Dairy foods and ovulation

Lactose is the main carbohydrate in milk and other dairy foods. In the intestine, lactose is broken down into two sugars, glucose and galactose, which are further broken down and used by the body. The ability to breakdown these sugars is an important factor in normal ovarian development. for example, women with a disease that prevents the breakdown of galactose often develop premature ovarian failure. Some researchers have suggested that high intake of milk and dairy products may increase the risk of infertility due to ovulatory dysfunction in otherwise healthy women. However, study results are not consistent. In fact, there are reasons to believe that dairy foods may be beneficial to ovarian function, and that not all dairy foods have the same relation to fertility. For example, dairy food intake has been associated with a lower risk of developing insulin resistance, which is a cause for not ovulating. In order to try and better answer these question, researchers conducted a study to determine whether the intakes of low-fat dairy foods, high-fat dairy foods, lactose and other nutrients concentrated in dairy foods were associated with infertility due to ovulation problems in a large group of otherwise healthy women. The Nurses Health Study II started in 1989 when more than 116 000 female registered nurses aged 24–42 completed and returned a mailed baseline questionnaire. The nurses have been followed every 2 years since then with additional questionnaires. Only married women, with available dietary information and without a history of infertility, were analyzed for this study. In total, 18 555 women were found without a history of infertility who tried to become pregnant or became pregnant during the 8-year period that the study was conducted.

Dietary assessment

The nurses were asked about more than 130 food items and 11 individual dairy foods. Participants were asked to report how often, on average, they consumed each of the foods and beverages during the previous year. This questionnaire had previously been validated in other studies as an accurate way to estimate a persons intake of dairy foods and nutrients concentrated in them. The women differed in several important areas. Women who consumed more low-fat dairy foods were less likely to smoke or drink high amounts of coffee and exercised more. Women who consumed more high-fat dairy foods were more likely to drink alcohol, more likely to have previously been pregnant and were less likely to exercise. In addition, women consuming more dairy foods, regardless of fat content, were also more likely to use multivitamins and less likely to use oral contraceptives.

Infertility assessment

During the eight year study, 26 971 pregnancies or pregnancy attempts were reported by among 18 555 women. 438 women reported developing infertility, underwent medical evaluation and were diagnosed with an ovulation problem.

Results of this study

Intake of total dairy foods was not associated with risk of ovulatory problems. However, women who had a high intake of low fat dairy food were more likely to experience ovulation problems. An increase in low-fat dairy foods of 1 serving per day was associated with an 11% greater risk of an ovulation problem resulting in infertility. Women who had a high intake of high fat dairy foods were less likely to have ovulation problems. Interestingly, the researchers were not able to demonstrate a correlation between intake of lactose and the development of ovulation problems. The results for specific dairy foods generally followed the same way. Among the low-fat dairy foods, adding one serving per day of yogurt or sherbet/frozen yogurt was associated with a greater risk of anovulatory infertility. Women who drank one or more servings per week of skim/low-fat milk also had a significantly higher risk compared with women who drank less than one serving per week. The researchers also tried to identify the type of woman that might be most affected by low fat dairy foods. The strongest associations were for women who were older than 32 years of age, had a lower body mass (less than 25 kg/m2) and who had been pregnant previously.

Results of previous studies

Only two previous studies have examined whether intake of milk or dairy foods could affect fertility in women. In 1994, one study found that the higher amount of milk women drank, the greater the chances for having fertility. However, a 2003 study found that women who drank three or more glasses of milk daily had a 70% lower risk of infertility when compared with women who did not drink milk at all.

Alternative conclusions

It is possible that the diets these nurses followed may have changed as a result of their having problems getting pregnant rather than the other way around. It is also possible that other ingredients found in low or high fat milk may be affecting the chances for ovulation problems The study authors state that they cannot exclude the possibility that lactose, calcium, phosphorus or vitamin D could have had some effect on ovulation in these nurses.

Don't draw too many conclusions

It is also important to point out what the study did not demonstrate. This study was not designed to detect if a specific length of time was necessary to develop problems with ovulation. It also did not show whether switching to high fat dairy consumption would reverse the problems in ovulation that seemed to have occurred in women who had higher intake of low fat dairy foods. The study controlled for the total amount of calories consumed. It would not be recommended that women increase their calorie intake as this could result in weight gain, which is another cause for ovulation problems. High fat diets are known to have other significant health issues such as weight gain and heart disease. Women should take these other factors into account before making changes to their diet.