Introduction

The prevalence of obesity in the United States has increased dramatically in the last 30 years (1). The observed increase has been reported in all age groups, including children. Even among 2- to 5-year-old children, the prevalence of overweight has increased 2-fold, from 5% in 1980 to 10.4% in 2000, with the highest prevalence (11.1% ) reported among Mexican Americans (1).

Overweight children are at increased risk for many health problems including diabetes, abnormal glucose tolerance, high blood pressure, and altered lipid profiles as well as psychosocial consequences (2, 3, 4). In addition, overweight children are at significantly increased risk for adult obesity (5).

Overweight occurs when there is an imbalance between energy intake and energy expenditure (6). In the United States, dietary intake patterns of children have changed dramatically since the 1970s, with increased consumption of energy-dense foods. Today, the daily food intake of children is excessively high in added sugar and discretionary fat, accounting for 40% of total energy intake (7). Sugar-sweetened beverages such as soft drinks and fruit drinks are the primary source of added sugar in a child's daily diet (8). In fact, total energy intake of 2- to 18-year-old children was 10% greater among children who consumed soft drinks than those who did not (9).

The rate of sugar-sweetened soft drink consumption by children has increased in parallel with child obesity in the past 30 years (10). In 1994, although total beverages contributed 20% to 24% of energy intake across all age groups, soft drinks provided an increasing percentage of energy intake with age, from 2.4% among 2- to 5-year-old children to 7.9% among adolescents (11). Furthermore, soft drinks contributed a higher proportion of energy for overweight than for non-overweight children. Even among 2- to 5-year-old children, soft drinks contributed 3.1% of energy intake in overweight children compared with 2.4% among non-overweight children (11).

A number of studies have examined the relationship of sugar-sweetened beverages to child overweight, but none have reported the relationship of soda alone. A school-based prospective study of 548 middle-school-age, ethnically diverse children found a 60% increased risk for obesity with each additional serving of sugar-sweetened drink (soda, fruit drink, iced tea) per day over a 19-month period [ adjusted odds ratio (OR),¹ 1.6; 95% confidence interval (CI), 1.1 to 2.2] (12). Using data from the Growing Up Today Study, a population-based prospective study, consumption of sugar-added beverages (soda, sweetened iced tea, non-carbonated fruit drinks) over a 1-year period was associated with a small increase in BMI among 9- to 14-year-old children (13). A retrospective cohort study of 10,904 primarily white children, 2 to 3 years old, reported consumption of naturally- and sugar-sweetened beverages, including vitamin C-containing juices, other juices, fruit drinks, and sodas, increased the risk for overweight 1 year later (14). Regardless of overweight status at baseline, the risk for overweight was increased approximately 2-fold with consumption of one or more sweetened beverages per day. Only one small study has examined the relation of sugar-sweetened beverage consumption with prevalence of overweight in Mexican-American children (15). Based on responses from 37 kindergarten-age participants, Mexican-American overweight children were significantly more likely than non-overweight children to consume sugar-sweetened beverages on a daily basis.

In the present study, we investigated the cross-sectional relationship of soda consumption and other dietary and physical activity factors with overweight status in 2-year-old children from low-income Mexican-American families who are participants in a birth cohort study in the Salinas Valley, CA.

Research Methods and Procedures

Sample

The Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) study is a longitudinal birth cohort study of the effects of pesticide and other environmental exposures on the health of pregnant women and their children living in the Salinas Valley, an agricultural region in California. Pregnant women were recruited between October 1999 and October 2000 through six prenatal clinics that serve a predominantly low-income, Spanish-speaking population. Eligible women were 18 years or older, <20 weeks gestation at enrollment, English or Spanish speaking, Medi-Cal eligible, and planning to deliver at the county hospital.

Of 601 women who were initially enrolled, 526 were followed through delivery of a live birth that survived the neonatal period, and 409 children completed the follow-up at 2 years of age. For this analysis, we excluded children who were preterm (n = 30) or very low birth weight (n = 1), missing 24-month height data (n = 23), or <23 months old at follow-up (n = 1). The final sample included 354 children. The study was approved by the Institutional Review Boards at participating institutions, and written informed consent was obtained from all mothers.

