PDF Summary:Fast Like a Girl, by Mindy Pelz

Fasting has gained popularity for offering a myriad of health benefits: It can improve your immune system, help you lose or maintain a healthy weight, and enhance your mental clarity.

In Fast Like a Girl, best-selling author, YouTuber, and holistic health expert Mindy Pelz suggests that many women fail to benefit from fasting because they don't understand how hormonal cycles impact their bodies. This lack of understanding often leads them to mistime fasts, resulting in hormonal imbalances and uncomfortable symptoms. Pelz argues that aligning fasting schedules with hormonal cycles can help women overcome these challenges and maximize the benefits of fasting.

In this guide, we’ll explore Pelz's ideas about fasting, covering the mental and physical health benefits of fasting, tips for prolonging and enhancing these benefits, and advice on how women can tailor their fasting schedules and eating habits to promote hormonal balance. Additionally, we'll supplement her ideas with research, advice, and practical tips from other health and nutrition experts.

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Part 2: How to Maximize Fasting Benefits

We've just discussed how fasting enhances overall health. While fasting doesn't seem complicated—it's just a matter of not eating for a certain amount of time—Pelz offers a couple of tips for maximizing the benefits of fasting: Incorporate different-length fasts into your routine and pay attention to the foods you eat when you're not fasting. Let's explore these two tips in detail.

(Shortform note: Pelz recommends that you consult a medical professional before adopting a fasting routine. She also suggests that you not fast if you're pregnant and limit your fasting duration to 13 hours if you're breastfeeding. Johns Hopkins Medicine adds to this advice, suggesting that the following people shouldn't attempt intermittent fasting: anyone under the age of 18, people with type 1 diabetes who take insulin, and those with a history of eating disorders. If you do intend to practice intermittent fasting, they recommend that you stay hydrated by drinking water or other zero-calorie drinks (such as black tea or coffee) during your fasting periods.)

Tip #1: Incorporate Different-Length Fasts Into Your Routine

According to Pelz, incorporating different-length fasts into your routine prevents your body from adapting to the fasting state, ensuring continued long-term benefits. To clarify how this works, we'll explore how your cells respond to both glucose and ketones, then explain how strategically switching between these two fuel sources (by varying fasting lengths) guarantees ongoing benefits.

Up until now, we've touched on the downsides of relying solely on glucose as an energy source. However, this doesn't mean you should always prevent your body from burning glucose. Both fuel sources are essential for your body to function optimally.

• When your body burns glucose, it activates mTOR signaling, a process that builds new cellular structures and contributes to hormone production, muscle building, and cell regeneration.
• When your body burns ketones, it activates autophagy, the cellular detox process we discussed above that breaks down and removes damaged components.

(Shortform note: Research expands on why your body needs both fuel sources. Glucose is the primary source of energy for all cells. However, relying solely on glucose for energy can cause hyperglycemia, which damages nerves, blood vessels, and organs. Further, glucose provides less cellular energy than ketones: Metabolizing 100 g of glucose generates 8.7 kg of energy-carrying molecules. Metabolizing 100 g of ketones generates between 9.4 and 10.5 kg of energy-carrying molecules. While ketones provide more cellular energy, relying solely on them for energy would result in no glucose in the bloodstream, which would affect the central nervous system and cause cellular dysfunction or death.)

Pelz explains that switching between these two states (by eating and fasting) creates hormetic stress, a beneficial form of stress that encourages cellular adaptation and resilience—like the way new exercises challenge muscles. However, sticking to the same fasting routine every day reduces this stress, causing you to miss out on the beneficial effects of switching between fuel sources. This is because your body gets too comfortable with routine—just as doing the same exercises every day stops having an effect. Therefore, Pelz argues that incorporating different-length fasts prevents your body from adapting to a specific fasting pattern, ensuring it benefits from fasting over the long term.

(Shortform note: While fasting experts such as Dave Asprey (Fast This Way) and Gin Stephens (Fast. Feast. Repeat.) agree that switching between different fasting durations helps overcome plateaus, other experts argue that sticking to consistent eating times is better for your health. They explain that eating at regular times helps you maintain a healthy circadian rhythm (the body's internal 24-hour clock) and that irregular eating can lead to obesity, high blood pressure, and type 2 diabetes.)

