Relationships Between Maternal and Child Cardiovascular Risk Factors: Ethnic Differences and Lack of Influence of Physical Activity

Objectives  To obtain information about the relationship between cardiovascular and diabetic risk factors of mothers and their children and to determine whether these relationships differ by physical activity or ethnicity.

Design  Prospective study.

Setting  The Texas site of the Studies of Child Activity and Nutrition Program.

Patients  A 1986 to 1989 triethnic sample (European American, African American, and Hispanic) of 133 mothers and their 6- to 7-year-old children.

Main Outcome Measures  Maternal and child fasting insulin levels; total, high-density lipoprotein, and low-density lipoprotein cholesterol levels; triglyceride levels; waist circumference; systolic and diastolic blood pressures; height; weight; and body mass index. Maternal physical activity was assessed by means of a 7-day recall. Child physical activity was assessed by heart rate monitoring. Correlational methods were used to describe the relationships among metabolic risk factors and physical activity; χ² tests of independence were used to examine the relationships between ethnic groups.

Results  Body mass index and waist circumference were significantly (P<.05) associated among Hispanic mothers and their children, but not in other ethnic groups. Insulin, high-density lipoprotein, and low-density lipoprotein levels were significantly associated among African American mothers and their children, but not in other ethnic groups. Maternal and child physical activity were not significantly associated with any of the risk variables or each other.

Conclusion  The relationships between the risk factors of mothers and children differed by ethnicity, but not by physical activity.

Elevated blood pressure, lipid levels, and insulin levels have been associated with cardiovascular disease and type 2 diabetes mellitus among children and adults.^1-3 Since obesity has been associated with increased blood pressure, lipid levels, and insulin levels among children⁴ and adults,⁵ obesity is a likely precursor for each of these risk factors. The increasing prevalence of obesity among US children⁶ suggests that there will shortly be a surge in the prevalence of these risk factors among children, as evidenced by the recent increases in the number of children diagnosed as having type 2 diabetes mellitus.^7,8 Low levels of physical activity also have been associated with an increased prevalence of risk factors^9,10 and obesity,¹¹ and increased physical activity ameliorates risk factors in both children^12-16 and adults.¹⁷

Epidemiologic studies have reported associations between children and their parents on overall adiposity, visceral adiposity, lipid levels, and blood pressure.^18-24 Parent-child correlations could exist because of genetic, other biological, or common environmental and lifestyle influences. Limited research has suggested a familial association of physical activity patterns,^25,26 but the evidence is weak, with some nonassociations also being reported.²⁷ The physical activity of the parent, the child, or a combination of both could moderate the relationship between parental and child risk factors. Since the published studies included only infants²³ or pubescent children,^21-24 there is a need for more information about the parent-child association for risk profiles and physical activity in the period between infancy and puberty. Furthermore, as a number of these studies have included participants from the biethnic (European American and African American) Bogalusa population,^18-20 there is also a need for information about the nature of this relationship among Hispanic children and their parents, and how this relationship may differ from that of other ethnic groups. This article examines these relationships among a triethnic sample of mothers and their children obtained before the description of the metabolic syndrome.²⁸

Data were collected as part of the Texas site of the Studies of Child Activity and Nutrition (SCAN) Program, a multicenter study funded by the National Heart, Lung, and Blood Institute that examined the development of cardiovascular risk factors and associated behaviors in families of young children.²⁹ Data were collected between the summers of 1986 and 1989. Although the data were collected more than 15 years ago, there has been no major change in the measurement of these variables since that time. Furthermore, analysis of these data provides an opportunity to assess these relationships early in the recent period of increases in obesity among children. Various methods, including newspaper advertisement, fliers, and word of mouth, were used to recruit families.³⁰

One hundred thirty-three mothers and their 3- to 4-year-old children (52% female) were enrolled in year 1 and followed up for 3 years, with only 1 child per family enrolled. The clinical data from the final (fourth) clinic conducted at the end of data collection year 3 (when the children were 6 to 7 years) are presented herein. Families with immediate members who had a history of chronic illness (cardiovascular disease, hypertension, stroke, etc) or who were taking antihypertension medication were excluded. Although it was intended that data would be collected from children and both their parents, most fathers did not complete assessments, and therefore only mother and child data are reported.

