Fasting: Is it different for women? A guide

Most people are striving to achieve some sort of weight-loss goal, to perform at a higher level or to simply slow down the dreaded aging process. Whether that is driven by a personal goal, or by some external factor, they’re doing everything they can to get there! Women in particular are jumping on the bandwagon of the latest trends like intermittent fasting, strict keto, one meal a day (OMAD), and even extended fasts, and all too often, they’re following right in the footsteps of their male counterparts. Humans are human, right?

No, humans are actually a rather diverse bunch. Each person has attributes and characteristics that are uniquely theirs. Biological sex is the most obvious, making men and women different in so many ways. People may have the same hormones and neurotransmitters, breathe oxygen and use the same fuels to function, but they differ in how they’re used in the body. It is the finer details that set women and men apart.

Women Are Unique

There aren’t many women who have always had a healthy relationship with their weight. Most women battle in one way or another with their body image, striving to achieve this idealistic representation of how they’re supposed to look, feel and perform. Sadly, these perceptions are primarily fueled by misinformation from the media, and a lack of understanding about the biological uniqueness of females.

The reality is that almost all of the information available is based on research done on men, that is if it’s based on research at all. Women are difficult to do research on! With the complexity of the fluctuating hormones during the menstrual cycle, conducting a research study on a group of women is expensive, and co-ordinating the timing of female’s menstrual cycles to control for variations in hormone levels is an insurmountable task. The answer seems to be to do it on men, and to extrapolate from there.

Too many women are hard-charging, type-A personalities who believe that the more they fast, train and restrict carbohydrates, the healthier and more successful they’ll be. They’re pushing themselves to the very limit, to the point where they override their body’s signals all too often. It’s as if it’s a competition out there! This excessive stress on the body is doing more harm than good, causing disrupted hormone levels which can lead to many issues including:

• Irregular menstrual cycle
• Complete loss of the menstrual cycle (amenorrhea)
• Infertility
• Hormone imbalances which leads to skin, mood, libido, energy levels and weight issues
• Bone, cardiovascular and neurological complication

While some may think not having a period is a convenience, think twice. The reproductive system and all of its hormones are responsible for a woman’s overall health and vitality! Bone mineral density, cardiovascular health and brain function are all negatively affected when sex hormones are out of alignment. Even if having babies isn’t a priority for someone, the female body is still wired to reproduce and healthy sex hormones are an important indicator or overall health.

As Stacey Sims puts it, “We are not small men.” This is exactly true, and to put some context to the relationship between reproductive hormone levels, and energy balance, let’s take a look at the female reproductive role and the menstrual cycle.

An overview of hormones of the menstrual cycle

A typical cycle lasts on average 28 days (but can range from 21-35 days) and consists of two main phases: the follicular phase, and the luteal phase. The key hormones involved in the menstrual cycle are luteinizing hormone (LH), follicle stimulating hormone (FSH), estrogen and progesterone. Day 1 marks the first day of menstruation and begins with the follicular phase which lasts until ovulation. Just prior to ovulation, LH, FSH and estrogen all rise. After ovulation, the luteal phase begins which is about 14 days and a drop in LH, FSH and estrogen occurs while progesterone rises. Estrogen spikes mid-luteal phase and if no egg is fertilized, menstruation occurs again, circling back to day 1.

This complex interplay of hormones is regulated by the hypothalamic-pituitary-gonadal (HPG) axis, and is tightly associated with the hypothalamic-adrenal axis and the thyroid! This means that even minor changes in one hormone can cause disruptions to the levels and function of several other hormones and bodily systems down the line. Things like adrenal fatigue and hypothyroidism are commonly seen when reproductive hormones are disturbed. Any disruptions to this complex and intriguing network in the body and brain can cause irregular periods, infertility and other health problems.

Fasting as a hormetic stressor

More and more research on fasting continues to reveal its impressive benefits for everything from neurodegeneration and cancer to longevity and heart health. One it’s biggest claims to fame is being a means of calorie restriction which is a great way to shed some weight; the very reason many women adopt some sort of fasting, whether it’s the 16/8 intermittent fasting, a 24-hour or longer extended fast.

Fasting is, by definition, a hormetic stressor – a healthy stress placed on the body that triggers an adaptive response making the body more resilient and healthier. Other means of hormesis are cold exposure, sauna and exercise. As with any of these, too large a dose – either too long a duration or too frequently – can become a real stress on the body that can cause damage to our defense mechanisms, or death! What’s more, the dose is relative and what is hormetic for one person may be a real stress to someone else.

