Intermittent Fasting & Hormones: EU Research Explained

How Intermittent Fasting Affects Hormones According to EU Research

Over the past decade, intermittent fasting (IF) has evolved from a relatively unknown concept to a widely recognised health trend. People try it to lose weight, feel better, and even live longer. The idea is simple: eat only during certain hours and give your body time to rest in between. But what happens inside our bodies when we fast, especially to our hormones?

Hormones are tiny messengers in the body. They control things like hunger, mood, energy, sleep, and even how our bodies grow and change. Some of the most important ones include insulin, cortisol, oestrogen, testosterone, and thyroid hormones. These hormones help run everything from our appetite to our metabolism.

Now, scientists in Europe are beginning to examine how intermittent fasting affects these hormones more closely. Because people eat different foods and have different lifestyles in each country, the results can vary. However, the research is showing that fasting can alter hormone levels in ways we didn't expect, sometimes beneficial, sometimes not.

This blog examines the latest studies from Europe to explore how IF affects our hormones. Whether you're fasting to lose weight, boost your focus, or improve your overall health, understanding how it affects your hormones can help you achieve better results, healthily and safely.

The Science Behind Intermittent Fasting and Hormonal Changes

What Is Intermittent Fasting?

Intermittent fasting involves eating at specific times and abstaining from food at others. It's not just what you eat but when you eat. Here are some common ways people do it:

• Time-Restricted Eating (TRE): You eat all your meals within a short time window, such as 8 hours, and fast for the remaining 16 hours.
  
  
• 5:2 Method: You eat normally for 5 days of the week, and on 2 days, you eat a lot less.
  
  
• Alternate-Day Fasting (ADF): You switch between days of eating normally and days of fasting.

 

These methods work with your body clock (called your circadian rhythm), which controls things like sleep, digestion, and hormone release.

Hormones That Change With Fasting

Fasting changes many hormones, the body’s chemical messengers. These changes can help or hurt, depending on the person. Here are some of the main hormones affected:

• Insulin: This helps your body use sugar. Fasting lowers insulin, which helps your body burn fat instead of sugar. This is beneficial for managing or controlling type 2 diabetes.
  
  
• Cortisol: This is your primary stress hormone. It can go up when you start fasting, especially for women. Too much cortisol for too long can mess with other hormones.
  
  
• Leptin and Ghrelin: These control your hunger. Leptin tells you when you’re full, and ghrelin tells you when you're hungry. Fasting often initially lowers leptin levels and raises ghrelin levels.
  
  
• Thyroid Hormones (like T3): These help control your metabolism (how fast your body uses energy). Long fasts might lower T3 and slow things down, but short or moderate fasts usually don’t cause problems.
  
  
• Sex Hormones (like oestrogen, progesterone, and testosterone): Fasting can help balance these in some people, but it might cause issues in others, especially depending on age, gender, and how often you fast.
  
  
• Growth Hormone (GH): Fasting increases this hormone, which helps burn fat and keep muscles strong.
  
  

Why Do Hormones Change?

Fasting affects two essential systems in your body:

• The Hypothalamic-Pituitary Axis (HPA): This is a pathway between your brain and hormone glands. It regulates functions such as stress, fertility, and thyroid activity. Fasting can alter how this system functions, depending on your stress levels and the nutrients you're consuming.
  
  
• Metabolic Switching: When you fast, your body switches from burning sugar to burning fat. This switch can help you lose fat and repair cells, but it might cause problems for your reproductive system if it goes too far.
  
  

What European Research Says About Intermittent Fasting and Hormones

Scientists in Europe and around the world have been studying how intermittent fasting (IF) affects hormones like testosterone, oestrogen, cortisol, and thyroid hormones. Here’s what they’ve found so far:

Testosterone and Other Male Hormones

A study has shown that intermittent fasting can lower testosterone levels in both men and women. It also raises a protein called SHBG, which helps control how much testosterone is active in the body.

• In women with obesity, this might help with PCOS (a condition that causes high male hormones).
  
