Fasting Mimicking Diets for Women: Fasting, Hormones, Menopause
What an FMD does, how it compares with water fasting and my experience with ProLon.
Fasting has become an increasingly interesting area of research for metabolic health, insulin sensitivity, cellular repair and healthy ageing — and this is what first drew me to fasting and
Fasting Mimicking Diets.
Weight loss can happen too, of course, but that’s very much a secondary benefit rather than the main reason to fast. If weight has become stubborn or difficult to shift, particularly through perimenopause and menopause, simply eating less isn’t always the answer. We need to understand why the body is holding onto weight in the first place.
Changes in oestrogen can affect insulin sensitivity, muscle mass and where we store fat, particularly around the middle. But hormones are only part of the picture. Blood glucose regulation, stress and cortisol, sleep, thyroid function, gut health, inflammation, loss of muscle and simply not eating enough of the right foods can all influence body composition and make weight loss more difficult.
That’s why the starting point shouldn’t automatically be “How can I eat less?” It should be looking at what has changed and what might be driving the weight gain or resistance to weight loss.
Fasting can be a useful metabolic tool for the right person, particularly where insulin sensitivity and metabolic flexibility need support, but it isn’t a shortcut around addressing those underlying factors.
I’ve water fasted before, including longer fasts, and I’ve now completed the 5-day ProLon Fasting Mimicking Diet (FMD) twice.
I wanted to give ProLon a try to understand how eating a small, structured meal plan designed to put the body into a fasting state would compare with water fasting, which can be difficult, isn’t suitable for everyone and can be particularly tricky for women. I wanted to experience the potential health benefits myself, while also understanding what it’s actually like to do an FMD so I can better guide the women I work with.
What makes fasting so interesting is what happens metabolically and at a cellular level when we temporarily change the body’s usual supply of energy, protein and nutrients — and how fasting might be used through our reproductive years, perimenopause, menopause and beyond.
Water fasting and the 24, 48 and 72-hour milestones
A longer water fast is very different from simply skipping breakfast or extending your overnight fast.
As a fast progresses, insulin falls, glycogen stores are gradually depleted, fat burning and ketone production increase and nutrient-sensing pathways begin to change.
Mindy Pelz, whose Fast Like a Girl approach focuses particularly on fasting for women, uses different fasting lengths for different purposes. Her framework includes longer fasts of up to around 72 hours, with commonly discussed milestones around:
24 hours: deeper ketosis and metabolic changes, alongside a period of digestive rest
48 hours: increasing activity in fasting-related cellular maintenance and autophagy pathways
72 hours: a deeper fasting state, with emerging research around immune and regenerative responses
These are useful markers for understanding what may be happening as a fast progresses rather than exact biological switches.
Autophagy, for example, doesn’t suddenly switch on at hour 48. These pathways change progressively and the timing will vary depending on the individual, their metabolic health, previous food intake, glycogen stores and activity.
A 3-day water fast is a significant intervention and needs to be treated as one.
That includes what happens before and afterwards.
During a water fast, the digestive system doesn’t literally shut down, but digestive activity changes significantly because no food is coming in. After a longer fast, going straight from water to a large or heavy meal can be uncomfortable, so food needs to be reintroduced gradually.
True refeeding syndrome is primarily a concern in people who are malnourished or have had little or no nutritional intake for substantially longer periods, rather than something that automatically happens after a short water fast. But even after a 48 or 72-hour fast, how you reintroduce food matters.
Preparation and refeeding are part of the fast, not an afterthought.
Fasting is different for women
There’s another important consideration with longer fasting: female hormones.
If you’re still cycling, your hormonal and metabolic environment changes throughout the month. Oestrogen and progesterone fluctuate, alongside changes in appetite, energy expenditure, sleep, mood, glucose handling, training and recovery.
Longer fasting shouldn’t necessarily be approached in exactly the same way throughout the month.
Mindy Pelz’s Fast Like a Girl framework uses what she calls “power phases”, placing longer fasts within particular parts of the menstrual cycle and being more conservative with fasting during the later luteal phase.
There is physiological reasoning behind this.
After ovulation, progesterone rises. Resting energy expenditure can increase slightly and many women notice increased hunger, changes in sleep, lower mood, fatigue or poorer exercise recovery in the days leading up to their period.
That may not be the best time to add the additional stress of a 72-hour water fast.