Procedure

Details of the study are presented elsewhere (16). Briefly, after informed consent was obtained, women were interviewed in English or Spanish by trained, bilingual, bicultural interviewers twice during pregnancy (13 and 26 weeks gestation), shortly after delivery, and when their children were 6, 12, and 24 months of age. During each interview, information was collected about family sociodemographic characteristics, household member work histories, maternal characteristics and personal habits, pregnancy and medical histories, child-based developmental milestones, and diet and behavioral information. The diet information was comprised of a 20-item modified food frequency questionnaire of child's intake of a variety of beverages, meats and other protein sources, fruits, and vegetables. Diet items were selected based on the potential for exposure to pesticides (the primary purpose of the study).

At the 24-month follow-up, standing height (centimeters) and weight (grams) were measured. Barefoot standing height was measured three consecutive times (to the nearest 1 mm) using a standard measuring board, and the three measurements were averaged. Standing weight was measured with clothing and diapers removed (to the nearest 0.1 kg) using a pediatric digital scale. In addition, medical records from prenatal visits, delivery, and well-baby checkups were abstracted by a registered nurse.

Exposure Measure

Soda consumption was the primary independent variable of interest. At the 24-month interview, mothers were asked "In the past 7 days, how often did your child drink sodas, like Coca Cola or Sprite?" and given the choices: never, 1 to 2 per week, 3 to 4 per week, 5 to 6 per week, 1 per day, 2 per day, or 3+ per day. We calculated average soda consumption per day for each child by assigning the midpoint of the category for each response.

Additional dietary variables of interest included fast food consumption, sweets consumption, and breast-feeding history. At the 24-month interview, mothers were asked, "In the past 7 days, how often did your child eat food from a fast food restaurant like McDonalds, Burger King, or Kentucky Fried Chicken?" and "In the past 7 days, how often did your child eat sweets like candy, cookies, cake, ice cream, or pan dulce?" For both questions, mothers were given the choices: never, 1 to 2 per week, 3 to 4 per week, 5 to 6 per week, 1 per day, 2 per day, or 3+ per day. For both variables, we then calculated average consumption per day for each child by assigning the midpoint of the category for each response. At the 6-, 12-, and 24-month follow-up interviews, mothers were asked whether they were still breast-feeding and, if not, how old their baby was when they completely stopped breast-feeding. From these questions, we created a variable for total duration (in months) of breast-feeding.

Physical activity (or inactivity) variables of interest were based on the following questions administered at the 24-month interview, "In the past 7 days, how many hours per day, on average, has your child spent playing outside?" and "On average, how many hours per day does your child spend watching television or videos?"

Outcome Definition

We calculated BMI at 2 years as weight in kilograms divided by height in meters squared. We then calculated age- and sex-specific BMI percentiles for each child using 2000 Centers for Disease Control and Prevention growth charts (17). Overweight was defined as at or above the 95th percentile of the sex-specific BMI for each child's age. At risk for overweight was defined as at or above the 85th percentile but less than the 95th percentile of sex-specific BMI for age.

Statistical Analyses

Statistical analyses were performed using Stata 8.0 (Stata Corporation, College Station, TX). Soda consumption and the other dietary variables (time breastfed, fast food, sweets) were analyzed both as continuous and as categorical variables. Measures of physical activity (time spent playing outside, watching television) were also analyzed both as categorical and continuous variables.

We used logistic regression modeling to examine the cross-sectional relation of soda consumption and other dietary and physical activity variables and overweight status. For all regressions, we report non-parametric (Huber, sandwich) standard errors, which are valid even when conventional assumptions for regressions are violated (18). We report the OR and 95% CI as the measure of effect.

For the multivariate logistic regression analysis of soda consumption and overweight status, we examined the effect of potential confounding variables identified in the child obesity literature (19). In addition to the dietary and physical activity variables described above, potential confounders considered included family socioeconomic status (poverty level, >poverty level); language spoken in the home (mostly Spanish, English/Spanish equally, mostly English); maternal education level (6th grade, 7th to 12th grade, high school); maternal parity (0, 1, 2+); maternal marital status (not married, married, or living as married); number of years mother had lived in the U.S. at the time of pregnancy (1, 2 to 5, 6 to 10, 11+); maternal country of birth (U.S., Mexico, other); maternal age (years); maternal pre-pregnancy BMI (report at initial interview); maternal BMI at 24 months (measured); child's age at 24-month interview (months); child's birth weight (grams, abstracted from medical record); child's sex; child's age at developmental milestones including standing, walking, and running (maternal report); and child's intake of various foods and beverages (maternal report). Additionally, the Home Observation for Measurement of the Environment score at 12 months, a measure of the home learning environment and parental responsiveness, was also included as a covariate. Covariates were kept in the model if they were statistically significant (p < 0.15). We reran the final model including children who were at risk for overweight (85th percentile to <95th percentile for age-specific BMI) in the overweight group.

