How to tell if fasting is helpful or harmful for you

by Dr. Shawna Darou, ND in Nutrition

Intermittent fasting and time-restricted eating have been very popular for several years now due to some significant potential health benefits. Benefits of fasting can include weight loss, visceral fat loss, mental clarity, better blood sugar control and insulin sensitivity, reduced inflammation and improved cellular repair. It all sounds very appealing, as a simple strategy for better health, wellness and longevity.

I do want to share some cautions however, as women’s bodies often respond differently to fasting, delayed eating and caloric restriction due to our hormones especially, and in many cases it can cause more harm than good.

Factors that impact your response to fasting

Every woman’s body responds differently to fasting due to age, hormones, stress levels, activity, and metabolic health. Understanding these factors can help you decide if intermittent fasting or time-restricted eating is appropriate for you.

Age and Hormonal Status

1. Women in reproductive years tend to me most prone to metabolic stress, and negative affects of fasting, as their bodies are always preserving fertility. There will be faster changes to the menstrual cycle with under-eating or prolonged fasting. Women post-menopause will likely respond more favourably to fasting, however this depends dramatically on exercise and activity level. For more on metabolic health and perimenopause.

Stress Levels

2. In general, fasting adds an additional stress to the body, and is more harmful when you’re already carrying a high stress load. If you are restricting food or delaying meals, your body will produce more stress hormones (cortisol and adrenalin) to function. During times of high stress, or if you have a very busy life, it is important to fuel yourself consistently.

Activity and Exercise

3. Fasting is never appropriate for athletes, or anyone who is exercising regularly at any level of intensity. Fasted workouts are not productive for women – we do better in a fed or fueled state in terms of better muscle response, building fitness, and maintaining healthy hormone balance. If you are largely sedentary, or unable to exercise due to injury, you may have a more positive response to time-restricted eating of fasting.

Hormonal State and Life Stage

4. Fasting and time-restricted eating are not recommended for women in the first 6 months post-partum, and definitely not if you are breastfeeding. In general it is not recommended if you are trying to get pregnant, with the exception of PCOS and insulin resistance where it may be helpful for fertility. During perimenopause, results will vary depending on stress, body fat, sleep, thyroid function and blood sugar levels.

Blood Sugar and Insulin Resistance

5. Time-restricted eating and intermittent fasting can be very favourable if you have pre-diabetes or insulin resistance, and will complement nutrition changes. If however, you are adding in significant exercise, or you have very high stress, it may negate the positive effects.

Fasting can lead to metabolic stress

I talk a lot about metabolic stress – this is what happens when you’re under-fueling for your lifestyle. Metabolic stress happens when you’re under-eating, not eating enough carbohydrates for your activity level, exercising in a fasted state, or fasting for too long in general. Women’s bodies when not fed properly go more quickly into a state of conservation which will affect the thyroid first, then sleep quality, and then the menstrual cycle.

Signs of metabolic stress include:

• Hair thinning

• Constipation and bloating

• Feeling cold

• Fatigue

• Weight has plateaued

• Mood swings

• Interrupted sleep

• Change in menstrual cycle – skipped, delayed, irregular

Tip: If you have 3 or more of these symptoms, and you have been intermittent fasting, this is a sign that your body is not adapting in a positive way.

How to check whether intermittent fasting and time-restricted eating are working well for you:

If you have been fasting regularly or using adopted time-resticted eating a lifestyle, there are some simple ways to check whether you are getting the desired benefits or causing metabolic stress.

1) Do you experience any of the symptoms listed above? Especially hair loss, constipation and weight plateauing.

2) Bloodwork to measure morning cortisol and thyroid hormones (especially free T3) are most important, and we may also include reproductive hormones if there has been a change in your menstrual cycle.

3) Blood-work to measure blood sugar and insulin to look for the positive effects of fasting.

4) How are you responding to your exercise program? Are you feeling great during workouts? Maintaining or building muscle? Recovering well?

