Intermittent Fasting: What the Science Says, What It Does Not, and Who Should Avoid It
Skipping breakfast or eating only in an eight-hour window is a popular weight-loss trend. Harvard Health and a major review of trials say it can work, but no better than simply eating less.
You may have heard the claim: "Do not eat after 7 pm" or "Skip breakfast and eat only between noon and 8 pm." This is called intermittent fasting. It is one of the most talked-about diet trends. But does it work, and is it right for everyone?
Quick answer
- Intermittent fasting means changing when you eat, not necessarily what you eat.
- Studies show it can lead to modest weight loss in adults with overweight, but not more than simply eating fewer calories.
- Most trials are short and small, so the long-term effects are not known.
- Some people should not try it, including people with diabetes who take medicines, people with an eating disorder, and pregnant or breastfeeding women.
What is intermittent fasting?
There are several styles. Two common ones are:
- 16:8 (time-restricted eating): you fast for 16 hours and eat only in an eight-hour window, for example 12 noon to 8 pm.
- 5:2: you eat normally for five days a week and eat much less on two days.
Harvard Health explains that intermittent fasting restricts when you eat, not what you eat. Its article looks mainly at time-restricted eating, which shortens your eating window each day.
Does it work better than eating less?
Harvard Health describes a one-year trial of 118 adults aged 18 to 75, all with overweight or obesity. All of them cut their calories by about 25 percent. One group also limited their eating to a time window, and the other group did not. At the end of the year:
- The time-restricted group lost about 18 pounds, and the other group lost about 14 pounds.
- That difference was not statistically significant, so it could be due to chance.
- Blood pressure, cholesterol and blood sugar improved by a similar amount in both groups.
The message from this trial is simple: restricting the eating window alone gave no clear advantage over cutting calories. Harvard Health also says that weight loss from time-restricted eating is usually modest, around 3 to 8 percent of body weight, mainly because people end up eating a few hundred fewer calories a day.
What does the bigger evidence say?
A review published in JAMA Network Open in 2021 looked at 11 summaries of trials, covering 130 trials. It checked 104 possible links between intermittent fasting and health outcomes related to obesity. Its findings:
- Only 28 of the 104 links were statistically significant, and most of these rested on very low or low-quality evidence.
- Some benefits showed up, including lower body weight, cholesterol, blood sugar and blood pressure.
- Only modified alternate-day fasting and the 5:2 diet were linked to significant weight loss in adults with overweight or obesity.
- Most of the research was in adults with overweight or obesity.
- The trials were short, with a median follow-up of about three months, and small, with a median of 38 people.
- The review could not judge safety, as earlier studies did not report side effects.
In plain words: there are signs of benefit, but the evidence is thin, and we do not yet know what happens over many years.
What are the downsides?
Harvard Health notes that weight loss often includes a loss of lean muscle, so strength training is strongly advised alongside fasting. It also stresses that fasting works only if your total food intake stays lower. If you overeat in your eating window, the benefit disappears. Fasting can also leave some people hungry or tired.
Who should avoid it?
Harvard Health lists these groups as people who should avoid it or be very careful:
- People with diabetes, especially if they take medicines that lower blood sugar
- People with an eating disorder, or a history of one
- Pregnant or breastfeeding women
- People who need to take food with their medicines
Children and teenagers also need enough food to grow. If you have any health condition, speak to your doctor first.
Many Indians fast for religious reasons, for example during Navratri. That is a personal choice, but the same caution applies: if you have diabetes or take regular medicines, ask your doctor how to fast safely.
A sensible way to think about it
- If an eight-hour window helps you eat less and you feel fine, it is one option, not a magic one.
- What you eat still matters. Fruit, vegetables, whole grains and dal beat fried snacks and sweets in any time window. See our guides to fibre and hidden sugar.
- Sleep and movement matter too. Try walking after meals.
This article is general information, not medical advice. Speak to a doctor before starting any fasting plan, especially if you have a health condition or take medicines.
Related reading
Sources
This article draws on the following reporting. Dates are when each source was originally published.
- Harvard Health Publishing: Intermittent fasting: surprising update
- JAMA Network Open (2021): Intermittent fasting and obesity-related health outcomes: an umbrella review of meta-analyses of randomized clinical trials ·
Written by
Stories, explainers and features published by The Daily, based on public sources that are listed with every article.
More from The Daily →Enjoyed this story?
Get the best of The Daily in your inbox every Sunday.
