Harvard Medical School on Intermittent Fasting

Home » Harvard Health Blog » Intermittent fasting: Surprising update – Harvard Health Blog

Intermittent fasting: Surprising update

POSTED JUNE 29, 2018, 6:30 AM , UPDATED DECEMBER 12, 2019, 12:00 AM

Monique Tello, MD, MPH

Monique Tello, MD, MPH
Contributing Editor

Intermittent fasting

There’s a ton of incredibly promising intermittent fasting (IF) research done on fat rats. They lose weight, their blood pressure, cholesterol, and blood sugars improve… but they’re rats. Studies in humans, almost across the board, have shown that IF is safe and incredibly effective, but really no more effective than any other diet. In addition, many people find it difficult to fast.

But a growing body of research suggests that the timing of the fast is key, and can make IF a more realistic, sustainable, and effective approach for weight loss, as well as for diabetes prevention.

The backstory on intermittent fasting

IF as a weight loss approach has been around in various forms for ages, but was highly popularized in 2012 by BBC broadcast journalist Dr. Michael Mosley’s TV documentary Eat Fast, Live Longer and book The Fast Diet, followed by journalist Kate Harrison’s book The 5:2 Diet based on her own experience, and subsequently by Dr. Jason Fung’s 2016 bestseller The Obesity Code. IF generated a steady positive buzz as anecdotes of its effectiveness proliferated.

As a lifestyle-leaning research doctor, I needed to understand the science. The Obesity Code seemed the most evidence-based summary resource, and I loved it. Fung successfully combines plenty of research, his clinical experience, and sensible nutrition advice, and also addresses the socioeconomic forces conspiring to make us fat. He is very clear that we should eat more fruits and veggies, fiber, healthy protein, and fats, and avoid sugar, refined grains, processed foods, and for God’s sake, stop snacking. Check, check, check, I agree. The only part that was still questionable in my mind was the intermittent fasting part.

Intermittent fasting can help weight loss

IF makes intuitive sense. The food we eat is broken down by enzymes in our gut and eventually ends up as molecules in our bloodstream. Carbohydrates, particularly sugars and refined grains (think white flours and rice), are quickly broken down into sugar, which our cells use for energy. If our cells don’t use it all, we store it in our fat cells as, well, fat. But sugar can only enter our cells with insulin, a hormone made in the pancreas. Insulin brings sugar into the fat cells and keeps it there.

Between meals, as long as we don’t snack, our insulin levels will go down and our fat cells can then release their stored sugar, to be used as energy. We lose weight if we let our insulin levels go down. The entire idea of IF is to allow the insulin levels to go down far enough and for long enough that we burn off our fat.

Intermittent fasting can be hard… but maybe it doesn’t have to be

Initial human studies that compared fasting every other day to eating less every day showed that both worked about equally for weight loss, though people struggled with the fasting days. So, I had written off IF as no better or worse than simply eating less, only far more uncomfortable. My advice was to just stick with the sensible, plant-basedMediterranean-style diet.

New research is suggesting that not all IF approaches are the same, and some are actually very reasonable, effective, and sustainable, especially when combined with a nutritious plant-based diet. So I’m prepared to take my lumps on this one (and even revise my prior post).

We have evolved to be in sync with the day/night cycle, i.e., a circadian rhythm. Our metabolism has adapted to daytime food, nighttime sleep. Nighttime eating is well associated with a higher risk of obesity, as well as diabetes.

Based on this, researchers from the University of Alabama conducted a study with a small group of obese men with prediabetes. They compared a form of intermittent fasting called “early time-restricted feeding,” where all meals were fit into an early eight-hour period of the day (7 am to 3 pm),or spread out over 12 hours (between 7 am and 7 pm). Both groups maintained their weight (did not gain or lose) but after five weeks, the eight-hours group had dramatically lower insulin levels and significantly improved insulin sensitivity, as well as significantly lower blood pressure. The best part? The eight-hours group also had significantly decreased appetite. They weren’t starving.

