Diets high in saturated fat are bad for waistlines, but they can also have a negative impact below the waist. They may lower sperm count and sperm concentration, according to a new study published in the journal Human Reproduction.

Researchers from Massachusetts General Hospital, Brigham and Women's Hospital and Harvard Medical School collected semen samples from 99 mostly overweight or obese men and assessed their diets by asking them how often over the previous year they had certain foods and beverages.

They found that eating a lot of saturated fat was associated with a lower total sperm count and concentration. Diets high in omega-3 polyunsaturated fats -- the fats commonly found in fish and plant oils -- were associated with better-quality semen, meaning the sperm cells were of a better size and shape. The study did not determine what particular kinds of saturated fats were linked to sperm count.

The study's lead author, Dr. Jill Attaman, now a reproductive endocrinologist at Dartmouth-Hitchcock Medical Center, said the study could lead to a better understanding of how lifestyle factors affect male fertility.

"There are few clearly identifiable lifestyle modifications that can be made to optimize natural fertility, especially for males," Attaman told ABC News in an email. "This is the first report of a relation between specific dietary fats and semen quality."

Experts not involved with the study have different opinions on the role diet plays in male fertility. Some say the research opens up an important door to future studies, while others say there are factors that play a much bigger role in fertility.

"The study explores an inadequately studied field in andrology and suggests associations between dietary habits and sperm parameters among subfertile, mostly overweight men," said Dr. Tamer Yalcinkaya, associate professor and section head of reproductive endocrinology and infertility at Wake Forest University School of Medicine.

Diet is an important fertility-related variable, other doctors told ABC News.

"Maintaining a healthy, well-balanced diet will be the key to optimizing sperm parameters," said Dr. John Petrozza, director of the MGH Fertility Center, a center involved in the study. "The concept of omega-3 fatty acids will be the key, since it has been well established as an important cell membrane stabilizer."

"We have been discussing diet with our female patients for quite a while," said Dr. Alan Penzias, associate professor of obstetrics, gynecology and reproductive biology at Harvard Medical School. Researchers at Harvard Medical School were also involved with the study, though Penzias was not. "This evidence is entirely plausible and affords us the opportunity to expand the discussion to the male partners of our female patients."

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Does Dietary Fat Affect Semen?