The evidence isn’t strong enough to prescribe one particular fasting day for every woman, so cycle timing needs to be considered alongside symptoms, energy, sleep, stress, training and recovery.
Earlier in the cycle may be an easier time for some women to undertake a longer fast.
Perimenopause becomes more individual again because cycles and hormone patterns can become unpredictable. At this stage, symptoms and how the body is coping can be more useful than simply counting cycle days.
Once we’re post-menopausal there is obviously no menstrual cycle to work around, but female physiology still matters. Muscle mass, protein intake, bone health, stress, sleep and recovery all need to be considered before adding significant nutrient restriction.
For women, more fasting isn’t necessarily better. The type of fast, its duration and its timing all matter.
So, why choose a Fasting Mimicking Diet rather than a water fast?
This is where the Fasting Mimicking Diet becomes particularly interesting as an alternative to a prolonged water fast.
Rather than completely removing food, an FMD provides a small, carefully calculated amount of energy and nutrients while still aiming to create many of the metabolic signals associated with fasting.
The standard FMD is followed for five days.
Calories are significantly reduced, but it isn’t just about calories. The balance of nutrients matters too.
Protein and certain amino acids are deliberately kept low, carbohydrate is controlled and a greater proportion of the available energy comes from fats.
As glucose coming in from food falls and glycogen stores are used, insulin levels reduce and the body starts relying more heavily on stored fat for energy. The liver also begins producing more ketones.
At the same time, changes in glucose and amino-acid availability influence nutrient-sensing pathways including mTOR, AMPK and IGF-1.
Put simply, the body temporarily moves away from a strong growth and nutrient-abundance signal towards one that favours adaptation, energy conservation and cellular maintenance. It’s this metabolic shift, rather than simply eating fewer calories, that makes an FMD different from a five-day low-calorie diet.
But there’s another important difference from a water fast: you are still eating every day, albeit very little.
This can make an FMD more manageable than a prolonged water fast, particularly for someone who struggles with complete food restriction or needs to continue with normal day-to-day life.
It also means the digestive system has continued to receive small amounts of food throughout the five days.
That changes the refeeding experience.
After a longer water fast, food needs to be introduced more carefully because the digestive system has had a period without food passing through it. With an FMD, digestion hasn’t stopped in the same way because small meals have continued throughout.
That doesn’t mean finishing Day 5 and immediately having a huge meal. With ProLon, Day 6 is still treated as a transition day. The advice is to keep food relatively light and predominantly plant-based, using simple foods such as vegetables, soups, healthy fats and complex carbohydrates before returning fully to normal eating.
For women in midlife, the days afterwards are also the time to start bringing adequate protein and nourishment back in, particularly when protecting muscle and bone is such an important part of long-term health.
So although water fasting and an FMD both use nutrient restriction to influence fasting physiology, the experience can be quite different.
With a water fast, there is complete food restriction. It can be more demanding, and after a longer fast more care is needed around reintroducing food.
With an FMD, a small amount of food continues throughout. This can make the five days more manageable and digestion continues, although a gentle transition back to normal eating is still recommended.
There’s also the practical side. With ProLon, the food and macronutrients are already calculated. There’s no deciding what to eat, wondering whether you’ve had too much protein or carbohydrate, or trying to calculate whether you’re still achieving the nutritional conditions the FMD is designed around.
That structure was one of the things I wanted to experience by trying ProLon.
What are the potential benefits of a Fasting Mimicking Diet?
Human research using repeated FMD cycles has found improvements in a number of cardiometabolic markers, including:
fasting glucose and insulin resistance
blood pressure
triglycerides and other lipid markers
waist circumference and body fat
IGF-1
There is also developing research looking at liver fat, immune ageing and biological ageing, making this an interesting area for longer-term metabolic health and healthy ageing.
Autophagy is another reason fasting attracts so much attention.
Autophagy is essentially part of the body’s cellular housekeeping system, allowing cells to break down and recycle damaged or unnecessary components. Nutrient deprivation is one of the signals that influences these pathways. This is sometimes oversimplified into the idea that autophagy suddenly “switches on” at a particular point or that fasting somehow cleans out all our damaged cells.
Human biology is much more complex than that.
A more useful way to understand fasting is that we temporarily change the nutrient signals the body is receiving, influencing pathways involved in growth, energy sensing, stress resistance and cellular maintenance.
Why is this particularly interesting for women in menopause?
