Intermittent Fasting for Women: 7 Research-Backed Side Effects

Written By


Expert Reviewed By

Dr. Lauryn Lax, OTD, MS

Dr. Lauryn, OTD, MS is a doctor of occupational therapy, clinical nutritionists and functional medicine expert with 25 years of clinical and personal experience in healing from complex chronic health issues and helping others do the same.

Intermittent Fasting for Women 101 

Intermittent fasting for women — or “time restricted eating”, within an 8 to 12 hour time frame— is so hot right now. A common approach intermittent fasters take is: stop eating at 8 pm, skip breakfast, eat the first meal sometime between 10 a.m. and noon, although some go as long as 20 or 24 hours without eating.

Intermittent Fasting For Women - Woman Measuring Her Waistline

Intermittent Fasting Benefits

Proclaimed benefits of breakfast-skippers and butter coffee drinkers include:

  • Allows your gut time to “rest and digest”
  • Decreased insulin
  • “Can help you lose weight”
  • Lowers cholesterol
  • Reduces oxidative stress and inflammation
  • Repairs cells and tissues
  • May help prevent cancer
  • Improves mental clarity and focus
  • May prevent Alzheimer’s
  • May extend your lifespan

While these are amazing benefits, what blog articles on Dr. Google DON’T tell you is that, despite the recent popularity of intermittent fasting and associated weight loss claims, the supporting evidence base in humans remains small and there is only one published systematic review examining the health benefits of this approach.

In other words: contrary to popular belief, intermittent fasting may not be the necessary ‘panacea’ that it’s made out to be for everyone (especially if you have two X-chromosomes…ahem, women).

Here are 7 surprising research-backed finds from the literature on the side effects of intermittent fasting for women (and men too) and why I don’t do intermittent fasting formally. 

Intermittent Fasting for Women: 7 Research-Backed Side Effects

  1. Can Disrupt Your Hormones
  2. Muscle Loss Frequently Occurs (Instead of Fast Loss)
  3. Can Deplete Energy
  4. Increases Cortisol
  5. It’s Better to Eat Earlier in the Day
  6. Exercise Mimics Intermittent Fasting
  7. Food Restriction Does Not Increase Lifespan

#1. Can Disrupt Your Hormones

Intermittent Fasting For Women - Woman Feeling Unwell

First things first, if you’re a woman, your hormones—the conductors of your body’s orchestra—are sensitive to meal timing and food intake. Intermittent fasting disrupts estrogen balance. This is because our hormones (especially estrogen and its right wingman, progesterone) are designed for fertility (nurturing and cultivating life). And a woman’s fertility is highly connected to her metabolic function.

So, anytime a woman sends a “starvation signal” to her body (like intermittent fasting), it goes into “protect my young”, where it holds onto weight (to survive the fast), increases production of the hunger hormones ghrelin (hunger hormone) and leptin resistance (fullness resistance) (so that you feel hungry and try to get food ASAP), and slows down non-essential functions like reproduction, metabolism, energy and cognitive function (so you can stay alive and not expend excess energy on growing a baby).

Basically, in order to safely grow a baby, estrogen and progesterone need nutrients. Without them, or consistency with nourishment, your hormones may go a bit bonkers (even if the fasting is not extreme). Animal studies highlight this phenomenon: in one trial, female models who did intermittent fasting for 12 weeks had decreased gonadal glands (ovaries) and increased insomnia compared to male mice who maintained their testes and sleep routine, and these shifts began as soon as two weeks after the females began intermittent fasting. Even if you are not trying to get pregnant or you’re post-menopausal, estrogen and progesterone still play an important role in maintaining good bone health, brain health and cognitive function, heart health, sexual health (libido, cycling), histamine regulation, detoxification and immunity.

Common signs of estrogen imbalance includes:

  • Poor blood sugar control
  • Osteopenia/osteoporosis
  • Brain fog
  • Low energy
  • Hair loss
  • Skin breakouts
  • Infertility
  • Poor heart health
  • Weight loss or weight gain resistance
  • Menstrual irregularities, PMS or amenorrhea

Additionally, a disruption in one hormone system in the body can trigger other hormone imbalances—especially cortisol (stress hormone) and thyroid hormone. Signs ad symptoms that you’ve got cortisol issues include: anxiety, low energy, feeling “wired and tired” at night, sugar or coffee cravings, blood sugar imbalances and insomnia, and thyroid issues tend towards: anxiety, depression, hair loss, weight gain, dry skin and hair, irregular periods, shortness of breath, hot flashes or cold really easily and overall, a sluggish metabolism.

