Intermittent fasting means eating within set windows and not eating outside them. It’s popular for weight loss and “metabolic health”, and some of the interest is earned. But the research is more modest than the headlines, and fasting isn’t safe for everyone. Here’s what the evidence actually says.
Quick take
- Popular styles are 16:8, 5:2 and alternate-day fasting.
- Weight loss mostly comes from eating less overall; it does about as well as ordinary calorie cutting.
- Fasting isn’t safe for everyone, so some people should check with a doctor first.
The common ways people do it
- Time-restricted eating: eating inside a daily window, often 8 to 10 hours (the “16:8” pattern is 16 hours without food, 8 hours eating).
- 5:2: eating normally five days a week and much less (often around 500 to 600 calories) on two non-consecutive days.
- Alternate-day fasting: very little or no food every other day. It’s the hardest to stick with.
What the research shows
- Weight: people often lose weight, mostly because a shorter window means eating less overall. In head-to-head trials, fasting has generally done about as well as ordinary calorie cutting, not clearly better. A one-year trial in China (NEJM, 2022) found time-restricted eating plus calorie limits gave about the same weight loss as calorie limits alone.
- Blood sugar and insulin: some small trials found better insulin sensitivity, especially when the eating window was early in the day. Results vary, and many studies are short.
- Blood pressure and cholesterol: modest improvements show up in some studies, usually alongside weight loss.
- Brain and mood: much of the excitement (for example about BDNF, a protein linked to brain cell growth) comes from animal studies. Good human evidence is still thin.
Bottom line: intermittent fasting is one reasonable way to eat less for some people. It isn’t magic, and the best eating pattern is the one you can keep up while eating real, mostly whole foods.
The best eating pattern is the one you can actually stick with.
Who should talk to a doctor first (or skip it)
- Pregnant or breastfeeding, or under 18.
- Diabetes, especially if you take insulin or tablets that can cause low blood sugar. Fasting with these can drop your blood sugar dangerously.
- Taking medicine that must be taken with food, or blood pressure medicine (you may get dizzy).
- Underweight, frail, or with a history of an eating disorder.
- Any ongoing health condition. Ask your doctor or pharmacist before changing how and when you eat.
If you try it
- Start gently: a 12-hour overnight gap (for example 7 pm to 7 am) before trying anything longer.
- Drink water through the fasting hours. Plain tea or black coffee are fine for most people (caffeine: stay under about 400 mg a day).
- Make the eating window count: vegetables, protein, fibre, healthy fats. Fasting doesn’t cancel out ultra-processed food.
- Stop and eat if you feel shaky, faint, confused or unwell. If symptoms are severe, get medical help.
Fasting fits naturally into the “lower the load” part of the SDM, but it’s optional, not required.
Try this today
Start gently: finish dinner a little earlier and push breakfast back a little. There’s no need to jump straight to 16 hours.
Related
Sources
- National Institute on Aging: Calorie restriction and fasting diets, what do we know?
- Liu D. et al. (2022), Calorie restriction with or without time-restricted eating in weight loss, New England Journal of Medicine
- Lowe D.A. et al. (2020), Effects of time-restricted eating on weight loss (TREAT trial), JAMA Internal Medicine
- Sutton E.F. et al. (2018), Early time-restricted feeding improves insulin sensitivity and blood pressure in men with prediabetes, Cell Metabolism
- de Cabo R., Mattson M.P. (2019), Effects of intermittent fasting on health, aging and disease, New England Journal of Medicine
Educational only, not medical advice. This information has not been evaluated by the FDA or Health Canada and is not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before starting any remedy, supplement, or detox, especially if you are pregnant, nursing, taking medication, or managing a health condition. Read our full Medical Disclaimer.