Oestrogen does much more than regulate the reproductive cycle. It influences glucose metabolism, insulin sensitivity, fat distribution, muscle, bone and cardiovascular health.
As oestrogen starts to fluctuate through perimenopause and eventually declines after menopause, the body can begin responding differently.
We may start to see:
reduced insulin sensitivity
more fat stored around the middle
changes in fasting glucose, insulin or triglycerides
loss of muscle or greater difficulty building it
changes in energy and exercise recovery
increasing cardiovascular and metabolic risk
And this can happen even when a woman feels she is eating and exercising in much the same way she always has. Research showing improvements in insulin sensitivity and other cardiometabolic markers following repeated FMD cycles is particularly relevant here.
An FMD doesn’t treat menopause, but it may provide another useful tool for addressing some of the metabolic changes that can accompany it.
But don’t menopausal women need MORE protein?
Yes — absolutely.
As we age, our muscles become less responsive to smaller amounts of protein, known as anabolic resistance. Combined with declining oestrogen, this makes maintaining muscle increasingly important.
It’s why my usual advice for women in midlife includes:
enough good-quality protein, spread across the day
resistance training and progressively lifting heavier weights
supporting bone health
eating enough to recover from exercise
prioritising sleep and recovery
So why deliberately reduce protein for five days?
Because it’s temporary.
One of the nutrient-sensing pathways affected by fasting is mTOR. mTOR is sometimes portrayed as something we should continually try to suppress because lower mTOR signalling is associated with some of the cellular responses to fasting and energy restriction.
But mTOR isn’t bad. We need it for growth, muscle protein synthesis and repair.
When I resistance train and then eat a good serving of protein, particularly one providing sufficient leucine, I want that anabolic signal. At 54 and post-menopause, maintaining and building muscle is one of my priorities.
The important part is being able to cycle between different metabolic states.
Most of the time the priority is being well nourished, eating enough protein, resistance training, maintaining muscle and bone and recovering properly.
Occasionally, in the right person, an FMD creates a short period where protein and energy intake fall and insulin, IGF-1 and mTOR signalling are reduced. Then the FMD finishes. Food and protein come back up, glycogen and cellular water are restored, training resumes and those growth and repair pathways are stimulated again.
For women in midlife, fasting shouldn’t become another version of chronic dieting or eating less and less. It’s a short intervention followed by proper nourishment.
Is an FMD good for weight loss?
Most women looking at fasting will understandably be interested in weight loss, particularly when weight that was once relatively easy to manage suddenly becomes much more stubborn through perimenopause and menopause. But if weight has become difficult to shift, simply eating less isn’t always the answer. We need to understand why the body is holding onto weight in the first place.
Changes in oestrogen can affect insulin sensitivity, muscle mass and where we store fat, particularly around the middle. But hormones are only part of the picture.
Blood glucose regulation, stress and cortisol, sleep, thyroid function, gut health, inflammation, loss of muscle and not eating enough of the right foods can all influence body composition and make weight loss more difficult.
That’s why the starting point shouldn’t automatically be “How can I eat less?”
It should be looking at what has changed and what might be driving the weight gain or resistance to weight loss. Fasting can be a useful metabolic tool for the right person, particularly where insulin sensitivity and metabolic flexibility need support, but it isn’t a shortcut around addressing those underlying factors. And although weight is likely to drop during a five-day FMD, it’s important to understand what that number actually represents.
I deliberately didn’t weigh myself either time because weight loss wasn’t why I was doing it.
When food and carbohydrate intake fall significantly, glycogen stores are depleted. Glycogen is stored with water, so as glycogen falls we also lose some of that associated water.
When normal eating resumes, glycogen is replenished and that cellular water comes back.
That’s normal physiology, not failure or sudden fat regain.
There may also be some loss of body fat, particularly when FMD cycles are repeated, but the more interesting potential lies in metabolic flexibility, insulin sensitivity and how periodic fasting might be used alongside nutrition and exercise to support longer-term metabolic health.
Who might an FMD be suitable for?
An FMD can be one tool within a much wider health programme rather than something everyone needs to do.
It may be worth considering where we are beginning to see changes such as:
increasing waist circumference or abdominal fat
declining insulin sensitivity
fasting glucose or insulin beginning to increase
less favourable triglycerides or other metabolic markers
increasing blood pressure
other signs of declining metabolic flexibility
There is also emerging research around FMD and metabolic dysfunction, including fatty liver, and FMD has been studied in carefully selected people with type 2 diabetes under medical supervision.
