Who should not do intermittent fasting?
Intermittent fasting may harm pregnant or nursing women, people with eating-disorder history or type 1 diabetes, underweight people and children, who should avoid it.
Intermittent fasting has real benefits, but it is not appropriate for everyone. Some people should avoid it entirely, and everyone else should start gradually. Ignoring this advice can cause genuine harm.
Who Should Not Fast
Pregnant or nursing women. Growing a baby or producing breast milk requires consistent energy and nutrient availability. Fasting can reduce nutrient transfer and affect milk supply. This is not the time to experiment with meal timing1.
People with a history of eating disorders. IF can reinforce harmful patterns around food restriction, perfectionism, and control. Research has raised concerns that fasting protocols may trigger or worsen disordered eating behaviours, particularly in individuals with a history of anorexia nervosa or bulimia3. If your relationship with food is complicated, talk to a professional before trying any fasting protocol.
People with type 1 diabetes. Type 1 diabetics rely on exogenous insulin and are at risk of dangerous hypoglycaemia during fasting periods. The clinical management of fasting in diabetes requires close medical supervision and is not something to self-prescribe from a blog post2.
Underweight individuals. If your BMI is below 18.5 or you're already struggling to maintain weight, fasting will work against you. You need caloric surplus, not restriction.
Children and adolescents. Growing bodies need regular fuel. There is no evidence supporting IF for anyone under 18, and the risks — nutrient deficiency, growth impairment, disordered eating patterns — are real.
What you can do
- If none of these apply, ramp up: 12-hour fasts in weeks 1–2, 14 hours in weeks 3–4, then 16:81.
- Break a fast gently with nuts, bone broth or protein and vegetables, then wait 20–30 minutes before a full meal.
- Stay hydrated with water, black coffee or plain tea, and add electrolytes on longer fasts; dehydration causes much fasting headache and fatigue2.
Be careful if
- If you take blood pressure or blood sugar medicines, talk to your doctor before fasting, because doses may need adjusting2.
- Eat if you feel dizzy, shaky or genuinely unwell; fasting should feel sustainable, not heroic.
- If you are pregnant, nursing, a teenager or have an eating-disorder history, talk to a professional before trying any fasting protocol1,3.
Go deeper
How it works What it does in your body, in plain words 1 min
If none of the above apply to you, the best approach is a gradual ramp-up. Jumping straight into 20-hour fasts is unnecessary and usually backfires.
Week 1–2: 12-hour fast. Stop eating after dinner at 8 PM, don't eat again until 8 AM. Most people already do something close to this. It's the training wheels version — your body acclimates to a defined eating window.
Week 3–4: 14-hour fast. Push breakfast to 10 AM. You'll start noticing that morning hunger is more habit than need. Stay hydrated — water, black coffee, and plain tea are fine during the fast.
Week 5+: 16-hour fast. The classic 16:8 window. Eat between noon and 8 PM (or whatever 8-hour block fits your life). At this point, most people report that the adjustment is behind them and the benefits are noticeable1.
Listen to your body. If you feel dizzy, shaky, or genuinely unwell (not just hungry), eat. Fasting should feel sustainable, not heroic. If a 14-hour window works for you and 16 doesn't, stay at 14.
The evidence The studies, what they found, and how much to trust them 2 min · 3 sources
How you break your fast matters more than most people realize. After 16+ hours without food, your digestive system has downregulated acid and enzyme production. Dumping a huge, heavy meal into an empty stomach can cause bloating, cramping, and blood sugar spikes.
Start small. Break your fast with something gentle — a handful of nuts, some bone broth, a small portion of protein with vegetables. Give your gut 20–30 minutes to wake up before eating a full meal.
Prioritise protein and healthy fats. These trigger satiety hormones and provide stable energy. Avoid breaking your fast with refined carbohydrates or sugary foods, which cause a rapid insulin spike followed by a crash1.
Stay hydrated throughout the fast. Most of the headaches and fatigue that new fasters experience are from dehydration, not hunger. Electrolytes (sodium, potassium, magnesium) matter — especially during longer fasts2.
Grajower and Horne's 2019 clinical review emphasised that patients attempting fasting protocols should be educated on recognising hypoglycaemia symptoms, maintaining hydration, and adjusting any medications in consultation with their healthcare provider2. This is especially relevant for anyone on blood pressure or blood sugar medication, even if they don't have type 1 diabetes.
The bottom line: intermittent fasting is a powerful tool, but like any tool, it works best when used correctly and by the right person. Start slow, pay attention to how you feel, and don't treat it as an identity.
More on Intermittent Fasting
- What does intermittent fasting do to your body? 3 min An introduction to intermittent fasting — the different protocols, the science of autophagy, and why it's not the same as starving yourself.
- What are the proven health benefits of intermittent fasting? 3 min The evidence-backed benefits of intermittent fasting — from fat loss and insulin sensitivity to brain health and longevity.
- Does the fasting-mimicking diet improve metabolic health and slow aging? 7 min A 5-day monthly eating protocol that triggers fasting's cellular benefits — autophagy, stem cell renewal, IGF-1 reduction — without complete food abstinence.
Sources
- Intermittent Fasting and Human Metabolic Health. Patterson RE, Sears DD. Journal of the Academy of Nutrition and Dietetics, 2015. PubMed 25857868
- Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Grajower MM, Horne BD. Nutrients, 2019. PubMed 31003482
- Intermittent fasting: Describing engagement and associations with eating disorder behaviors and psychopathology among Canadian adolescents and young adults. Ganson KT, Cuccolo K, Hallward L, Nagata JM. Eating Behaviors, 2022. PubMed 36368052