Men who eat foods containing a lot of saturated fats - think burgers and chips - and monounsaturated fats may be harming their sperm, a new study by Harvard researchers suggests.
Men consuming a lot of these types of fats may be producing fewer and less active sperm.
Conversely, men who consume foods containing healthier, polyunsaturated fats such as omega-3 and omega-6 fatty acids (fish, whole grains) may be producing healthier sperm, the study suggested.
"We observed significant relationships between dietary fat and semen quality," said lead researcher Dr Jill Attaman, an instructor in obstetrics, gynecology and reproductive biology. "This shows an association between modifiable lifestyle factors, specifically nutrition, and male fertility potential."
The reasons for the apparent connection aren't clear, Attaman said. But, she added, different types of fats are treated differently in the body.
"Polyunsaturated fats are important components of sperm cell membranes and may influence the ability of sperm to fertilise an egg," she explained, adding that they "may [also] stimulate hormone production."
For the study, Attaman's team analysed the sperm of 91 men who were attending the Massachusetts General Hospital Fertility Center in Boston. The men also answered questions about their diet and the types of fats they ate. A number of the men also had levels of fatty acids in their sperm and semen measured, the researchers said.
The researchers found that men with the highest intake of saturated fat had 41% fewer sperm than men who ate the lowest amount of saturated fat. And men with the highest intake of monounsaturated fat had 46% fewer sperm compared with men with the lowest intake of monounsaturated fat.
On the other hand, men who had a higher intake of omega-6 polyunsaturated fats had greater sperm motility, and a higher intake of omega-3 polyunsaturated fats was related to better sperm "morphology" - the size and shape of sperm.
"Whether these finding are important may depend on an individual male," Attaman said.
For example, a 40% difference is large, but may be important to some men and not to others. If a man has a marginal sperm concentration, such as 25 million sperm per milliliter, a 40% reduction could bring his sperm count down to 15 million per milliliter, which is abnormal, Attaman said.
But, if a man's sperm count is closer to 100 million per milliliter, a 40% decrease would still maintain a normal sperm count of 60 million per milliliter, she said. "That would be unlikely to make a difference in fertility potential."
Saturated fat is the main dietary source of high blood cholesterol and can be found mostly in foods from animals and some plants, such as beef, veal, lamb, milk and cheese. Monounsaturated fats are found in vegetable oils such as olive oil, canola oil, peanut oil, sunflower oil and sesame oil. Other sources include avocados, peanut butter, and many nuts and seeds, according to the American Heart Association.
Polyunsaturated fats are found in fish, whole grains and some seeds and nuts.
Dr David Katz, director of the Prevention Research Center at Yale Unioversity School of Medicine, said that "this is a small study of association between variations in dietary intake, and variations in sperm quantity and function. It does not directly prove cause-and-effect."
Some of the findings, such as an inverse association between monounsaturated fat intake, which is a staple of the Mediterranean diet, and sperm count are counter-intuitive, he noted. The association between polyunsaturated fat intake, and in particular balanced intake of omega-6 and omega-3 fat, with sperm vitality is in accord with general health knowledge, he said.
"Overall, the study makes this provocative suggestion: Not only is it true that we are what we eat, but we start to be so before we are ever conceived," Katz said. "The dietary pattern of a father-to-be is affecting the composition of the sperm that will be delivering half of the genes to a future son or daughter. So, it's one more reason to choose our foods wisely."
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