Intermittent fasting: Surprising update

Monique Tello, MD, MPH

Contributing Editor

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-based, Mediterranean-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.

Why might changing timing help?

But why does simply changing the timing of our meals to allow for fasting make a difference in our body? An in-depth review of the science of IF recently published in New England Journal of Medicine sheds some light. Fasting is evolutionarily embedded within our physiology, triggering several essential cellular functions. Flipping the switch from a fed to fasting state does more than help us burn calories and lose weight. The researchers combed through dozens of animal and human studies to explain how simple fasting improves metabolism, lowering blood sugar; lessens inflammation, which improves a range of health issues from arthritic pain to asthma; and even helps clear out toxins and damaged cells, which lowers risk for cancer and enhances brain function. The article is deep, but worth a read!

So, is intermittent fasting 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.

Sources

Effects of intermittent fasting on health, aging, and disease. de Cabo R, Mattonson MP. New England Journal of Medicine, December 2019.

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.

 

Comments:

  1. Jeff

    Every 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. Christine

    I just started intermittent fasting. Can I take diatomaceous earth while in my fasting state?

  3. ZK

    I 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. Anna

    Hey, 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. Enzo

    Why 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.html

    Anectodally 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. Maggie

    The timeframe doesn’t apply to people who work overnight/graveyard shifts.

  7. Michael

    Advice for us night workers wouldn’t go amiss.

  8. Martin Jenkins

    I 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. Isaac Morales

    60 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. SKGarriott

    I’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!

    • Monique Tello, MD, MPH
      Monique Tello, MD, MPH

      Wow, Impressive results, skgarriot! More and more evidence supports an overnight fast, though sixteen hours works very well for most folks.

  11. Michele

    Basically, we have to skip dinner. A very old trick.

  12. T.

    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. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Hi 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. Cameron 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. Matilde

    A dumb question possibly, but is drinking water, tea or coffee (no sugars or milk or..) during fasting or in between meals ok?

  16. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Thanks for this! It’s even more popular recently.

  17. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Thanks Shane!

  18. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Hi April, that’s wonderful for you, thanks for reading and sharing!

  19. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    This 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. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Thank 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. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Thanks 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. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Thanks, Zeeb, for sharing!

  23. Monique Tello, MD, MPH
    Monique Tello, MD, MPH

    Hi 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