Six Fasting Lengths and Their Specific Health Advantages

In addition to encouraging healthy hormetic stress, practicing diverse fasting durations provides a spectrum of health advantages. Pelz explains that, generally, your body shifts to burning ketones after eight hours of fasting, and the specific health benefits vary based on the duration of the fast.

• 12 to 16 hours: aids in weight loss, enhances mental clarity, and boosts energy levels
• At least 17 hours: detoxes cells, improves memory and cognition, increases resilience against viruses, and balances female sex hormones
• At least 24 hours: creates stem cells that repair gut lining damage caused by unhealthy bacteria, antibiotics, or birth control
• At least 36 hours: reduces cholesterol and maximizes weight loss by converting excess glucose into ketones
• At least 48 hours: calms anxiety and resets dopamine levels, making it easier to experience joy
• At least 72 hours: maximizes the production of stem cells that rejuvenate the immune system, heal injuries, and reverse the effects of chronic illnesses or age-related decline

Opinions Differ on the Health Benefits of Different Fasting Durations

While Pelz ties different fasting lengths to specific health advantages, it's important to note that various factors, such as physiology, lifestyle, and health conditions can influence the way these benefits unfold. In essence, a fasting duration that creates advantages for one person might not yield the same results for another.

Further, opinions vary on how different fasting lengths impact your health. For example, some experts suggest the following timeline:

• 4 to 16 hours: depletes glucose reserves, transitions the body into ketosis, and activates autophagy

• 16 to 24 hours: burns fat, detoxes cells, and increases growth hormone levels

• 24 to 72 hours: increases the rate at which the body burns fat and enhances mental clarity

• 72 hours +: triggers stem cell production and supports the creation of new neural networks

Meanwhile, other experts propose a different timeline:

• 8 to 12 hours: triggers glycogen metabolism, stabilizes blood sugar levels, and transitions the body into the early stages of ketosis—effects that can help lower blood pressure and increase insulin sensitivity

• 12 to 18 hours: enters full ketosis, burns body fat, suppresses appetite, and boosts BDNF levels to enhance mental clarity

• 24 hours: triggers autophagy, which detoxes cells and reduces inflammation

• 36 to 48 hours: increases growth hormone production, which promotes muscle growth and repair

• 72 hours +: triggers stem cell production, which helps rejuvenate the immune system

These differing opinions suggest that the link between specific fasting durations and health advantages may not be as clear-cut as Pelz suggests.

Tip #2: Pay Attention to What You Eat

While fasting alone offers benefits, paying close attention to your post-fast meals can elevate those gains. Pelz explains that being mindful about what you eat can prolong and enhance the positive effects of fasting. She suggests two key pieces of advice:

1) Prioritize healthy, organic foods over processed, chemical-laden ones, such as those containing hydrogenated oils, preservatives, or artificial dyes and flavors. Consuming nutrients in their natural form enables efficient absorption without the burden of processing harmful additives, thereby minimizing the cleanup workload for your body. (Shortform note: Michael Greger (How Not to Diet) adds that you should avoid highly processed foods because they're never good for your health—they cause numerous diseases and can desensitize your palate, making natural foods less appealing. They're also engineered to encourage overeating, which disrupts your body's hunger and fullness cues.)

2) Maintain stable blood sugar levels by balancing your intake of complex carbohydrates (including fiber), grass-fed proteins, and “good” fats (such as olive oil, nut butter or oil, or grass-fed butter). According to Pelz, this supports a smooth transition from the fasting state and prevents energy crashes by slowing the absorption of glucose into your bloodstream.

