The study led by Dr. Jorge E. Chavarro of the Harvard School of Public Health in Boston and colleagues found that the infertility risk for a woman who eats trans fat was dose dependent, that is, the more trans fat a woman eats, the higher her risk of becoming infertile.

Trans fat, also known as partially hydrogenated vegetable oil, is widely used in processed foods such as fried food, baked products and packed snacks because of its long shelf life, unique flavor and or texture.

Trans fats can suppress the activity of a cell receptor involved in inflammation, glucose metabolism and insulin sensitivity, according to Chavarro and his team.  This suppression could lead to low fertility, which manifests as polycystic ovary syndrome, a fertility condition in women, can be improved by drugs that activate the receptor.

In the study, the researchers analyzed data from 18,555 healthy women participating in the Nurses' Health Study to see if there was any association between intake of tarns fat and infertility. The participants were married and trying to get pregnant between 1991 and 1999.

A women's risk of infertility increased by 73 percent for every 2 percent of energy she took from trans fat instead of carbohydrates, the researchers found. 

Similarly, the risk of infertility increased by 79 percent for every 2 percent of energy from trans fats instead of omega-6 polyunsaturated fats. The risk more than doubled for every 2 percent of energy from trans fat instead of monounsaturated fat.

Two percent of energy is equivalent to 4 grams of trans fat for a woman who consumes 1800 calories per day.  According to the Food and Drug Administration, an average American eats 6 grams of trans fat a day. This means that the effect of trans fat on infertility can be substantial..

Other Harvard scientists have said earlier that trans fat has no known physiological functions other than serving as a source of energy. They also said there is no safety threshold for trans fat; meaning any amount in the blood can affect cellular physiology. 

It is well-known that trans fat increases bad cholesterol and reduces good cholesterol, raising risk of heart disease, stroke and diabetes.  Harvard scientists have earlier linked dietary trans fat with about 100,000 deaths from cardiovascular disease a year in the U.S.

In addition, trans fat   can lower the quality of human milk, reduce birth weight in human infants, negatively affect immune response,   decreases testosterone in male animals, increase the level of abnormal sperm, and interferes with gestation in females, negatively affect detoxification enzymes, alter physiological function of cell membranes, adversely interact with conversion of plant omega-3 fatty acids to elongated omega-3 tissue fatty acids, according to Dr. Mary G. Enig, a noted researcher on trans fat, from Nutritional Sciences Division Enig Associates, Inc.

The FDA requires that trans fat content be labeled for processed food. But when a food contains no more than 0.5 gram per serving, the food processor is allowed to make a claim as "trans fat: zero", which may mislead some consumers. Because of this, consumers may want to check the ingredient list to make sure there is no partially hydrogenated vegetable oil listed if they want to avoid trans fat.

However, it may be impossible to absolutely avoid trans fat, a scientist affiliated with foodconsumer.org suggests, as natural vegetable oils are subject to transformation into trans fat at high temperature used to cook food.