Study overview
Background
Plasticizers, mainly phthalates, can interrupt endocrine system even if at very low level of exposure, and there is evidence of plasticizers depositing in animal brain. Concerns have been rising in pediatricians and scientists. Obesity, which is a major public health problem across the world, is also associated with plasticizer exposure. First fat and now sugar have been blamed for causing obesity, while increasing evidence shows that plasticizer pollution is also playing an important role. However, effects of plasticizers on bodyweight within different age and gender groups are different. With more detailed exposure monitoring data of plasticizer exposure available at NHANES datasets, we can now explore the relationship between body weight and plasticizer exposure.
Data source
All data was retrieved from the the National Health and Nutrition Examination Survey. The National Health and Nutrition Examination Survey (NHANES) is a program of studies designed to assess the health and nutritional status of adults and children in the United States.
In the study, we uesd demographics data (DEMO2011-2012, DEMO2013-2014 and DEMO2015-2016), phthalates metabolites data (PHTHTE2011-2012, PHTHTE2013-2014 and PHTHTE2015-2016) and body measures data (BMX2011-2012, BMX2013-2014 and BMX2015-2016).
Analysis and results
Our study mainly consists of the following two parts:
Analyzing phthalate exposure pattern
Phthalate exists in human body in different types. In the original dataset, eight types of phthalates in participants’ urine were recorded. Therefore, we analyzed percent contribution of each type of phthalate to the sum-total phthalate and visualized it. Then, we made some plots to compare phthalate exposure in different subgroups of people.
Assessing phthalate-obesity relationship
First, we made some plots and perceived association between phthalate exposure body mass index. We also noticed that the effects of phthalate exposure on BMI are defferent between different age and gender groups.
Then, we fitted logistic regression model to analyze the relationship.We defined a categorical variable, where being overweight/obese was 1 and being normal weight/underweight was 0. Then, the logistic regression model is given by (log(p/(1-p)) ~ log_value + age + race + poverty_status), where p stands for the probability of being overweight/obese. Since the association between phthalates and obesity might be modified by gender and age, we stratified the result by gender and age. The result is recorded in the table below. It shows odds of obesity changes associated with unit phthalate exposure change in each age and gender group, and their 95% percent confidence intervals.
| group | female | male |
|---|---|---|
| adult | 1.18(1.09, 1.29) | 1.11(1.02, 1.2) |
| children | 1(0.89, 1.11) | 1.15(1.02, 1.29) |
Among adults, for a one-unit increase in the sum-total phthalate exposure, we expect to see about 18% and 11% increase in the odds of being overweight/obese for females and males, respectively, adjusting for age, race and poverty status.
Among children, there is no association between phthalate and obesity in females statistically. While in males, for a one-unit increase in the sum-total phthalate exposure, we expect to see about 15% increase in the odds of being overweight/obese, adjusting for age, race and poverty status.
Conclusions and discussions
Conclusion
- Mono-ethyl phthalate (MEP) contributed most to the sum-total exposure among our study population.
- Non hispanic blacks and people who are living in poverty are at higher exposure levels of phthalates, compared to other groups.
- Phthalates and obesity are positively correlated among adults, and the influence of phthalates tends to be stronger on females than that on males. While among children, the positive correlation between phthalates and obesity is only observed among males, statistically.
Discussion
Previous research shows that phthalates have greater influence on BMI for females than for males. Our findings is consistent with this conclusion only for adult. Actually, our analysis demonstrates that the situation is exactly the opposite for children – – phthalates only affect boys’ BMI but not girls’. That’s possibly because phthalates has similar effects on human body as estrogen does. Just like estrogen, it causes fat accumulation in human body. However, for girls, increased weight and body fat may lead to precocious purberty, which limits the degree of weight gain but cause other health problems in future life.
Though plastic products have brought huge convenience to modern life, the phthalates contained in them not only does harm to animals but also to ourselves. Our stusy shows that phthalate exposure varies with a number of demographic factors, including gender, race/ethnicity, and poverty status. People may have different levels of phthalate exposure due to different living habits.It is well worth the effort to regulate the application of such chemicals in industry and mitigate the harm in population who have been exposed .