  
• In fit, lean men, testosterone levels also decreased, but muscle and strength remained unchanged.
  
  
• Fasting earlier in the day (before 4 pm) intensified these effects.

 

What this means: IF may help women with hormone problems, but men might notice lower testosterone, which could affect things like energy and libido (sex drive).

Oestrogen, Progesterone & Period Health

Some studies have found no significant changes in oestrogen or progesterone (the main female hormones) in most people.

But a newer study warned that fasting can mess with hormone timing in women. This may affect menstrual cycles, although IF can help with weight loss and improve insulin levels.

In older women (after menopause), fasting didn’t significantly change estrogen levels. However, it did slightly lower DHEA (a hormone associated with ageing) and helped alleviate stress and regulate blood sugar.

What this means: IF doesn’t seem to harm oestrogen, but young women should watch their periods, especially if fasting a lot.

Cortisol and Stress Hormones

A study in Europe found that a 24-hour fast made cortisol (a stress hormone) rise earlier in the day. Its strength also went up. Another hormone, DHEA, also increased slightly.

Other studies show different results depending on when you fast:

• Skipping dinner: Lowered cortisol at night but raised it in the morning.
  
  
• Skipping breakfast: Lowered morning cortisol.
  
  
• Ramadan-style fasting (eating only at night) Often messes up expected hormone timing.
  
  
• What this means: Your fasting schedule matters. Skipping breakfast or dinner can affect your stress hormones in different ways.
  
  

Thyroid and Growth Hormone

A study on men who lift weights found that IF caused a slight drop in a thyroid hormone called T3. But their metabolism and energy levels stayed normal.

Many health sites also say that IF boosts growth hormone (GH). This helps your body burn fat and keep muscle during fasting.

What this means: IF is safe primarily for your thyroid. It may even help with fat loss and muscle health.

Quick Summary Table

Hormone	What Happens with Intermittent Fasting (IF)
Testosterone / FAI	Levels tend to decrease in both premenopausal women and men. This can help women with PCOS but may lower sex drive in men.
SHBG	Increases in women with obesity. This helps regulate and balance male hormones, such as testosterone.
Oestrogen / Progesterone	These hormones usually stay the same. However, women who are still having periods should track any changes in their menstrual cycle.
DHEA	Slight decreases in post-menopausal women. The change is minor and may not have a significant impact.
Cortisol	Fasting can change the timing and levels of this stress hormone. The effects depend on whether you skip breakfast or dinner.
T3 (Thyroid hormone)	It may drop slightly with some types of fasting, but most people still keep a normal metabolism and energy level.
Growth Hormone (GH)	Increases during fasting. This helps the body burn fat and keep muscle, especially during longer fasts.

 

Intermittent Fasting and Hormones at Different Life Stages

Intermittent fasting (IF) doesn’t work the same way for everyone. Your age, gender, and hormone levels can all change how your body reacts to fasting. Here’s how fasting affects people at different stages of life:

Women of Childbearing Age

Fertility, Periods & Hormone Changes

Women who are still menstruating may react more strongly to fasting. That’s because hormones like oestrogen and progesterone go up and down during the month.

A study showed that fasting too much or eating too little can lead to irregular periods, especially in women who are very thin or exercise a lot.

But gentle fasting, like eating in a 10-14 hour window (for example, eating from 8 am to 6 pm), may help with hormone balance, especially for women with PCOS, a condition that affects fertility.

Tip: Women should try to avoid long fasts after ovulation when the body needs more energy (the second half of the cycle).

Pregnancy & Breastfeeding

Not Safe for Fasting.

Fasting is not recommended during pregnancy or breastfeeding. At these times, your body needs more food and nutrients to grow a healthy baby and make milk.

Experts in Europe suggest that fasting can lead to a deficiency in essential nutrients, such as iron and folate, which are crucial for babies' growth and development.

Important: Don’t fast during pregnancy or breastfeeding unless a doctor says it’s okay.