The NOVA Food Quality Guidelines

While Pelz offers strict dietary guidelines, Stephens (Fast. Feast. Repeat.) stresses that restricting your diet makes it more difficult to stick to, and that meal timing is ultimately more important than what you eat. However, she adds that this doesn't mean that food quality is unimportant—what you eat directly impacts how quickly your body enters and benefits from ketosis during fasting. Stephens suggests using the NOVA classification system, a set of guidelines devised by researchers from the University of São Paulo, Brazil:

• Category 1: Unprocessed foods, including whole fruits, vegetables, and meats

• Category 2: Processed culinary ingredients, such as salt, flours, butter, and oils

• Category 3: Processed foods, including bread, cheese, pickles, and canned foods

• Category 4: Ultra-processed foods, such as sugary cereals, ice cream, soft drinks, and “instant” meals like cup ramen

Stephens advises sticking mainly to categories 1 through 3, though she allows for selective enjoyment of your favorite category 4 items. In contrast, the researchers suggest avoiding them altogether, if you can.

Break Your Fast With Foods That Support Your Goals

To further enhance the benefits of fasting, Pelz suggests that the first foods you eat to break your fast should complement your fasting goals:

• If you're attempting to lose weight, break your fast with “good” fats, such as those found in avocados, bone broth, nuts, or olives. These fats stabilize blood sugar levels and don't disrupt the fasted state, allowing your body to continue burning body fat.
• If you're trying to build muscle, break your fast with protein-rich foods, such as eggs, beef jerky, chicken, broccoli, mushrooms, chickpeas, or plant-based protein shakes. This will provide your body with amino acids essential for muscle repair and growth.
• If you're aiming to balance gut microbiomes, break your fast with foods that are rich in probiotics, prebiotics, or polyphenols. Probiotic-rich foods introduce beneficial bacteria into your gut, prebiotic foods nourish these bacteria, and polyphenol-rich foods help repair the gut's lining. Suitable foods include broccoli, seeds or seed oils, and fermented foods like yogurt.

Guidelines for Breaking a Fast

Pelz bases these recommendations on feedback from her fasting community. While research substantiates the correlations between the foods and the health benefits she describes, there's limited scientific evidence supporting the argument that the first foods consumed to break a fast can contribute to specific goals.

However, the first meal you consume does impact how you benefit from fasting and how easily you transition back to regular eating. Specifically, experts point out that starting with a large meal can put too much pressure on your digestive system, causing you to feel sluggish and bloated. They offer the following guidelines for easing yourself out of the fasting state:

• Begin by drinking water or broth to help rehydrate your body and prepare your digestive system for food.

• Consume a light snack, such as a piece of fruit or a handful of nuts. This will help reintroduce food to your digestive system without overwhelming it.

• Wait 30 minutes to an hour to give your body time to adjust.

• Eat a balanced and nutritious meal, including a source of lean protein, cooked vegetables, and a small portion of complex carbohydrates.

• Resume your regular eating schedule—stay hydrated and pay attention to your body's hunger and fullness cues.

Part 3: What Women Can Do to Maximize Fasting Benefits

We've just covered how to practice and benefit from fasting. Now, let's narrow our focus to how women can further enhance their fasting experience. Why focus on women specifically? Pelz says that, unlike men, women experience monthly and menopausal hormonal cycles that significantly impact how fasting affects them. (Shortform note: Asprey (Fast This Way) suggests that fasting impacts women differently because their bodies are designed to support reproduction—they're more sensitive to dietary changes because a lack of food or nutrients could endanger a potential child.)

In this final part of the guide, we'll first explain why women need to pay more attention to their fasting and eating habits. Then, we'll explore Pelz's advice for tailoring a fasting schedule that aligns with your hormonal cycle.

Mistimed Fasting Creates Hormonal Imbalance

Pelz explains that a woman's hormonal cycle spans the time from the onset of menstruation (the first day of bleeding) to the beginning of the next period. On average, this takes 28 days, but it's normal for cycles to be a bit longer or shorter.

Throughout each cycle, the levels of three key hormones—estrogen, progesterone, and testosterone—rise and fall. Pelz emphasizes that each of these hormones thrives on specific blood sugar and insulin levels. When you provide the necessary environment for each hormone, you enhance your overall health. However, mistiming your fasting or consuming the wrong foods can deprive these hormones of the nourishment they require to keep your body in balance, leading to a spectrum of unpleasant symptoms.

