Explaining the Positive Relationship Between Fourth-Grade Children's Body Mass Index and Energy Intake at School-Provided Meals (Breakfast and Lunch)

Childhood obesity has increased dramatically in the US over the last several decades.1 Data from 2008 showed more than one third of children ages 6 to 11 years had a body mass index (BMI) for age ≥85th percentile.2 Childhood obesity has been linked to adult health problems including cardiovascular disease, type 2 diabetes, and cancer.3 On each school day in the US, millions of children participate in the School Breakfast Program (SBP)4 and the National School Lunch Program (NSLP)5 which are federally assisted meal programs administered by the United States Department of Agriculture (USDA). Policy makers and researchers have expressed concern that participation in school-meal programs may contribute to childhood obesity.6–8 Research concerning the relationship of school-meal participation and childhood obesity has produced conflicting results.7 For example, Schanzenbach9 found that after 2 years of participating in school lunch, children were 2 percentage points more likely to be overweight than non-participants, whereas Hofferth and Curtin10 found no relationship between school-meal participation and childhood overweight. In analyses of data from the School Nutrition Dietary Assessment Study III (SNDA-III), Gleason and Dodd11 found no association between NSLP participation and BMI; however, those analyses did show an inverse relationship between SBP participation and BMI. Analyses by our group have suggested that it is energy intake at school-provided meals, instead of participation in school meals, that is related to obesity. Specifically, in analyses of data from a dietary-reporting validation study12 (which used direct meal observations of school-provided meals to assess the effects of retention interval on 4th grade children’s dietary recall accuracy), Baxter et al13 examined the relationship of BMI and school-meal participation as well as the relationship of BMI and energy intake at school-provided meals. Those results, published in 2010,13 showed that children’s BMI was not significantly associated with school-breakfast participation, school-lunch participation, or combined participation (both meals on the same day). However, there was a significant and positive relationship between children’s BMI and energy intake at school lunch, and there was a marginally significant and positive relationship between children’s BMI and energy intake at school breakfast.13 Aspects of school-provided meals that could influence children’s energy intake at those meals include the USDA’s “offer-versus-serve” provision, the availability of flavored milk, and trading during school meals. These are briefly discussed below. The “offer-versus-serve” provision began in elementary schools in 1981; the goals were to reduce food waste at school-provided meals and to allow children to select foods they prefer.14 Each school district decides whether to implement “offer-versus-serve.” Most districts do implement this provision; according to the SNDA-III, during the 2004–05 school year, 78% of elementary schools implemented “offer-versus-serve”.15 In school districts where “offer-versus serve” is implemented, children may decline one of the 4 required food items offered in the reimbursable breakfast and up to 2 of the 5 required food items offered in the reimbursable lunch.14 The 4 items that must be offered at breakfast are one serving of juice/fruit/vegetable; one serving of milk; and either 2 servings of grains/breads, 2 servings of meat/meat alternate, or one serving of each. The 5 items that must be offered at lunch are one serving of meat/meat alternative, one serving of grains/breads, 2 servings of vegetables/fruits, and one serving of milk.14 Depending on which items a child selects and declines, the meal components (eg, vegetable, beverage) on trays may differ from child to child. As these meal components can vary considerably in energy content, components selected when “offer-versus-serve” is implemented may impact children’s energy intake at school meals. Flavored milk is a popular choice among children; for example, data from 2005 showed that nearly 70% of children participating in the NSLP selected flavored versus unflavored milk.16 Data from 2006 showed that 32% to 36% of states required or recommended that schools offer at least 3 different types of milk (eg, 1% chocolate, skim plain) each day for breakfast and for lunch.17 Because sugars added to flavored milk (eg, chocolate, strawberry) add kilocalories to the milk,18 it is intuitive that flavored milk has the potential to impact children’s energy intake at school meals. Trading during school meals, which occurs when children give and/or receive items,19 may also influence children’s energy intake at school meals. Results from a retrospective analysis of school-lunch observations for 1st grade and 4th grade children indicated that: (1) trading varied by grade, with more 4th than 1st grade children trading and (2) most food items were traded as full servings.19 The purpose of the current analyses was to identify aspects of school meals that may contribute to the positive relationship between 4th grade children’s BMI and energy intake at school meals that was published in the 2010 article.13 It was hypothesized that the following 7 aspects of school meals would be significantly related to BMI: (1) energy content of items selected, (2) number of meal components selected, (3) number of meal components eaten, (4) amounts eaten of standardized school-meal portions, (5) percentage of energy intake from flavored milk, (6) percentage of energy intake received in trades, and (7) percentage of energy content given in trades. The hypotheses were that BMI would be positively related to each of the first 6 variables, and that BMI would be negatively related to the seventh variable.