These types of hormetic stressors can be controlled, but there’s a whole lot more stress that people don’t have full control over like financial stress, broken relationships, poor sleep, and work stress!

Often, the problem is that those who practice some sort of fasting are likely doing other things simultaneously that add to their total stress load, which is called the ‘allostatic load’. Caloric restriction, and carbohydrate limitation often go hand in hand with fasting, and are stressors on their own. And because women tend to want to do everything at once, they’re likely to be doing frequent HIIT workouts before a full day of work, followed by the sauna and perhaps some cold exposure after that. That’s a lot of stress!

The hormones mentioned above are very sensitive to all kinds of external stressors. This is exactly why females need to pay special attention to their allostatic load, or the amount of stress their body is exposed to at one time. Negative energy balance, especially when combined with other sources of stress, is known to cause imbalances in women’s hormones, which could very well lead to irregular menstrual cycles or even amenorrhea, and have other negative consequences on women’s health.

Females are in fact more sensitive to changes in dietary intake compared to men. And so when they follow in the footsteps of their male counterparts, who are fasting, restricting carbohydrates, exercising, doing the sauna and the ice bath, and performing better than ever, they find themselves fatigued, in a bad mood, having low libido and maybe even gaining weight, and think they’re failing. To fix this, they continue to add more; fast for longer, exercise harder, etc., pushing for those same results.

The link between allostatic load & reproductive function

Reproduction in females requires a whole lot more energy and resources than it does for males! Pregnancy, lactation and the nurturing of the baby consume tons of energy – any mother would agree.

Under a state of metabolic stress, it is a natural instinct for animals to invest all of their energy into survival rather than reproduction. This is why reproductive function is dependent on energy availability. When the body isn’t getting enough calories and perceives a stressful environment too often, the reproductive system is the first to shut down in attempt to conserve energy and resources that can be used for more important bodily functions.

This is where the concept of the allostatic load is so important to be aware of for women. Reproduction, although necessary for propagation of the population in the long run, is not essential for survival, and so menstrual cycle dysfunction are clear signals that something is wrong.

What causes disruptions to the menstrual cycle?

There are many peripheral hormones like leptin, ghrelin, estradiol, and neuropeptides like NPY and kisspeptin that play a role in regulating reproductive function in humans.

There is a lot to be discussed but this article will focus on how 3 key players: leptin, neuropeptide-Y (NPY), and kisspeptin play a pivotal role in energy balance and reproductive function. The reality is that all of these hormones are intertwined, and each have some sort of downstream or upstream effect within the HPG and HPA axis.

Leptin is a hormone that assesses your body’s energy availability. It is produced by fat cells as well as in the small intestine and has receptors on the hypothalamus, which is why it is involved in the HPA and HPG axis. Energy or caloric restriction has been shown to decrease levels of leptin since there is a weak or no signal of being fed. A decline in leptin causes levels of NPY to increase and kisspeptin to decrease, which, as energy sensors of the body, inhibit the release of gonadotropin-releasing hormone (GnRH). NPY is a neuropeptide that  stimulates feeding behavior and kisspeptin is responsible for metabolic control of the reproductive axis by controlling GnRH release. GnRH is responsible for signalling to the ovaries to secrete sex hormones like LH, FSH, estrogen and progesterone and so low levels cause hormonal imbalances.

In summary:

CALORIC RESTRICTION → ↓ leptin → ↑ NPY ; ↓ kisspeptin → ↓ GnRH → ↓ LH = AMENORRHEA / IRREGULAR CYCLES

Although there are more hormones and neuropeptides involved in energy homeostasis, this is a basic outline of how energy restriction sends a signal of scarcity to the brain and causes physiological changes in the body.

What the research shows

The majority of studies on fasting have been conducted on rats, with very few being done on humans. While many experts claim to know the ‘how, when and who’ of fasting, there really is no clear consensus on what is ‘good’ or ‘bad’ for women. One study that looked at the effect of intermittent fasting (IF) on young female rats showed that IF dietary restriction negatively impacted the hypothalamic-gonadal axis (HPG). The results showed that body weight, ovarian weight, LH, GnRH, kisspeptin were all reduced in females.

Another study done on humans examined the effects of alternate day fasting on blood glucose and actually showed that women’s, but not men’s, blood glucose tolerance was impaired. The length of the fast was 36 hours, every other day, and the average weight loss was 2.1kg which showed that they could not eat enough to maintain their weight. This is important, as perhaps by choosing a shorter fasting period in which you are able to maintain an adequate energy intake, might be ok.

An interesting study showed a shift towards sympathetic dominance indicating an increase in stress after a two day fast. Men, on the other hand, showed more parasympathetic dominance appearing to be more relaxed, and their cognitive performance improved.