How IF Helps Women in Menopause

When women reach menopause, their oestrogen levels drop. This can lead to weight gain, increased blood sugar levels, and mood changes. Fasting may help with some of these issues.

A study found that women who fasted for 16 hours a day had better blood sugar levels and less belly fat.

Fasting might also reduce inflammation and oxidative stress, which are common after menopause.

Tip: Combine fasting with exercise and a balanced protein intake to maintain strong muscles and bones.

Men & Hormones

What Happens to Men Who Fast?

For men, IF mostly has positive effects. However, testosterone (a male hormone) may decrease slightly if fasting is too strict.

One study found that, despite a drop in testosterone levels, muscle strength remained unchanged in active men.

Fasting boosts growth hormone and IGF-1, which help burn fat and keep muscles strong.

Who Should Be Careful With IF?

Life Stage	Is Fasting Safe?	What You Should Know
Women who get their period	Yes, but be careful	Avoid long fasts after ovulation. Track your cycle and eat enough food.
Pregnant or breastfeeding	No	Not safe unless a doctor says it's okay. The baby needs more food and nutrients.
Women after menopause	Yes	Fasting may help with weight, blood sugar, and energy levels.
Men	Yes	Fasting helps with fat loss and energy. Testosterone may drop slightly.

 

What Influences Hormonal Response to Intermittent Fasting?

Intermittent fasting (IF) can help with hormones, but how well it works depends on how you do it and what your body is like. It's not just about whether you fast but when, how, and who is doing the fasting.

Timing: When You Eat Matters

When you eat during the day, it can change how your hormones behave, especially hormones like insulin, cortisol (related to stress), and testosterone.

 

• Early Eating (7 am–3 pm): This aligns with your body's natural rhythm. It can help regulate blood sugar levels and support hormonal balance.
• Late Eating (12 pm–8 pm): Easier for busy people, but might not be as good for hormones. It can raise stress at night and mess with your sleep hormone, melatonin.

 

Tip: Eating earlier in the day usually leads to better hormone results.

Different Fasting Styles

Not all types of fasting affect your body in the same way. Here's a simple guide:

Fasting Type	Eating Window	What It Does to Hormones
16:8	Eat for 8 hours daily	Boosts growth hormone and insulin control. It may affect periods if overused in women.
14:10 or 12:12	Eat for 10–12 hours	Gentler on the body. Suitable for women and people under stress.
5:2 Diet	Eat normally for 5 days, and eat minimally for 2 days	Helps with insulin and hunger hormones. Slight stress hormone change.
Alternate-Day Fasting (ADF)	Eat every other day	It's a big growth hormone boost, but it's more stressful. Not suitable for younger women.
OMAD / 20:4	One meal a day	Powerful effects, but can harm the thyroid and sex hormones if done too often.

 

Warning: Extreme fasting styles (like OMAD) can cause problems with your thyroid, mood, or periods.

Gender & Sleep Patterns

• Women have more sensitive hormone systems. Fasting should be done carefully to avoid disrupting the menstrual cycle.
  
  
• Men can usually last longer, but may notice a decrease in testosterone levels.
  
  
• Morning vs. Night People: If you're a night owl, fasting early may disrupt your sleep or stress levels.
  
  

Stress, Sleep & Body Fat

Fasting works best when you’re sleeping well and not under too much stress.

• If you're already highly stressed, fasting may exacerbate the issue by increasing cortisol levels.
  
  
• Inadequate sleep also disrupts the hormones that regulate hunger and blood sugar levels.
  
  
• People (especially women) with very low body fat may stop getting periods or feel very tired if fasting too much.
  
  

Tip: If your period stops, you lose interest in things (like eating or relationships) or feel super tired. Eat more, shorten fasting, or take a break.

Health & Genetics

Some medical conditions change how fasting affects your hormones:

• Thyroid Problems (like Hashimoto’s): Long fasts can lower thyroid hormones and make you feel worse.
  
  
• Type 2 Diabetes: Fasting often helps manage blood sugar levels, but it should be done under the guidance of a doctor.
  
