Ketogenic diet and hormones: why your body reacts differently than your friend's

✓ Women's Health. Published: August 2026. Reading time: 11 minutes.

Your body is constantly talking to you. Through the fatigue after lunch. Through the cravings that appear three days before your period. Through the night you lie awake at three in the morning, for no reason. Through the energy that simply doesn't come, even though you're doing everything right.

You stuck with keto for two weeks. Then came the headaches, that foggy feeling in your head, and a fatigue that doesn't feel like a well-earned rest after work. Your friend has been doing the same thing for four months and talks about energy she hasn't felt in years. You ate almost identical meals. Your bodies gave two different answers.

What happens when you cut out carbohydrates

Your body normally runs on glucose. You eat bread, rice, or an apple; your digestive system turns it into blood sugar; your pancreas releases insulin; and the insulin delivers the sugar to your cells. Your body has known this system since you were born.

With a ketogenic diet, almost none of that arrives. The guideline is under 30 to 40 grams of carbohydrates per day, with fat making up about 75 to 85 percent of calories. For your metabolism, this is an adjustment, not a disorder.

When glucose is absent, your body falls back on its fat reserves. The liver breaks down fatty acids and produces ketone bodies from them, such as beta-hydroxybutyrate. These molecules circulate in the blood and supply your brain with energy as well, entirely without sugar. This state is called ketosis, and it is a metabolic mode that your body is fundamentally capable of.

Because there is hardly any glucose entering the blood, the pancreas also hardly needs insulin. In everyday life, you notice this as: no steep blood sugar spikes after meals and, for many women, significantly fewer cravings in the hours that follow.

Why the first days are the hardest

Ketosis does not happen overnight. Your body first depletes the glycogen stores in your muscles and liver, i.e., the stored sugar, and that takes a few days. It is precisely during this transitional period that the symptoms occur, which many know as the keto flu: fatigue, headaches, irritability, and sometimes muscle cramps.

What you feel during this week is the adjustment itself.

Why the 30-gram rule doesn't solve your problem

The rule sounds precise. Under 30 grams of carbohydrates per day, then you are in ketosis, then it works. This number comes from studies that measured what happens on average to a group.

An average says little about you. Your reaction to the same amount of carbohydrates shifts depending on where you are in your cycle, how you slept the nights before, how much stress you are under, and what stage of life you are in. These four factors do not appear in any keto rule, and yet they are decisive.

That is why the same week brings energy for your friend and headaches for you. Not because one of you is doing something wrong. You have two different hormonal environments in which the same food ends up.

Four hormones, one by one

Insulin

Insulin drops when hardly any carbohydrates arrive. For women with insulin resistance, this is the real lever: fewer insulin spikes mean more stable energy and fewer cravings in the hours after eating.

Cortisol

Cortisol reacts not only to psychological pressure but also to energy deficiency. If you reduce too aggressively or eat too little overall, your body can switch to an alarm state and cortisol rises. You rarely see this in blood tests, but you notice it in your sleep.

Leptin

You may know leptin as the satiety hormone. But it also influences how regular your cycle is. Leptin levels drop on a ketogenic diet, especially with rapid weight loss, and if it drops sharply, your body may start to downregulate reproductive functions.

Estrogen

Estrogen is partly produced in body fat, and fats are also the building blocks from which your body creates estrogen, progesterone, and testosterone in the first place. With keto, high-quality fats are therefore not just energy; they are raw materials.

The HPO axis, the system above the four

Behind these hormones is a regulatory circuit that coordinates them: the HPO axis, i.e., the interplay of the hypothalamus, pituitary gland, and ovaries. It controls when which sex hormones are released, and it is sensitive to a single question: is there enough energy?

If your body receives too little over weeks, the HPO axis can throttle its signals. The cycle becomes irregular or pauses. Your body then prioritizes survival over reproduction, which from its point of view is the more reasonable decision.

The numbers behind keto

290 studies in five years

Between 2019 and 2024, 290 studies on the ketogenic diet were published worldwide. Keto is no longer a blog trend, but a research field. For you, this means: there are reliable answers, they just don't answer all questions.

About one-fifth is about weight

The largest single topic is obesity. This is followed by epilepsy, type 2 diabetes, and athletic performance. Women-specific hormonal issues are just starting to be added. If you wonder why you find so little on your situation when searching: that is the reason.

