3 Reasons Women Shouldn't Fast: Luteinizing Hormone, Cortisol & Metabolic Health

Sims contrasts this pattern with popular time-restricted eating that delays the first meal until 11 a.m. or later. She notes that late fasting provides no improvements in glucose tolerance, telomere length, or parasympathetic tone in either sex, but for women, the hormonal consequences are particularly damaging. By eating early and regularly, the hypothalamus perceives abundance rather than scarcity, allowing the full cascade of gonadotropin-releasing hormone, LH, FSH, and sex steroids to proceed without interruption. This, in turn, supports stable energy, mood, sleep, and metabolic health. She frames this not as a novel protocol but as a return to evolutionary normalcy—what humans did before the modern fasting trend. For active women, coupling this eating pattern with fed exercise (see protocol below) optimizes body composition and performance.

Sims directly challenges the common belief that fasted exercise is superior for fat burning or metabolic flexibility. Because women are already metabolically flexible, fasting before training provides no additional advantage and instead compounds physiological stress. When women fuel with protein before exercise, they provide amino acids that can be used for repair, mitigating cortisol spikes. Post-exercise protein and carbohydrate hit the muscle when it is most receptive, promoting glycogen replenishment and muscle protein synthesis. The result is a higher EPOC—the afterburn effect—meaning metabolism stays elevated longer. Sims also notes that fed women experience more incidental movement throughout the day (non-exercise activity thermogenesis) and better appetite regulation, avoiding the rebound hunger and simple carbohydrate cravings that often follow fasted sessions. This protocol, combined with the normal eating pattern, creates a synergistic environment for muscle gain, fat loss, and hormonal stability.

Sims explains that holding a fast until noon or later—what she calls a “late fast”—shows no improvement in glucose tolerance, telomere length, or parasympathetic activity even in men, and in women it specifically disrupts luteinizing hormone (LH) pulse and cortisol responses. She contrasts this with the pattern of eating breakfast within 30 minutes of waking to offset the cortisol awakening response, eating at regular intervals, and stopping food with dinner to achieve a 12-13 hour overnight fast. This “normal eating” rhythm keeps the hypothalamus happy because it receives regular nutrient signals that signal abundance, enabling robust endocrine function, circadian alignment, and proper LH, estrogen, and testosterone pulses. She argues that the cultural push toward prolonged daily fasting ignores the fact that the female body already possesses metabolic flexibility and that withholding nutrients creates a hypothalamic stress state, leading to sympathetic overdrive, sleep disruption, and elevated baseline cortisol. For women, especially as they age, this stress cascade can promote chronic disease. Thus, resetting to a simple, consistent eating pattern is foundational.

Sims flatly states that women are already metabolically flexible by the nature of being born XX. The menstrual cycle creates a built-in shift in fuel preference: during the follicular phase, estrogen enhances fat oxidation, while the luteal phase favors carbohydrate utilization. This natural variability means women do not need to impose additional fasting stress to “teach” the body to switch between fuels. Attempting to do so is redundant and may backfire. Instead, when women fuel appropriately before and after exercise, they see greater training adaptations, higher excess post-exercise oxygen consumption (elevated metabolism), more incidental daily movement, better satiety, and no cravings for simple carbohydrates. The claim that fasted training improves metabolic flexibility ignores female physiology entirely, and Sims presents this as a critical reason why women should abandon fasted protocols.

Sims details that when women hold a fast and perform fasted training, the body interprets this as a compound stressor. The luteinizing hormone (LH) pulse—critical for ovulation and estrogen production—becomes dysregulated, and cortisol spikes disproportionately. Over time, this overdrive pushes the nervous system into a sympathetic-dominant state, impairing the ability to shift into parasympathetic (rest-and-digest) mode for sleep and recovery. A chronically elevated baseline cortisol further contributes to metabolic syndrome, inflammation, and cardiovascular risk. This is especially harmful as women age and hormonal resilience naturally declines. The hypothalamus, sensing low nutrient density, interprets the situation as a threat and downregulates the entire endocrine cascade, including testosterone and estrogen. In contrast, consistent nutrient intake signals abundance, allowing these hormone pulses to flow normally. This makes fasting not just unnecessary but actively counterproductive for female hormonal health.

Sims mentions that there is a “big push” for postmenopausal women to adopt ketosis or fasting for weight control, but the literature indicates that calorie restriction paired with sufficient protein yields better outcomes. While she does not elaborate extensively in this segment, the statement aligns with her broader thesis that women’s bodies respond better to consistent nutrition than to deprivation stress. Postmenopause already carries a decline in estrogen, which affects muscle mass and insulin sensitivity; adding fasting may exacerbate metabolic and hormonal dysregulation. The brief note underscores that the anti-fasting message applies across the female lifespan.