Thursday August 5 2010
BPA is found in bottles with this symbol, not all plastic containers
“Gender bending chemical in food tins may cut male fertility,” according to the Daily Mail. The article is based on research examining levels of bisphenol A (BPA), a chemical found in commercial plastic packaging (including many food and drink products) in the urine of 190 men recruited from an infertility clinic.
The study found BPA was present in 89% of samples and that there was a trend for higher BPA levels to be associated with poor sperm quality, as well as damage to sperm DNA.
This small cross-sectional study has several limitations. The results were not statistically significant, the study design cannot show cause and effect and there was no comparison group of men who were not attending an infertility clinic.
As such, the newspapers have exaggerated the significance of this research, which does not provide evidence that BPA causes sperm damage or poor sperm quality in humans. As the researchers say, this observed association needs further research using larger and varied population samples, and taking multiple urine and semen samples. As BPA is so widespread in the environment and as lab studies have shown the chemical can affect animals, it makes sense to carry out this research.
A risk assessment of BPA was published by the European Food Safety Authority (EFSA) in 2007. It found that intakes of BPA through food and drink were well below the Tolerable Daily Intake (TDI), which is the estimate of the amount of a substance that can be ingested daily over a lifetime without appreciable risk. The EFSA is publishing a new report on BPA in September that takes into account the latest scientific evidence. It has indicated it will maintain the current TDI but says it has identified “areas of uncertainty which merit further consideration”.
Where did the story come from?
The study was carried out by researchers from the University of Michigan, Harvard School of Public Health, Massachusetts General Hospital and the Centres for Disease Control and Prevention, in the US. It was funded by the National Institutes of Health. The study was published in the peer-reviewed journal: Reproductive Toxicology.
The news reports have implied a more definite link between BPA and sperm quality than this preliminary study actually suggests, with the Mail claiming that BPA “can cause infertility in men”, which this cross-sectional analysis cannot prove.
However, both the Mail and the Metro quoted opinions from independent experts, including the Food Standards Agency, saying that exposure to the chemical is “well below levels considered harmful”.
What kind of research was this?
This cross-sectional study examined levels of bisphenol A (BPA) in men’s urine and the quality of their semen and sperm. BPA is a chemical used in the manufacture of plastic packaging, including many food and drink products. Its use over many years in such a wide variety of products has resulted in most of the population being exposed to the chemical.
Laboratory studies have shown that BPA can affect hormones in animals while other studies have found it to impair sperm development in rats. This is the first research to look at its possible effects in adult human males.
This type of study cannot prove that BPA causes male infertility. As the urine and semen samples were taken at roughly the same time, it is not possible to say that BPA exposure, as shown in the men’s urine, had an effect on sperm quality.
What did the research involve?
Between 2000 and 2004, the researchers recruited 190 men between the ages of 18 and 55 into an ongoing study of environmental agents and reproductive health. All the men were recruited at fertility clinics where they had been seeking treatment with their partner. The men gave urine samples and sperm samples on the same day as their clinic visit. As a single urinary measure is likely to reflect only very recent exposure to BPA, some men also gave second and third samples between a week and two months after the first sample.
Levels of BPA in the urine were analysed in the laboratory and semen samples were analysed for sperm concentration, movement, sperm shape, size and total sperm count. Any DNA damage to sperm was assessed.
The researchers used standard statistical methods to assess the relationship between levels of BPA in the urine, sperm quality and DNA damage. They adjusted their results for other factors that can affect sperm quality such as age, body mass index, current smoking status and the time of the urine sample.
What were the basic results?
The researchers detected BPA in 89% of the urine samples. Higher BPA levels in urine were associated with a slightly higher (but not statistically significant) chance of a man having below average sperm quality.
Compared with men who had the lowest levels of BPA (bottom 25%), men with the highest levels (top 25%) had, on average:
- 23% lower sperm concentrations
- 7.5% lower motility
- 13% decline in size and shape of sperm
- 10% increase in sperm DNA damage
The researchers pointed out that the participants’ average BPA levels were lower than average levels in adult men in the US general population.
How did the researchers interpret the results?
The researchers say high levels of BPA in urine may be associated with a decline in semen quality and sperm damage. They point out that their findings are consistent with previous animal studies suggesting adverse effects on sperm production. However, they stress that further studies are needed to confirm this link.
This small cross-sectional study has several limitations, the main ones being that the results were not statistically significant, the study design cannot show cause and effect and there was no comparison group of men who were not attending an infertility clinic.
As such, the newspapers have overblown the significance of this research, which does not provide evidence that BPA causes sperm damage or poor sperm quality. As the researchers themselves say, this observed association needs further research. As BPA is so widespread in the environment and as lab studies have shown the chemical can affect animals, it would seem prudent to carry out this research.
Important points to consider:
- Although the results did show a trend for an association between high BPA levels and markers of poorer sperm quality (count, concentration, motility and shape of sperm), these associations were not statistically significant. This is important as it means that there is a real possibility this these results are due to chance only.
- This was a cross-sectional study, a design that cannot indicate cause and effect between factors as it does not show which came first. As such, it cannot tell us if BPA causes damage to sperm. It cannot be assumed that higher BPA preceded a change in sperm quality and was the cause of it. In most cases the urine and semen samples were only taken on a single occasion on the same day; the observations of BPA levels and sperm quality may not have been consistent if other samples had been taken. In the subset of men who gave more than one urine sample, BPA levels varied between samples. This suggests that the results based on only one urine sample may not be reliable.
- The participants had all attended a fertility clinic and there was no comparison group from the general population. It would be beneficial to see if a more random sample of men had similar BPA levels and associations between the chemical and sperm quality.
- Many factors, both female and male, can affect fertility and these participants may have been attending the clinic for different reasons that have not been reported. Additionally, several speculated lifestyle factors such as smoking and drinking can affect semen and sperm quality, though the researchers did try to adjust for some of these factors in their analyses.
Overall, it is not possible to know for certain what caused some men in this sample to have poorer sperm quality and DNA damage than others.