A Breakdown of Intermittent Fasting Types
IF is not a one-size-fits-all concept. There are multiple ways to implement the regime, so that one can adapt it better to their lifestyle.
Alternate-Day Fasting (ADF): Alternating between a full day of fasting and a day of eating freely.
Modified ADF (MADF): Similar to ADF, but fasting days allow a limited intake of 0–600 calories, usually for between 3-5 days/week.
Time-Restricted Eating (TRE): Eating within a specific window, and fasting for the remaining 12–24 hours each day. In this category fall the most popular IF approaches, such as 16:8 (16 hours fasting, 8 hour eating window).
Numerous clinical trials and meta-analyses have found that IF can improve:
Weight loss (reduced waist circumference and fat mass);
Insulin sensitivity;
Cholesterol levels (↓ LDL or bad cholesterol, ↑ HDL or good cholesterol);
Blood pressure;
Inflammation.
For example, a vast review containing 130 high-quality studies showed that 1–2 months of fasting followed by MADF led to significant improvements in healthy adults and individuals with overweight, obesity, or nonalcoholic fatty liver disease when compared to a regular diet. Apart from its benefits for metabolic parameters, IF has also been investigated for its influence on mental health. One study suggested a potential positive impact of IF on depressive symptoms and the responsible mechanisms include enhanced neuroplasticity and hormonal regulation.
The Hormonal Caveats for Women
Despite the potential upsides, women often hesitate to try intermittent fasting due to concerns about its effects on reproductive hormones, disruptions in the menstrual cycle and fertility issues. Here's what science tells us:
1. Concerns from Animal Studies
One widely cited rodent study (Kumar et al., 2013) found that young female rats fasting every other day for 12 weeks had increased estrogen and disrupted luteinizing hormone (LH) levels, leading to irregular cycles. But it’s important to note: these rats were 3 months old, which is equivalent in age to 9-year-old girls. Generally, fasting is not recommended for children under the age of 12, since it can negatively impact puberty and growth. Therefore, this study doesn't reflect outcomes in adult women.
2. How Fasting Affects the HPG Axis
In women, very low calorie intake can interfere with the hypothalamic-pituitary-gonadal (HPG) axis, the hormone system responsible for regulating the menstrual cycle. Disruption here can lower luteinizing hormone (LH) and follicle-stimulating hormone (FSH), affecting ovulation. Because of this, it is essential for women to ensure that they are consuming an adequate amount of food in the shorter eating windows to avoid hormone imbalances.
Estrogen, Weight, and PCOS
Estrogen is the main female reproductive hormone and is involved in the regulation of the menstrual cycle and the development of female reproductive tissues. Overweight and obese women have been found to have higher levels of estrogen, which could result in increased risk of polycystic ovarian syndrome (PCOS), anovulation and breast cancer. Weight loss interventions have proven to be beneficial in reducing excessively high estrogen levels among females with obesity. Research shows that shifting calorie intake later in the day (e.g., eating a big dinner instead of breakfast) can increase estrogen levels in women with PCOS. Therefore, a change of the eating window earlier in the day could help avoid a further rise in estrogen in women with PCOS.
Fasting and Androgens: A Win for Women with PCOS?
Hyperandrogenism, or elevated male hormones like testosterone and DHEA-S, is common in PCOS. In women, it is portrayed by excessive hair growth, acne and irregular periods. The good news? Fasting—particularly early TRE (eating between 8 a.m. and 4 p.m.)—has been shown to:
Decrease testosterone;
Improve insulin sensitivity;
Lower inflammation.
Interestingly, in some studies, these improvements occurred without weight loss, suggesting that meal timing alone can make a big difference.
The Role of SHBG
Sex hormone-binding globulin (SHBG) is a protein that regulates the availability of estrogen and testosterone in the body. Low SHBG is linked to insulin resistance and type 2 diabetes, independent of hormone concentrations. Moreover, low levels of SHBG are frequently found in females with PCOS and contribute to symptoms such as acne, and androgenic alopecia (male pattern baldness). Similar to the previous hormones discussed, weight loss has been shown to increase SHBG and improve insulin sensitivity in women with obesity and PCOS. Studies have shown that:
Early TRE can significantly increase SHBG;
The 5:2 diet also improves SHBG levels;
Morning-heavy eating patterns improve SHBG even without weight loss.
This suggests IF—particularly when paired with an earlier eating window—may help restore hormonal balance.
Gut Health & Hormones: An Overlooked Link
Studies have shown that alterations in the gut microbiome can significantly affect reproductive hormones. Research has found that IF can improve the composition and diversity of the gut microflora and reduce gut permeability, decreasing systemic inflammation. The gut microbiome can still utilize substrates during fasting, resulting in the production of metabolites beneficial to the gut bacteria.
The Circadian Rhythm Connection
Our bodies thrive on rhythm—and hormones are no exception. Eating in alignment with your circadian clock (i.e., during daylight hours) may optimize hormone production. That’s why early TRE may outperform other IF schedules when it comes to hormonal health in women.
Bottom Line: Should Women Fast?
It depends. While IF has many potential benefits for women, especially those with PCOS or insulin resistance, it’s not ideal for everyone. Furthermore, if the eating pattern is not sustainable for your lifestyle, the stress of trying to adapt your schedule accordingly might cause more harm than good to your hormonal health.
Promising strategies:
Early TRE (eating most calories between 8 a.m. and 4 p.m.);
Focusing on sufficient quantity and quality nutrition during eating windows.
Proceed with caution if you:
Have a history of eating disorders;
Are underweight or trying to conceive;
Experience cycle irregularities with fasting.
What’s Next?
The evidence in this area is still very limited. Future studies should explore how fasting affects progesterone, and include perimenopausal and postmenopausal women for a more complete picture.
Sources:
Cienfuegos S, Corapi S, Gabel K, Ezpeleta M, Kalam F, Lin S, Pavlou V, Varady KA. Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients. 2022 Jun 3;14(11):2343. doi: 10.3390/nu14112343. PMID: 35684143; PMCID: PMC9182756.
Gupta, Priya & Mahapatra, Archisman & Suman, Anjali & Singh, Rahul. (2021). Effect of Endocrine Disrupting Chemicals on HPG Axis: A Reproductive Endocrine Homeostasis. 10.5772/intechopen.96330.
Sun ML, Yao W, Wang XY, Gao S, Varady KA, Forslund SK, Zhang M, Shi ZY, Cao F, Zou BJ, Sun MH, Liu KX, Bao Q, Xu J, Qin X, Xiao Q, Wu L, Zhao YH, Zhang DY, Wu QJ, Gong TT. Intermittent fasting and health outcomes: an umbrella review of systematic reviews and meta-analyses of randomised controlled trials. EClinicalMedicine. 2024 Mar 11;70:102519. doi: 10.1016/j.eclinm.2024.102519. PMID: 38500840; PMCID: PMC10945168.