Results

Table 1 presents maternal and child characteristics of the CHAMACOS birth cohort by overweight status at 2 years of age. The majority of mothers were Mexican-born (87.6% ), Spanish-speaking (94% ) women who were married or living as married (81.9% ), had not completed high school (81.6% ), and were living in poverty (58.2% ). At the time of the pregnancy, mothers were an average of 26.0 (standard deviation 5.2) years old. Before pregnancy, 64.4% of mothers were overweight or obese [ mean BMI = 27.4 (5.3) kg/m²] ; by the 24-month follow-up, this number had increased to 80.5% [ mean BMI = 29.6 (5.6) kg/m²] .

Table 1. - Maternal and child characteristics by overweight status at 2 years of age, CHAMACOS (Salinas, CA), 2002 to 2003.

Full table

At birth, children weighed an average of 3508 ( 448) grams, about one-third were first born, and 51.1% were girls. At the 24-month follow-up, children were, on average, 24.6 (1.0) months old.

At the 24-month follow-up, 55 (15.5% ) of the 354 children were classified as overweight (95th percentile for age-specific BMI), and 51 (14.4% ) were at risk for overweight (85th percentile and <95th percentile for age-specific BMI). Thus, a total of 29.9% of children were overweight or at risk for overweight at 2 years of age. As presented in Table 1, overweight children were more likely to have an obese mother at pre-pregnancy (p = 0.001) and 24-month follow-up (p < 0.01). There was no difference in overweight status of children by maternal sociodemographic indicators, including mother's country of birth, years lived in the U.S., education, poverty, marital status, or child characteristics such as birth weight, gender, or age at 24-month follow-up.

Soda consumption and other dietary and physical activity characteristics of the children are presented in Table 2. The majority of children (56% ) consumed soda, with an average consumption of 0.5 (0.5; range, 0.2 to 3) sodas/d among consumers (data not shown). Most children ate fast food at least once per week (60% ), and almost half (44% ) of the children consumed one or more sweets per day. Almost all mothers (96% ) initiated breast-feeding, and the median length of breast-feeding was 6 months. On average, children played outside 2.1 (0.9) h/d and watched television 2.6 (1.1) h/d. Only 10% of children watched television <2 h/d, as recommended by the American Academy of Pediatrics (20).

Table 2. - Dietary and physical activity characteristics and crude odds ratios and 95% confidence intervals for overweight status at 2 years of age, CHAMACOS (Salinas, CA), 2002 to 2003.

Full table

Descriptive statistics of children who drank soda are presented below (complete data not shown). Children who drank soda were of higher birth weight (p = 0.05). They were also older (p < 0.01), consumed more fast food (p = 0.005) and more sweets (p = 0.01), and watched more television per day (p = 0.004). Children who drank soda consumed less 100% juice (p = 0.06), but there was no difference in consumption of other foods including milk (p = 0.89), fruits (p = 0.40), or vegetables (p = 0.59). Mothers of children who reported drinking soda in the last week had a higher BMI both at pre-pregnancy (p = 0.001) and at the 24-month interview (p = 0.01) and reported living in the U.S. >5 years (p = 0.10).

The crude OR and 95% CI for overweight status by dietary and physical activity characteristics are presented in Table 2. In crude analysis, soda consumption and fast food intake, but not sweets intake, were positively associated with overweight status. Children who breastfed longer were less likely to be overweight than children who had breastfed 0 to 2 months, but not significantly. Overweight status, however, was not significantly related to any of the physical activity characteristics including hours of outside play or television watching.

