The Research Behind Intermittent Fasting and Weight Loss

Jodi Helmer
by Jodi Helmer
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The Research Behind Intermittent Fasting and Weight Loss

When it comes to moving the needle on the scale, dieters are turning to intermittent fasting to cut calories and lose weight. Among the 1/3 of millennials who followed a specific eating pattern, intermittent fasting was the most popular, beating out options like the Paleoketogenic and Whole30 diets, according to the International Food Information Council Foundation 2018 Food and Health Survey.

Intermittent fasting incorporates brief periods of fasting with unrestricted eating. The diet can take several forms: Some followers fast for a certain number of hours each day; others eat regular meals for five days per week and restrict calories two days of the week or engage in a 24-hour fast one day per week.

“The proposed theory is that people may consume fewer calories if they are significantly restricting food intake on certain days or times of day, which might alter their metabolism in beneficial ways,” explains Whitney Linsenmeyer PhD, RD, instructor in the Department of Nutrition and Dietetics at Saint Louis University and spokesperson for the Academy of Nutrition and Dietetics.


Some studies have shown intermittent fasting can help with weight loss: A 2018 study found that following the 16:8 diet where dieters fasted for 16 hours and ate regular meals for 8 hours helped obese participants consume 350 fewer calories and lose 3% of their body weight over 12 weeks.

Recent research published in the American Journal of Clinical Nutrition found intermittent fasting was no more effective than traditional diets. The 150 overweight or obese women who followed a 5:2 diet (unrestricted eating for five days and two days with significant calorie restrictions) lost more weight during the 12-week study period than the control group. At the one-year follow-up, all dieters lost about the same amount of weight.

“Calorie balance and weight control seem to be most important for good metabolic health [but] the methods to achieve weight loss or maintenance make no difference,” says researcher Tilman Kühn of the German Cancer Research Center. “It’s the amount of net calories that matters.”


You’ll need to cut 500–1,000 calories per day — or burn at least that many more calories than you’re consuming — to lose 1–2 pounds per week, which is considered a safe rate of weight loss, according to Linsenmeyer. Both intermittent fasting and traditional weight-loss programs advocate eating less and moving more, but fasting is believed to alter metabolism, allowing you to burn more calories at rest. Older research found a 48-hour period of fasting helped increase metabolism by 3.6%; another small study found a 3-day fast led to a 14% increase in metabolism.

Linsenmeyer warns, “The truth is that an extended fast may cause your metabolism to actually slow down in an effort to conserve energy, which is counterproductive to weight loss.”

A lack of current research and no standardized recommendations for fasting — thanks to the multiple forms this eating pattern can take — makes intermittent fasting a controversial approach to weight loss. It also comes with risks such as fatigue or low blood sugar, notes Linsenmeyer.


Despite research showing intermittent fasting is no better than a traditional diet, the eating pattern might still be an effective option for weight loss, according to Kuhn.

“Even if [intermittent fasting] is not superior over daily moderate calorie reduction, it’s not worse,” Kuhn says. “For some, it may be a very good alternative.”

About the Author

Jodi Helmer
Jodi Helmer

Jodi Helmer writes about health and wellness for publications like WebMD, AARP, Shape, Woman’s Day, Arthritis Today and Costco Connection among others. She often comes up with the best story ideas while hiking with her rescue dogs. You can read Jodi’s work or follow her on Twitter @helmerjodi.


28 responses to “The Research Behind Intermittent Fasting and Weight Loss”

  1. Avatar Shari says:

    If you check out “The Obesity Code” by Dr. Jason Fung you will have all the scientific research showing the benefits of intermittent fasting as opposed to just calorie restriction. On a fast your body revs up. It doesn’t slow down because you need to be fast and alert in order to go and find food.

    • Avatar Golda Smith says:

      I agree with you Shari. Dr. Fung has presented a lot of evidence-based science that shows the effectiveness of intermittent fasting not only for weight loss but other health challenges too.

      I think some of the confusion regarding IF is the number of different ways some people do it and the notion of eating whatever they want when they are eating. Quality of calories always matter (in my opinion).

    • Avatar Carole says:

      After reading Dr. Fung’s Obesity Code, I began using this technique and am no longer Obese. It was sensible and I no longer have sugar cravings, or the desire to eat every 2 hours like I used to. Keeping blood sugar stable is the key as Dr. Fung states.

      • Avatar JGray1 says:

        I recall that many years ago when I was in my twenties my friend’s friend did this and was very successful with it and happy doing it. I am wondering about what unrestricted eating means however. are we talking about cookies and potato chip and ice cream eating along with food eating. that would have to put weight on me. for sure. what is unrestricted mean? I remember myfriend said this girl was eating junk food. but that was in another lifetime.

