This study sought to examine the contribution of diet and physical activity patterns on the metabolic syndrome and its components in children. A greater intake of energy from carbohydrate was related to a greater waist circumference and higher concentrations of triglyceride particularly among AA. Fat intake was associated with a lower waist circumference and with lower concentrations of triglyceride in EA and AA, but not HA. Total physical activity was associated with few improvements in metabolic syndrome risk factors. The results of this study indicate that diet composition was more closely related to the components of the metabolic syndrome than was physical activity, with dietary fat inversely associated with reduced metabolic syndrome components and carbohydrate intake being adversely related to waist circumference, triglycerides, and glucose.
The identification of nearly 9% of children meeting the criteria for the metabolic syndrome is particularly alarming, since these children were “healthy” volunteers. This study adds to the literature indicating adiposity and distribution of fat may yield the greatest influence in predicting risk for future progression of disease. Of those meeting the criteria for the metabolic syndrome, 74% exceeded the 90th
percentile cut-off (28
) for waist circumference. HA, who as a group had the greatest total adiposity, were most likely to meet the criteria of the metabolic syndrome. It is also important to note that lower incidence of metabolic syndrome in AA could be attributed to lower triglycerides and higher HDL-C concentration, as has been previously reported in adults (29
), and that, according to these results, this is evident even at early stages of development. Special attention is needed relating to the consequences of categorization of individuals as having the metabolic syndrome in the prevention of metabolic conditions in children of diverse racial/ethnic backgrounds.
Diet composition and physical inactivity may exacerbate genetic or physiologic differences between racial/ethnic groups in regards to metabolic outcomes even among healthy youth. Both diet and physical activity have been suggested as the first line of intervention for the metabolic syndrome and its components (6
). However, there were few differences between ethnic groups in dietary intake or engagement in physical activities in this sample. In addition, the macronutrient profile observed in this sample of children is quite similar to what has been reported nationally (8
) and is in line with current USDA recommendations (7
); yet the quality of the diet in this sample and that reported nationally (e.g. 40% of energy from simple carbohydrates) likely intensifies the increases in the obesity epidemic. Due to a wide variance in the individual nutrients (e.g. sugar, saturated fat), significant associations were not established. However, a trend towards a positive relationship between a higher intake of simple sugars and waist circumference was noted in the total sample (data not shown) and more robust dietary measures (e.g. increased number of 24-hour recalls) may confirm the relationship between multiple components of the diet and metabolic syndrome components. Physical activity patterns were also similar to those observed in nationally representative samples of children (8
), and likewise fall short of recommendations (8
). The data supports that in this sample, dietary intake had a greater influence on the individual components of the metabolic syndrome than did physical activity.
In this sample, dietary components influenced many of the individual components of metabolic syndrome. The most recent recommendations of the International Diabetes Federation have suggested that waist circumference is the key component of the metabolic syndrome in children (33
). Higher carbohydrate intake was associated with a greater waist circumference whereas higher fat intake (and thus lower carbohydrate intake) was associated with a lower waist circumference and lower triglycerides. Relationships among diet and metabolic syndrome outcomes were stronger among AA. As no difference in fat or carbohydrate intake was noted between the racial/ethnic groups, a positive association primarily in AA may indicate a greater physiological sensitivity to the diet. An augmented response to carbohydrate (e.g. a glucose challenge measured by an intravenous tolerance test) has been previously documented (34
). Inherent differences in physiology and metabolism may alter the pathways associated with metabolism uniquely among AA. It ahs been previously demonstrated that AA compared to EA have 42% lower insulin sensitivity and 135% higher AIRg, even after adjusting for body composition (36
). The physiological relevance of differences in the pathways of carbohydrate metabolism and the potential to translate these differences into increased risks for obesity, CVD, and T2D later in life has not been clearly established.