Similarly, 40% caloric restriction caused female rats to become emaciated, stop the menstrual cycle, undergo endocrine masculinization, and increase their stress response and activity significantly. Clearly these responses show that the rats diverted their energy production from reproduction towards brain and neuromuscular function to increase their chances of survival. Interestingly, males didn’t show these results and in fact maintained higher body weight.

Another catch to this topic is that women go through phases where hormones are shifting dramatically from pre- to peri-, to post-menopausal states, making blanket statements for women even further inaccurate. Mark Sisson explains his take on how intermittent fasting may affect females at different phases of menopause: “I’d be inclined to agree that pre-menopausal (and perhaps peri-menopausal) women are more likely to have poor—or at least different—experiences with intermittent fasting (at least as a weight loss tool). That said, it appears to be a potentially gender-neutral therapeutic tool for chemotherapy, cancer, and age-related neurodegeneration patients.”

Fasting & the female athlete

Studies suggest that even male athletes who eat regularly throughout the day struggle to consume enough calories to meet their energy demands. Also, there is no evidence that shows that fasting improves performance at all, and so female athletes training in a fasted state for 2-3 hours at a time might be harming their performance.

Getting back to the concept of fasting being a hormetic stressor, it is important to remember that the dose is relative. What is hormetic for one person, might be a plain stressor for another.

Many female athletes are already lean and possibly already in a calorie deficit. Being lean (depending on how lean) is probably sending a message of scarcity to the body, and being in a calorie deficit is definitely sending a message of scarcity to the body. Fasting, in this case, may stimulate a far too powerful response in the body. As an athlete, recovery, building strength and adapting to exercise is a priority, and a body in survival mode is definitely not going to achieve any of those goals.

There are questions women should consider before following a strict fasting routine, like how often do they exercise, what is the macronutrient ratio in their diet, do they have any hormonal imbalances already, and are they training for performance or weight loss?

Training in a fasted state, especially early in the morning can cause cortisol levels to rise, and perhaps stay elevated for a while. Cortisol, when chronically elevated, can lead to muscle catabolism and a heightened stress response.

Women who probably shouldn’t be fasting

While there are many positive health outcomes to be had from fasting, not everyone is in a position to benefit from it. Food restriction can be very detrimental in some cases. Here are some situations where people should probably be avoiding fasting.

• lean (<18%), or underweight
• high-performing athletes
• young women/teenagers
• pregnant or lactating mothers
• those with a history of eating disorder
• those with hypothyroidism
• those with amenorrhea or an irregular menstrual cycle

There are various types of fasting that you can choose from that will suit different lifestyles. To the hard-charging women out there, it is important to take a cautious and healthy approach based on what has been discussed above. Here are three types of fasting that are likely to place the least amount of metabolic stress on women:

1. Intermittent Fast (16/8): Two to three days of intermittent fasting on low-activity days, with regular eating on high activity days
2. ‘Eat-stop-eat’: Practice a 24-hour water-fast
3. Caloric-liquid fast: Practice a longer three to four day fast while consuming low calorie beverages like bulletproof coffee, bone broth or teas.

A few points women should remember while fasting

• Avoid HIIT, prolonged exercise, especially in the morning
• Prioritize sleep
• Ensure adequate nutrients, protein and overall calories in the refeeding period
• Assess overall stress load and make sure to have time for rest and recovery

There is no clear cut answer on exactly when during the menstrual cycle is the best time to fast, however, an idea can be had based on the rise and fall of certain hormones. Everyone will respond differently at certain times of the month, making bio-individuality king here. The key is self-experimentation to determine when someone is most ready for a fast.
Fasting can certainly be done by healthy individuals when done correctly. The benefits it provides can be extremely supportive of overall health and obese individuals may benefit most given their metabolic state as things like insulin and leptin sensitivity as well as metabolic flexibility are improved.

One of the greatest benefits of fasting is improving the body’s ability to switch between burning carbohydrates and fats for energy. Peter Attia explains that “metabolic switching, refers to the preferential shift from the use of glucose as a fuel source to the use of fatty acids and ketone bodies. Your ability to toggle back and forth between these two metabolic states is an indicator of your metabolic flexibility.”

The great thing is that there is no need to fast ALL the time, and for too long to improve metabolic flexibility. Making sure to reach a 12 hour fast overnight is more than sufficient, and probably a safer approach to improving metabolic flexibility for most women than doing daily 16 to 18 hour fasts or frequent 24+ hour fasts.

Follow to part 2 of this article where we cover Keto diet for women.