To our knowledge, this is the first study to demonstrate the effects of leptin on the decline in physical activity in a sample of minority peri-pubertal girls. Baseline leptin levels were related to both cross-sectional and longitudinal physical activity levels. These results suggest that leptin levels at the beginning of puberty may be a salient factor in the steady decline of physical activity levels in girls.
Findings from previous studies on the relationship between leptin and physical activity have been inconsistent. Romon and colleagues 28
reported leptin was negatively correlated to pedometer-calculated steps per day in girls in a cross-sectional sample of 510 White youth aged eight to 18 years. Conversely, in a sample of 125 Pima Indian children (mean weight= 23.2 kg) Salbe and colleagues 29
found that plasma leptin concentrations were positively associated with physical activity levels (measured as the ratio of TEE:RMR). However, this relationship was not found in a longitudinal study of 213 healthy children, where accelerometer-measured physical activity did not significantly correlate with leptin levels 30
. The results from the current study support the findings of Romon and colleagues, suggesting that higher circulating leptin concentrations are associated with lower levels of physical activity in girls. The correspondence between the findings from this study and those from Romon may be due to the fact that both studies employed device-based measures of physical activity, which provides more robust estimates of activity in youth than other measures. Also, although sex differences in leptin levels during puberty have been shown 31
, Salbe and Metcalf did not stratify by sex, while our study and that of Romon were conducted in girls of similar ages.
The decrease in physical activity with higher leptin levels in this sample is not congruent with the expected relationship between leptin and physical activity. Based on known biological mechanisms, high leptin levels signal the brain that there is excessive energy intake, which results in decreased intake and increased energy expenditure 32
. However, the results indicate higher baseline leptin levels predicted a decline in physical activity. One possible reason for this unexpected result may be because of higher than normal leptin levels in our sample. Normal leptin levels for peri-pubertal normal weight girls averages 5.0 ng/mL 33
, while this sample’s average was 15.2 ng/mL. The higher than normal baseline leptin levels suggest that this sample may more prone to energy imbalance 34
. Furthermore, girls are more prone to increasing fat mass at the initiation of puberty, which may act to raise leptin levels 31
and determine a decrease in physical activity 35
. The findings from this study support a biological basis for the age-related decline in physical activity.
This study has a number of limitations. This analysis included only baseline leptin concentrations and therefore prevented the assessment of the simultaneous change in leptin and physical activity over one year. Collecting leptin at more frequent intervals may help improve the ability to detect simultaneous changes in leptin and physical activity during puberty when hormone levels fluctuate. A majority of the sample (62.0%) was overweight or obese, and therefore these findings may not be generalizable to normal weight youth. Also, the small sample size precluded stratified analysis based on race/ethnic group, although African Americans recorded more physical activity than Latinas. Previous studies have reported that there are race/ethnic differences in physical activity levels 9
and future studies should attempt to discern whether these differences are influenced by variations in leptin levels between groups. Also, while these accelerometers are an excellent device-based measure of physical activity in youth 36
, they do not capture all types of activities (e.g.: swimming, load carrying) and may bias our physical activity estimates.
In conclusion, leptin levels promote a decline in physical activity over one year. In minority youth, girls are consistently less active than boys of all weight status categories 9
. Entering puberty with high leptin levels puts them at a greater disadvantage during the observed decline in physical activity that may have negative health consequences 37
. To our knowledge this is the first study to find this relationship in a longitudinal model in minority girls. These findings add to the growing support for the biological basis of declining activity levels in pubertal girls and may explain in part why interventions to increase physical activity based solely on behavioral and psychosocial theories have shown little success to date. Future studies are needed to further investigate how biological mechanisms, such as leptin concentrations, impact physical activity throughout puberty in order to better understand how these mechanisms contribute to the age-related physical activity decline.