Mothers’ self-reported ethnicity and household income were recorded. The sample was triethnic: 37% of participants were European Americans, 26% were Hispanic, and 37% were African American. The University of Texas Medical Branch at Galveston’s institutional review board approved the study, and written informed consent was obtained for all participants.

Height and weight were measured to the nearest centimeter and the nearest 0.1 kg with a stadiometer (CDC Prospective Enterprises Stadiometer, Atlanta, Ga) and balance-beam scale (Detecto Enterprises Inc, Roslyn, NY), and participants’ body mass index (BMI) was calculated as weight in kilograms divided by the square of height in meters. Waist circumference was recorded to the nearest 0.1 cm at the umbilicus. This process was performed 3 times and the mean of the 3 recordings was used in all analyses.

The methods used to collect and analyze the blood samples have been previously described.³¹ Briefly, participants were screened at a laboratory and were asked to report whether they were fasting or pregnant. Serum blood samples were collected only from fasting, nonpregnant participants in a sterile Vacutainer specimen tube (Becton Dickinson Vacutainer Systems, Rutherford, NJ) by means of the Lipid Research Clinics’ protocol for sample collection and storage.³² All samples were immediately chilled and spun in a centrifuge within 30 minutes of collection. The samples were analyzed to measure triglycerides, high-density lipoprotein cholesterol (HDL-C), total cholesterol, calculated low-density lipoprotein cholesterol (LDL-C), and insulin by a Centers for Disease Control and Prevention standardized laboratory with a random-access chemistry analyzer (Technicron RA-500; Miles Inc, Diagnostics Division, Tarrytown, NY). Resting systolic and diastolic blood pressure and heart rate were obtained by means of the fifth Korotkoff phase on an automated adult-pediatric vital signs monitor (Dinamap model 845 XT; Critikon Inc, Tampa, Fla). The appropriate-size cuffs were used for all participants. Participants reported to the laboratory early in the morning, having fasted overnight and abstained from exercise. After a 5-minute rest period, 5 automated measurements were taken on the right arm. The first blood pressure value was discarded and the mean of the 4 remaining measurements was used in all ensuing analyses.

On an appointment basis, a research technician arrived at each child’s home at approximately 7 AM. A telemetry heart rate monitor (Quantum XL; AMF American, Jefferson, Iowa) that was preprogrammed to record for the entire day was attached to each child’s chest and removed by a technician at approximately 7 PM. Previous analyses have shown that this procedure provides a reliable assessment of children’s physical activity.³³ Four days of monitoring were attempted during observation years 1 and 2, and 3 days were attempted during observation year 3. To obtain the most stable indication of each participant’s habitual activity patterns, aggregate data obtained during the 3-year period were used for each participant (mean ± SD, 6.6 ± 2.2 days).

Acceptable heart rate values were between 50 and 220 beats/min; the 4.0% of values outside this range were treated as outliers. A valid day consisted of at least 504 minutes (70%) of usable heart rate values. Heart rate data above a threshold were used to assess the children’s physical activity. Two thresholds were defined: (1) number of minutes above 50% of maximal heart rate reserve and (2) number of minutes the heart rate was at or above 140 beats/min. Maximal heart rate reserve was calculated as the individual’s mean resting heart rate plus 50% of the individual’s heart rate reserve. Heart rate reserve was defined as 200 less the individual’s mean resting heart rate.³⁴ The 50% maximal heart rate reserve method was selected to take into account the sex and maturational factors that can influence children’s heart rates,³⁵ while the 140-beats/min threshold was selected for ease of comparison with other studies.³⁶

Maternal physical activity was assessed by means of a previously validated 7-day retrospective directed interview during which participants were asked to recall all physical activities, including household chores and gardening, in which they engaged during the previous week along with the duration of participation.^37,38 Each activity was then converted to a metabolic equivalent intensity by means of the compendium of physical activities.³⁹ The mean number of minutes of moderate to vigorous physical activity per day, which was defined as activity greater than 3.5 metabolic equivalents, was then calculated for each mother.

