Will dairy negatively affect your fertility? Should you cut it out if you’re trying to get pregnant?
Short answer? No, you should not.
There seems to be a trend lately toward demonizing particular food groups. “Sugar is toxic.” “Gluten is a hormone disruptor.” “Dairy is very bad for fertility.” And the solution is to remove as much as possible of the offensive food from your daily fare. However, when you look at the scientific literature, there is rarely evidence to back up these claims, or if there is, it is MUCH less black and white than what gets spread online. Let’s look at the claims on dairy. “VERY BAD for fertility” according to a recent podcast a member of my Facebook support group listened to. “In this article we will examine the link between milk and infertility and why this beloved, popular food can be so detrimental to your fertility.” from natural-fertility-prescription.com. Oh, and how about this gem, “Dairy can actually hurt everyone’s fertility because of the fertility harming toxins it contains, and for people who have lactose intolerance, or a casein (milk) allergy or sensitivity, it can actually cause infertility” from tomakeamommy (emphasis mine). And you can find a ton more if you google “is dairy bad for fertility?”
What all these sources have in common is an almost complete lack of scientific support for their claims. Or they take studies in mice or that find hormones/chemicals in milk and then make all kinds of assumptions to translate the studies to humans, or hypothesize the downstream effects of the hormones/chemicals – but this gets reported as FACT, not the speculation it actually is (hello, pseudo-science.)
There aren’t actually a lot of studies on the role of dairy in fertility, so let’s take a look at those that are out there. (I searched on Pubmed using “dairy infertility not cow not cattle” because most of the articles that came up initially when I used “dairy infertility” were related to cow physiology and menstrual cycles which is not super relevant.
Dairy intake and IVF outcome – no effect
The most relevant article, IMHO is this one, “Dairy intake in relation to in vitro fertilization outcomes among women from a fertility clinic.” The study population was women attending the Massachusetts General Hospital fertility clinic and enrolling in the Environment and Reproductive Health (EARTH) study, an ongoing prospective cohort started in 2006 to identify environmental and nutritional determinants of fertility among couples. There was no criterion for cause of infertility, meaning that the study group should be a representative cross-section of infertile women in general.
The findings? The researchers divided 232 women into four groups based on amount of dairy products consumed. After adjusting for possible confounding factors there was not a statistically significant difference between the group with the lowest dairy intake and the group with the highest. Of note, prior to adjustment there was a 21% higher live birth rate in those consuming the most diary (significant with a p-value of 0.02), after adjustment, there was a 16% higher live birth rate among those consuming the most dairy, but not significant, with a p-value of 0.10. One can definitely say there is no detrimental effect of dairy consumption in this group.
The researchers looked at other intermediate outcomes during the IVF cycle and found no relationship between dairy intake and uterine lining thickness, estradiol levels at retrieval, or number of eggs retrieved. They also looked at different age groups and found no association between dairy intake and live birth rate in women under 35, but to their surprise, there was a positive association in women older than 35 (meaning higher pregnancy rates in groups with more dairy intake). Again, no evidence for a detriment to fertility – if anything suggestion of a positive effect.
It’s possible that women who are infertile have different characteristics from those who do not have difficulty conceiving, and that in the latter population dairy does cause higher rates of infertility (those women would then likely attend fertility treatment centers like this one and increased dairy would thus show a negative effect…) but I would surmise that women who are having trouble conceiving would be MORE sensitive to dairy than the general population, if in fact dairy was causing infertility/trouble conceiving.
Changes in reproductive hormones with dairy consumption
The next study, “Dairy Food Intake Is Associated with Reproductive Hormones and Sporadic Anovulation among Healthy Premenopausal Women” tested hormone levels at four times during two menstrual cycles (eight tests in total) in 259 healthy, regularly menstruating women and assessed correlation with overall dairy intake, high fat, low fat, different individual types of dairy (e.g. milk, butter, cream, yogurt, ice cream), as well as looking at ovulatory versus anovulatory cycles (assessment based on hormone measurements in the luteal phase). In this study, a ~5% decrease in estradiol was found for each serving of dairy consumed, but also a 3% increase in luteinizing hormone (LH). I’m not in the least convinced that either of these changes is meaningful given the wide range in inter-woman hormone variability as in the figure below (from https://en.wikipedia.org/wiki/Menstrual_cycle), and also the very wide ranges given. I looked at the supplementary data where hormonal changes were broken down by menstrual cycle change, and the only time when the range of decrease in estradiol did not include “no change” was during late luteal phase and menses. (and then the range was from -10 to -3.2% during late luteal phase, to -8.3 to -1.2% during menses). Of note the decreases were larger with low-fat dairy than full fat.
The researchers also assessed whether there was an association between consumption of different types of dairy and anovulatory cycles. Their model which adjusted for various factors found that cream and yogurt were both associated with an increased likelihood of anovulatory cycles, but… with cream (7.5% anovulatory cycles with no cream consumption versus 9.8% with cream), the 95% confidence interval includes 1.0 which means the statistical significance is marginal (i.e. if they were to do another study, they could very well get a different outcome). With yogurt, there does seem to be a difference (6.1% anovulatory cycles with no yogurt consumption, 13.2% with).