The results of adjusted logistic regression analysis are presented in Table 3. Soda consumption remained significantly positively related to overweight status in adjusted analysis. After adjusting for fast food, breast-feeding, and maternal pre-pregnancy BMI, <1 soda/d compared with no soda consumption was not related to overweight status (adjusted OR, 0.97; 95% CI, 0.47, 1.99), but 1 soda/d was significantly associated with a 3-fold increased odds of overweight (adjusted OR, 3.39; 95% CI, 1.43, 8.07). The test for trend was significant (p = 0.02).

Table 3. - Adjusted odds ratios (95% confidence interval) for overweight status at 2 years of age, CHAMACOS (Salinas, CA), 2002 to 2003.

Full table

As presented in Table 3, in the adjusted analysis, fast food intake was no longer significantly associated with overweight. Compared with no fast food consumption, 1 to 2 fast food meals/wk was associated with a non-significant increased odds of overweight (adjusted OR, 1.54; 95% CI, 0.77, 3.10), and 3 fast food meals/wk was associated with a non-significant 4-fold increased odds of overweight (adjusted OR, 4.08; 95% CI, 0.93, 17.94). The test for trend, however, was significant (p = 0.02).

Breast-feeding and maternal pre-pregnancy BMI remained independently associated with child's overweight status in the final multivariate model (see Table 3). The adjusted OR for overweight per month of breast-feeding was 0.96 (95% CI, 0.92, 1.00). Higher maternal pre-pregnancy BMI was significantly related to overweight status of the child at 2 years. Compared with normal/underweight maternal pre-pregnancy BMI, overweight maternal pre-pregnancy BMI was associated with a >2-fold increase in odds of overweight (adjusted OR, 2.27; 95% CI, 0.99, 5.22), and obese maternal pre-pregnancy BMI was associated with a >5-fold increase in odds of overweight (adjusted OR, 5.14; 95% CI, 2.13, 12.38).

We repeated the final model including the covariates that had been found to be related to soda consumption (age at follow-up, birth year, birth weight, time spent watching television, consumption of sweets, consumption of 100% juice, and living in the U.S. >5 years), and the results were not different (data not shown). When we reran the final model including children who were at risk for overweight in the overweight group, the results for soda consumption were similar, yet diminished. Compared with no soda consumption, consumption of 1 soda/d was still significantly associated with increased odds of overweight (adjusted OR, 2.49; 95% CI, 1.17, 5.32).

Discussion

At 2 years of age, we found the prevalence of overweight in the CHAMACOS birth cohort of 354 Mexican-American children living in an agricultural community was 15.5% . This prevalence is higher than that (11.1% ) reported for the 259 Mexican-American 2- to 5-year-old children in U.S. National Health and Nutrition Examination Study, 1999 to 2000, which is already considerably higher than that reported for other ethnic groups (non-Hispanic whites, 10.1% ; non-Hispanic blacks, 8.4% ) (1). With more than half of the 2-year-old children consuming soda and 12% consuming one or more per day, our findings suggest soda consumption could be an important contributory factor to the obesity epidemic, even in children as young as 2 years of age. Compared with children who consumed no soda, the odds of being overweight increased more than 3-fold among children who consumed at least 1 soda/d.

Other studies indicate that high soda consumption in children as young as 2 years is not limited to Mexican-American populations. In a study of white and African-American children, 51% of preschool children consumed any soda, and 12% consumed 9 ounces of soda/d (9). Harnack et al. (9) reported soda consumption differed significantly by race, and white children were 2 times more likely than African-American children to consume soda. The findings of this study suggest that soda consumption patterns of Mexican Americans may be more similar to whites.

Our finding of an increased risk for overweight with higher soda consumption is consistent with previous studies of sweetened beverages including soda in school-aged (12, 13) and preschool-aged (9, 14) children. An advantage of the present study over the previous study of preschool-aged children is that we were able to control for other important confounding factors, including maternal BMI and breast-feeding (14). Our findings are also consistent with the small, but only other, study in young Mexican-American children, which found an association between sweetened beverage consumption and overweight status (15).

Others have suggested that consumption of soft drinks may also displace more nutritious beverage choices, particularly milk (9, 10). An inverse relationship between soda consumption and healthier options including milk and 100% juice has been reported among preschool-age children (9). In the current study, children who drank soda consumed similar amounts of milk but less 100% juice than children who did not drink soda.