Almost 80 percent run for a maximum of six months

Only about ten percent of studies observe people for longer than a year. What keto does in the short term is well documented. What happens after two years, especially for women with hormonal peculiarities, no one currently knows. The FH Münster explicitly names this as a central research gap.

10,000 female metabolic profiles

That is how many real CGM profiles are in the model behind Hello Inside. This is why our assessments calculate with cycle phases and hormonal fluctuations instead of averaging them out.

Your carbohydrate needs change with your cycle

Almost all German-language texts on the ketogenic diet name carbohydrate needs as a fixed value. Yet the factor that is most decisive for women is missing: your body is not a constant system. It changes throughout the month because your hormones change throughout the month.

This is not a plan you have to follow. It is the reason why the same week runs completely differently twice a month.

First half of cycle

From your period to ovulation, your estrogen rises, and estrogen makes cells more sensitive to insulin. Your body therefore needs less insulin to process the same amount of glucose. Your energy metabolism runs more efficiently in this phase, and many women report that a low-carb week feels significantly easier here: fewer cravings, more stable energy, clearer head.

Second half of cycle

After ovulation, progesterone takes over. Your basal metabolic rate rises slightly, your brain asks for more glucose, and mood and sleep react more sensitively to cortisol.

It is precisely in the days before your period that hunger for sweets or starch often makes itself known. Strict restriction can further raise cortisol levels in this phase, and that hits PMS symptoms directly: irritability, sleep problems, exhaustion. What feels like a lack of willpower is often just an unmet energy requirement on these days.

Keto and PMOS, formerly PCOS

As of May 2026, PCOS is officially called PMOS, polyendocrine metabolic ovarian syndrome. The new name shifts the focus to exactly what this chapter is about: metabolism. What changes with this can be found here.

The research situation has improved. A systematic review and meta-analysis of ten studies, published in Reproductive Biology and Endocrinology in 2025, shows positive effects for women with a BMI over 25: body weight, insulin sensitivity, and the reproductive hormone profile improved in the groups studied.

Why this doesn't apply to everyone

The group of authors points out themselves that the differences between the included studies were large. Participants, study durations, and the exact dietary forms differed significantly, which is why the results cannot simply be transferred. The German Health Portal emphasizes the same point in its analysis.

And if your BMI is not over 25? Current research provides no clear answer. A distinction that disappears in the BMI is important here: insulin resistance also occurs at normal body weight. The question that matters to you is therefore not your BMI, but how your insulin metabolism works.

PMOS is not a uniform picture

Some women primarily have insulin resistance. Others show elevated androgen levels, i.e., hyperandrogenism. Many have both, in varying degrees.

Keto attacks via insulin metabolism. If insulin resistance is at the forefront, this can help directly. In cases with a strong androgen-dominant progression without pronounced insulin problems, the effect is less clear. A single dietary recommendation cannot do justice to these differences, and that is why forums are so full of women reporting conflicting experiences.

Perimenopause: same carbohydrates, different effect

You might notice it by feeling suddenly tired after lunch, even though you ate the same thing as always. Or that sweets, which used to be unproblematic, now hit you differently.

As estrogen drops, the way your cells react to insulin changes. They become somewhat more resistant, your body produces more insulin for the same effect, and carbohydrates you previously processed easily now trigger stronger blood sugar fluctuations. Your metabolism has changed the rules without asking you.

Where fewer carbohydrates can help

It is precisely here that a carbohydrate-reduced approach becomes interesting: fewer insulin spikes, more stable energy throughout the day, fewer cravings in the evening. Researchers are also discussing possible effects on hot flashes, because insulin spikes and hot flashes can be linked via common hormonal pathways. The evidence for this is still being built, but the direction is plausible.

Why rigid rules become a problem here

Perimenopause is not a stable phase. Estrogen and progesterone fluctuate daily, sometimes hourly. What supports your energy on Monday can act differently on Friday. A rigid "under 30 grams, always" ignores exactly that.

And if a strict protocol itself becomes a stress factor, i.e., raises cortisol, it works against the very system it is supposed to support.

The cycle of cortisol, sleep, and blood sugar

You are sleeping poorly. The next morning, you reach for a snack sooner, your energy doesn't really get going, and staying on track feels harder than it did last week, even though you haven't changed anything.

Your brain does not interpret a very drastic reduction in carbohydrates as a conscious choice, but rather as potential food scarcity. The stress axis reacts, and cortisol rises. It is the same reaction triggered by sleep deprivation, conflict, or work pressure, because your body does not differentiate between the triggers. Nutritional stress and emotional stress follow the same path.

