Obesity breaks up sperm DNA
Overweight men have reduced fertility due to increased fragmentation of DNA in their sperm, suggests a new US study.
The research by William Roudebush and colleagues at Reproductive Biology Associates in Atlanta, Georgia, is the first to examine the impact of body weight on the quality of sperm in terms of its actual DNA structure.
They compared men's body mass index (weight in kilograms divided by the square of height in metres) with a measure of the fragmentation of the DNA in their sperm.
"As BMI goes up, the DNA fragmentation rate goes up, and we could see a dramatic reduction of sperm quality," says Roudebush. High rates of DNA fragmentation in sperm are associated with reduced fertility and a higher risk of miscarriages in any pregnancies achieved, he says.
Fragmentation rose as BMI's passed 25, defined as overweight, and became acute over 30, defined as obese. Most Americans - 61 per cent - have a BMI above 25 and Roudebush suggests that rising obesity levels in the western world might be partly responsible for the decline in male fertility in western countries.
Smoking and warmth
The sperm fragmentation is being increasingly used to assess male infertility.
Other factors believed to increase sperm DNA fragmentation are being over 50 years old, smoking or being exposed to air pollution, prolonged sexual abstinence or exposure of the testicles to greater warmth than normal.
Roudebush speculates that in the case of obese men, excess fat in the genital area could raise the temperature of the testicles.
Sperm motility
Roudebush's team analysed 30 semen samples, recording the men's BMI on the day they were collected. The average BMI was 26.9, and the average DNA fragmentation index (DFI) was 24.7 per cent.
"Typically, a male presenting with a DFI over 30 per cent will have reduced fertility and an increase of miscarriages," says the team. Men with high BMI are already known to have lower sperm motility, which may compound fertility problems.
Obesity has also been shown to have a negative effect on women's fertility. A high BMI gives a greater risk of polycystic ovary syndrome and insulin resistance, both linked to poor IVF outcomes.