Just changing the timing of meals, by eating earlier in the day and extending the overnight fast, significantly benefited metabolism even in people who didn’t lose a single pound.

So, is this as good as it sounds?

I was very curious about this, so I asked the opinion of metabolic expert Dr. Deborah Wexler, Director of the Massachusetts General Hospital Diabetes Center and associate professor at Harvard Medical School. Here is what she told me. “There is evidence to suggest that the circadian rhythm fasting approach, where meals are restricted to an eight to 10-hour period of the daytime, is effective,” she confirmed, though generally she recommends that people “use an eating approach that works for them and is sustainable to them.”

So, here’s the deal. There is some good scientific evidence suggesting that circadian rhythm fasting, when combined with a healthy diet and lifestyle, can be a particularly effective approach to weight loss, especially for people at risk for diabetes. (However, people with advanced diabetes or who are on medications for diabetes, people with a history of eating disorders like anorexia and bulimia, and pregnant or breastfeeding women should not attempt intermittent fasting unless under the close supervision of a physician who can monitor them.)

4 ways to use this information for better health

  1. Avoid sugars and refined grains. Instead, eat fruits, vegetables, beans, lentils, whole grains, lean proteins, and healthy fats (a sensible, plant-based, Mediterranean-style diet).
  2. Let your body burn fat between meals. Don’t snack. Be active throughout your day. Build muscle tone.
  3. Consider a simple form of intermittent fasting. Limit the hours of the day when you eat, and for best effect, make it earlier in the day (between 7 am to 3 pm, or even 10 am to 6 pm, but definitely not in the evening before bed).
  4. Avoid snacking or eating at nighttime, all the time.


Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical TrialJAMA Internal Medicine, May 2017.

Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolismAmerican Journal of Clinical Nutrition, January 2005.

The Obesity Code, by Jason Fung, MD (Greystone Books, 2016).

Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database of Systematic Reviews and Implementation Reports, February 2018.

Metabolic Effects of Intermittent FastingAnnual Review of Nutrition, August 2017.

Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with PrediabetesCell Metabolism, May 2018.

  • 40.4k


Related Posts:

COMMENTS41TOPICSDiet and Weight Loss | Health


Put an end to diet disappointment with Harvard Medical School’s Ultimate Guide to
Weight Loss

