Keto and the Menstrual Cycle: Is There Reason To Worry?

Every “keto for women” forum abounds with stories about menstrual cycles gone haywire in the first few months of keto. Common complaints include:

• Irregular menstrual cycles
• Breakthrough bleeding
• Sudden changes in menstrual cycle length, especially periods lasting much longer than normal

Keto critics love to cite these stories as evidence that keto isn’t good for women. After all, for premenopausal women, menstrual cycle activity acts as a barometer for overall health. Menstrual cycle disruptions are usually a sign that your body is under some kind of stress.

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Keto can be stressful depending on your approach, or at least the body can perceive it as such. Premenopausal women’s bodies are especially sensitive to dietary changes. The reproductive system’s job is to ensure that a potential pregnancy would be safe for parent and fetus. Any signs that could portend food scarcity or nutrient deficiencies, and the body responds by turning down the dial on reproductive capacity.¹

Keto diets require you to strictly limit or remove high-carb foods, including some nutrient-rich offerings like fruits, beets, and sweet potatoes. Keto dieters very often restrict calories as well, intentionally or not. Thus, it’s reasonable to hypothesize that women might have a tougher time adapting to or sustaining a ketogenic diet. Maybe this so-called “keto period” phenomenon is a sign that (pre-menopausal) females shouldn’t be doing keto. Or maybe menstrual changes aren’t a big deal in this context. 

What does the science say? What is it about keto that affects the menstrual cycle?

Can Keto Affect Your Period?

Let me reassure you from the get-go that there is no evidence that keto diets cause any systematic or lasting harm to menstruating folks. Anecdotally, many people don’t experience any menstrual changes at all, while others find that PMS symptoms improve and their cycles become more regular as soon as they start keto. Even if you’re one of those people whose cycle becomes wonky (that’s the accepted scientific term, right?), chances are good that things will return to normal, or even improve, after a few months. Still, it’s natural to feel alarmed any time your bodily functions change unexpectedly.

One statistic you’ll see floating around online is that “45 percent of females experience irregular menstrual cycles on keto.” This statistic comes from a single small study of adolescent girls using a therapeutic ketogenic diet to treat epilepsy.² Six of the twenty girls experienced amenorrhea (loss of period) and three were diagnosed with delayed puberty. That sounds bad! Don’t rush to judgment, though. It would be a mistake to conclude that nearly half of teenagers, much less females of all ages, are likely to experience keto-related menstrual problems based on this one study.

The ketogenic diet used for epilepsy is different and much stricter than the typical (non-medical) keto diet most people follow. Moreover, epilepsy is frequently associated with menstrual dysfunction, regardless of diet.³ I’m unable to find any studies documenting menstrual or reproductive issues in healthy females, or even in female mice for that matter, on keto diets. In fact, the (admittedly scant) research seems to point to the opposite—keto diets having positive effects on menstruation and reproductive health.

However, we do have abundant stories from people whose periods changed for the worse when they started a keto diet. The question is why.

What Is a Normal Menstrual Cycle?

Let’s briefly review what constitutes a healthy menstrual cycle, understanding that everybody’s normal will be a little different.⁴ A typical cycle lasts from 21 to 24 days on the short end to 31 to 35 days on the long end, with 28 days being the median. Day 1 is the first day of your period and begins the follicular phase, which lasts until ovulation. Just before ovulation, levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and estradiol (a form of estrogen) spike.

Next comes the luteal phase covering the approximately 14 days from ovulation to menses. LH, FSH, and estradiol drop, while progesterone rises. Estradiol bumps up again in the middle of the luteal phase. If a fertilized egg is not implanted, menstruation commences, and the whole cycle starts over again. All this is regulated by a complex communication network under the hypothalamic-pituitary-gonadal (HPG) axis, which is closely tied to the actions of the adrenal and thyroid glands.

It’s perfectly normal for blood glucose levels and body weight to fluctuate throughout the month as hormone levels change. Generally, blood glucose levels are lower at the beginning of the cycle and higher in the pre-menstrual period (the second half of the luteal phase). Insulin-dependent diabetics often find that they need to adjust their dose at different times of their cycles to keep their blood sugar in check. You may also get lower ketone readings at certain times of the month—usually coinciding with a period of (transient) weight gain and carbohydrate cravings.

Rest assured that these fluctuations reflect normal physiology and don’t mean that you’re doing something wrong. Most individuals experience these ups and downs every month but don’t notice them until they start measuring blood glucose and ketones. They’re mostly tied to the cyclical nature of estrogen and progesterone and aren’t anything to worry about. What’s potentially more worrisome is when you start a keto diet and all of a sudden your period is longer or shorter than normal, your cycle becomes irregular, or you skip a period altogether.

What Causes Menstrual Cycle Changes on Keto?

The many factors that affect sex hormones and the menstrual cycle include:

• Other hormones, such as metabolic hormones
• Gut health and microbiome
• Metabolic health, e.g., insulin sensitivity)
• Environmental toxins
• Stress
• Sleep
• Immune health
• Nutrient deficiencies
• Activity level and energy expenditure
• Age

It’s no surprise, then, that menstrual changes and reproductive issues can be extremely difficult to pin down. Unfortunately, there’s almost no research into keto diets and their effects on the female reproductive system. There is, however, some evidence regarding carbohydrate restriction, which is a defining characteristic of keto, as well as other correlated factors like weight loss.

