Intermittent fasting isn’t just changing your weight: Expert warns it may also be affecting your hormones

Fasting is no longer limited to religious practice or crash dieting. Over the last few years, fasting patterns such as intermittent fasting, time-restricted eating, and alternate-day fasting have become part of everyday wellness conversations. Many people now skip breakfast, limit eating windows, or avoid food for long stretches in the hope of improving metabolism, losing weight, or feeling mentally sharper.

But behind the visible changes on the weighing scale, another story quietly unfolds inside the body, a hormonal one.

Hormones are chemical messengers that regulate hunger, stress, sleep, metabolism, energy, mood, and reproductive health. Even small shifts in eating patterns can influence how these hormones behave. That is why fasting can feel energising for some people, while for others it may trigger fatigue, mood swings, headaches, or disrupted menstrual cycles.

According to Ms Twincy Ann Sunil (Bachelors in Clinical Nutrition & Dietetics), Apollo Spectra Hospital, Bengaluru, “These diets are not merely focused on when to eat, but also affect hormones that control the appetite, metabolism, stress, sleep, and reproduction.”

One of the first hormones affected during fasting is insulin. This hormone helps the body move glucose from the bloodstream into cells for energy. When food, especially carbohydrates, is eaten frequently, insulin levels remain elevated for longer periods.

Fasting gives the body a break from constant insulin release.

Ms Twincy Ann Sunil explains, “When the body is not being regularly supplied with glucose, the insulin levels start to drop slowly. Lower insulin levels enable the body to burn fat for energy and enhance insulin sensitivity.”

This is one reason why fasting is often linked to weight loss and improved metabolic health. Some studies suggest that time-restricted eating may help improve blood sugar control, particularly in people with obesity or insulin resistance.

​Research supported by the National Institutes of Health has explored how intermittent fasting may improve insulin sensitivity and metabolic markers in certain individuals.

There is also growing evidence that meal timing affects circadian rhythm, the body’s internal clock. Eating earlier in the day appears to support better glucose regulation compared to late-night eating habits.

Not every fasting experience is calming for the body. In some cases, fasting can activate cortisol, the body’s primary stress hormone.

Short fasting periods may help the body become more adaptable between fed and fasting states. But prolonged fasting, extreme calorie restriction, and intense workout routines without proper recovery can push the body into a stress response.

“Extended fasts and diets may increase cortisol levels,” says Ms Twincy Ann Sunil. “Excessively high levels of cortisol may also lead to fatigue, increased irritability, sleeping disorders, and loss of muscle mass.”

This explains why some people feel anxious, restless, or emotionally drained during aggressive fasting plans. The body begins to interpret prolonged food restriction as a survival challenge.

A government-backed resource from the US National Library of Medicine also highlights the relationship between fasting, stress hormones, and circadian biology.

Sleep can also suffer when cortisol remains elevated. Many people unknowingly blame stress from work or lifestyle, while irregular eating patterns may quietly contribute to hormonal disruption.

Hormonal responses to fasting are not always the same for men and women. Female hormones are deeply connected to energy availability, which means the body can become sensitive to prolonged calorie restriction.

Ms Twincy Ann Sunil notes, “Prolonged fasting and low-calorie consumption can cause disruptions in the menstrual and ovulation cycles, while excessive fasting can result in hormonal imbalance because of excessive fluctuations of the estrogen hormone.”

For some women, fasting may lead to delayed periods, irregular cycles, mood fluctuations, or worsening fatigue. Women with conditions such as Polycystic Ovary Syndrome or thyroid disorders may respond differently compared to others.

The body is designed to protect reproductive health during periods of perceived starvation. When calorie intake drops too low for too long, the brain may reduce signals linked to ovulation and estrogen production.

This does not mean fasting is unsafe for all women. It simply means fasting should not be approached as a one-size-fits-all trend copied from social media routines.

The thyroid gland plays a major role in controlling metabolic rate, energy production, and body temperature. Extreme fasting patterns may slow down thyroid activity in some people, particularly when nutrition becomes inadequate.

“Frequent fasting can lead to a slowing of the metabolism and subsequently, to loss of energy and increased susceptibility to cold,” explains Ms Twincy Ann Sunil.

This is often why people initially lose weight quickly during strict fasting routines but later feel constantly tired or struggle with low energy levels.

Moderation appears to be the key. Balanced meals, proper hydration, sleep, and realistic fasting schedules are less likely to place excessive hormonal stress on the body.

The conversation around fasting is becoming more nuanced now. Instead of asking whether fasting is good or bad, experts are increasingly asking a more important question: who is fasting appropriate for, and under what conditions?

This article includes expert inputs shared with TOI Health by:

Ms Twincy Ann Sunil (Bachelors in Clinical Nutrition & Dietetics), Apollo Spectra Hospital, Bengaluru.

Inputs were used to explain how fasting diets influence hormonal balance, affect metabolism, stress response, reproductive health and insulin function, and why following fasting patterns without proper medical guidance may negatively impact overall health.