Click to get access to exclusive content.Click here to learn more

  1. POSTED AUGUST 25TH, 2018 AT 7:09 PMJeffEvery study seems to support cognitive and health benefits for IF. Studies are coming out showing it may help stave off heart disease and it’s even been shown to halt or possibly reverse brain-related diseases such as Alzheimer’s. If in doubt, check out Jason Fung’s youtube videos along with a couple of youtube researchers who do wonderful analytics, an American who lives in Japan who goes by, “Things I’ve Learned” and Thomas DeLauer’s IF material. I’ve been doing IF myself for a few months now and I feel better, more energy, better sleep, and controlled weight.
  2. POSTED AUGUST 23RD, 2018 AT 8:11 AMChristineI just started intermittent fasting. Can I take diatomaceous earth while in my fasting state?
  3. POSTED AUGUST 21ST, 2018 AT 2:22 PMZKI started IF a month ago and I feel great. I lost weight, I feel “lighter,” more awake, and more energetic. I plan to make IF a part of my life.I eat from 12PM to 8PM. I feel that skipping breakfast is the easiest thing to do. I used to go to a bakery every morning – I’ve stopped that. If I get a little hungry in the morning, I use coffee as an appetite suppressant. On most days I don’t even think about breakfast and before I know it, it’s noon!I’ve been good about not eating past 8PM. Okay, If I’m out for dinner or at a party, I might go past 8PM…but I don’t kill myself over it. Some days if I have a big lunch (crispy duck with basil or cheeseburger), I won’t eat until the following day.I’m 43 years old and not a big guy (5’7″) but I could use some trimming around the gut. When I started IF I was 169 lbs and I’m now at 165 lbs…so losing 1-2 lbs per week.One important thing: I herniated a disc in my lower back a couple months ago so I’m not exercising. I don’t know how things will change (appetite, blood sugar, fatigue, etc) once I resume weights and start riding my bike to work again.In summary, I’m going to use some form of IF for the rest of my life and I highly recommend it. As with anything check with your doctor and make sure it’s the right thing for you. Good luck!
  4. POSTED AUGUST 20TH, 2018 AT 7:02 PMAnnaHey, Im 16 years old. Should I try this? I also use special app on my phone to count protein, carbohydrates and fat, i understand that its really important for my body to work well and to have everything for it. It won’t be so harmful to try if in my age as well? Some tips?
  5. POSTED AUGUST 18TH, 2018 AT 5:06 AMEnzoWhy do you conclude that evening fasting is better? Both the groups started the feeding time window at 7am, so the different results are not related to the timing but to the duration of fasting. Nothing new for us… this simply remarks the benefits of restricted time feeding, indipendently from time of the day.As for eating early there are no evidence of the benefits and also some studies that go against this advice http://www.ergo-log.com/emphasisonbreakfast.htmlAnectodally I find more difficult to fast in the evening (have you ever tried to go to bed on an empty stomach?  )… without count that in the evening my self-control is really depleted after a day of work and I want just to relax and enjoy a good dinner.For me, the best and simplier to stick advices are:
    – skip breakfast for intermittent fasting;
    – eat ever at the same time like a swiss clock;
    – eat fat at lunch and carbs at dinner.
  6. POSTED AUGUST 18TH, 2018 AT 12:21 AMMaggieThe timeframe doesn’t apply to people who work overnight/graveyard shifts.
  7. POSTED AUGUST 16TH, 2018 AT 1:41 AMMichaelAdvice for us night workers wouldn’t go amiss.
  8. POSTED AUGUST 8TH, 2018 AT 12:15 PMMartin JenkinsI am a 65-year-old male who started IF seven weeks ago. I only eat between noon and 8pm. I am obese, but losing about a pound a week so far. Notably, except for time, I have not changed what I eat at all. My diet was never terrible or great, and now it is the same, a mix of raw fruit sometimes and a donut another time. But I only eat it during the appointed hours. Remarkably, I do not feel hungry. I used to eat comfort breakfasts like pancakes or waffles, and I thought I would miss them. But no, I truly am not hungry in the mornings. I often delay lunch, but I still stop eating at 8. That alone probably has cut many calories of desserts. Bottom line: works for me so far.
  9. POSTED AUGUST 6TH, 2018 AT 11:16 AMIsaac Morales60 year old and just started IF a week ago. I eat from noon to 8pm. The noon start works for me because I’m not starting my day with the thought of food! I LOVE FOOD AND LOVE TO EAT! I am moving away from some bad habits and it doesn’t seem that difficult for me with IF! Just one week in and I do feel better. Can’t wait till I’ve got a month under my belt.
