Can Fasting Cause Menstrual Cycles Longer Than 35 Days?
Yes, fasting can cause menstrual cycles longer than 35 days, particularly in lean women and those who fast for extended periods, though the effect varies significantly based on body composition, duration of fasting, and baseline nutritional status.
Evidence from Fasting Studies
Short-Term Fasting (72 hours)
The impact of brief fasting depends critically on body composition:
Lean women (≤20% body fat) are significantly more vulnerable to fasting-induced menstrual disruption. A 72-hour fast in lean women resulted in anovulation in 2 of 7 cycles studied, and among the 5 women with ovulatory cycles, all experienced significant lengthening of the follicular phase (14.4 vs. 13.2 days), which would extend total cycle length 1.
Normal-weight women show remarkable resilience to short-term fasting. Multiple studies demonstrate that 72-hour fasts during the follicular phase do not affect cycle length, follicle development, or ovulation in women within 10-15% of ideal body weight, despite causing alterations in LH pulse frequency 2, 3.
Prolonged Fasting (15-21 days)
Ramadan fasting (repeated daily fasting cycles) shows a dose-response relationship: women fasting more than 15 days experienced significantly more menstrual abnormalities including oligomenorrhea (cycles >35 days) compared to those fasting fewer than 15 days 4.
Three-week medically supervised fasting (400 kcal/day) in extremely obese women (BMI >41) with average weight loss of 12.5 kg did not disrupt gonadotropin pulsatility or cause menstrual cycle abnormalities 5.
Clinical Context: Energy Availability
The mechanism underlying fasting-induced menstrual disruption relates to energy availability (EA), defined as energy intake minus exercise energy expenditure normalized to fat-free mass 6:
Optimal EA: 45 kcal·kg⁻¹ FFM·day⁻¹ maintains normal menstrual function 6.
EA <30 kcal·kg⁻¹ FFM·day⁻¹ discriminates between amenorrheic versus eumenorrheic status, though subclinical disturbances occur at higher EA levels 6.
Eumenorrhea is defined as regular cycles occurring at 21-35 days intervals 7. Cycles >35 days represent oligomenorrhea, a form of menstrual dysfunction.
Key Physiological Mechanisms
Low energy availability from fasting disrupts the hypothalamic-pituitary-gonadal axis through several pathways 8, 7:
- Suppression of gonadotropin-releasing hormone (GnRH) pulsatility
- Decreased LH pulse frequency (19% reduction observed in lean fasted women) 1
- Alterations in metabolic hormones including cortisol, TSH, T3, insulin, and growth hormone 1, 2
- Disruption of follicular development leading to anovulation or prolonged follicular phase
Clinical Assessment Considerations
When evaluating a woman with cycles >35 days and fasting history 9, 10, 8:
Measure mid-luteal progesterone (day 21-23 of cycle): levels <6 nmol/L indicate anovulation 9.
Assess LH/FSH ratio (days 3-6 of cycle): ratio >2 suggests polycystic ovary syndrome, which can be triggered or exacerbated by weight changes 9.
Evaluate body composition: lean women (body fat ≤20%) are at higher risk for fasting-induced menstrual disruption 1.
Quantify fasting duration and pattern: intermittent daily fasting >15 days or continuous fasting in lean individuals carries higher risk 4.
Rule out other causes: thyroid dysfunction, hyperprolactinemia, hyperandrogenism, and chronic illness can all cause oligomenorrhea independent of fasting 9, 8.
Important Caveats
The relationship between energy deficit and menstrual disturbances is linear rather than threshold-based in many women, meaning even modest energy deficits can cause subclinical disruptions 6.
Psychological stress, exercise, and dietary composition interact with low energy availability to amplify effects on menstrual function 6, 7.
Recovery of normal cycles after fasting may be delayed: menstrual abnormalities peak during Ramadan but remain elevated three months post-fasting 4.
Women with pre-existing low body fat stores or those experiencing rapid fat mass reduction are particularly vulnerable to menstrual dysfunction even with relatively brief fasting periods 7, 1.