Independent t tests were used to test for sex differences in children’s risk values. To describe the participants and facilitate comparison with other studies, the percentage of mothers who were overweight (BMI >25), were obese (BMI >30), or possessed large waist circumferences (>88 cm), high total cholesterol level (>200 mg/dL [>5.17 mmol/L]), low HDL-C level (<40 mg/dL [<1.03 mmol/L]), high LDL-C level (>130 mg/dL [>3.36 mmol/L]), high triglyceride level (≥150 mg/dL [1.70 mmol/L), or high blood pressure (≥130/≥85 mm Hg) was determined by means of the National Cholesterol Education Program Adult Treatment Panel III criteria.⁴⁰ The percentage of children who were at risk of overweight (BMI >85th percentile) or overweight (BMI >95th percentile) was assessed by Centers for Disease Control and Prevention age and sex growth charts.⁴¹ The percentage of children with high total cholesterol level (>171 mg/dL [>4.42 mmol/L]) or LDL-C level (>110 mg/dL [>2.84 mmol/L]) was determined by means of National Cholesterol Education Program criteria.⁴² The percentage of children with high-normal systolic or diastolic blood pressure (>90th percentile for age, sex, and height) was assessed by means of the National High Blood Pressure Education Program criteria.⁴³ Pearson product moment correlations were used to describe the relationships between risk factors and physical activity among both mothers and children. Partial correlations were conducted between maternal and child risk factors, controlling for ethnicity. The correlations between parental-child risk factors were then examined separately by ethnicity. Significant differences in correlations were tested by Fisher z transformations and the χ² test statistic.⁴⁴ To maximize the sample available, analyses were performed using pairwise deletion for missing data.

Means and standard deviations for the metabolic risk factors, physical activity, age, and household income are presented in Table 1 for all participants and separately for each of the 3 ethnicities. The mean ages of the children and mothers were 6.5 and 32.1 years, respectively. There were significant (P<.05) sex differences in children’s diastolic blood pressure, with higher blood pressure among the girls (60.1 ± 4.9 vs 58.1 ± 5.4 mm Hg), and in children’s insulin levels, with higher insulin level among the girls (6.2 ± 4.2 vs 4.5 ± 2.1 μIU/mL [43.1 ± 29.2 vs 31.3 ± 14.6 pmol/L).

Pearson product moment correlations showed no significant associations between either of the 2 expressions of children’s physical activity and any of the children’s risk variables. There also was no significant association between mothers’ physical activity and any of the maternal risk factors.

Fifteen percent of the children were classified as at risk of overweight (BMI >85th percentile) and 10% were overweight (BMI >95th percentile) (Table 2). Although a large number of children (46%) had high total cholesterol levels, few children had high LDL-C levels (6%) or blood pressure (9% systolic, 3% diastolic). There was a high incidence of obesity among the mothers, with 26% being obese, 51% being overweight, and 23% exhibiting large waist circumference. A number of mothers also had high total cholesterol (59%) and LDL-C (33%) levels, but a relatively small number had low HDL-C levels (4%), high triglyceride levels (15%), or hypertension (6%).

Pearson correlations were conducted between maternal and child anthropometric factors, metabolic risk factors, and physical activity variables while controlling for ethnicity (Table 3). Significant positive associations were detected for BMI (r = 0.25, P = .002), waist circumference (r = 0.24, P = .007), HDL-C level (r = 0.36, P = .001), LDL-C level (r = 0.37, P = .001), systolic blood pressure (r = 0.20, P = .048), and insulin level (r = 0.20, P = .049). There was no significant (P<.05) association between the physical activity levels of mothers and children. Partial correlations controlling for mother’s activity level alone, and for both mother’s activity and child’s activity, resulted in no changes in the relationships between mother’s and child’s risk variables (data not shown).