  10. POSTED AUGUST 3RD, 2018 AT 6:53 AMSKGarriottI’m 63 years old and I have been following a daily 19 hour protocol called Fast 5, fast5.org for two years. I eat lunch at 3pm and dinner at 7pm close my eating window at 8pm. I’ve lost 43 lbs and kept it off, feel great and I am no longer pre diabetic. I eat what I want and don’t track anything. I belong to a Facebook Intermittent fasting group called Fast Club and would to have you check it out. Fasting is free and it works!
    • POSTED AUGUST 3RD, 2018 AT 1:19 PMMonique Tello, MD, MPHWow, Impressive results, skgarriot! More and more evidence supports an overnight fast, though sixteen hours works very well for most folks.
  11. POSTED JULY 30TH, 2018 AT 5:14 AMMicheleBasically, we have to skip dinner. A very old trick.
  12. POSTED JULY 25TH, 2018 AT 7:52 AMT.I function well with iF. 71 year old male. 151 lbs 5 foot 9. But maintaing my weight is tough. The large meal appears to crowd my stomach to the point of moderate to extreme discomfort. What is the solution? Thanks.
  13. POSTED JULY 23RD, 2018 AT 12:26 PMMonique Tello, MD, MPHHi Matilde, Yes, you are encouraged to drink fluids while fasting. Water, tea, coffee, seltzer, and even broth are all fine. You can put a dollop of cream in your coffee if you like as well. Check out Jason Fung’s book Obesity Code (and just FYI, there are no benefits to me for recommending others’ writing…)
  14. POSTED JULY 23RD, 2018 AT 11:20 AMCameron J.Yep. Also good article here too Prediabetes Symptoms – Lark (https://www.web.lark.com/prediabetes-symptoms/) (“Having prediabetes puts you at risk for developing type 2 diabetes. As you might expect, prediabetes is a condition with higher blood sugar, or blood glucose, than normal, but lower levels than in diabetes. It happens as your body develops insulin resistance and is less able to regulate blood sugar levels properly. Every year, 5 to 10% of people with prediabetes develop diabetes”)
  15. POSTED JULY 23RD, 2018 AT 10:44 AMMatildeA dumb question possibly, but is drinking water, tea or coffee (no sugars or milk or..) during fasting or in between meals ok?
  16. POSTED JULY 20TH, 2018 AT 5:00 PMMonique Tello, MD, MPHThanks for this! It’s even more popular recently.
  17. POSTED JULY 20TH, 2018 AT 4:59 PMMonique Tello, MD, MPHThanks Shane!
  18. POSTED JULY 20TH, 2018 AT 4:59 PMMonique Tello, MD, MPHHi April, that’s wonderful for you, thanks for reading and sharing!
  19. POSTED JULY 20TH, 2018 AT 4:58 PMMonique Tello, MD, MPHThis is a new area, but the research that has come out since this article is also positive, and promising. One example: In this June 2018 study of 23 people with obesity, 12 weeks of 8-hour time-restricted feeding resulted a 2.6% decrease in body weight and a 7 point decrease in systolic blood pressure, which was significant when compared to controls: https://www.ncbi.nlm.nih.gov/pubmed/29951594
  20. POSTED JULY 20TH, 2018 AT 4:51 PMMonique Tello, MD, MPHThank you Dr. De Foa, for the metabolic details! We generally would not go into this kind of specific physiologic explanation, rather, we go big-picture, for clarity.
  21. POSTED JULY 20TH, 2018 AT 4:50 PMMonique Tello, MD, MPHThanks Tom and Wendy and Jim, Yes, this particular study emphasized earlier time-restricted feeding, but I agree that for many people a 12 pm- 8 pm feeding window is more realistic, and if it’s working, then why not?
  22. POSTED JULY 20TH, 2018 AT 4:47 PMMonique Tello, MD, MPHThanks, Zeeb, for sharing!
  23. POSTED JULY 20TH, 2018 AT 4:47 PMMonique Tello, MD, MPHHi Thea, That’s wonderful that IF has worked for you. Diets, and particularly fasting, can be very triggering for others with a history of an eating disorder. People who have been in remission can relapse. For more about what concerns and problems others have had, there is alot of information out there, and for starters I recommend this thorough article from Psychology Today: https://www.psychologytoday.com/us/blog/hunger-artist/201411/the-fast-diet-fast-route-disordered-eating
  24. POSTED JULY 11TH, 2018 AT 1:11 PMZeeb Ralstongreat stuff, I agree wholeheartedly, posted it on my facebook site and emailed it to most of my mailing list. I am also a personal patient of Dr. Fung and can heartily vouch for his qualifications, his focus and his abilities. Win Win Win! and yes, eating disorders I believe are easily conquered with fasting, both Intermittent and extended.
