Women should avoid fasting during pregnancy, while breastfeeding, when trying to conceive, when underweight, when managing diabetes, or when recovering from an eating disorder. Fasting affects women differently than men because of its direct impact on reproductive hormones, iron stores, and thyroid function. The situations below carry real physiological risks that make fasting a poor choice regardless of its potential benefits in other contexts.
During Pregnancy
Fasting during pregnancy restricts the steady supply of nutrients a developing fetus needs. Animal research published in PLOS One found that intermittent fasting during pregnancy caused fetal growth restriction, reduced placental transport efficiency, and altered fetal amino acid profiles, with particular drops in branched-chain amino acids that are critical for tissue growth. Placental weight was also reduced. Even when birth weight appeared normal, organ development told a different story: kidney weight was reduced in both male and female offspring, and the ratio of brain to liver weight suggested that brain growth had been compromised relative to other organs.
These aren’t abstract lab findings. They reflect what happens when a fetus doesn’t receive consistent energy and protein during key windows of development. The Mayo Clinic lists pregnancy as a clear reason not to practice intermittent fasting.
While Breastfeeding
Breastfeeding demands roughly 500 extra calories per day. Compressing your eating window or skipping meals makes it harder to meet that need consistently. While some animal research suggests milk quality may not drop immediately, the mother’s own nutrient reserves take the hit. If you’re already in a caloric deficit from the demands of nursing, adding fasting on top increases the risk of depleting iron, calcium, and B vitamins that your body needs to recover postpartum. The Mayo Clinic groups breastfeeding alongside pregnancy as a time when intermittent fasting is not recommended.
When You’re Trying to Conceive
Fasting can shut down ovulation by creating an energy deficit your brain interprets as a signal that conditions aren’t safe for reproduction. The threshold is specific: when energy availability drops below 30 calories per kilogram of fat-free mass per day, the probability of menstrual disorders exceeds 50%. Each one-unit drop in energy availability below that threshold increases the likelihood of menstrual disruption by about 9%.
At 20 calories per kilogram of fat-free mass per day, the frequency of luteinizing hormone pulses (the hormonal signal that triggers ovulation) drops by 16%. At 10 calories per kilogram, it drops by 39%. You don’t need to be starving for this to happen. A daily energy deficit of just 470 to 810 calories, or about 22 to 42% below your normal intake, is enough to cause both clinical and subclinical menstrual disorders. Women who spent more hours per day in an energy deficit were significantly more likely to develop irregular cycles than women who stayed closer to energy balance throughout the day.
This is why time-restricted eating can be particularly risky for fertility. Even if you eat enough total calories, compressing them into a short window means you spend more of the day in a catabolic, energy-depleted state. If your periods have become irregular, lighter, or have stopped since you started fasting, that’s a direct sign your reproductive system is being suppressed.
If You’re Underweight or Have Low Body Fat
Most clinical research on intermittent fasting has been conducted on people with a BMI of 25 or higher. If your BMI is in the normal or underweight range, fasting creates an energy deficit your body can’t afford. Research on functional hypothalamic amenorrhea suggests that restoring menstrual function often requires a body fat percentage above 22%. Fasting when you’re already lean pushes you further from that threshold and increases the risk of losing your period, weakening bones, and impairing immune function.
Energy availability below 30 calories per kilogram of fat-free mass per day is considered equivalent to basal metabolism, the bare minimum your body needs to keep organs functioning. Dropping below that level triggers a cascade of protective shutdowns, starting with reproduction and extending to thyroid function and bone maintenance.
If You Have a History of Disordered Eating
Fasting involves the same behaviors that characterize many eating disorders: rigid food rules, extended periods of not eating, and preoccupation with timing and control. Research published in Nutrients found that fasting can trigger or worsen disordered eating behaviors, especially in people with a prior history. The study’s authors directly recommended that healthcare providers screen for signs of orthorexia or eating disorder symptoms before suggesting any fasting protocol, and that alternative approaches be used if those signs are present.
The line between “disciplined fasting” and restrictive eating can blur quickly. If you find yourself feeling anxious about breaking a fast early, using fasting to compensate for eating “too much,” or feeling a sense of moral accomplishment from not eating, those are warning signs that fasting is reinforcing harmful patterns rather than supporting your health.
If You Have Diabetes, Especially Type 1
Fasting increases the risk of hypoglycemia, which is when blood sugar drops below 70 mg/dL. Severe hypoglycemia, below 54 mg/dL, can cause confusion, seizures, and loss of consciousness. This risk is especially high in people with type 1 diabetes, where the body produces no insulin on its own and blood sugar management depends entirely on external insulin dosing. Skipping meals while on insulin or certain oral medications creates a dangerous mismatch between medication timing and food intake.
Menstruation itself can also affect blood sugar levels, making fasting even more unpredictable for women with diabetes. The combination of hormonal fluctuations and extended periods without food creates a moving target that’s difficult to manage safely.
If You Have Thyroid Issues
Fasting directly suppresses your body’s ability to convert stored thyroid hormone into its active form. Even a 24-hour fast reduces active thyroid hormone (free T3) by about 6% while increasing the inactive form (reverse T3) by 16% in healthy people. In animal studies, the liver enzyme responsible for this conversion dropped by 54% during fasting. The body does this deliberately: when food is scarce, it slows metabolism to conserve energy.
If you already have an underactive thyroid, this metabolic downshift compounds the problem. You may feel more fatigued, cold, or mentally sluggish. Your metabolism slows further, which can make weight management harder, often the opposite of what people hope fasting will achieve. Women are five to eight times more likely than men to have thyroid conditions, making this a particularly relevant concern.
If You Have Low Iron Stores
Women of reproductive age lose iron every month through menstruation, making iron deficiency far more common than in men. Research found that even two days of food restriction significantly decreased serum iron, ferritin (stored iron), hemoglobin, and red blood cell counts in healthy women. The iron intake during the restricted period dropped to about 50% of the recommended daily allowance. But the researchers noted that the changes in iron markers were more dramatic than the reduction in iron intake alone would explain, suggesting that the fasting state itself disrupts iron metabolism.
The same study found a significant correlation between dropping ferritin levels and increased depression symptoms in the fasting women. If you already experience heavy periods, fatigue, or have been told your iron is on the low side, fasting is likely to make all of those worse.
How Much Fasting Is Too Much
There’s no single “safe” fasting window for all women, but the research points to some practical boundaries. Common intermittent fasting protocols range from 6 to 16 hours of fasting per day. The Mayo Clinic notes that intermittent fasting may affect menstrual cycles even in otherwise healthy women. The fertility research is more specific: staying in an energy deficit for extended portions of the day increases the risk of hormonal disruption, even if your total daily calories are adequate. A range of 30 to 45 calories per kilogram of fat-free mass per day is considered reduced energy availability and is only recommended for short periods, such as during a brief fat-loss phase.
If you’re going to fast, shorter windows (12 to 14 hours, including sleep) carry less risk than longer ones. Pay attention to your cycle, energy levels, and mood. Irregular periods, persistent fatigue, hair thinning, feeling cold all the time, or worsening mood are all signals that your body isn’t tolerating the energy restriction well.