A man's fertility can be affected by being too fat or too thin, according to scientists.
A study found that men of average weight had a higher volumes of healthy sperm than those outside the normal range. The work builds on previous research which showed an association between obesity and DNA damage in sperm.
Researchers looked at the results of seminal fluid tests on 5,316 men attending Aberdeen Fertility Centre after having difficulties conceiving.
Body Mass Index (BMI) measures a person's weight relative to height. The "normal" BMI range is between 20 and 25, while someone with a figure of 30 or above is classified as clinically obese. A BMI of 19 or below is considered too low.
The study found that men with a "normal" BMI had more healthy-looking sperm – as judged by its appearance under a microscope – than men in the other groups. They also produced greater quantities of sperm.
Dr Ghiyath Shayeb, from the University of Aberdeen, who led the study, said: "Adopting a healthy lifestyle, a balanced diet, and regular exercise will, in the vast majority of cases, lead to a normal BMI."
The findings were presented at the annual meeting of the European Society of Human Reproduction and Embryology (Eshre) in Barcelona.