Why Eating Enough Fat Matters for Hormones and the Menstrual Cycle (Episode 606)

Full Transcript: Episode 606

Lisa Hendrickson-Jack:

If you’ve been a longtime listener to the podcast, I’m sure you’ve heard me talk about the importance of eating fat, the importance of balancing macronutrients. We need sufficient protein, fat, and carbohydrates in order to make hormones, in order to have a healthy menstrual cycle. But in today’s episode, we are diving into a really interesting study that provides a more specific insight as to why we actually need to consume sufficient fat if we’re really looking to optimize our hormones and maintain a healthy menstrual cycle. So without further ado, let’s go ahead and jump into today’s study.

Like I always say, the title of these studies leaves something to be desired. So I will read a portion of the title for the study, but if you’re wanting to take a look at the abstract and take a peek at what we’re talking about today, head over to fertilityfriday.com/606 for details. So the title of today’s study: Effects of a Low-Fat, High-Carbohydrate Diet on Plasma Sex Hormones in Premenopausal Women.

So this study is so interesting for a number of reasons, which we’ll get into. But I think what you should know about this study is that they were not trying to improve menstrual cycle health. They were not trying to show the importance of fat for hormone production. This study was actually designed to try to figure out if putting women on a low-fat diet might reduce their risk of cancer — because of the hypothesis that estrogen causes cancer to grow. And so if we find a way to suppress estrogen production, then we might actually reduce cancer risk.

And so again, think about what I just said. So the researchers actually, within the design itself of the study, specifically were trying to reduce hormone production. And the way that they did that was to induce a low-fat diet into a population. So again, really really interesting to think about why they were doing the study, who was in the study, what the researchers might have been trying to ascertain and dissect from their research. But also again, look at what they did to induce a low hormone state in women — they specifically eliminated fat.

So I feel like I’ve already foreshadowed what the results of the study were, but that’s the context. So a couple other things about the study that are really interesting. One is that this was not a short-term study. A lot of the times when I’m looking at different research and different studies, they might be doing like a control trial for a two-month period or even sometimes like a two-week period or something like that. Sometimes the intervention is quite short. But in this particular study, the intervention period was two years.

So they actually had a control group and they had their test group. And their test group — they showed them and gave them information about reducing fat in their diet — and essentially had these women in the test group reducing their fat for a two-year period. And they followed the participants during that period, which is really interesting.

In order to achieve the reduced fat intake in the intervention group, the participants were encouraged to reduce their dietary fat intake to 15% of their total energy intake, which is pretty significant. That’s a very small amount. And to put that into perspective, the study results after the two years: the control group on average was consuming about 34% of their total dietary intake from fat. And the test group, although they significantly reduced their fat intake, was around 20% of total energy intake. So it’s kind of hard to reduce fat intake that drastically. And the kind of normal tendency of the control group was to be right around 30-ish percent, which is interesting.

So again, a couple of things. I’m always looking at how they measure the hormones and how they measure the cycle. And so essentially what the researchers did was they tested hormone levels at the very beginning of the study and then they tested them again at the very end of the study. So they didn’t test the hormones throughout the two-year period. They tested them before and they tested them at the end.

And what the researchers found was that in the test group, who had significantly reduced their fat intake, the test group’s fat intake was significantly lower than the control group. So they saw about a 20% drop in estradiol and they saw about a 35% drop in progesterone at the two-year mark.

And so if anything, this study shows us that fat intake is heavily correlated with our hormone output. If we think about how our bodies make hormones — well, our hormones, estrogen, progesterone, testosterone, cortisol, even vitamin D — are synthesized from cholesterol. And so it stands to reason that fat intake plays a significant role.

And in Real Food for Fertility, in our hypothalamic amenorrhea chapter, we cite a significant amount of really interesting research about HA women. So in this study, we’re not talking about women who have under-nourished or under-exercised to the point that they’re losing their cycle. But interestingly, women who are trying to reduce their intake of calories and things like that will often specifically try to exclude fat. One of the differences between women who have full-blown HA or are on the HA spectrum is that at times they’re significantly reducing their fat. In fact, there was one study that we talked about in Real Food for Fertility where we had two different groups of women — one who had HA, so they weren’t menstruating, and the other who were menstruating — and their caloric intake was actually the same. But it turns out that the women with HA, even though they were eating the same caloric intake, were eating 50% less fat. So they were eating significantly more carbohydrates but significantly less fat. And even though the calories were the same, that was enough to be the tipping point of them not being able to have a menstrual cycle.

So in this study, it’s really interesting because again, over the two-year period of the intervention group eating a low-fat diet specifically — and again, the researchers were specifically trying to reduce estrogen levels as a way to reduce cancer risk — they did that by having these women intentionally eat less fat.