How to integrate fasting and time-restricted eating safely

If you are still intrigued by the potential health benefits of time-restricted eating and intermittent fasting, there are some safer ways to get started. Remember that women who are post-menopause, or have significant insulin resistance or pre-diabetes, or who have a more sedentary lifestyle are the ones who will respond more favourably.

Start slow and see how you feel:

• Prioritize 12 hours overnight fasting, especially stopping eating 3 hours before bedtime. This is helpful for anyone, regardless of age, hormones, stress and activity.

• Extend to 14 hours overnight if you’re feeling good – this depends on your morning schedule and whether or not you’re working out in the mornings. If you are exercising early in the morning, my suggestion still is to fuel your body, not workout fasted. In this case, eat dinner earlier to extend your fasting window.

• Extend beyond 14 hours, with guidance and bloodwork to check in with your body’s response.

• Remember that more isn’t better – a longer overnight fast and shorter eating window for women especially don’t lead to better results.

What’s Next?

If you are interested in learning more about fasting, time-restricted eating and how it applies to you, or if you recognize the symptoms of metabolic stress, please book in for an appointment to discuss in more detail. We will likely start with some bloodwork, and a close look at how to best fuel your body for performance and longevity.

• Start with bloodwork to assess your hormones, thyroid, and blood sugar.

• Get personalized guidance for safe fasting and optimal nutrition.

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Frequently Asked Questions About Fasting and Time-Restricted Eating

Is intermittent fasting safe for women?

Intermittent fasting can be safe for some women, but it is highly individual. Age, stress levels, activity level, hormone status, and metabolic health all influence how a woman responds. For many women—especially those under high stress, exercising regularly, or in their reproductive years—fasting can create metabolic stress rather than benefit.

How can I tell if fasting is harming my metabolism?

Signs that fasting may be harmful include hair thinning, constipation, bloating, feeling cold, fatigue, sleep disruption, mood changes, weight loss plateaus, and changes in your menstrual cycle. If you experience three or more of these symptoms while fasting, it may indicate metabolic stress rather than metabolic improvement.

Can intermittent fasting help with insulin resistance?

Yes, intermittent fasting and time-restricted eating can be helpful for people with insulin resistance or pre-diabetes, particularly when paired with appropriate nutrition. However, the benefits can be reduced or reversed if fasting is combined with high stress, intense exercise, or under-fueling.

Should women work out in a fasted state?

In most cases, fasted workouts are not ideal for women. Exercising without adequate fuel can increase stress hormones, impair recovery, and negatively affect hormone balance. Women generally perform better, recover more effectively, and maintain healthier metabolism when exercising in a fed state.

Is fasting recommended during perimenopause or menopause?

Women in menopause may respond more favorably to fasting than younger women, but results still depend on stress, sleep, exercise, thyroid function, and blood sugar regulation. During perimenopause, fasting outcomes are highly variable and should be approached cautiously and individually.

Can fasting affect my menstrual cycle?

Yes. Prolonged fasting, delayed eating, or under-eating can disrupt the menstrual cycle by signaling metabolic stress to the body. This may result in skipped periods, delayed cycles, heavier bleeding, or worsening PMS symptoms.

What is the safest way to start time-restricted eating?

For most women, the safest starting point is a 12-hour overnight fast, especially avoiding food within three hours of bedtime. If this feels supportive, some women may extend to 14 hours. Longer fasting windows should only be considered with careful monitoring and professional guidance.

Is longer fasting better for metabolic health?

No. More is not better when it comes to fasting, especially for women. Longer fasting windows and shorter eating windows do not necessarily lead to better metabolic outcomes and may increase stress on the body.

What tests can help determine if fasting is working for me?

Bloodwork can be very helpful, including fasting glucose, insulin, HbA1c, thyroid hormones (especially free T3), and morning cortisol. Reproductive hormones may also be assessed if menstrual changes are present.

Who should avoid fasting altogether?

Fasting is generally not recommended for women who are pregnant, breastfeeding, within six months postpartum, actively trying to conceive, experiencing high stress, or engaging in regular or intense exercise. Athletes and highly active women usually require consistent fueling for optimal health.

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