It is well known that obesity, glucose intolerance, hypertension, and hypercholesterolemia in adulthood increase mortality rates. We conducted the present study to determine whether the presence of these risk factors in childhood predicts premature death. The rate of death from endogenous causes in the highest quartile of childhood BMI was more than double that in the lowest quartile, and the rate in the highest quartile of childhood two-hour plasma glucose levels during a 75-g oral glucose-tolerance test was 73% higher than that in the lowest quartile. Although neither blood pressure nor cholesterol level in childhood, when included as a continuous variable, significantly predicted premature death, childhood hypertension increased the risk of premature death from endogenous causes by 57%.
The absence of an association between premature death and cholesterol levels may be due partly to the low proportion of deaths due to cardiovascular disease in this cohort (13.3%). Treatment for any of the predictor traits during childhood or during adulthood did not appear to explain the pattern of association (data not shown). No childhood risk factor that was examined significantly predicted rates of premature death from external causes.
Childhood obesity predicted premature death from endogenous, but not external, causes. The study was not powered to analyze effects on more specific categories of cause of death. Including only liver-related causes of death in the analysis reduced the magnitude of the association of premature death with childhood BMI and with the 2-hour glucose level, but the direction and pattern of associations were similar to those observed when all endogenous causes of death were included.
We considered whether the relationship between childhood BMI and premature death reflects associations with adiposity or some other component of body mass. Our study began before the availability of modern adiposity measures such as dual-energy x-ray absorptiometry. However, we previously reported relationships between BMI and adipose mass and between adipose mass and the cardiovascular risk factors in this population19
; in that study, BMI and adiposity were strongly correlated (r>0.96), varying little with age and sex, and BMI and adipose mass were similarly correlated with the cardiovascular risk factors. Thus, the observations for childhood BMI reported here are likely to reflect a positive association between adiposity and rates of premature death.
In a study involving 508 U.S. adolescents (13 to 18 years of age) who were born between 1922 and 1935, overweight (>75th percentile of the sample distribution) was associated with increased rates of death due to coronary heart disease.20
Two studies have assessed the relationship between body weight and mortality in European birth cohorts from the early 20th century.21,22
In a study of 2299 Welsh children born between 1937 and 1939, there was no association between childhood BMI and death from cardiovascular causes.21
However, there was an association between childhood BMI and death from all causes; the lowest rate of death was seen in the next-to-lowest BMI quartile and the highest rate of death in the highest quartile, suggesting that, as in the case of adult Pima Indians,23
a U-shaped relationship exists between obesity and mortality. In the second European study, involving 504 overweight children and adolescents admitted to hospitals in Stockholm between 1921 and 1947, weight gain between puberty and young adulthood was associated with cardiovascular disease, diabetes, and death from all causes.22
A limitation of these studies is that obesity was uncommon during the study period. For example, of the 2299 children in the Welsh study,21
only 92 (4.0%) had a BMI above the 90th percentile for the age-specific and sex-specific distributions of the 1990 British population, and British children in 1990 were leaner than their contemporary counterparts.24
In the Arizona Pima Indians, unlike most other ethnic groups, childhood obesity has been common for decades.25
It has been estimated that at the turn of the 21st century, approximately 15% of U.S. children between the ages of 6 and 19 years (11 million children) were overweight or obese,26
a prevalence that is unlikely to decline in the near future27
and that is triple the prevalence among children of the same age in the 1960s.28,29
In the present study, 1394 children (28.7%) were obese (BMI, ≥95th percentile on the 2000 CDC growth charts). This prevalence is similar to that observed in contemporary Hispanic and African-American children.27
Thus, although we studied a population with high rates of obesity and diabetes, our findings may reflect the future burden of premature death among contemporary children from other ethnic groups and may be more generalizable than the findings in previous studies.
In this study, we compared mortality rates with several clinical risk factors as variables. Adjusting the obesity models for the development of diabetes in adulthood did not significantly alter the risk estimates, whereas adjusting the glucose models for subsequent diabetes did attenuate the association between childhood glucose levels and premature death. Hence, dysregulated glucose metabolism in childhood may be a mediator of the effects of childhood obesity on mortality rates, but it does not appear to be the sole or dominant factor; however, the association between childhood glucose intolerance and premature death does appear to be mediated by the development of subsequent diabetes.
The pattern of the relationships between the risk factors and observed mortality supports the view that childhood obesity is an early metabolic derangement, whereas most of the other risk factors evolve later. In fact, the predictive power of a risk score for type 2 diabetes (including measures of obesity and insulin, blood-pressure, glucose, and lipid levels) in children is almost entirely dependent on abdominal obesity, whereas in adolescents, the risk profile has evolved to include obesity, hyperglycemia, and dyslipidemia.30
Our findings complement those in our previous study, which showed that type 2 diabetes, when it occurs during adolescence in this population, strongly predicts subsequent renal failure and death.2
Although there was no significant association between childhood hypercholesterolemia and death before 55 years of age in this young cohort, an elevated cholesterol level in childhood may emerge as a significant risk factor and other causes of death may predominate if the cohort is followed to older ages. Cholesterol levels, however, are lower in American Indians than they are in most other ethnic groups,31
a finding that may partially explain the absence of association for this trait. The relationship between BMI and high-density lipoprotein (HDL) cholesterol is relatively strong in Pima children (r = −0.3 to −0.6), but the relationship between BMI and total cholesterol is weaker (r = 0.1).19
The effect of BMI on premature death might be attributable in part to low HDL-cholesterol concentrations, which were not measured in most of the study participants. Nevertheless, we speculate that low HDL-cholesterol levels are likely to mediate rather than confound this relationship.
It is possible that the relationship between childhood BMI and mortality is confounded by unmeasured lifestyle factors. Nevertheless, obesity can be both the cause and the consequence of adverse lifestyle factors such as physical inactivity, excessive caloric intake, and specific nutrient preferences. Thus, such factors may be important components of the causal pathway between obesity and death. It is also possible that genetic factors have pleiotropic effects on BMI and mortality.
Childhood obesity is predictive of excess mortality in several divergent settings,20-22
indicating that obesity itself is causally related to either death or other commonly related factors. Even if preventing childhood obesity does not affect the risk of death, increased physical activity and modification of diet are likely to have long-term benefits. The lack of specific data on such factors is a limitation of this study.
In summary, obesity in children who do not have diabetes is associated with an increased rate of death from endogenous causes during early adulthood, an association that may be partially mediated by the development of glucose intolerance and hypertension in childhood. In contrast, the cholesterol level in childhood is not a major determinant of premature death in this population. Childhood obesity is becoming increasingly prevalent around the globe. Our observations, combined with those of other investigators, suggest that failure to reverse this trend may have wide-reaching consequences for the quality of life and longevity. Such evidence underscores the importance of preventing obesity starting in the early years of life.