The correlations between maternal-child risk factors were examined separately by ethnicity (Table 3). Significant differences in correlations were tested by means of Fisher z transformations and the χ² test statistic.⁴⁴ The relationship between the BMI of mothers and children was significant among Hispanics (r = 0.56, P = .001), but not the European American (r = 0.00) or African American (r = 0.14) groups. This χ² value for differences approached significance (P = .06). Similarly, maternal and child waist circumference were significantly associated among the Hispanics (r = 0.68, P<.001), but not among the European American (r = −0.04) or African American (r = 0.22) groups.

The insulin values of mothers and children were significantly positively associated among the African Americans (r = 0.47, P = .009), but not the European American (r = −0.05) or Hispanic (r = −0.09) groups. The χ² value for these differences approached significance (P = .051). Results also yielded significant ethnic differences (P = .05) among the maternal-child resting heart rates, with a significant negative association evident among the Hispanics (r = −0.39, P = .047). Bivariate plots indicated that outliers did not account for this difference. There were no significant ethnicity differences in the parental-child relationships for HDL-C level, LDL-C level, systolic or diastolic blood pressure, or physical activity variables.

In an attempt to explain the ethnicity-related differences in resting heart rates and insulin level, we repeated the correlations by ethnicity while separately controlling for mother’s physical activity, child’s physical activity, mother’s and child’s physical activity, and child’s BMI, but there was no substantial change in any of the associations (data not shown).

To understand the influence of ethnicity on maternal-child risk factor relationships in a period less confounded by high levels of obesity, we examined these associations among a triethnic sample collected in the late 1980s. The prevalence data (Table 2) indicated that only 10% of the children in our sample were overweight. This is less than the 15% of childhood overweight reported in the National Health and Nutrition Examination Survey (NHANES) 1999 to 2000 and the 11.3% reported in NHANES III (1988-1994).⁶ Similarly, 26% of the mothers in our sample were obese, which is less than the 34% reported in NHANES 1999 to 2000, but almost identical to the 25.9% reported in NHANES III.⁴⁵ The prevalence of hypertriglyceridemia and low HDL-C levels among the mothers was also lower than the 24.7% and 39.3% reported in NHANES III.⁴⁶ Thus, the participants in our sample had lower levels of adiposity and other associated risk factors than those in recent national surveys. This allowed us to explore the relative importance of ethnicity and physical activity on the relationship between the risk factors of mothers and their children in a sample that was less confounded by obesity than current samples.

In our sample, the maternal-child relationships between risk factors were not uniform across ethnicities. Although there were significant correlations for BMI and waist circumference between Hispanic mother-child pairings, no such relationship was evident for either the African American or European American samples. In previous studies, adult Hispanics were more likely to accumulate visceral adiposity than European Americans were,⁴⁷ and Mexican-Hispanic children had higher levels of obesity than European American or African American children did.⁴⁸ Because genetic factors accounted for approximately 60% of the variance in the central adiposity of British women,⁴⁹ and children were likely to share a large number of genes with their mothers, genetic factors undoubtedly account for some of the relationship between the adiposity of Hispanic mothers and children. However, the lack of such an association between European American and African American children and their parents suggests that the genetic effects do not manifest at this early age among children in these other groups, or that other environmental factors such as dietary intake and physical activity may play an equally important role.⁵⁰ However, we found no support for differences in physical activity accounting for these differences in risk factor relationships.

There was a significant positive association between the maternal-child insulin, HDL-C, and LDL-C levels among the African American group, but not among the Hispanic or European American groups. Higher fasting insulin levels have been reported among African Americans⁵¹ but were not found in this sample. Higher HDL-C levels have been reported among African American children and adults^52-54 and were found in this sample. These findings suggest that maternal-child risk factor relationships varied by ethnicity. Possible explanations for these differences (including BMI and physical activity) were tested but not found, suggesting that other factors including diet and shared genetic factors may account for these findings.

Physical activity was not related to any of the individual risk factors of either mothers or children. This is in contrast to reported associations among physical activity levels, individual risk factors, and the combined metabolic syndrome factors among multiethnic groups of women.^55-57 Associations between physical activity and metabolic syndrome risk factors have also been reported among children.^58-60 Accounting for this difference in findings with other published studies is a challenge. The objective measure (heart rate) of physical activity across multiple days was a particular strength of the present study. Perhaps the lower prevalence of abnormal metabolic syndrome risk factors in the late 1980s (when the data were collected) limited the ability to detect meaningful relationships with physical activity. Since dietary habits have also been related to the level of metabolic risk factors,^61,62 dietary factors may have been more closely related to the risk profiles and overwhelmed the effect of physical activity. As the relationship between physical activity and risk factors increased with age among children and adolescents,⁶³ it may be that this relationship changes during the developmental process and no relationship exists at this early age.