That doesn’t mean an FMD treats these conditions. It’s about looking at the underlying metabolic pathways and risk factors and deciding whether periodic fasting has an appropriate place alongside nutrition, movement, muscle building, sleep and stress management.
The person always needs to come before the protocol.
Are they already under-eating? Are they getting enough protein? What is their muscle mass like? How well are they sleeping? How much stress are they under? What exercise are they doing and how well are they recovering? Are they taking medication? What is their relationship with food?
Five days of fasting can’t be separated from what someone is doing for the other 360 days of the year.
There are also people for whom an FMD may not be appropriate, including during pregnancy or breastfeeding, if someone is underweight or malnourished, or where there is a current or previous eating disorder. Certain medical conditions and medications also need individual consideration, particularly medication affecting blood glucose or blood pressure.
Caution is also needed if someone is already chronically under-eating, highly stressed, depleted, losing muscle or recovering from illness.
Sometimes the body doesn’t need another stressor. It needs nourishment.
My experience of the 5-day ProLon Fasting Mimicking Diet
This is where the science meets real life.
I chose ProLon because the calories and macronutrients have already been calculated. There’s no trying to work out whether you’ve eaten too much protein or carbohydrate — each day is provided for you.
Having previously water fasted, I wanted to see how it compared.
I’ve now completed the ProLon FMD twice and the two experiences weren’t exactly the same.
The first time, Day 1 was surprisingly easy. Compared with a water fast, it felt much more like a gentle transition. There is more food on the first day and I experienced very little hunger.
Day 2 was noticeably different. My energy dropped and by Day 3, thinking was hard. I wasn’t desperately hungry or unable to function; I just wasn’t mentally sharp and everything seemed to require a little more effort.
By Day 4 something changed. My energy came back, my head felt brighter and I was genuinely surprised by how good I felt considering how little I was eating.
Day 5 was similar, with good energy and very little hunger. I felt amazing!
The second time was slightly different.
Day 2 was hard. I had a headache, very low energy and generally felt pretty rubbish. Day 3 was similar, although not quite as bad.
By Day 4, once again, I’d turned a corner. My energy was back and physically I felt good, although my brain was still slightly frazzled and by day 5 i felt great but ready to eat properly again.
That difference between the two experiences is important because our response to fasting doesn’t happen in isolation.
How those five days feel can be influenced by:
what you’ve been eating beforehand
carbohydrate intake
caffeine and alcohol
hydration
sleep
stress
exercise and recovery
Preparation matters.
And just as the refeed matters after a longer water fast, the rebuild after an FMD matters too.
Five days of low protein and low energy intake shouldn’t simply be followed by returning to poor-quality food and forgetting about protein, muscle and recovery.
The fasting period is only one part of the process.
Can you do a Fasting Mimicking Diet without ProLon?
Yes — but this is where understanding the nutritional composition of an FMD becomes important.
The big benefit of ProLon is that everything has already been calculated for you. You don’t have to work out how much protein, carbohydrate or fat you’re eating or whether your food choices are taking you away from the fasting-mimicking environment you’re trying to create.
An FMD isn’t simply about eating 700 or 800 calories of whatever you fancy.
The macronutrient balance matters.
Protein and certain amino acids need to remain sufficiently low to influence nutrient-signalling pathways such as mTOR and IGF-1. Carbohydrate also needs to be controlled, while the right balance of fats and nutrients makes the five days more manageable.
And this is exactly what I’m currently working on.
I’m developing my own Functional You Fasting Mimicking Diet programme, using real food and carefully calculated calories and macronutrients so you can follow an FMD without relying on a pre-packaged box.
But it will go further than simply giving you five days of recipes.
The programme will cover how to prepare beforehand, the nutritional balance during the FMD, what you may experience as your metabolism adapts, specific considerations for women and menopause, and how to reintroduce food and properly nourish your body afterwards.
Whether it’s a longer water fast or a Fasting Mimicking Diet, the fast itself isn’t an isolated event.
The preparation matters.
The fast matters.
And the refeed and rebuild matter.
Prepare. Fast. Rebuild.
I’m working on the programme now.
Functional You Fasting Mimicking Diet Programme for Women.
Good luck and good health.
Diane
Comments