So if you’re wondering about the conclusion of the researchers — they were looking at the reduced hormone levels and they hypothesized that maybe this was because these women were having longer cycles — but the irony is that they weren’t actually testing the menstrual cycles. They weren’t tracking them in any way. So the only thing that they did is at the end of the two years, they asked the women to identify the first day of their last period. And so they really weren’t tracking the cycle lengths to kind of see if there was any changes or any disruptions in the menstrual cycle whatsoever. That was not their intention.

And also, they didn’t track cancer rates. So even though they were trying to determine if this intervention would have an impact in cancer rates, they didn’t actually track that. So that was not one of the parameters that they tracked. So they actually weren’t able to draw any conclusions whether or not this diet actually made a difference for the stated goal. Instead, they were kind of able to say, okay, well, when we reduce the fat, the estrogen reduced, and therefore there could be a benefit. And that was kind of how they concluded.

So I mean, what can we take from this study? I think there’s a few things. One of the things I think to mention is that when you’re trying to find data on hormone levels or things like that, you do have to be creative sometimes in terms of where you’re going to go — like who is actually doing this research, who is doing a type of study where we’re actually having women on a sustained low hormone diet so that we can get any information about how this is impacting their hormones and what can that data show us about the overall picture. So it doesn’t mean we can draw a direct conclusion and say this is exactly what causes it, but it’s really interesting to see how consuming a low-fat diet impacted hormone production. And also that of course the researchers kind of knew and expected that that would do it.

So that’s something to think about as well. So when we’re looking at what would be the optimal diet for a woman who is actively trying to balance her hormones, actively trying to reduce negative hormone symptoms, things like PMS — or if she’s trying to conceive and she’s trying to optimize ovulation and support optimal follicular development.

So going a little bit into the physiology of the menstrual cycle — if we’re looking at how the menstrual cycle is correlated with hormone production, then what we’d have to understand is that we produce estrogen as our ovarian follicles develop as we move towards ovulation. And so if we’re seeing a 20% drop in estrogen, that typically means that follicular growth and development is less than optimal. And one of the ways that can be tested would be through ultrasound. You can actually test the size, measure the size of the ovarian follicle. And there is an ideal size of an ovarian follicle that would indicate optimal development and growth in that pre-ovulatory phase.

And so when we’re seeing a significant drop and reduction in estrogen levels, it does provide that indication that follicular development is being affected. And the follicle that, once it’s fully developed, that’s the follicle that ovulates at ovulation. And once you ovulate, that follicle then turns into the corpus luteum, which then produces progesterone for the rest of the cycle.

So it stands to reason that if the follicle is not optimal moving towards ovulation, that the corpus luteum may also not be optimal. And so one is going to affect the other. You’re not going to have a menstrual cycle where you have suboptimal estrogen but then perfect progesterone, because these things are related — one does affect the other. So it’s no surprise that there was a 35% drop in progesterone as well.

And so there’s a study that I’ve referred to before, again, that I covered in Real Food for Fertility. In that study, it wasn’t specifically fat — the researchers were actually reducing overall caloric intake. They had a control group of women who were eating enough overall — eukaloric — and they had a group of women who were given 20 to 30% less calories than what they would require. And there was a similar result: you saw a drop in estrogen and progesterone levels. So again, it was really interesting to see this correlation between what we’re eating and the hormone output.

And so whether we’re eating overall too little calories, or whether we’re specifically restricting fat — neither of these scenarios are optimal if we’re really looking to support menstrual cycle health, if we’re looking to support follicular health, egg quality, if we’re looking to reduce symptoms associated with hormone imbalances.

And again, this study at hand, where they’re having the participants eat significantly less fat, shows us what happens — and so reliably that this was the researcher’s whole plan. They were like, how do we reduce estrogen? Well, we just make these women eat less fat. And then that’s exactly what’s going to happen. So definitely something to consider, and certainly shows us how important fat intake is in overall hormone balance and supporting optimal, healthy menstrual cycles.

And so this certainly doesn’t mean that we need to go overboard. And one thing to consider as well was the control group naturally consumed an average of about 34% fat. So without really any intervention, they were consuming the amount of fat that they needed to.

If you’re wanting an even deeper dive into the research as to why fat is important, if you’re wanting to understand more of the logistical side of the role that fat plays in supporting hormone production, Lily and I dive into that more deeply in Real Food for Fertility. We have an entire section on fat in chapter 2 and we talk about it throughout. And there are some really interesting discussions of fat in our chapter on HA. Even if that is not a huge issue for you at this moment in time, it is really fascinating to read through how restricting fat plays such a role, and for many women with HA, how introducing fat and actually restoring that more natural macronutrient balance — where they’re actually getting sufficient fat — is often one of the key factors to returning and resuming normal ovulatory cycles. You can find details at realfoodforfertility.com, and of course you can download the first chapter of the book free if you haven’t done so already.

So have a wonderful week, weekend, whenever you’re tuning into the show. And this is the last episode of 2025. Wishing you a very blessed and prosperous new year. Best wishes for 2026. And as always, until next time, be well and happy charting.