(Shortform note: Since research on how fasting and nutrition affect women's hormones is still in the early stages, it's difficult to say exactly how estrogen, progesterone, and testosterone depend on different blood sugar and insulin levels. However, research shows that each of these three hormones plays a role in regulating blood sugar and insulin levels: Balanced levels of estrogen and progesterone can help reduce blood sugar levels and enhance insulin sensitivity. Meanwhile, unusual testosterone levels, whether high or low, have been associated with increased insulin resistance.)

Align Your Fasting Schedule With Your Hormonal Cycle

Pelz suggests that you can establish and maintain hormonal balance by following her fasting schedule. If you have a regular cycle, start the schedule on your first day of bleeding and follow it until your next period. If your cycle is irregular or nonexistent due to factors like birth control, stress, or menopause, start when you want and follow a 30-day continuous schedule. (Shortform note: Some nutritionists have raised concerns about Pelz's fasting advice due to its lack of supporting research and references. Further, these critics warn that Pelz oversimplifies hormonal health, which could inadvertently lead her followers to compromise their health.)

Pelz divides the hormonal cycle into four phases, providing fasting and dietary advice for each. Let's explore the specifics of each phase.

Phase #1: Days One to 10

On day one of your hormonal cycle, when bleeding begins, estrogen, testosterone, and progesterone levels are at their lowest. Your body starts building its estrogen stores to prepare for a new round of ovulation.

Pelz says that estrogen supports the production of:

• Collagen, which helps maintain the connective tissue that supports skin, teeth, bones, cartilage, and organs
• Pleasurable neurotransmitters such as serotonin and dopamine that help you feel calm and positive

(Shortform note: Estrogen also impacts cholesterol levels, circulation and blood flow, and the ability to focus. Symptoms of consistently low estrogen levels include breast tenderness, vaginal dryness, brittle bones, night sweats, irregular periods, headaches, fatigue, sleep problems, and depression. Signs of consistently high estrogen levels include weight gain, irregular periods, heightened premenstrual symptoms, and decreased sex drive.)

Estrogen production improves when blood sugar and insulin levels are low. Pelz suggests that you can keep these levels low and support the production of estrogen by following the following fasting routine: On days one to four, fast for 13 hours; on day five, fast for 15 hours; and on days six to 10, fast for 17 hours.

(Shortform note: While Pelz asserts that estrogen production thrives when blood sugar and insulin levels are low, research paints a mixed picture. One study aligns with her argument, suggesting that higher blood sugar levels alter hormone metabolism, resulting in decreased estrogen production. However, some nutritionists argue that the relationship is the other way around: Higher blood sugar and insulin levels generate more fat cells—and fat cells secrete estrogen—therefore, increased blood sugar and insulin levels can increase estrogen levels.)

During each of these days, consume a maximum of 50 grams of complex carbohydrates (to keep blood sugar and insulin levels low) and up to 75 grams of protein. Pelz also suggests that more than 60% of your food should be in the form of “good” fats, specifically those naturally high in cholesterol.

(Shortform note: Pelz refers to this diet as "ketobiotic," the term she coined for her adaptation of the standard ketogenic diet. While maintaining the high-fat, moderate-protein, and low-carb principles, she distinguishes her version of the diet by including complex carbohydrates and prebiotic foods that support a healthy gut microbiome—a component not typically prioritized in the standard ketogenic diet. Yet, while Pelz recommends eating “good” fats high in cholesterol, she doesn't specify what these are. According to experts, you need to aim for monounsaturated fats (sources include nuts, nut oils, avocados, and olives) and polyunsaturated fats (sources include seeds, walnuts, fatty fish, fish oil, soybean, soymilk, and tofu).)

Phase #2: Days 11 to 15

During days 11 to 15 of your cycle, when ovulation occurs, your estrogen levels reach their peak and your testosterone levels elevate. Testosterone supports muscle growth and increases motivation and energy. According to Pelz, testosterone production thrives when you feel calm; therefore, removing potential stressors from your life helps maintain healthy levels. (Shortform note: Research supports the beneficial effects of testosterone that Pelz describes. However, while Pelz suggests that stress reduction leads to healthy testosterone levels, studies show that individual reactions to stress vary, with some women experiencing increases in testosterone levels and others experiencing decreases.)