The inverse relationship between dietary fat and triglyceride has received recent attention. Recent reports suggest that high carbohydrate (and therefore low fat) diets elicit a reduction in fatty acid oxidation and result in greater triglyceride concentration than a an isocaloric low carbohydrate (high fat) diet (37
). However, this finding did not carry across groups. The inverse relationship between dietary fat and triglycerides and the positive relationship between carbohydrate intake and triglycerides in EA and AA, but not HA may suggest the hypertriglyceridemia noted among HA may be attributable to a factor other than diet, plausibly increased amounts of adiposity. Previous studies have demonstrated a relationship between triglycerides and dietary intake (24
). The lack of association in HA is puzzling and might suggest differential mechanisms mediating the relationship of dietary fat and triglycerides in this population. As such, a reduction in adiposity would likely improve metabolic and cardiovascular risks (6
Blood pressure was not associated with any measure of diet or physical activity. Higher rates of elevated blood pressure particularly among AA may be attributable to genetic predisposition. Although other factors (social factors, SES, adiposity, etc) contribute to increased blood pressure, research has demonstrated that AA have greater systolic and diastolic blood pressure than EA and HA and these differences exist after controlling for age, BMI, diet, physical activity, social structure, and SES (42
Racial/ethnic differences in substrate utilization during bouts of exercise have been observed in adults (43
). Total daily physical activity was positively associated with greater HDL-C among EA; whereas moderate and hard physical activity was inversely related to triglyceride concentration in AA. Absolute HDL-C concentration was significantly lower in HA; however, when TG was included in the model, the ethnic difference disappeared, implying lower HDL-C concentrations are mediated by TG. The discrepancy between the contribution of physical activity to HDL-C in EA but not HA may be attributable to the lack of association between physical activity and TG in HA. According to these findings, the modest engagement in physical activity observed in this sample of HA offers little benefit to improving the lipid profile.
Greater fat and protein intake were inversely associated with glucose, whereas increased carbohydrate was positively associated with glucose. These relationships existed regardless of race/ethnicity. Therefore, it appears that modifications in macronutrient profile and diet quality may be successful for improving glucose metabolism in the pediatric population. Engagement in physical activity did not influence glucose concentration in this sample. On the other hand, sedentary behavior was positively with glucose concentration. This suggests that decreasing sedentary behavior may also be a means to improve glucose metabolism, especially among HA. According to these results, half of the HA children met the criteria for elevated blood glucose.
Although a clear threshold for physical activity in children has not been established, numerous recommendations indicate that 30–60 minutes of moderate to high intensity physical activity may be beneficial in reducing adiposity and improving metabolic outcomes in children (6
). Although diet was a stronger predictor of the components of the metabolic syndrome than physical activity, it is important to note that this particular sample was fairly inactive. Few participants engaged in bouts of hard and very hard activity exceeding 6 minutes per day. As such, the potential benefit of physical activity may not have been attained by this modest level of activity. It does, however, highlight the need to find ways to motivate children to increase duration and intensity of daily physical activity, especially among HA.
The strengths of this study included robust measures of body composition by DXA and physical activity by accelerometry. A limitation of this study was its cross-sectional nature preventing the establishment of cause and effect relationship; longitudinal data will be required to determine the long-term contribution of diet and physical activity to the metabolic syndrome and its components in children. Second, although steps were taken to improve reliability (i.e. multiple pass method, presence of parent), reliance on self-reported dietary intake lends way for inaccuracy. In addition, although both the dietary and physical activity patterns of this sample closely mimic national averages, the sample was relatively small and included only participants from a small geographic area limiting the generalizablity. For example, the HA children included in this study were primarily of Mexican descent and recent immigrants to the United States. Finally, the low association detected between diet and components of the metabolic syndrome in HA does not infer that the associations do not exist and it is plausible that there was not enough power to detect the associations. Although this does not appear to be the case, based on the p-values (>0.5) it is plausible that associations may be noted with a larger sample size.