Also, they only followed women for two cycles and do not report the number of women with 0, 1, or 2 anovulatory cycles, nor did they confirm ovulatory cycles before the study commenced. So… grain or two of salt on this one. Regardless, given a (non-significant) decrease in anovulation upon consumption of cheese (11.6% in those consuming no cheese versus 7.9% in those who did eat cheese), butter (9.3% vs. 6.8%), and only a miniscule increase in those consuming ice cream (8% versus 9%), and no difference in ovulation based on overall dairy consumption, low-fat or high-fat dairy consumption, one cannot say unequivocally that “dairy causes infertility” or “dairy is bad for fertility.”
Dairy products and anovulation – high fat dairy seems to be protective
Since the previous study brought up anovulation, the next study I want to look at is the one we refer to in No Period. Now What?, “A prospective study of dairy foods intake and anovulatory infertility.” Researchers evaluated questionnaires from participants in the Nurses Health Study II, and found 18,555 women who were attempting to get pregnant. Within the eight years the subjects were followed, 3,430 women reported infertility of more than a year’s duration, with 438 of those reporting “ovulatory disorder.” Unfortunately the authors do not report an analysis of dairy intake with general infertility. The researchers do examine correlations between dairy intake and ovulatory disorders, and overall find no association.
They go on to further examine association of low-fat (skim/low-fat milk, sherbet, yogurt and cottage cheese) or high-fat (whole milk, cream, ice cream, cream cheese and other cheese) dairy with ovulatory disorders. In this case, they find an 11% increased risk of anovulation per serving of low-fat diary, and an almost 50% decreased risk with one serving of full fat dairy per day versus <1 per week. Interestingly, they also find a strong correlation between yogurt and anovulation (as well as sherbert and frozen yogurt), with a 30-60% higher risk (depending on the statistical model) with an increase of 1 serving yogurt per day. I assume that they put yogurt into the low fat category as most of what is available in stores is fat-reduced. I would suspect that full fat yogurt would not show the same association. I do find, after looking at the data, that their assertions of effect might be a little strong… with low fat diary ovulatory disorder occurs in 1.1% of women consuming <1 serving per week, 1.6% of those consuming 2-4 servings per week, 2.3% of those consuming 5-6 servings, 1.9% of those consuming LF dairy once per day, and 1.4% of those consuming more than one serving low-fat dairy per day. Adjustment for confounding factors makes the differences more significant… but in general you can see that the percentages are quite low.
Once again, one cannot draw the conclusion that “dairy is bad for fertility.” As far as anovulatory infertility goes, it seems that there might be a correlation between consumption of low-fat dairy products and ovulatory disorders, but high fat dairy seems to be protective (and why we encourage women with HA to consume high fat dairy products!!)
Phew! Time is getting short here, so I’m going to throw out a few other things I’ve come across without going into quite so much detail.
In 1994 a study was published ostensibly looking at correlation between dairy intake and fertility on a global scale, “Adult hypolactasia, milk consumption, and age-specific fertility.” The authors compare rates across countries of hypolactasia (lacking the enzyme lactase that digests the sugar galactose in milk), amount of milk consumed per person in each country, and number of births across age groups, using that as a proxy for fertility. They state that “decline in fertility with aging is steeper in populations with high per capita consumption of milk” and suggest that might be due to the effect of galactose on ovarian function. There are so many reaches in this paper I don’t even know where to start (for example using number of live births per age group as a proxy for fertility). I will point out that when they restricted their analysis to European and North American countries where economic development is similar, “the correlation between milk consumption and decline in fertility is not present.”
Just to throw in a little something on the male side, this study, “Impact of men’s dairy intake on assisted reproductive technology outcomes among couples attending a fertility clinic” likewise found no effect of dairy intake on live birth rates in couples based on the males’ dairy intake.
Finally, this study, “Low Starch/Low Dairy Diet Results in Successful Treatment of Obesity and Co-Morbidities Linked to Polycystic Ovary Syndrome (PCOS)” found improvement in some hormonal measures of PCOS when women diagnosed with PCOS were put on a low starch/low dairy diet… but A) hello, you obviously can’t assign that improvement solely to low starch or low dairy, that would require another study where the two factors are separated, B) there is no evidence in the paper that this leads to improvement in fertility and C) even if it is the case for women with PCOS, no reason on earth to then go and say that dairy is “very bad” for *everyone*. PCOS is a specific hormonal condition, so in the absence of a similar study in women without PCOS showing that dairy harms fertility (which in fact we have the opposite of, see above), one simply cannot make that claim.
In conclusion, if you like dairy, eat/drink dairy without fears of it affecting your chances of getting pregnant. If you do want to consume dairy, it seems that the full fat versions are probably better for you, especially if you are already missing periods. If you don’t like dairy, or are allergic, don’t have lactase, then don’t consume them (although Shanta in my facebook support group says, “I eat LOADS of lactose free dairy and full fat yogurt and cheese, in fact i go through more than a liter of yogurt a day, and it REALLY helped my recovery, so you can always go the lactose-free route.)
If there are others studies you’re aware of that support diary as detrimental, please let me know, I’d love to take a look.