Increased cortisol makes falling asleep more difficult and causes you to wake up more frequently at night. Poor sleep further elevates cortisol levels the next day. This creates a cycle that has nothing to do with discipline.

Sleep is a metabolic factor

Poor sleep increases insulin resistance the following day, even if you eat exactly the same thing as usual. Your breakfast affects your blood sugar differently after four hours of sleep than it does after seven.

Studies on very-low-calorie ketogenic diets show that this form of nutrition has measurable effects on the stress axis that extend far beyond your plate.

What this means for everyday life

• → The start plays a role. For most women, a lower-carb week is easier during the first half of the cycle than the second. If your first attempt began in the luteal phase, you had drawn the tougher version.
• → The same meal is not the same meal. After four hours of sleep, your blood sugar reacts differently than it does after seven. Your breakfast hasn't changed, but your context has.
• → Cravings in the days before your period are a need for energy. Not a willpower problem. Your basal metabolic rate has increased, and your brain is demanding more glucose.
• → Sleep, stress, and exercise are part of the equation. A change in diet always enters a hormonal environment that shifts daily.
• → Your metrics are more meaningful than any average. What happened in a study with 40 women says nothing about what is happening with you today.

Frequently asked questions about Keto and hormones

Why am I so tired in the first few weeks of Keto?

Your body first empties the glycogen stores in your muscles and liver before switching to ketones. This takes a few days, during which fatigue, headaches, and irritability occur.

How long does it take to reach ketosis?

Usually a few days, depending on how quickly your glycogen stores are depleted. It is not a switch that flips, but a system that recalibrates itself.

Can a ketogenic diet change my cycle?

Yes, that is possible. Leptin levels drop with rapid weight loss, and the HPO axis is sensitive to available energy. An irregular cycle is a signal that your body is expecting more energy than it is receiving.

Is Keto sensible for PMOS, formerly known as PCOS?

For PMOS with insulin resistance and a BMI over 25, studies show positive effects on weight, insulin sensitivity, and hormonal profile. For PMOS without an elevated BMI and for cases with strong androgenic symptoms, the data is lacking. The decisive factor is not so much the BMI as it is how your insulin metabolism is functioning.

Does Keto work differently in perimenopause?

As estrogen drops, your cells become less sensitive to insulin, which is why carbohydrates have a stronger impact. At the same time, hormones fluctuate daily, which is why fixed limits often do not work during this phase.

What is the difference between Keto and Low Carb?

Keto is the strictest form: under 30 to 40 grams of carbohydrates per day to get the metabolism into ketosis. Low Carb is the umbrella term and allows for significantly more flexibility, without ketosis as the goal.

Do I have to count carbohydrates for Keto?

Counting shows you what you have eaten. You can only see what your body does with it by its reaction: energy, sleep, satiety, blood sugar.

How do I know if Keto isn't right for me?

If the fatigue doesn't subside after two to three weeks, sleep gets worse instead of better, your cycle becomes irregular, or your mood consistently dips, these are signals that suggest an adjustment is needed. If symptoms persist, you should seek medical advice.

What you can do now

You don't have to change anything to learn something. Looking closely for one week is enough to start.

• → Energy between 2 PM and 5 PM. Does the slump happen, and exactly when?
• → Sleep. How did you fall asleep, and did you wake up during the night?
• → Satiety after breakfast. How many hours until the first thought of food arises?
• → Cravings for sweets. When did they come, and how strong were they?

Add a note to this: where you are in your cycle. After two cycles, you will have something that no study knows about you. Your pattern.

If you have PMOS, endometriosis, a thyroid condition, or are in perimenopause, discuss any major changes with your doctor beforehand. Then you will know what to look out for.

Your body doesn't have an opinion on Keto. It has reactions, and those are worth reading. 💜

Discover Hello Inside now →

Note: This information does not replace medical advice. If you have existing medical conditions or persistent symptoms, please consult your doctor.

Sources: FH Münster, analysis of 290 studies on ketogenic diets 2019 to 2024. Meta-analysis on ketogenic diets for PCOS, Reproductive Biology and Endocrinology, 2025. Stiftung Gesundheitswissen, Thieme, LMU Klinikum München, Deutsches Gesundheitsportal.

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• PCOS is now called PMOS, and that changes more than just the name
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• Mental health and menopause