Hormone imbalances explain why eating less calories or eating less frequently (such as 1 meal per day) slows down metabolism. You need fuel to rev hormones and metabolism back into gear. 

#2. Muscle Loss Frequently Occurs (Instead of Fast Loss) 

Have we been wrong about intermittent fasting all along?

That’s what research in JAMA seems to suggest. The study assigned 29 subjects to intermittent fast and eat their meals between noon and 8 pm each day for three months, while 57 participants followed a normal dietary schedule. Subject in the intermittent fasting group only lost 2 to 3.5 pounds, not much different than the control group. Even more, the majority of their weight loss was lean muscle mass—not body fat.

One explanation for this may have to do with the decreased protein and overall energy intake (muscle needs fuel). Additionally, fat mobilization in adipose tissue (AT) has a specific timing in accordance with circadian rhythms. Delayed meal timing earlier on in the day, accompanied by later day and nighttime eating for the bulk of nutrients disrupt both the circadian clock and body’s tendency to hold onto body fat. 

#3. Can Deplete Energy

Intermittent Fasting For Women - Woman Feeling Tired

Accidental dieting or under eating is a common occurrence in intermittent fasting—primarily because intermittent fasters shift from 3 meals and/or snacks to 1 or 2 meals per day, and perhaps a morning cup of coffee.

Intermittent fasting studies in healthy Ramadan fasting subjects (folks who are “expert” intermittent fasters during the 30 day holiday) reveal significantly decreased fitness performance—especially for anaerobic (quick twitch, power output) activities and increased sleepiness. One study in healthy, athletic basketball players also showed both males and females experienced a sharp decrease in their heart rate  variability (HRV)—associated with stress and fatigue, along with decreased quality of sleep, pronounced low energy and increased muscle soreness.

You need food and glucose in your bloodstream to fuel your body for energy, so if you’re fasting long enough and on an empty stomach, you will feel tired. Additionally, intermittent fasting can throw off your body’s circadian rhythms (your biological clock), enhancing fatigue and dysregulating cortisol. 

#4. Increases Cortisol

Speaking of cortisol—our stress hormone, stress is inevitable in life, however excess stress—without proper recovery—increases oxidative stress and inflammation in the body. (Think: suppressed immunity, increased energy, hormone imbalances, gut imbalances, impaired detoxification and more).

Just like running a marathon seven days in a row, trying to squat 500 pounds on a barbell or working out 3 to 4 times per day would be stressful on the body, meticulously avoiding fuel if your body needs fuel for energy is stressful.

Intermittent fasting is “dose dependent” depending on the lifestyle you lead.

If you lead a slower paced lifestyle, you sleep a solid 7 to 9 hours each night, your work is low stress, you eat a variety of nutrient dense foods, you’re hydrated and maintain an amazing mind-body and detox practices (deep breathing, yoga, sauna, etc.), intermittent fasting may serve you well.

However, if you’re running on 5 to 7 hours of sleep, waking up at 5 or 6 a.m., trying to scale a business or working 2 jobs or hitting the books in school, you’re recovering from chronic illness or an eating disorder, you love training and working out, and/or you’re managing other humans (ie. A busy parent), intermittent fasting may not be your BFF (at least from a stress perspective). 

#5. Alters Circadian Rhythms

Contrary to popular belief, human physiology and circadian rhythms are wired to eat fuel earlier on in the day—when the sun is up, followed by tapering food intake down as the day goes on. The thermic effect food (TEF) and metabolic “fire” is greater in the morning compared to the evening, as is digestive “fire” and hormones that govern blood sugar and body balance, like Insulin, Leptin Cortisol, Ghrelin and Melatonin.

Intermittent fasting schedules can hijack the “normal” circadian rhythms of our digestion, metabolism and hormones. While the circadian clock can seemingly adapt to your schedule, if you keep a consistent schedule (such as coffee at 8 am, meal 1 at noon, meal 2 at 5 or 6 pm), this does not necessarily benefit your body more or mean your timing is ideal.

Delaying meals by 4 to 6 hours (such as waking up at 7 or 8 a.m. and eating at noon) results in the significant delay of PER2—the primary circadian pacemaker in our brain, which regulates the daily rhythms of energy, hormones, metabolism, and behavior, and is suspected to be the cause of the many health implications associated with late night or irregular meal timing (like insomnia, weight gain or weight loss resistance, bloating and constipation and fatigue), even if you eat the same amount of calories or less in a day.

