Fertility doctors yesterday advised obese men to lose weight if they want to start a family, after research linked a dramatic fall in sperm quality to rising body weight.
Obese men produced substantially less sperm than average and had higher levels of abnormalities, which can reduce chances of conceiving and increase the risk of miscarriage, researchers said.
Soaring rates of obesity have already led to a rise in female infertility. Last year the British Fertility Society advised clinics to put obese women on weight-loss programmes before treating them, to improve their chances of getting pregnant.
In the study, doctors examined patient records for 5,316 men attending Aberdeen Fertility Centre between 1990 and 2007 and identified 2,037 who had listed their body mass index. A BMI above 30 is classed as clinically obese.
Tests revealed that obese men produced 60% less seminal fluid than men with a healthy BMI, and had 40% higher levels of abnormal sperm.
The study also found that severely underweight men, with BMIs less than 19, also had poorer sperm quality than men in the healthy weight range.
Speaking at the European Society of Human Reproduction and Embryology meeting in Barcelona, Ghiyath Shayeb, who led the study, said: "The bigger the problem we have with obesity, the bigger it will be with fertility."
A quarter of British adults are clinically obese, according to the NHS.
In a separate study presented at the meeting, researchers at Queen's University in Belfast found that diabetes, which is closely linked to obesity, independently damages fertility.