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Should Women Fast?

Intermittent Fasting has been in the headlines for the last few years now and there is lots of conflicting information around it. Should you even fast? Should menstruating women fast? What about those with thyroid conditions? These are questions I get asked a lot. Lets delve deeper into what Intermittent Fasting actually is and if it would be beneficial for you.

What is Intermittent Fasting?

Intermittent fasting (IF) is any time period where you have chosen to go without food. It is not technically a "diet" as you do not restrict your food, you just restrict the times you eat. These times are commonly known as an "eating window" and a "fasting window".

Common methods include:

  • Time restricted eating includes 16:8 in which only eat during an 8-hour period each day, and fast for the remaining 16 hours (for instance, you may eat between noon and 8 PM, and then fast between 8 PM and noon the next day)

  • 5:2 (where on 5 days of the week you ear normally but restrict calories to 500-600 on two other days).

  • OMAD is an acronym for One Meal A Day. This is when you eat one large meal a day that consists of a variety of foods to ensure you get adequate nutrients.

  • ADF is an acronym for Alternate Day Fasting means eating one day and fasting the next day.

The time restricted eating method can be altered to any time you decide eg; 19:5, 20:4, 22.2. The 16:8 method is the most popular, as most people find it to be easier to stick to, and more sustainable in the long term. For example, if you fast for 16 hours your "eating window" would be 8 hours long.

Isn't this just skipping meals?

The short answer is NO. Skipping meals means reducing your calorie intake for the day. Intermittent Fasting means eating your full calorie requirement during a shorter time frame, eg the eating window. Skipping meals deprives your body of nutrition which your body needs to give you energy. By eating your normal amount of food in a shorter eating window does not deprive you of these much needed nutrients.

Some people may naturally reduce their calorie intake on a shorter window, and this is where planning your meals becomes important. If you are going to reduce your intake, you must make sure the food you eat is nutritious and your body has an adequate supply of vitamins, minerals and macronutrients.

What does Intermittent Fasting Do?

Research suggests that the beneficial metabolic changes associated with fasting start at around 12 hours, though the benefits increase the longer you fast for. Sixteen hours seems to be the ideal amount for optimal benefits – but you can build up to that over time, or you can stay at 12 or 13 hours. Your body will benefit, and you will notice big changes in how you look and feel over time.

There are a number of benefits of fasting, which can include:

  • Insulin levels in the blood drop, which helps the body to burn fat

  • There are positive changes in genes which can impact longevity and protect against disease

  • Cellular repair processes are initiated (Autophagy)

  • Levels of human growth hormone increase, leading to muscle gains and fat burning

  • Increased brain function

  • Weight loss

Is Fasting Safe for Women?

There have been concerns raised on the potential negative impacts of intermittent fasting on women and their reproductive cycles. There have been studies using female rats with extreme calorie restriction that have show menstrual irregularities.

The first study on albino rats showed irregular cycles after excessive fasting. The long-term caloric restriction reduced the satiety hormone leptin in female rats disrupting the menstrual cycle. As a result, kisspeptin production in the hypothalamus was inhibited. Since the peptide hormone regulates the release of GnRH, the fertility hormones LH and FSH were also reduced (Kumar et al. 2013).

The second study showed a 31% increased chance of irregular periods in female rats subjected to the same fasting schedule, with minimal reduction in estrogen levels. In contrast, periods were utterly absent in 91% of those female rats that ate daily but reduced calories by 40%. In addition, estrogen levels decreased markedly (Martin et al. 2007).

It should be noted that the rats in the first study were only three months old, which would correlate to nine years of age in humans. The rats also were subjected to ADF, eating one day and fasting the next. The human equivalent is eating for one week, then fasting for a full week! However, this study raised concerns about the potential negative impact of this or other fasting regimens on cycling women

In short, extreme periods of starvation or other stressors put offspring’s health at risk. Therefore, the body focuses on survival instead of reproduction.

How should Women Fast?

In a meta analysis of over 44 peer reviewed studies completed on women and fasting, the study concluded that “fasting can be prescribed as a safe medical intervention, as well as a lifestyle regimen which can improve women’s health in many folds.” Specifically, they found 5 different categories where fasting helps women. These categories included Cancer, Reproductive health, , Musculoskeletal health, Mental Health & Metabolic Health.

Dr Mindy Pelz is a well known expert on fasting for women. She recommends that menstruating women under the age of 40, do not attempt longer fasts during the last week of the luteal phase of your cycle (Day 21-28). This is when your body needs to make high levels of progesterone. (longer fasts means 24+ hours). You need to be eating adequate carbohydrates to focus on building your progesterone levels up.

Women aged 40-55 experience a time of shifting hormones. Progesterone and oestrogen levels are starting to decline. Your adrenal glands will be making your sex hormones once your ovaries stop producing them. It is vital to lower your stress levels so your adrenals are not pumping out high levels of stress hormones, ultimately reducing the output of your sex hormones. You can still fast the same as a women under the age of 40 but be aware of your changing hormone patterns and adjust accordingly.