  
• PCOS (in women): Fasting can reduce male hormones and help with weight, but it must be balanced with enough nutrients and rest.
  
  
  

How to Support Hormone Balance During Intermittent Fasting

Fasting can be great for your health, but if you do it the wrong way, it can upset your hormones. The good news? A few smart changes can help you get the benefits of fasting without hurting your body.

Here’s how to fast safely and feel your best.

Start Slow and Easy

Jumping into long fasts (like 16:8 or eating once a day) can be too hard on your body, especially if you’re new to fasting, under stress, or a woman.

Try this instead:

• Start with 12:12 or 14:10 (eat for 10–12 hours a day and fast for the rest).
  
  
• Don't fast every day; try 3 to 5 days a week.
  
  
• Don't fast for more than 16 hours unless a doctor advises otherwise.

Tip for women: In the second half of your period cycle (after ovulation), your body needs more food, so use shorter fasts during that time.

Eat with Your Body Clock

Your body runs on a 24-hour clock (called your circadian rhythm). Eating at the correct times can help regulate your hormones.

• The best time to eat is between 8 am and 4 pm or 10 am and 6 pm.
  
  
• Try to avoid late-night Eating—it can disrupt your sleep and increase stress.
  
  

Eat the Right Foods

Fasting doesn’t mean skipping essential nutrients. Your hormones need vitamins, minerals, and healthy fats to work properly.

Eat whole, healthy meals with:

• Protein (like fish, eggs, or chicken)
  
  
• Healthy fats (like olive oil or avocado)
  
  
• Vegetables with fibre
  
  
• Good carbs (like quinoa or sweet potato)

 

Drink water and get enough electrolytes (magnesium, potassium, sodium), especially during longer fasts or hot weather.

Listen to Your Body

Your body will often tell you if fasting is working or not.

Good signs:

• Better sleep
  
  
• Stable energy and mood
  
  
• Clear skin and good digestion
  
  
• Regular periods (if you're a girl)
  
  

Warning signs:

• Missed periods
  
  
• Feeling tired, cold, or weak
  
  
• Hair falling out
  
  
• Low interest in food or relationships
  
  

Supplements That May Help

Fasting changes how much of some nutrients your body gets. In some cases, supplements can help your hormones stay balanced:

• Magnesium: Helps you relax and reduces stress
  
  
• Omega-3s: Good for mood and hormone balance
  
  
• Vitamin D: Supports the thyroid and hormones
  
  
• Adaptogens (like ashwagandha or rhodiola) – May help manage stress

 

Important: Talk to a doctor before using any supplements, especially if you have health problems like PCOS or thyroid issues.

Make It Work for You

You don’t have to fast the same way all the time. Changing your fasting routine is healthier.

• Women: Fast more during the first half of your cycle (days 1–14), and take it easier in the second half (days 15–28).
  
  
• Men & older women: You can fast during the week (like 16:8 from Mon to Fri) and relax on weekends.
  
  
  
• Feeling sick, stressed, or travelling? Take a break from fasting.
  
  

Is Intermittent Fasting Safe for Your Hormones?

Intermittent fasting can be a great way to improve your health, but when it comes to hormones, it doesn’t work the same for everyone. It can help regulate hormones such as insulin, cortisol, oestrogen, testosterone, and thyroid hormones, but this depends on how and when you fast.

European research indicates that age, gender, and health all contribute to the risk. For example, women who still have periods might need to fast gently and pay attention to their cycle. Men and older women often benefit from regular fasting schedules. But no matter who you are, good sleep, low stress, and healthy food are just as important as when you eat.

The most important thing? Personalise your fasting plan. There’s no single approach that works for everyone, so listen to your body and make adjustments as needed.

 

If you’re not sure how to start, or if you feel tired, moody, or if your periods are changing, Mobi Doctor can help. They offer online appointments with doctors and hormone specialists all across Europe. You can receive personalised advice, hormone tests, and a customised fasting plan tailored to your body and lifestyle. Waiting rooms are needed.