Intermittent Fasting for Specific Groups: Who Should (and Shouldn't) Fast
Who should try intermittent fasting and who should avoid it? Evidence-based guidance for women, older adults, athletes, teens, and people with specific health conditions.
Quick Answer: Intermittent fasting is safe and effective for most healthy adults, but it's not one-size-fits-all. Women, older adults, athletes, and people with conditions like diabetes, PCOS, or thyroid disorders may need modified approaches. Some groups — including pregnant or breastfeeding women, teens still growing, and people with a history of eating disorders — should avoid fasting or proceed only under medical supervision.
Intermittent fasting has strong evidence behind it for metabolic health, weight management, and even cognitive function. But the research participants in most studies are typically healthy adults aged 20 to 50. If you don't fit that profile, you need more specific guidance.
This page covers what the science says about intermittent fasting for different groups of people — who benefits, who needs modifications, and who should skip it entirely.
Intermittent Fasting for Women
Women's hormonal systems are more sensitive to caloric restriction signals than men's. The hypothalamic-pituitary-gonadal (HPG) axis in women responds to energy availability, meaning aggressive fasting can disrupt menstrual cycles, particularly in women who are lean or physically active.
A 2021 study in Obesity Reviews found that while intermittent fasting produced similar weight loss in men and women, women were more likely to report menstrual irregularities on protocols with fasting windows longer than 18 hours. The practical takeaway: most women do well with 14:10 or 16:8 protocols. More aggressive approaches like OMAD (one meal a day) or alternate-day fasting warrant caution.
Other considerations for women include:
- Cycle syncing. Some women find fasting easier during the follicular phase (days 1-14) and harder during the luteal phase when progesterone rises and caloric needs increase slightly.
- Iron and nutrient timing. Women with heavy periods need to ensure adequate iron and nutrient intake during their eating windows.
- Perimenopause and menopause. Hormonal shifts during these transitions can affect hunger, metabolism, and sleep, all of which interact with fasting.
For the complete picture, read intermittent fasting for women.
Fasting Over 40
Turning 40 doesn't change the fundamental biology of fasting, but it does shift the context. Metabolic rate begins a gradual decline — roughly 1-2% per decade after age 20 — and muscle mass decreases (sarcopenia). Intermittent fasting can actually help counteract both trends when done correctly.
Research published in Cell Metabolism (2018) showed that time-restricted eating improved metabolic markers in middle-aged adults, including insulin sensitivity, blood pressure, and oxidative stress. The key for this age group is ensuring adequate protein intake during the eating window (at least 1.2-1.6 g/kg of body weight) to preserve muscle mass.
Joint health, recovery from exercise, and sleep quality also become more relevant considerations. A moderate 16:8 approach tends to work well for most people over 40 without requiring dramatic lifestyle changes.
Detailed guidance in intermittent fasting over 40.
Fasting Over 50
By 50, the stakes around muscle preservation get higher. Sarcopenia accelerates, bone density concerns emerge (especially for women post-menopause), and chronic disease risk rises. Intermittent fasting remains beneficial for metabolic health at this age, but the approach needs to be more intentional.
A 2020 study in The Journals of Gerontology found that time-restricted eating in adults over 50 improved insulin sensitivity and reduced inflammatory markers without significant muscle loss — provided protein intake was adequate and participants maintained resistance training.
Priority considerations for people over 50:
- Protein distribution. Aim for 25-30g of high-quality protein at each meal within your eating window to maximize muscle protein synthesis.
- Bone health. Ensure adequate calcium and vitamin D intake. Fasting itself does not harm bone density, but inadequate nutrition during the eating window can.
- Medication timing. Many people over 50 take medications that require food. Your eating window needs to accommodate this.
Read more in intermittent fasting over 50 and our guide for beginners over 60.
Fasting for Athletes
Athletes have higher caloric and nutrient demands, which makes fasting a more nuanced proposition. The concern is straightforward: can you get enough fuel in a restricted eating window to support training, recovery, and performance?
For most recreational athletes and fitness enthusiasts, the answer is yes. The Moro et al. (2016) study on resistance-trained males found that an 8-hour eating window was compatible with maintaining strength and muscle mass while reducing fat. Endurance athletes, however, may struggle with long fasting windows on heavy training days.
The practical approach for athletes:
- Time your eating window around training. Eat your largest meal within two hours of your hardest session.
- Don't fast on competition days. Performance is the priority; fasting is a tool, not a religion.
- Monitor recovery markers. If sleep quality, HRV, or performance metrics decline, your eating window may need to expand.
Full breakdown in intermittent fasting for athletes.
Fasting and Diabetes
This is an area where intermittent fasting shows genuine promise — and where medical supervision is essential.
Type 2 diabetes is fundamentally a disease of insulin resistance. Intermittent fasting reduces insulin levels and improves insulin sensitivity, which addresses the root mechanism. A 2022 meta-analysis in The Journal of Clinical Endocrinology and Metabolism found that intermittent fasting reduced HbA1c by an average of 0.3-0.5% in people with type 2 diabetes — comparable to some medications.
However, the risks are real. People on insulin or sulfonylureas face hypoglycemia risk during extended fasts. Medication timing often needs adjustment. No one with diabetes should start intermittent fasting without discussing it with their doctor or endocrinologist first.