  25. POSTED JULY 10TH, 2018 AT 9:50 AMRichard BarnardJason Fung has a new book out: The Diabetes Code.Dick
  26. POSTED JULY 9TH, 2018 AT 9:09 PMTheaThe disclaimer cautioning that people with a Hx of eating disorder shouldn’t attempt IF without medical supervision is ubiquitous in articles like this one. However I have never seen anyone cite any evidence as the foundation for this recommendation, nor even provide clinical anecdotes or a thorough clinical rationale.I believe this is a disservice to those, like me, with a history of eating disorder. It has made experimenting with IF unnecessarily stressful. Despite my worry about what might happen (reading all these baseless cautions), I went ahead and experimented. In my experience, contrary to this “expert advice”, IF has been the most profoundly effective intervention I’ve experienced for my bulemia.It has totally regulated my appetite and normalised my relationship with food. My obsessive thoughts have completely subsided, my black and white thinking around food has gone, and I no longer binge! This is amazing. For the first time in my adult life I feel like I know what it is like to have a normal relatinoship with food. I eat when I eat, a range of healthy whole foods and occasional less healthy foods. In normal amounts. In manageable amounts. And when my meal is over, I stop! Normal for others, a seeming impossibility for me (and, I’m guessing, others with eating disorders).I have struggled iwth the hell of an eating disorder for over 25 years, and I think it is negligent to say that eating disorder is a contraindication for IF when there is NO EVIDENCE that this is the case, and in fact I think IF has amazing potential as a therapy.At least modify the disclaimer to state something like “people with a Hx of ED should approach IF with caution, as there is no research as yet to show any potential adverse impact of IF. Seek clinical supervision if in doubt.”
  27. POSTED JULY 9TH, 2018 AT 1:32 PMHerman HamotI’m reading this on iPhone 6C. I get nothing when clicking REPLY. How to resolve this?
  28. POSTED JULY 5TH, 2018 AT 1:22 PMTomJerimiah, the linked study in the article (https://www.sciencedirect.com/science/article/pii/S1550413118302535) specifically studied “eTRF”(Early Time-Restricted Feeding) from 8am – 2pm, and implies that eating earlier is better than later. I haven’t read the study (it’s behind a damn Elsevier pay-wall), so I don’t know how strongly they feel about early vs late, though. For me, personally, 12-8 is doable, and skipping dinner (given the existence of a family and the desire to have dinner with said family) isn’t doable, so I’m pleased to hear from you and April above that it’s working. Just starting!
  29. POSTED JULY 5TH, 2018 AT 6:24 AMWendyHi JerimiahAs a starting point you might like to start looking at the work of Professor Satchin Panda at the Salk Intitute.Personally, I have tried both and found the later eating window (12:00 – 18:00) easier to maintain with good benefits.https://panda.salk.edu/
  30. POSTED JULY 4TH, 2018 AT 10:25 PMJ Lance De Foa, MDYou wrote, “But sugar can ONLY ENTER our cells WITH INSULIN, a hormone made in the pancreas. INSULIN BRINGS SUGAR IN the fat cells and keeps it there.”Are you unaware of insulin-INdependent GLUT1 channels present on MOST cells which let glucose freely enter, driven by high concentration outside the cells to the lower concentration inside?Also, hypo/aninsulinemic patients who NEED injected INSULIN to survive don’t suffer from lack of glucose entering cells. The main role of insulin for them is to shut off the flood of glucose and ketones from the liver.Insulin doesn’t “BRING” glucose in, it merely opens the insulin DEPENDENT GLUT4 channels on muscle and fat cells to make it easier for glucose to flow from the higher serum to lower intracellular concentration. Just like opening all the doors on a stadium let the fans flow in.In so far as insulin promotes de novo lipogenesis and suppresses lipolysis in adipocytes it DOES help keep the fat inside. But in Hyperinsulinemia / Insulin Resistance with Impaired Glucose Tolerance lipolysis may not be sufficiently reduced and fatty acids and glycerin can be spilled at the same time that Triglycerides are being formed & stored. In the liver the glycerin gets converted to glucose producing hyperglycemia.The system is dysregulated. There is no lack of insulin. There is just too much work for it to do, and the system resists it also.
  31. POSTED JULY 4TH, 2018 AT 1:41 PMHåkon DahleI skip breakfast and have no issues with that. I also heard that eating in the evening could be an advantage, because the body is at rest and all energy can be used for digestion. That is similar to animals that rest and sleep after hunting and eating.
