Carbohydrates and fertility: just the tip of the (fertility) iceberg

Regular readers of The American Journal of Clinical Nutrition will undoubtedly recognize the importance of diet on shaping human health throughout the life course. AJCN readers will also be familiar with the notion that nutrition's influence on health begins even before birth, as clearly signaled by AJCN's section on Women's Nutrition. What may be surprising to some readers is that nutritional influences on our health—and perhaps the very fact that we were born—may have been influenced by our parents’ dietary choices. Over the last decade the literature on the relation between nutrition and fertility has experienced a dramatic expansion and found a primary audience in o and gynecology journals rather than nutrition journals, with some exceptions (1, 2). Interestingly, this literature seems to suggest a substantial overlap between the dietary factors recommended for the prevention of major chronic noncommunicable diseases and those related to a variety of markers of fertility, from semen quality parameters in men and markers of ovulation or ovarian reserve in women, to risk of infertility and time to pregnancy among couples attempting conception, to outcomes of infertility treatment among couples requiring medical assistance to conceive (3). Despite the apparent consistency of this expanding literature, there are still some important gaps to fill. Replication of findings is one of them; not so much because findings fail to replicate when replication is attempted (they do for the most part), but rather because very few studies have even attempted replication, either because relevant data are not available in many existing studies or because studies expressly designed to evaluate the relation of diet and fertility have only recently been able to do so.

In this issue of the Journal, Willis et al. (REF) make important progress on the replication problem using data from 2 prospective cohort studies, 1 from Denmark and 1 from North America, expressly designed to examine how diet and other lifestyle factors affect fertility. In their evaluation of the relation between intake of carbohydrates and fertility among couples attempting conception without medical assistance, they find that different measures of carbohydrate quality, including glycemic load, intake of added sugars, fiber intake, and the ratio of carbohydrate to fiber, are related to how long couples take to get pregnant (REF). In analyses using pooled data from both cohorts, they find that couples where the woman is in the top intake category for glycemic load (>141) have 14% lower chances of becoming pregnant than couples where the woman is in the lowest category of glycemic load (≤100), with a similar association observed for intake of added sugars (17% lower fecundability). On the other hand, fiber intake was associated with a faster time to conception, whereby couples where the woman had a fiber intake ≥25 g/d had 13% higher chances of achieving conception than couples where the woman had a fiber intake ≤16 g/d. As mentioned by the authors, this particular hypothesis had strong prior evidence, with previous work suggesting that carbohydrate quality could have an impact on fertility (4, 5). Studies have identified possible biological intermediates, including work showing that insulin sensitivity and long-term measures of glycemia also play an important role in fertility (6, 7). Although additional replication is still desirable, this article represents an important and welcome addition to the literature, shedding additional light on how diet may affect human fertility and help prevent infertility—an exceedingly common yet often overlooked disease.

Still, much work remains to be done and there are many other questions that deserve attention—including questions that fertility patients discuss passionately in the blogosphere but for which there are much weaker prior points of data and the evidence is not as clear, as is the case for the relation of dairy intake with fertility (8–10), for example. As the literature on the relation of diet with fertility has grown, so has the interest of reproductive scientists in nutrition, the article by Willis et al. being a clear example of this. Ideally, this article will spark the interest of nutrition scientists in reproduction and thus accelerate the pace of discovery for this important issue. I look forward to reading many more articles on how diet may affect fertility in the pages of AJCN because the study by Willis et al. is likely just pointing to the tip of the iceberg.

Notes

Supported by National Institute of Diabetes and Digestive and Kidney Diseases grant P30 DK046200.