For type 1 diabetes, fasting is generally not recommended without very close medical management due to the risk of both hypoglycemia and diabetic ketoacidosis.
Detailed discussion in intermittent fasting and diabetes.
Fasting While Breastfeeding
The short answer: most experts advise against intermittent fasting while breastfeeding, or at minimum recommend very conservative approaches.
Breastfeeding increases caloric needs by approximately 300-500 calories per day. Caloric restriction can reduce milk supply and may alter milk composition. While a mild 12:12 eating pattern (which most people follow naturally) is unlikely to cause issues, more restrictive protocols like 16:8 or beyond carry more risk.
A 2009 study in Acta Obstetricia et Gynecologica Scandinavica found that acute fasting during Ramadan was associated with changes in breast milk macronutrient composition, though the clinical significance was debated. The precautionary principle applies here: your infant's nutrition comes first.
More nuance in intermittent fasting while breastfeeding.
Fasting for Teens
Teenagers are still growing. Their brains are developing. Their hormonal systems are establishing patterns. This is not the time for caloric restriction experiments.
While there is limited research on intermittent fasting in adolescents, the American Academy of Pediatrics has expressed concern about any restrictive eating patterns in teens due to the risk of triggering disordered eating behaviors. Adolescents have higher nutrient requirements per kilogram of body weight than adults, and the developing brain is particularly sensitive to energy availability.
If a teenager is interested in healthier eating patterns, the focus should be on food quality, regular meals, adequate sleep, and physical activity rather than meal timing restriction.
Read our position in intermittent fasting for teens.
Fasting and PCOS
Polycystic ovary syndrome (PCOS) affects roughly 10% of women of reproductive age and is strongly linked to insulin resistance. Because intermittent fasting improves insulin sensitivity, it may be particularly relevant for this group.
A 2021 pilot study in the Journal of the American Nutrition Association found that women with PCOS who followed a 16:8 fasting protocol for 6 weeks showed improvements in insulin levels, androgen profiles, and inflammatory markers. Larger studies are needed, but the mechanistic rationale is sound.
The caveat: women with PCOS who are underweight or have concurrent eating disorders should not fast. And as with any hormonal condition, fasting protocol changes should be discussed with a healthcare provider.
Full details in intermittent fasting and PCOS.
Fasting and Thyroid Health
Thyroid function is sensitive to energy availability. Prolonged caloric restriction can reduce T3 (the active thyroid hormone) levels, which some interpret as evidence that fasting harms thyroid function. The picture is more nuanced than that.
Short-term fasting (16-24 hours) causes a transient dip in T3 that normalizes when eating resumes. This is a normal adaptive response, not thyroid damage. However, chronic severe caloric restriction can lead to sustained T3 suppression, which is problematic.
For people with hypothyroidism (including Hashimoto's thyroiditis), moderate intermittent fasting like 16:8 appears to be safe as long as total caloric intake is adequate. The timing of thyroid medication is the main practical concern — levothyroxine should be taken on an empty stomach, which actually pairs well with a fasting morning.
People with hyperthyroidism should be cautious, as their metabolic rate is already elevated and fasting may exacerbate weight loss or nutrient depletion.
More in intermittent fasting and thyroid.
Who Should Not Fast
To be direct, the following groups should avoid intermittent fasting or proceed only with explicit medical guidance:
- Pregnant women. Caloric restriction during pregnancy can affect fetal development.
- People with active eating disorders or a history of anorexia or bulimia. Fasting can reinforce restrictive patterns.
- Children and adolescents who are still growing.
- People who are severely underweight (BMI below 18.5).
- Anyone on medications that require food at specific times who cannot adjust their schedule.
Fasting is a powerful tool, but tools need to be matched to the user.
How Fasted Helps
Fasted is designed to support whatever protocol fits your life and health situation. Whether you're doing a conservative 14:10 or a more aggressive 18:6, the app lets you customize your fasting schedule and adjust it as your needs change. The weight tracking and stats features help you monitor your progress objectively, and meal logging ensures you're hitting your nutrition targets during your eating window.
For the science behind how fasting works at a cellular level, visit our science pillar. If you're ready to start, our getting started guide walks you through choosing the right protocol for your situation.
Frequently Asked Questions
Is intermittent fasting safe for women?
Yes, for most women. However, women tend to do better with moderate fasting windows (14:10 or 16:8) rather than extended fasts. Women who are pregnant, breastfeeding, or have a history of amenorrhea should avoid fasting or consult their doctor first.
Can I do intermittent fasting if I have diabetes?
People with type 2 diabetes may benefit from intermittent fasting due to its effects on insulin sensitivity, but medical supervision is essential. Medication adjustments are often needed to prevent hypoglycemia. People with type 1 diabetes should only fast under very close medical management.
Is intermittent fasting appropriate for people over 60?
Yes, with some modifications. Adequate protein intake (1.2-1.6 g/kg body weight), resistance training, and attention to medication timing are important. A moderate 16:8 or 14:10 protocol is generally well-tolerated. Always discuss new dietary approaches with your healthcare provider, especially if you take multiple medications.
Should teenagers try intermittent fasting?
Generally, no. Adolescents have high nutrient requirements for growth and development, and restrictive eating patterns can trigger disordered eating behaviors. Teens interested in healthier eating should focus on food quality and regular meals rather than time restriction.