Although we asked about consumption of healthier beverages including milk and 100% juice, a limitation of this study is that we did not ask about consumption of fruit drinks (non-100% juice drinks). Thus, we cannot examine the relation of all sugar-sweetened beverages with overweight status in this study. In this young age group, fruit drinks may represent a significant proportion of total sweetened beverage intake. Among preschool-age children, Welsh et al. (14) reported daily consumption of fruit drinks averaged 0.7 drinks/d, whereas soda averaged 0.3 drinks/d. The impact on our estimate of risk for overweight from soda consumption by not measuring fruit drink consumption could go in either direction. If non-consumers of soda consumed more fruit drinks instead, we may have underestimated the risk due to soda. If soda consumers were also more likely to consume fruit drinks, then we may have overestimated the risk due to soda. Another limitation of this study is that we did not specifically exclude diet soda consumption in the food frequency question about consumption of soda. If consumers of diet soda were misclassified as soda consumers, we may have underestimated the risk for overweight from soda consumption in this population.

Although the cross-sectional nature of the data limit interpretation of whether soda consumption preceded overweight status, the observed association is biologically plausible. It is believed that sugar-sweetened beverages such as soft drinks promote energy intake and excessive weight gain because compensation for increased calories consumed in liquid form is less complete than for calories consumed in solid form (12, 21). That is, children who drink sugar-sweetened beverages do not adequately reduce their intake of other foods and beverages, leading to increased caloric intake and subsequent weight gain.

Our findings also suggest that fast food consumption is related to overweight status as young as 2 years of age. To our knowledge, no study has examined the relation of fast food consumption with overweight status of very young children. Since the 1970s, fast food consumption by children in the United States has increased, with 30% of children reporting consuming fast food on a given day (22). Children who eat fast food have been shown to consume more total energy, more total fat, more total carbohydrates, more added sugar, more sugar-sweetened beverages, less fiber, less milk, and fewer fruits and vegetables (22). We found that children who ate any fast food meals consumed significantly more soda (mean = 0.35 per day) than children who did not eat fast food meals (mean = 0.20 per day) (p < 0.05).

Consistent with the growing body of literature suggesting that breastfeeding is protective against child overweight (23, 24), we found a small but significantly decreased risk for overweight with breastfeeding compared with children who were never breastfed. In this Mexican-American population, 96% of the mothers initiated breastfeeding, which is much higher than national estimates of between 46% and 69% (25). Continued breastfeeding in this population should be encouraged.

Inconsistent with previous studies in school-aged children (26, 27, 28, 29, 30), we did not find an association between number of hours children watched television and overweight status. However, the children in this study did watch a lot of television (average, 2.6 h/d), and 90% of children exceeded the recommendation of the American Academy of Pediatrics to watch <2 h/d (20). A recent study of 3- to 6-year-old children suggested that 6 years was a critical age when television may have a significant affect on BMI (31).

In summary, the results of this study highlight the high prevalence of overweight among children of low-income Mexican-American families living in an agricultural community in the United States. We found that the prevalence of overweight at 2 years of age among the CHAMACOS cohort is higher than the national prevalence estimate for Mexican-American 2- to 5-year-old children (1) and that this high rate is significantly related to current soda consumption. Because the CHAMACOS study is a longitudinal study, it will be important to continue to monitor the overweight status and examine the role of total sweetened beverage consumption and physical inactivity, such as television and other media use, on overweight status as the cohort ages. Our findings suggest that interventions designed to reduce consumption of soda in preschool-aged Mexican-American children could be instrumental in reducing childhood obesity in this population. Interventions may need to consider cultural perceptions that the availability of fast foods and soft drinks is a manifestation of improved economic circumstances and even affluence (32).

Notes

¹ Nonstandard abbreviations: OR, odds ratio; CI, confidence interval; CHAMACOS, Center for the Health Assessment of Mothers and Children of Salinas.

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Acknowledgments

This work was supported by the U.S. Environmental Protection Agency (Grant RD 8317100), by the National Institute of Environmental Health Sciences (Grant PO1 ES009605), and by the National Institute of Occupational Safety and Health (Grant RO1 OH007400). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Institute of Environmental Health Sciences, National Institute for Occupational Safety and Health, NIH, or Environmental Protection Agency. We gratefully acknowledge the CHAMACOS staff, students, community partners, and, especially, the CHAMACOS participants and their families, without whom this study would not be possible.