Monday, December 24, 2007

Diet and infertility: is there a link?

Oh, Walter. Say it isn't so!

I first became aware of Walter Willett when I worked at Harvard School of Public Health a few years ago. He is the chair of the Nutrition Dept. there, and in addition to doing research, wrote a number of books on diet that I found really useful. The first is Eat, Drink and Be Healthy: The Harvard Medical School Guide to Healthy Eating, and the second is Eat, Drink, and Weigh Less: A Flexible and Delicious Way to Shrink Your Waist Without Going Hungry which he co-wrote with Mollie Katzen of Moosewood fame. Willett promotes a very healthy diet, with lots of fruits and vegetables, whole grains, low fat proteins, low fat dairy, and even a little chocolate and wine for good measure.

On the other hand, there are a few things that Willett is doing that I don't agree with. One, that I mentioned in a previous post, is research that has to do with the connection between diet and cancer. While I believe that there is some connection, I don't think the risk is as high as Willett professes.

And now Willett has come out with a book professing that improved diet will improve your fertility. Judy Foreman takes a look at this book in today's Globe. Her critique, which I agree with, is as follows:

Observational studies like the Nurses' Health Study do not prove that a behavior causes or prevents a health problem, only that it might do so. To prove that diet affects fertility, researchers would have to take a group of women with diagnosed infertility and randomly put half on a special diet and half on a regular diet and compare conception rates.

This would be an interventional study, which would have come closer to the "gold standard" of medical research and would have provided the solid ground to offer advice. Such a study could also show how long a woman would need to eat this way to affect her fertility.

So what she's saying is that you can't take data like the Nurses' Health Study, which asks a huge group of nurses to fill out questionnaires about health issues over their lifetime, and then analyze that data and go back and say that because this group of nurses has a healthier diet, they had better fertility. This type of study only shows correlation, not causation. In other words, there seems to be a link between this type of diet and fertility, but we can't say definitively that the diet improves fertility. For example, it could be that women who are more fertile and who have more kids, eat a healthier diet because they are feeding their kids a healthier diet. So it could be that their diet had nothing to do with their fertility.

In the end, there is nothing wrong with eating the diet that Willett promotes. It's a great, healthy diet. The problem is that he's saying that it will increase your fertility. I wish he would just stick to promoting a healthy diet, and stop claiming that it will reduce your chances of cancer and increase your fertility. These things are sexy and sell books, but it isn't good practice or good science to promote statements like these that haven't been proven.

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