Does Carbohydrate Restriction Affect the Menstrual Cycle?

Based on the available evidence, the answer seems to be yes, and the effects are beneficial.

By definition, ketogenic diets restrict carbohydrate intake, usually below 50 grams per day. While no studies have focused on keto diets, a handful have examined the effects of low-carb-but-not-keto diets on markers of reproductive health among overweight women. A meta-analysis concluded that in four out of four studies, low-carb diets improved menstrual regularity and/or ovulation rates. Furthermore, out of six studies that measured reproductive hormones, five reported significant improvements.⁵

Polycystic ovarian syndrome (PCOS) is one of the leading causes of female infertility and a frequent trigger of menstrual irregularity. People with PCOS have high rates of hyperinsulinemia and insulin resistance. Restricting carbohydrates decreases the insulin burden in one’s diet, so a lot of people are experimenting with keto to overcome their PCOS. Only one small study has so far directly tested the effectiveness of a ketogenic diet to treat PCOS, but the outcome was promising.⁶

Weight Loss and Period Changes

Many folks lose weight rapidly when first starting a keto diet. Weight loss can impact menstruation through a variety of pathways, one of which is by reducing the hormone leptin. Leptin’s main job is to communicate energy availability to the hypothalamus. High levels of leptin tell the hypothalamus that you have enough energy on board, so it’s safe to reproduce. Low leptin can disrupt the menstrual cycle and is linked to hypothalamic amenorrhea.⁷

Body fat loss can also affect estrogen levels since estrogen is both stored and produced in adipocytes (fat cells). While fat loss in the long term will decrease estrogen production, it is possible that rapid fat loss might temporarily raise estrogen levels and can also affect estrogen-progesterone balance. These transient changes in estrogen levels might underlie some of the menstrual irregularities people report.

Stress

Stress can impact the menstrual cycle in myriad ways.⁸ Cortisol acts on the hypothalamus and pituitary glands, affecting hormone levels, sleep, immune function, and gut health, to name a few. Diets can be a source of stress, both on a physiological and psychological level. Keto has a reputation for being especially stressful because it is more restrictive than other low-carb diets, but you can mitigate diet-related stress by following the Keto Reset tips for women.

Thyroid Function

Thyroid dysregulation is another common cause of menstrual irregularities, and there remains a pervasive belief that keto is bad for thyroid health. Indeed, the thyroid is sensitive to nutrient deficiencies and caloric restriction, and thyroid hormones, especially T3, do frequently decline on keto. However, as Mark has discussed in a previous post, changes in T3 levels might not be a problem, especially in the absence of other problematic symptoms. Moreover, many practitioners now use keto as a cornerstone in their treatment of thyroid disorders. If you’re experiencing persistent menstrual issues, though, it’s always a good idea to ask your doctor to test your thyroid function.

What Should I Take From These Findings?

The first takeaway is there isn’t cause for alarm. If anything, studies suggest that low-carb diets improve some aspects of menstruation and reproductive health. Much more research is needed, but ketones themselves have important physiological properties, such as being directly anti-inflammatory, which might positively impact women’s reproductive health.⁹

Second, factors that potentially disrupt the menstrual cycle—namely weight loss and stress—aren’t unique to keto, they’re common to any diet. Furthermore, many people combine a ketogenic diet with calorie restriction and fasting, both of which can independently lead to weight loss, cause stress, and affect the menstrual cycle and reproductive health. That makes it incredibly difficult to conclude that keto per se causes “keto periods.”

That said, people do need to be cognizant of the signals they are sending their bodies when it comes to energy availability and stress. Women who come to the keto diet with a history of adrenal, thyroid, metabolic, and reproductive issues should be extra careful about how they approach keto. I encourage anyone who’s dealing with other health issues to work with a medical practitioner to tailor a keto diet to their unique needs.

But I’m Telling You, Keto Made My Period Go Haywire!

I believe you! Remember, though, changes do not necessarily equal dysfunction. It is normal to experience hormone fluctuations when you make a massive—or even a relatively small but profound—change to your diet. Sometimes those fluctuations are unpleasant or unwanted, such as a period that lasts 14 days or one that arrives a week early while you’re on vacation. However, that doesn’t make them bad from a health perspective. We need to respect that our bodies are dynamic systems. Changing an input will invariably change the output, and the system might need a few months to adapt to a new normal.

If your cycle becomes irregular but you’re otherwise feeling good, give it a few months to sort itself out. In the meantime, check to make sure you’re not short-changing yourself nutritionally or calorically. Scale back on fasting efforts, and consider shifting more toward a traditional Primal way of eating. If after a few months it’s still all over the place, talk to your doctor. Definitely do so immediately if you’re having other concerning symptoms.

At the end of the day, if keto isn’t working for you, stop. Keto gets a lot of hype, much of it deserved, but it doesn’t work for every body at every time. You can always try again later. It might be that your first attempt at keto didn’t work, but with a few adjustments and some self-experimentation, you can find a version of keto that works for you.

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