Studies show that low-carb diets can cause weight loss and improve metabolic health ().
However, even though low-carb diets are great for some people, they may cause problems for others.
For example, following a very low-carb diet for a long time may disrupt hormones in some women.
This article explores how low-carb diets may affect women's hormones.
Low-Carb and Low-Calorie Diets May Affect Women's Adrenals
Your hormones are regulated by three major glands:
- Hypothalamus: located in the brain
- Pituitary: located in the brain
- Adrenals: located at the top of the kidneys
All three glands interact in complex ways to keep your hormones in balance. This is known as the hypothalamic-pituitary-adrenal (HPA) axis.
The HPA axis is responsible for regulating your stress levels, mood, emotions, digestion, immune system, sex drive, metabolism, energy levels and more.
The glands are sensitive to things like calorie intake, stress and exercise levels.
Long-term stress can cause you to overproduce the hormones cortisol and norepinephrine, creating an imbalance that increases pressure on the hypothalamus, pituitary and adrenal glands ().
This ongoing pressure may eventually lead to HPA axis dysfunction, sometimes controversially referred to as "adrenal fatigue" ().
Symptoms include fatigue, a weakened immune system and greater risk of long-term health problems such as hypothyroidism, inflammation, diabetes and mood disorders.
Many sources suggest that a diet too low in calories or carbs can also act as a stressor, causing HPA dysfunction.
In addition, some evidence suggests that low-carb diets can cause increased production of cortisol ("the stress hormone"), making the problem worse ().
One study found that, regardless of weight loss, a low-carb diet increased cortisol levels compared to a moderate-fat, moderate-carb diet ().
Bottom Line: Eating too few carbs or calories and experiencing chronic stress may disrupt the HPA axis, causing hormonal problems.
A Low-Carb Diet May Cause Irregular Menstrual Cycles or Amenorrhea in Some Women
If you don't eat enough carbs, you may experience irregular menstrual cycles or amenorrhea.
Amenorrhea is defined as a woman's menstrual cycle being absent for 3 months or more.
The most common cause of amenorrhea is hypothalamic amenorrhea, caused by too few calories, too few carbs, weight loss, stress or too much exercise ().
Amenorrhea occurs due to the drop in levels of many different hormones, such as gonadotropin-releasing hormone (GnRH), which starts the menstrual cycle ().
This results in a domino effect, causing a drop in the levels of other hormones such as luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, progesterone and testosterone ().
These changes can slow some functions in the hypothalamus, the region of the brain responsible for hormone release.
Low levels of leptin, a hormone produced by fat cells, is another potential cause of amenorrhea and irregular menstruation. Evidence suggests that women need a certain level of leptin to maintain normal menstrual function (, ).
If your carb or calorie consumption is too low, it can suppress your leptin levels and interfere with leptin's ability to regulate your reproductive hormones. This is particularly true for underweight or lean women on a low-carb diet.
However, evidence on amenorrhea on low-carb diets is scarce. Studies that report amenorrhea as a side effect were usually only done in women following a predominately low-carb diet for a long period of time ().
One study followed 20 teenage girls on a ketogenic (very low-carb diet) diet for 6 months. 45% experienced menstrual problems and 6 experienced amenorrhea ().
Bottom Line: Following a very low-carb (ketogenic) diet over a long period of time may cause irregular menstrual cycles or amenorrhea.
Carbs Can Be Beneficial for Thyroid Function
Your thyroid gland produces two hormones: thyroxine (T4) and triiodothyronine (T3).
These two hormones are necessary for a wide range of bodily functions.
These include breathing, heart rate, the nervous system, body weight, temperature control, cholesterol levels and the menstrual cycle.
T3, the active thyroid hormone, is very sensitive to calorie and carb intake. If calorie or carb intake is too low, T3 levels drop and reverse T3 (rT3) levels increase (, ).
Reverse T3 is a hormone that blocks the action of T3. Some studies have shown that ketogenic diets reduce T3 levels.
One study found that T3 levels dropped by 47% over 2 weeks in people consuming a no-carb diet. In contrast, people consuming the same calories but at least 50 grams of carbs daily experienced no changes in T3 levels ().
Low T3 and high rT3 levels can slow your metabolism, resulting in symptoms such as weight gain, fatigue, lack of concentration, low mood and more.
One study found that, after 1 year, a diet consisting of moderate carbs (46% of total energy intake) had more positive effects on mood than a long-term diet of very low carbs (4% of total energy intake) in overweight and obese adults ().
Bottom Line: Very low-carb diets may cause a drop in thyroid function in some people. This may result in fatigue, weight gain and low mood.
How Many Carbs Should You Eat?
The optimal amount of dietary carbs varies for each individual.
Many experts in the field recommend that you consume 15–30% of your total calories as carbs.
For most women, this usually equates to around 75–150 grams daily, although some may find a higher or lower carb intake to be more beneficial.
A Moderate Carb Intake May Be Better for Some Women
Certain women may do better consuming a moderate amount of carbs, or around 100–150 grams daily.
This includes women who:
- Are very active and struggle to recover after training
- Have an underactive thyroid, despite taking medication ()
- Struggle to lose weight or start gaining weight, even on a low-carb diet
- Have stopped menstruating or are having an irregular cycle
- Have been on a very low-carb diet for an extended period of time
- Are pregnant or breastfeeding
For these women, benefits of a moderate-carb diet may include weight loss, better mood and energy levels, normal menstrual function and better sleep.
Other women, such as athletes or those trying to gain weight, may find a daily carb intake of more than 150 grams appropriate.
Bottom Line: A moderate carb intake may benefit some women, including those who are very active or have menstrual problems.
A Low Carb Intake May Be Better for Others
Certain women may do better sticking to a low-carb diet that is under 100 grams per day.
This includes women who:
- Are overweight or obese
- Are very sedentary
- Have epilepsy ()
- Have polycystic ovarian syndrome (PCOS), fibroids or endometriosis ()
- Experience yeast overgrowth
- Have insulin resistance ()
- Are diagnosed with type 1 or type 2 diabetes ()
- Have a neurodegenerative disease such as Alzheimer's or Parkinson's ()
- Have certain forms of cancer ()
Here is more info about how many carbs you should eat.
Bottom Line: A lower carb intake may benefit women with obesity, epilepsy, diabetes, polycystic ovarian syndrome (PCOS) and other conditions.
Take Home Message
Evidence suggests that women's hormones are sensitive to energy availability, meaning that too few calories or carbs can cause imbalances.
Such imbalances can have very serious consequences, including impaired fertility, low mood and even weight gain.
However, most evidence suggests these effects are generally seen only in women on a long-term, very low-carb diet (under 50 grams per day).
Everyone is different, and the optimal carb intake varies greatly between individuals. There is no one-size-fits-all solution in nutrition.
Some people function best on a very low-carb diet, while others function best on a moderate- to high-carb diet.
To figure out what works best for you, you should experiment and adjust your carb intake depending on how you look, feel and perform.