No association was detected between maternal and child physical activity levels. This finding is in contrast to several studies that reported such an association.^25,26 This difference may be because the studies that did report an association relied on self-report assessments of physical activity, which suffer from common self-report biases.^64,65 Also, stronger associations were reported between the physical activity levels of children and their fathers²⁵ than for mothers in previous research. The lack of an association detected in this study could also be the result of substantial differences in the occupational activity levels of mothers (included in the 24-hour activity recalls) and the school or recreational activity levels of their children, or other environmental factors.

The strengths of this study include the triethnic sample of mothers and their children, which facilitated the comparisons of associations of risk across the 3 ethnicities. Since the children were 6 or 7 years of age, those associations were examined before the onset of puberty and the complications from pubertal hormones. The objective assessment of childhood habitual physical activity, obtained from multiple days of heart rate assessment, provided a stable assessment of childhood physical activity. Also, these data were collected between 1986 and 1989, which provides an important opportunity to examine these relationships very early in the surge in childhood obesity.⁶ Limitations are the relatively small sample of each of the ethnic groups, limiting the ability to compare associations across the 3 groups, and the lack of dietary data. Although heart rate monitoring is a valid and reliable measure of physical activity,⁶⁶ other factors including stress can result in elevated heart rates.⁶⁶ This could have resulted in an overestimation of the amount of physical activity in which the participants engaged.

Maternal and child risk factors were associated among a triethnic population, but the associations varied by ethnic group. Controlling for either the mother’s physical activity or mother’s and child’s physical activity did not change the relationship between the risk factors of mothers and their children. Furthermore, as these data were collected in a period in which there were lower levels of obesity, which reduced the confounding effect of adiposity, they support a more biological interpretation of the relationship between the risk factors of mothers and their children. The ethnic differences in the nature of the relationship between the risk factors of mothers and their children could be genetic or environmental in origin and require further examination.

Previous research showed associations among the risk factors between very young children or adolescents and their mothers, suggesting that family-based interventions may be feasible to prevent the development of cardiovascular disease, type 2 diabetes mellitus, and associated conditions such as the metabolic syndrome. Previous research, however, has not examined the nature of these relationships in the period before puberty or among triethnic samples to test whether these relationships differed by ethnicity or physical activity.

This study showed that, in a sample characterized by lower levels of obesity than in recent national surveys, the relationships between the metabolic risk factors of mothers and their 6- to 7-year-old children was not uniform across ethnicities. Adiposity and, particularly, visceral adiposity (as indicated by waist circumference) were significantly positively related among Hispanic mothers and their children, whereas insulin, HDL-C, and LDL-C levels were positively associated among African American mothers and their children. These relationships were not influenced by BMI or physical activity. Therefore, future family-based chronic disease prevention efforts should tailor interventions to the participants’ ethnicity, focusing specifically on visceral adiposity among Hispanics and on insulin, HDL-C, and LDL-C levels among African American families. Adopting such an approach should enhance the likelihood of preventing the development of the metabolic syndrome.

Correspondence: Russell Jago, PhD, Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, 1100 Bates St, Houston, TX 77030-2600 (rjago@bcm.tmc.edu).

Disclaimer: The contents of this publication do not necessarily reflect the views or policies of the USDA, nor does mention of trade names, commercial products, or organizations imply endorsement from the US government.

Accepted for Publication: June 11, 2004.

Funding/Support: This research was funded by grant HL35131 from the National Heart, Lung, and Blood Institute, Bethesda, Md. This work is also a publication of the US Department of Agriculture (USDA/ARS) Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Tex, and was funded in part with federal funds from the USDA/ARS under Cooperative Agreement 58-6250-6001.