If symptoms such as irregular spotting, insomnia, weight gain, anxiety, irritability, PMS or PMDD occur this may be due to low progesterone. This is when you may need to reduce the amount of hours you are fasting and start increasing the foods that will help increase your progesterone levels.

In my experience of Intermittent Fasting, I have found 19:5 to generally be the best fasting and eating window. I intuitively break my fasts earlier during my luteal phase to 16:8. I change my fasting times according to my cycle and I am never too strict with myself. The rigidity can cause more stress and I am at the age where my adrenals need to be nourished.

In women over the age of 55 or have experienced early or surgical induced menopause, you have more flexibility in the hour you fast but you still need to be eating hormone building food and be aware of any negative changes in how you feel.

General Guideline for Menstrual Cycle Fasting: (according to Dr Mindy Pelz)

Follicular Phase is the first two weeks/Luteal Phase is the 2nd 2 weeks. These are further broken down into:

Days 1-10 - Menstrual phase 13-72 hours fasting (trying for minimum of 17 hours)

Days 11-15 - Ovulation Phase 13-15 hours fasting, increasing hormone boosting foods

Days 16-18 - Early Luteal Phase 13 - 72 hours fasting (trying for a minimum of 17 hours)

Days 19-beginning of Bleed - Late Luteal Phase - No Fasting increasing hormone boosting foods

Every person's cycle is different and most will not be exactly the same as the days above. This where tracking your hormones and working with a health practitioner that understands hormonal fasting is important. A Dutch Hormone test can help give you insight into how your hormones are doing and help you figure when to fast and when to be supporting hormone production.

What about Under-active Thyroid or Hashimoto's?

With usual low calorie diets you are consuming less food to lose weight, but even in healthy people, this works only short term. Your body has to work economically. The less you eat, the less energy your body can spend. For this reason, metabolism can slow down on calorie-reduced diets, making it harder to lose weight. For people with Hypothyroidism or Hashimoto's, their metabolism is already slow and the situation is compounded by reducing calories.

Intermittent fasting is, however, different from the typical calorie-reduced diet. The body is able access stored energy in the form of body fat. Once we access this energy there is no shortage. When practiced correctly, intermittent fasting does not slow down metabolism or energy expenditure.

However, there are still not many studies assessing the effect of (intermittent) fasting on hypothyroidism. In one study, prolonged fasting led to a decrease in T3. But this was only temporary. As soon as the study participants ate, T3 was back to normal.

In another study, Ramadan fasting showed no effect on thyroid hormones. During Ramadan fasting, people only eat and drink between sunset and sunrise, for one month. Usually people will eat a big meal before sunrise or alternatively eat a meat after sunset.

Intermittent fasting has shown to reduce both insulin levels and inflammation, which is beneficial to anyone with Hashimoto's or hypothyroidism. You may want to start slowly by pushing your breakfast back by an hour a day until you get to the 16:8 fasting/feasting windows. Make sure your meals are nourishing and you are not reducing your carbohydrate intake to less than 50g per day.

Key Takeaways

  1. If you are aged 40 or under, adopt a flexible fasting schedule. Track your cycle and as every other day or a few days a week, so that you don’t risk the possibility of throwing off your menstrual cycle. This approach is in contrast to a more regular fasting schedule that can be followed by a woman north of fifty five and have are in menopause.

  2. Fasting is beneficial in specific circumstances. Women with Thyroid issues, PCOS, Type 2 Diabetes can benefit with intermittent fasting.

  3. Listen to, feel and acknowledge your stress levels. If you’re under a lot of stress, it may be best to postpone fasting until your situation becomes more manageable. Remember, when you fast, cortisol goes up, which can lead to imbalances in both oestrogen and progesterone. It can even lead to a loss of your period. Not getting your period is a sign that your body is under too much stress to fast! To manage stress proactively.

  4. Get plenty of nutrients during your feeding window—and at other times when you are not fasting. Do not focus on calorie restriction.

I use Cyclic Fasting with my clients on my 12 week Skin Hormone Program with great success. You can find out more here

For further reading on everything to do with Intermittent Fasting these are my favourite books:


Van Gaal LF, Vansant GA, De Leeuw IH. Factors determining energy expenditure during very-low-calorie diets. Am J Clin Nutr. Jul 1992;56(1 Suppl):224S-229S. doi:10.1093/ajcn/56.1.224S

Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. Oct 2017;39:46-58. doi:10.1016/j.arr.2016.10.005

Azizi F. Effect of dietary composition on fasting-induced changes in serum thyroid hormones and thyrotropin. Metabolism. Aug 1978;27(8):935-42. doi:10.1016/0026-0495(78)90137-3

Sheikh A, Mawani M, Mahar SA. Impact of Ramadan Fasting on Thyroid Status and Quality of Life in Patients with Primary Hypothyroidism: A Prospective Cohort Study from Karachi, Pakistan. Endocr Pract. Oct 2 2018;24(10):882-888. doi:10.4158/EP-2018-0038

Sulimani RA. The effects of Ramadan fasting on thyroid functions in healthy male subjects. Nutrition Research. 1988/05/01/ 1988;8(5):549-552. doi:

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