Intermittent Fasting: Does It Work, Is It Safe and Who Should Avoid It?

Intermittent fasting (IF) has become one of the most popular ways to lose weight. Instead of changing what you eat, it limits when you eat. Some people find it simple and effective, while for others it is difficult or unsafe. Here is what the evidence shows.

Common methods

• Time-restricted eating (16:8): eat within an 8-hour window (for example 11 am to 7 pm) and fast for 16 hours, including sleep.
• 14:10 or 12:12: gentler versions with a 10-12 hour eating window.
• 5:2 diet: eat normally for 5 days and restrict to about 500-600 kcal on 2 non-consecutive days.
• Alternate-day fasting: a very low-calorie day (about 500 kcal) alternating with normal eating days.
• One meal a day (OMAD): a single meal in 24 hours, which is hard to sustain and not recommended for most people.
• Religious fasting such as Ramadan, Poya days or Navratri follows different patterns and has its own medical considerations.

Does it work for weight loss?

Most studies suggest IF can produce weight loss of about 3-8% over 8-24 weeks, mainly because people eat fewer calories. Compared directly with standard daily calorie restriction, results are generally similar. Long-term data beyond one to two years are limited. The best diet is the one you can maintain, and some people find the simple rules of IF easier to follow than counting calories.

Possible benefits

• Weight and waist reduction
• Improved insulin sensitivity and blood sugar in people with prediabetes or type 2 diabetes, mostly through weight loss
• Lower blood pressure and triglycerides in some studies
• Possible benefits for fatty liver (see fatty liver disease)
• Simplicity: fewer meals to plan

Claims that IF "detoxes" the body, extends lifespan in humans or cures disease are not proven.

Possible downsides and risks

• Hunger, headache, irritability, poor concentration and tiredness, particularly in the first 1-2 weeks
• Overeating or eating unhealthy food in the eating window
• Muscle loss if protein intake and strength training are inadequate
• Constipation, acid reflux, dizziness
• Low blood sugar in people with diabetes taking certain medicines
• Disrupted sleep and mood in some people
• Triggering or worsening eating disorders
• Gallstones with rapid weight loss
• Limited long-term evidence on heart health and bone health

Who should NOT try intermittent fasting without medical advice

• Pregnant and breastfeeding women
• Children and teenagers
• People with type 1 diabetes, or type 2 diabetes treated with insulin or sulfonylureas such as gliclazide, glibenclamide and glimepiride
• People with a history of eating disorders
• Underweight people (BMI below 18.5) or those with malnutrition
• Frail or older adults at risk of falls and muscle loss
• People with kidney, liver or heart disease, and those who take medicines that must be taken with food
• People with gout, since fasting can raise uric acid. See high uric acid and gout
• Anyone who needs to drive or work in safety-critical jobs while fasting and feeling unwell

Diabetes and fasting

If you have diabetes, do not begin fasting without a plan. Your doctor may need to reduce insulin or tablet doses, and you should check blood sugar more frequently. Break the fast if your sugar falls below 70 mg/dL (3.9 mmol/L) or you feel unwell. Read low blood sugar for symptoms and treatment. For people with prediabetes, weight loss by any method helps. See how to reverse prediabetes.

Tips for safer, more sustainable fasting

1. Start gently: begin with a 12-hour overnight fast and extend gradually to 14 hours or more if comfortable.
2. Stay hydrated: water, plain tea and black coffee are fine. Avoid sugar, milk and sweeteners that trigger hunger.
3. Break your fast with a balanced meal: vegetables, protein (dhal, fish, egg) and a moderate portion of rice or whole grains, not a huge feast of sweets and fried food.
4. Prioritise protein and fibre to protect muscle and keep you full. Check needs with our protein calculator.
5. Include strength exercise twice a week.
6. Sleep well and avoid fasting on days of heavy physical or mental demands.
7. Do not overcompensate. Eating double afterwards undoes the benefit. Our TDEE calculator shows your calorie needs.
8. Stop if you feel unwell: dizzy, faint, shaky or extremely weak.
9. Track progress over weeks, using weight, waist, energy and blood sugar if relevant.

Alternatives that work equally well

• Reducing portion sizes and using the plate method
• Cutting sugary drinks and fried foods
• Eating a regular, balanced diet with a moderate deficit
• Walking after meals

See how to lose weight safely for a complete plan.

When to see a doctor

Speak to your doctor before starting IF if you have a medical condition, take regular medicines, are over 65, or have a history of disordered eating. Seek help if you feel faint, confused, have palpitations, or develop an unhealthy preoccupation with food.

References

• Varady KA et al. Clinical application of intermittent fasting for weight loss. Nature Reviews Endocrinology, 2022
• Lowe DA et al. Effects of time-restricted eating on weight loss. JAMA Internal Medicine, 2020
• American Diabetes Association: consensus on fasting and diabetes
• International Diabetes Federation and DAR: diabetes and Ramadan practical guidelines