On these days, Pelz recommends that you help your body metabolize the hormones it has been producing. This ensures your cells effectively assimilate these hormones and prevents the accumulation of unmetabolized hormones in your tissues—which, according to Pelz, is the primary cause of uncomfortable premenstrual symptoms like mood swings, weight gain, night sweats, and sore breasts.

(Shortform note: Research clarifies why it's important to metabolize hormones. When hormones are not properly metabolized, they don't exit the body. Instead, they recirculate in the bloodstream and gradually accumulate in various tissues. This accumulation can overstimulate hormonal receptors, creating hormonal imbalances that can lead to a variety of health issues—from the premenstrual symptoms Pelz describes to more severe conditions such as fertility problems and hormone-dependent cancers.)

Two organs responsible for hormone metabolism are the liver and gut. Therefore, Pelz recommends prioritizing foods that nourish these organs, such as root vegetables and fruits. Stick to a daily allowance of 150 grams of complex carbohydrates (the increase allows for more fruit and vegetables), 50 grams of protein, and as much “good” fat as you want.

(Shortform note: Your liver and gut play a key role in many bodily functions, including hormone metabolism. Dieticians clarify that root vegetables such as beets and carrots contain plant flavonoids and antioxidants that support your liver and gut by reducing cell damage and inflammation. Meanwhile, fruits such as grapefruit and blueberries are high in vitamin C and polyphenols, which help to neutralize toxins and protect cells.)

Additionally, Pelz advises against fasting for longer than 15 hours on each of these five days. (Shortform note: While research on how fasting impacts hormone metabolism is in the early stages, existing studies indicate that fasting can disrupt hormonal circadian rhythms, which might interfere with hormone metabolism. Therefore, limiting the duration of fasts during this phase may help maintain these rhythms and support hormone metabolism.)

Phase #3: Days 16 to 18

During days 16 to 18 of your cycle, your hormone levels dip, which might cause your energy levels to drop. Pelz suggests that you fast for 15 hours each of these days, and follow the dietary guidelines provided for Phase #1. (Shortform note: In medical terms, this phase of the menstrual cycle is commonly referred to as the early luteal phase. During this time, insulin sensitivity starts to decrease. Therefore, Pelz's nutritional and fasting advice should keep your blood sugar in balance, preventing blood sugar spikes that decrease energy levels even further. Additionally, experts note that your body temperature may be high from ovulation—they recommend reducing your exercise intensity to prevent overheating.)

Phase #4: Day 19 Until Bleeding Begins

On day 19 of your cycle, your body begins producing progesterone. This hormone prepares your uterus to receive a fertilized egg. If fertilization doesn't occur, progesterone and estrogen levels decrease as your uterus readies itself to shed its lining and start menstrual bleeding (the beginning of the next cycle).

Pelz emphasizes that insufficient progesterone can lead to uncomfortable symptoms, such as irregular menstrual cycles, spotting, heightened irritability, and sleep difficulties. (Shortform note: It's worth noting that the same symptoms can also signify excessively high progesterone levels.)

To support healthy progesterone levels during this phase, she recommends that you avoid fasting until your next cycle begins (or, if you don't have a cycle, until the 30 days are up). There are two reasons for this. First, stress decreases progesterone levels (fasting creates hormetic stress). Second, progesterone thrives on higher blood sugar and insulin levels, which fasting tends to decrease. Additionally, she suggests that you follow the dietary guidelines provided for Phase #2.

(Shortform note: While research in this area is limited, dieticians tend to agree that it's best not to fast during this phase to prevent stress and encourage progesterone production. They also caution against restricting calories, as that too can create stress that disrupts progesterone production. Therefore, when following Pelz's dietary guidelines, ensuring you don't drastically cut your calorie intake might help keep your progesterone levels in balance.)