Our goal was to characterize abdominal fat deposition and to evaluate the behavioral correlates of abdominal fat and its sub-compartments, VAT and SAAT, in subjects at menarche, at 4 years after menarche, and over the first 4 years following menarche. We found no evidence to suggest that physical activity was associated with abdominal fat deposition. Our findings do suggest that the risk behaviors of cigarette smoking and alcohol use may contribute to the deposition of abdominal fat over the pubertal period.
We observed significant increases in percent body fat, SAAT, and abdominal fat in subjects during the pubertal period. Although VAT increased over this period, the absolute changes were of borderline significance. Our findings of only small changes over the pubertal period are consistent with findings from a small study of 16 obese boys and girls in which significant increases were seen in SAAT, but not in VAT, over the 4 years following the start of puberty (mean[SD] age: 12.8 [1.4] years) (9
). In a study that included only boys, VAT was found to increase more than SAAT over this period (6
). These discrepant findings suggest that sex differences in VAT deposition may begin as early as puberty.
Several studies of adults using anthropometric measures to assess abdominal fat have suggested that abdominal fat deposition may be modified by behavioral factors, such as physical activity (13
), alcohol use (16
), and smoking (11
). Smoking is generally associated with lower BMIs (27
), presumably due to the appetite suppressant effects of nicotine (28
). However, after adjustment for total fatness by BMI, higher waist circumferences or waist/hip ratios have been observed in smokers (11
). Furthermore, a dose response association between pack-years of cigarette smoking and waist-to-hip ratio in a population sample of adults (29
) suggests a direct effect of smoking on fat distribution. Because all of these studies relied on anthropometric measures to assess abdominal fat, none could distinguish between subcutaneous and visceral fat compartments.
More recently, the impact of behavioral risk factors has been explored using more direct measures of VAT. We (30
) reported that adults who were moderately fit had less VAT than those who were less fit for a given BMI or level of total body fat. Saelens et al. studied 8-year-olds deemed at high risk for obesity by virtue of having a BMI >75th
). In their cross-sectional study, physical activity assessed by accelerometry was related to VAT, but not to SAAT. In a 4-month intervention trial that enrolled obese children, those who received physical training showed significantly smaller increases in VAT than did children in the control group (31
). In our study, we did not identify a significant relationship between physical activity and VAT, SAAT, or abdominal fat or the 4-year changes in these fat depots at the time points examined. The absence of a significant relationship may reflect our small sample size, the imprecision of our self-reported physical activity measure, or both.
To our knowledge, the only published study to assess the influence of smoking on directly measured VAT was a cross-sectional study of Japanese-American men who had normal glucose tolerance, impaired glucose tolerance or diabetes. Smoking history was not related to VAT in this study (32
). Our results suggest that smoking and alcohol each are related to deposition of VAT and SAAT in young females, but the extent to which these associations are specific is not established by our analyses. Smoking and drinking are moderately correlated (r = 0.51) and VAT and SAAT changes even more so (r = 0.85). Despite the high correlation of VAT and SAAT (0.82 in our study), distinct regulatory mechanisms are presumed to determine growth of each compartment (10
). Although specific mechanisms have yet to be elucidated, the effects of tobacco smoke and alcohol on sex hormones and on adipocytes are suggested (12
). Although it is not clear why effects on VAT (or on SAAT, if they are independent) would be apparent at the low levels of smoking and alcohol observed in our study, the period following menarche may be particularly sensitive to environmental stresses on sex hormones. Whereas the importance of central adiposity in relation to cardiovascular risk factors is increasingly well established (3
), whether VAT per se
is responsible for these health effects is less clear (38
). With the exception of VAT effects on lipids outcomes, Gutin et al. failed to establish the independent effect of VAT on other cardiovascular risk factors (2
). SAAT may be metabolically important in its own right, or may merely serve as a marker for VAT deposition during the pubertal period in girls.
Our findings should be interpreted cautiously for several reasons. First, although subjects were seen annually, physical activity, smoking and alcohol use were evaluated only at the 2 developmental time points, and smoking and drinking were too rare at menarche to study meaningfully. When we examined annual physical activity changes our conclusions were unchanged: physical activity level was not related to abdominal adiposity or its sub-compartments. When we examined the association between smoking and alcohol use in the prior year (3 years after menarche) in relation to abdominal fat at 4 years after menarche, associations were not statistically significant. We lack measures of abdominal fat in the years between menarche and 4 years after menarche, so this analysis is not complete. Second, the relationships we explored were based on smoking and drinking assessed at 4 years after menarche, with a reference period of the 30 days prior to the study visit, thus the temporal separation of these risk factors was small. However, intermediate reports indicate that for three-quarters of these subjects these factors were present at visits prior to the visit at 4 years after menarche. For the remaining subjects, it seems unlikely that smoking and drinking occurred during the reporting period only. Third, despite our standard protocol and fixed parameters, the change in MRI instrumentation mid-way through the study may have introduced error. We do not believe the machine to machine differences would exceed the scan to scan variation over time with the same machine. Fourth, because the main study included only girls who were not overweight at enrollment, when their mean age was 10 years old, there were no very overweight girls in the sub-sample studied. Finally and importantly, our limited sample size restricted our ability to fully tease out independent effects of smoking and alcohol use, and precluded our assessment of potential modifying effects by race/ ethnicity.
Despite these limitations, our data gathered on a small cohort of girls over a precisely defined developmental period suggest that in addition to their established health risks, smoking and drinking, even at low levels, may be associated with increased VAT deposition in girls over the pubertal period. Our hypotheses were formulated a priori based on the adult literature. The relationships we identified appear to be specific; these factors were unrelated to percentage body fat. These associations, if confirmed, would have substantial public health significance, given the high rates of smoking and alcohol use in teens, especially smoking in adolescent girls.
Several areas of future research on this topic are warranted. Larger studies would provide a sample with more statistical power, to assess if smoking and alcohol use are independent, to allow for assessment of hormonal mediation of significant effects, as well as to permit stratification by any effect modifiers identified. More frequent measures of the smoking and drinking behaviors and fat deposition (e.g., from annual assessments) would permit better characterization of these behaviors over time, perhaps including the role of cessation on visceral adipose tissue. Studying this relationship in a cohort with higher prevalence of these behaviors might further elucidate the dose-response relationship between the behaviors and the outcome. Finally, it would be important to investigate these relationships in a cohort of girls and boys to explore differences by sex.