    • Avatar Safiya Oni says:

      Agreed, Dr. Jason Fung and his research provided the 1st solid research for me to share the amazing benefits of fasting for those that ate not ready for the conversatuin about the spiritual, energetuc benefits. It has been pivotal in my practice and the lives of my clients. A true Gem!

  2. Avatar Alex says:

    This article is incorrect on a key point. Intermittent fasting is not about calorie restriction. It works on insulin, the body’s fat-storing hormone. When we eat constantly, our bodies are constantly producing insulin. Fasting allows the body to stop producing insulin (several hours after eating), which in turn stops turning food to fat, and instead uses the fat stored in cells for fuel. Calories in/out is what slows metabolism as your body adapts to ever decreasing caloric input. Because the body gets fed often with IF, it does not have this long-term adaptation (as in the Biggest Loser study, wherein people ate less and moved more and have become worse for wear over time).

    • Avatar Lanny says:

      Alex hit the nail on the head. I would add that I want to keep insulin down while eating, so I’m combining a ketogenic diet with IF. I’m down 37 lbs in 4 months, bloodwork looks great and I’ve never felt better.

      • Avatar LisaH says:

        Congratulations, Lanny! I also have combined IF with a ketogenic lifestyle, after 4 and a half years on a ketogenic diet, alone. I lost 140 pounds, plateaued, IF was recommended on advice of a surgeon, and I’ve lost an additional 51 pounds since then. Bloodwork is stellar and I have had two total knee replacements in the last 9 months.

    • Avatar Bart says:

      Absolutely. Calories are important, but explain NOTHING about the real, METABOLIC EFFECT, which is a reduction in INSULIN, the fat storage hormone. To quote Dr. Fung, “Calories explain weight loss like “gravity” explains a plane crash. It’s just stupid.” Dr. Fung is also correct that weight loss is dependent on solving the two compartment problem! Our bodies are hybrids. You simply cannot burn any fat while insulin is circulating. Do keto and IF and you will be free from being “hangry” all the time—AKA, the carb roller-coaster.

  3. So, I’m confused is IF a good thing or bad?

    • Avatar Safiya Oni says:

      Good on all counts, if you read Dr. Jason Fungs book or even watch his youtubes you will learn a lot and feel more comfortable to take the next step towards balancing your health, increasing your energy and so much more.

    • Avatar LisaH says:

      IF is good if you do the research and understand what you’re undertaking, AND you understand the impact of the foods you choose when not fasting.

      I know a number of people who use IF as a tool for weight loss and for maintenance. None of them do unrestricted eating. Honestly, if you eat something that triggers an insulin response when eating, you’re going to struggle with hunger while fasting because of it.

    • Avatar Jack Faessler says:

      Yes! Fasting is widely believed to alter metabolism, thereby burning more calories during rest. Insulin resistance is the sensitive factor altered by fasting, which also has led to further study regarding its effects upon obesity and diabetes. Simply stated, American society eats “too much of the wrong food too often, and with too LITTLE water” (my layman’s quotes). I have lost 38 pounds since January 2018, as I continue my IF regimen — and as I maintain my target weight. These three YouTube channels continue to inspire me:
      Fledge Fitness (Edward V)
      Dr. Jason Fung
      Dr. Nick Zyrowski
      They may inspire you. Good luck!

  4. Avatar LisaH says:

    Two comments:

    1. Dr. Jason Fung provides evidence-based wide range research studies that are more than adequate; indeed, there is PLENTY of evidence that shows IF is an effective method not only of controlling weight, but more so, controlling insulin resistance, which is a huge factor for those with metabolic syndrome. As others have suggested, read: The Obesity Code.

    2. Quality of calories always matter. If we just look at calorie restriction, 1000 calories of cupcakes = 1000 calories of lettuce, and we know that’s not true at all; the glycemic response is greatly altered by one and barely moves the needle with the other. Those doing IF and using “unrestricted eating” may have limited success, but if the cause of obesity is insulin resistance, IF is MUCH more effective combined with a low carb, high fat/keto plan of eating, because it reinforces keeping insulin response in check.

    My own experience: I lost 140 pounds with low carb (Atkins-style) and needed both of my knees replaced, but was refused surgery until I could lose more weight. My surgeon recommended The Obesity Code to me. Despite having previously been on low carb for over 4 years at the time, adding IF allowed me to push through a plateau and continue losing. My A1C and fasting blood sugar have both improved, my cholesterol numbers are beyond perfect (164), I have lost 191 pounds to date, and have successfully had both knees replaced. I now view IF as a useful tool for moving into maintenance, as I have roughly 15 pounds to go.

