, 4, 6, 8, 10, 12, 16, 20 and 24 months). Because of the IBD, the number of times anthropometric measurements were obtained for any one infant ranged from three to five times in the 2-to-10 month and two to four times in the 12-to-24 month period. Use of the IBD maintained the statistical power of the total group (n = 94) without the expense and effort of all subjects being interviewed at each time period.
At each interview, mothers reported infants food intake (24 hour recall), including breast, formula or a combination of the two feeding modes. The energy/nutrient intakes from breast milk, formula and supplementary foods have been reported for 2 to 24 months [11]. Mothers reported the infants ages when cereal, juice, fruits, vegetables, meat, mixed food and table foods were first added to the diet.
Data were collected at each interview about the childrens current medical care, number of visits to the physicians office, number of illnesses by type and medication regimen prescribed, and days of hospitalization. Interviewers probed for indicators of health status and health care. Medical records were reviewed retrospectively to identify any pregnancy and neonatal health problems that could affect food intake and growth from birth to 24 months. Based on these reviews, all infants were eligible for the study. At each interview, mothers also reported sources of information about infant feeding practices, and a standard protocol was used to measure weight and recumbent length for the 2-to-24 month period [27,28]. The major advantage of longitudinal data (compared to cross sectional studies) is that intra-individual variability is reduced. Over time, the study group at different ages has the same genetics.
Analyses
Interview assignments, data preparation and analyses were performed using the SAS System for Statistical Analysis [29]. A simple linear regression model was used to obtain a measure of growth for each infant using weight (kg/month) and length (cm/month) as dependent variables and age of the infant (month) from birth to eight months as the independent variable. This methodology produced a slope (a regression coefficient) for weight or length regressed on age of the infant when measurements were made that represented the rate of change in weight and in length relative to a unit change in age. A similar procedure was used to compute weight and length slopes for individual infants from birth to 12 and from 12 to 24 months.
Prior to the stepwise multiple linear regression analyses, correlation coefficients between changes in weight and in length and potential influencing factors were computed. Variables with p = <0.15 were retained for further analyses.
Stepwise multiple linear regression was used to determine the relationship between length and weight slopes (dependent variables) and the age of infants when supplementary foods were first added to the diet. The independent variables included the infants age (to the nearest 0.25 months) when cereal, juice, fruit and vegetables were each first added to the diet. For meats, mixed foods and table foods, a food cluster was formed because all of these foods were rarely introduced to the diet before the infants were eight months of age. The food cluster was included as a dichotomus variable of zero or one indicating that the foods were absent or present for the two to eight months. The variable was used in the regression modeling as an independent variable along with cereal, juice, fruit and vegetables. Gender, illness scores and breast feeding (duration in months and ever breast fed) were included as covariates in the regression modeling to control for factors that could influence changes in weight and length (birth to eight and 12 to 24 months).
Stepwise multiple linear regression was also used to determine the relationship between unit changes in the weight slope relative to those in the length slope and the infants age when supplementary foods were first added to the diet. In these analyses, length slope, gender, illness scores and breast feeding were independent variables used in the regression modeling for the period of two to eight months when all mothers had first added supplementary foods. Preliminary analyses indicated that males were longer than females, and the slope from the regression of length on age of the infant was considered a control variable for changes in size. Although growth is not linear, a curvilinear function has linear components within restricted ranges of time. Hence, a linear function is adequate to model a portion of a curvilinear function for the period from two to eight months when supplementary foods were added and for 12 to 24 months.
Epi Info (Version 6.03, 1995) was used to compute individual Z-scores by age (months) and gender for recumbent length (cm), weight (kg), and weight-for-length (kg/cm) [30]. Group mean Z-scores were compared to normalized NCHS/CDC reference weight (kg), length (cm), and weight-for-length Z-scores.
Frequency information about illnesses, medications and hospitalizations was summarized by frequency counts. Illness categories that either increased caloric needs and/or influenced food intake were assigned points (pts) and weighted according to severity. Skin infection/rashes/thrush received 1 point (1 pt), cold = 2 pts, ear/eye infections = 3 pts, respiratory/allergies = 4 pts, gastrointestinal problems/colic/diarrhea = 5 pts and an immobilizing injury that influenced nutritional status and food intake = 6 pts. For each day of hospitalization, one point was added to give a total illness score for each child. This weighting was suggested by medical consultants to the study who ranked the severity of the illnesses.