While there is no such thing as the “perfect time” to eat, we do know that eating protein in particular within the first two hours of waking sets the tone for your blood sugar regulation and metabolic function (energy) for the entire rest of the day. The same thing happens when you eat an earlier dinner (6 pm vs 9 pm) after a normal eating schedule of breakfast and lunch—balancing blood sugar the entire next 24 hours.

Additionally, eating the majority of your fuel earlier in the day allows for both optimal liver detoxification and muscle tissue repair to occur while you sleep. Furthermore, circa- dian clock gene regulation in peripheral tissues is likely to be responsible for reducing nutrient absorption in the evening, allowing for more readily available fuel to prepare for the onset of fasting.

What this means for you? Fueling up earlier on with balanced meals—particularly protein—mitigates overeating and binging, hypoglycemia, fatigue, caffeine, carb and sugar cravings as the day progresses. Late night eating and snacking is a natural response to reduced glucose regulation and avoidance of calories earlier on in the day, if not emotionally intertwined. 

#6. Creates Unnecessary Food Rules 

Intermittent Fasting For Women - Woman Not Eating

“No eating until noon.”

“Nothing in your coffee—just black.”

“No oxalates or starch or omega 6 fatty acids in chicken and eggs.”

Intermittent fasting is often accompanied by its fair share of rules, regulations and expectations—enough to make you feel overwhelmed, anxious or guilty if you “break the rules.”

“Clean eating” rules can give way to eating disorders—especially orthorexia. 

#7. May Provoke Dysbiosis & Constipation

Dysbiosis is a fancy word for “gut imbalance.” Under eating and altered circadian meal timing are two of the leading triggers of gut disturbances. Your gut bacteria actually have their own (ideal) circadian rhythms, which affect optimal meal timing and digestion too.

Intermittent fasting can either positively or negatively impact your gut biome—depending on how extreme you go with it. For some folks, IF helps them get on an eating schedule for the first time, whereas for others, IF inadvertently inspires them to restrict fuel  intake (necessary for feeding a healthy gut biome), promoting dysbiosis. Constipation may also closely follow because, without appropriate amounts of fuel and “bulk” in the stool, we can’t poop.

While intermittent fasting can allow time for your body and digestive superpowers to repair, this is best accomplished  by simply eating earlier on in the day and fasting over night for approximately 12 hours—just enough to rest and digest. 

Why I Don’t Do Formal Intermittent Fasting 

After over 20 years of disordered eating and a funky relationship with food, I can confidently say that today, me and food are finally on “good terms.” While fasting adopters report “euphoric” experiences, I’ve had plenty of those to last three lifetimes and “balance” is my motto of the moment—no extremes.

Additionally, the reason why intermittent fasting may be so popular today is because its a 180-difference in many people’s former way of eating and lifestyle. A vast majority of people have erratic eating habits—with many eating within a 15 to 20 hour time frame each day and consuming the majority of their calories (70%) after noon and in the evening hours. Hence, the “intermittent fasting rules” force many folks to shift these habits to an 8 to 12 hour window. Intermittent fasting is sort of like the Dorito-loving, Big Mac eating diehard gone vegetarian—of course they feel better with the abrupt change in lifestyle.

When you no longer live in extremes and you lead and enjoy a balanced lifestyle, militant or meticulously practiced IF is simply not necessary. Sure I may naturally go 12 to 13 hours between my last meal and morning breakfast shake, but simply call this what it is “break-fast”—breaking my fast in order to fuel my brain, my energy and my fitness for the day.

Oh yes, fitness. I love training and working out and know that fuel is required to support my endeavors—CrossFit, strength training, dance, hiking, power yoga, HIIT. I need more fuel not less. (And fun fact: did you know that exercise promotes autophagy—cell waste removal—just like intermittent fasting? In other words, exercise may replace or mimic the role of intermittent fasting in the body. 

Tips for Making Intermittent Fasting Work for You

Intermittent fasting for women can be accomplished healthfully to reap the digestive and detoxification benefits it can bring without the other side effects we’ve discussed. Here are several tips to make it work for you: 

  • Eat in a 12 hour window (such as 8 am to 8 pm)
  • Eat enough fuel and balanced meals (approximately 1800-2200 calories for women and 2400 to 2800 calories for most men)
  • Eat breakfast, within the first 1 to 2 hours of waking and eat an earlier dinner
  • If you aren’t a “breakfast person”, consider starting your day off with just a little protein—such as a scoop of protein power, collagen or bone broth—to set the tone for blood sugar for the day
  • If you workout and aim to improve fitness, lean muscle and performance, fuel your body (don’t fear food)
  • Listen to your body, more than a clock
  • Unfollow obsessive fasting influencers and social media

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