  32. POSTED JULY 4TH, 2018 AT 1:30 PMJimJeremiah. Insulin is higher towards the evening, add to that your more sedentary when winding down.
  33. POSTED JULY 4TH, 2018 AT 11:56 AMEmma GlaisherI’ve been doing Moseley style 5:2 IF for 5 or 6 years now. I was attracted to it primarly because of the suggested health benefits, aside from the weight loss. My weight has pretty much maintained, but my main hope is to avoid heart disease and cancer for as long as possible – 58 and still here!
    Are the health benefits still borne out by the research does anyone know?
  34. POSTED JULY 4TH, 2018 AT 6:52 AMAprilI started IT about 6 weeks ago. I eat between 12 noon and 8 pm. This works best for me and I have found easily sustainable. The results so far have blown my mind. I have an autoimmune disease and struggled with bloating, multiple food intolerance, gut pain, frequent urination, sugar cravings. All of these symptoms are gone. My hunger is controlled and I can enjoy lovely family dinners again. I think ideally eating earlier in the day would be better, but due to my schedule this works better for me and I am happy with the results.
  35. POSTED JULY 4TH, 2018 AT 3:17 AMSteveIF was popular years before 2012 by Martin Berkhan and his Leangains diet.
  36. POSTED JULY 3RD, 2018 AT 2:38 PMKeithJeremiah, I don’t think the author is suggesting that TRF in the later hours of the day is bad, but rather that it is DIFFICULT. The key finding in this study is that the 07:00-15:00 eaters had a reduced appetite (in other words, didn’t find it very hard to follow this regimen), whereas other approaches have been found to be kind of difficult for some.It definitely makes sense for you to stick with a system that is working well for you, although if you are finding it hard maybe it would be worth it to experiment with shifting your eating window to earlier in the day just to see how that goes.
  37. POSTED JULY 3RD, 2018 AT 12:32 PMLucyHi, I’ve heard about intermittent fasting and was thinking about trying it. I’ve also been looking at this red tea diet recently and was wondering what you thought about it.
  38. POSTED JULY 2ND, 2018 AT 12:43 PMJerimiahI would like to know what led you to the conclusion to recommend eating in the morning and fasting in the evening instead of the other way around. You do not link any studies here that show TRF in the morning is better than TRF in the evening. You do state “Nighttime eating is well associated with a higher risk of obesity, as well as diabetes.” but I would hazard a guess that alot people that snack into the evening have many other factors at play that could effect their risk of obesity and diabetes and are possibly not fasting at all. I have been doing TRF from 12-8pm every day for almost a year and have seen vast improvements in my health, not least of which is a loss of 70 lbs, so it seems odd to read items 3 and 4 on your 4 ways to use this information for better health. If you have evidence that supports the idea that TRF in the evening is bad then I would like to see it and perhaps change my dieting habbits.
  39. POSTED JULY 2ND, 2018 AT 11:40 AMShaneNice simple advice here, easy to understand and consistent with most general dietary advice. Thanks for this article.

10 thoughts on “Harvard Medical School on Intermittent Fasting

  1. Pingback: Harvard Medical School on Intermittent Fasting — Rain Coast Review | Timeless Mind

  2. Long Ago, I read a great book called, DIET SIMPLE, and I took some good ideas from there and didn’t diet. I lost weight. I don’t believe in diet foods. To me the best thing is moderation, smaller portions and staying away, as mush as possible from processed foods. (Or any foods that have more than six or seven ingredients. Look at your ingredients list, and it probably has thirty to forty ingredients –that’s a no ).

    Liked by 3 people

  3. I really enjoyed your article summing up what I have been looking into and thinking about. Been IF since the beginning of the year. Lost 5 kg have 20 kg to go. Funny thing is I used to naturally eat like this 30 years ago and didn’t have a weight problem. Every body used to chastise me for not eating properly. After the birth of my first daughter I thought I owed it to her to eat properly. That is when I steadily gained weight despite exercising. I put it down to aging. But I think I was doing the right thing pre-kids. Anyway I found your post most encouraging. Thank you.

    Liked by 1 person

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.