When this study was initiated, iron-fortified cereals were recommended as a first food and could be given by four months of age; however, adding cereal earlier than recommended can result in a higher incidence of illnesses [22]. Therefore, two groups of infants were formed based on when cereal was added prior to
4 or >4 months, and a significant difference in illness scores was determined using a 2-tailed independent sample t test. Mothers sources of information about infant feeding were tabulated by primary and secondary sources, i.e., physicians compared to relatives, media, books.
| RESULTS |
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As shown in Table 1, results indicate that 13 mothers initially added supplementary foods at 2 months, 20 mothers at 3 months, and 27 mothers at 4 months. These data represent mothers newly adding foods at each time period and the number does not include those who started foods at an earlier time period. Except for juice, these are the amounts of food given by spoon and do not reflect foods that mothers may have provided by bottle. Of 94 mothers, 60 added solid foods by four months and another eight were feeding cereal via bottles.
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Results of the stepwise regression modeling for the 12-to-24 month period are not shown in table format because the most significant predictor of weight gain (adjusted for length) was the infants pre-existing weight (F = 109.76, p = .0001, 57% of the variability). Infants who weighed more in the period prior to 12 months continued to follow a similar growth pattern of weight gain relative to length in the second year of life, irrespective of gender. The age that supplementary foods were added to the infants diet, breast feeding and illness scores were not significant predictors of relative weight gain during the 12-to-24 month period. The addition of cereal
4 or >4 months was not significantly related to illness scores from 2 to 24 months.
Weight-for-age (WA), length-for-age (LA), and weight-for-length-for-age (WL) are shown in Fig. 1, Fig. 2 and Fig. 3. For 2 to 24 months, mean Z-scores of WA, LA, and WL are within 1.0 to 1.5 standard deviations of the reference population, indicating normal growth relative to their age and gender cohort. Mean Z-scores at birth show that the infants WL was greater than that of the reference population (Fig. 1). These results are consistent with characteristics of mothers in our study, i.e., upper socioeconomic status, educated, and access to health care [32].
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3 days). The type and frequency of gastro-intestinal illness reported in the first year of life and the addition of cereal were examined. In a subgroup of 19 mothers who added cereal by two months (including cereal in bottle), four infants had initial digestive problems compared to eight of 75 infants who were introduced to cereal after four months. Two mothers reported that their physicians diagnosed food-related allergies by 24 months, i.e., milk and eggs; these infants received supplementary foods between four and five months of age. Based on illness scores over time (2 to 24 months), no significant relationship was found between age of the infant when supplementary foods were added to the diet and illness scores. Mothers were more likely to ask their physicians advice about the timing for adding supplementary foods than to ask relatives. In some cases, a health care provider suggested that cereal be added to the diet prior to four months when a mother was concerned about the infants sleeping pattern and the babys size. Since the study began, the American Academy of Pediatrics has begun to recommend exclusive breast feeding for the first six months [2].
| DISCUSSION |
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Based on results of the stepwise regression analyses, the infants age when supplementary foods were first added to the diet was not significantly associated with weight or height slopes as indicators of rate of growth during two to eight months or 12 to 24 months. Gender explained 10% of the variability in weight gain and 3% of variability in length gain for the 2-to-8 month period. All mothers had introduced supplementary foods by eight months, and the inclusion of ten-month dietary data in the analyses did not alter the findings.
In the regression analyses to predict relative weight gain (length slope in the model), infants exhibited a strong linear relationship between gains in relative weight and gains in length. Although vegetables and the food cluster were retained in this model to predict relative weight gain, they were not retained when length slope was not in the model. Thus, the results were similar to those models with weight or height slope as the dependent variable in the regression modeling. In this model, gender explained <2% of the variance in relative weight gain (28 months).
In the 12-to-24 month period, weight prior to 12 months was the major factor predicting relative weight gain. During the birth-to-eight month period, females had a slower growth rate (i.e., unit of weight change relative to change in length for age) compared to males. Similar findings were reported by Guo et al. [33]. However, in the second year of life, gender was not a predictor of gains in weight or length.
Illness scores, as defined in this study, were not associated with the age that supplementary foods were first added to the diet, and they were not a significant predictor of weight or length gains from birth to eight months or 12 to 24 months. These findings in our study may reflect the fact that mothers sought the physicians advice more often than other sources and utilized appropriate health services for their children. The continuous data collection about illnesses, medications, hospitalizations and injuries in this study group of children allowed investigators to monitor factors that could contribute to decreased food intake and/or decelerated rates of growth over the 2-to-24 month period. Foman states, "Infants who remain free from illness and receive adequate intakes of energy and essential nutrients will be able to meet their growth potential" [34].
In a pooled US, Canadian, and European data set, 42% of the total sample introduced supplementary foods between four and five months and 92% added foods by nine months of age [26]. In that study, weight for age (p = .01) and weight for length (p = .02) were significantly related to timing for the introduction of supplementary foods. Infants who received supplementary foods before four months had lower weights than those given foods at six months or later [26]. Other recent studies about growth and addition of supplementary foods to infants diets did not include mothers who introduced foods as early as mothers in this study, i.e., 27 of 94 mothers introduced food by three months. However, Heinig et al. [15] and Mehta et al. [19] reported no significant relationship between growth in the first 12 months and the timing for introduction of supplementary foods as early as four months. Results of our study indicated a negative relationship between relative weight gain and adding vegetables and the food cluster, but none of the supplemental food categories were significant when length or weight gains were used as dependent variables in the regression modeling. No published studies were found that had comparable longitudinal growth data (2 to 24 months) and reported the infants ages when categories of supplementary foods were first added to the infants diets.
The weight for length Z scores (birth to 24 months) are compatible with the overall adequate nutrient intake previously reported [11]. Zinc intake at eight and ten months and vitamin D at eight, ten, and 12 months were below the Recommended Dietary Allowances [35], and low intakes continued in the 12-to-24 month period. For example, Adequate Intake (AI) value for vitamin D is 5 µg/day for children, 0 to 3 years of age [36], and mean dietary intakes of vitamin D in this study were less than the new AI for the period 8 to 24 months. Although these are major nutrients needed for growth, mean length and weight Z-scores by age and gender of children in this study compared to the reference population do not reflect nutrient intakes so low that growth was compromised.
Of all illnesses reported in the 2-to-24 month period, otitis media occurred most frequently. Otitis media has been linked to infant feeding practices by Duncan et al. [31]. In their retrospective medical record review of infants (n = 1,013) and the incidences of otitis media, those infants exclusively breast fed for four or more months had a protective effect from acute or recurrent otitis media. Infants who received formula or foods other than breast milk had more episodes of otitis media than those exclusively breast fed. Findings in our study did not show a significant relationship between illness scores, age of infants when supplementary foods were first added or duration of breast feeding.
Families in this study were educated and had adequate socioeconomic and environmental conditions to optimize their childs growth. Because of these characteristics, results cannot be extrapolated to groups with limited education, limited access to health services or few economic resources, to pre-term and low birth weight infants or to infants who have anomalies that interfere with food intake and growth.
| CONCLUSIONS |
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Our results have a number of implications about mothers infant feeding practices and the role of health givers, particularly the physicians help in meeting the current guidelines of breast feeding for six months or longer and delaying the introduction of supplementary foods. Mothers who are comparable to those in our study may need more individualized encouragement and instruction from their primary physician about infant feeding practices. Our data clearly showed that they chose the physician as their primary source, although they received information from multiple other sources throughout the first year.
Because of the increased employment of mothers outside the home [37], this advice must include strategies about how mothers work-related roles can be accommodated and about the subtle benefits of breast feeding that are not so apparent to mothers in developed countries [22]. Those mothers with prior experience may seek less information about feeding their new babies without acknowledging individualized growth patterns in well-born infants (e.g., gender, nutritional status, potential allergen reactions). Inexperienced primipara mothers may add supplementary foods earlier than recommended, based more on cultural patterns and perceived needs rather than on physiological needs of the infant. There is consensus about broad feeding guidelines for the infant population and less consensus about how to individualize feeding modes for healthy children in developed countries. As Garza and Frongillo suggested, there are likely optimal ages for adding supplementary foods that are determined by multiple factors, rather than a single optimal age [1].
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Received February 1, 1999. Revised March 1, 2000. Accepted March 1, 2000.
| REFERENCES |
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