    I suspect the slant in MFP covering this is that suggesting the importance of calorie counting (which I do not do at all) reinforces MFP’s existence and UA’s ad structure. /slightly cynical

  5. Avatar Tom Jones says:

    This misses the point. I echo everything said here in the comments. I did low carb for years… longer than most it seems… but eventually, I got sick of salads. IF is easier and more effective. I was 200 pounds and I am now 168, much more in my zone (I am 5’8″). I find the diet easy… and actually easier not eating every day. I go for 1-4 days and it gets easier after that first day. I just drink seltzer water. It is all about insulin. I can’t successfully calory restrict when I eat. Easier not to eat.

  6. Avatar MemeHermetic says:

    Barring the constant leaning on what Dr. Fung has said (good or bad it’s irrelevant to my point here) I feel the major benefit to IF that I have seen from everyone that has done it, myself included, is that it breaks horrible modern eating habits. Most specifically binge eating. It breaks the psychological addiction to food very quickly. It rapidly retrains you to stop when you’re full. Once you’ve done that, applying a good diet on top of that is extremely successful. If anything, I’d say that IF needs to be the precursor to any other weight loss diet for them to work long term.

  7. Avatar Craig Lafferty says:

    It seems that all of the people who respond are more educated about IF than the people in this blog article. IF does not slow down metabolism. A decrease in insulin causes an increase in Growth Hormone and adrenaline- this causes a rise in metabolism. This whole article is based on the CI-CO argument when in reality, IF is about hormones. I’m not one of these people who completely throw out the calorie argument and say “eat as much as you want” in your feeding window- it can play a role.

  8. Avatar Matthew Bell says:

    Long post, but hopefully useful:

    When I’m fasting, I put “Placeholder” calories in MyFitnessPal to keep it from griping about not eating enough.

    I think some of the confusion about IF and fasting in general is that some people/studies fail to recognize that fasting and eating 500 calories a day are completely different. Fasting is much better for you than eating 500 calories a day. Eating a small number of calories a day can kill you, or have a seriously damaging effect on your metabolism; actual fasting doesn’t. I have yet to see a study that makes that distinction and still shows fasting as bad. Before food was reliably plentiful, people fasted because they had no choice, and (surprise, surprise!) they survived. They still had to have the energy they needed to hunt down the next wildebeest, find the next fruiting tree/bush, or find and dig up that tuber. None of those things are “resting on the couch in front of the TV” or “laying in bed”.

    Dr. Fung makes logical, well-reasoned arguments for fasting. His “two-compartment” model of food makes complete sense, and blows away the “one-compartment” model that suggests that all that matters is calories in/calories out. I believed in the calories in/calories out idea [eat less/move more] (and lost 100 lbs doing so), but Dr. Fung’s arguments are persuasive. He also makes a good point that when you are eating, what and how much you eat still matters.

    I did a 10-day water-only fast in January, and another 10-day fast water-only (plus non-caloric electrolytes) in February. After a day or so, I didn’t feel hunger. I think what I did feel was an occasional “habit of eating” mental desire, but that went away, too. I never felt weak, tired, confused, or incapable of doing any normal activity either time. On day 7 of my January fast, I rode my bicycle 26 miles. Although I never felt like sprinting, my pace wasn’t slower than normal and I felt fine during and after the ride.

    My favorite paragraph of Dr. Fung’s is:

    Can’t eat meat? You can fast!
    Can’t eat wheat? You can fast!
    Don’t have any money? You can fast!
    Don’t have time? You can fast!
    Food allergies? You can fast!

    If you are overweight but otherwise healthy and not on any medication, you can fast. You always lose weight. It works every time.

    I lost 20 pounds (average 2 lbs/day) for both fasts. When I started eating again, I gained about 10 pounds back each time (so my net average was 1 lb/day). I attribute the weight that came back as a combination of refilling my gastrointestinal system, plus re-gaining the water weight I lost when I burned through my glycogen stores the first day or two of each fast.

    Dr. Fung’s comparison between fasting and starving is also classic. Fasting is like running as a recreational activity, such as a 5K run. Starving is like running because the lion is chasing you. They are NOT the same. If you can decide to stop fasting and eat at any time, you are not starving.

  9. Avatar Breezy111 says:

    This article is ridiculous. Please read up on Dr. Jason Fung’s findings if you’re interested in intermittent fasting. This article is not accurate.

  10. Avatar Marcia Gloekler says:

    You lost me at “calorie restriction”…
    Fasting is about so many things… but calorie restriction isn’t one of them. If you truly want to learn about IF and why fasting works read The Obesity Code or a book like Delay, Don’t Deny if you prefer a more conversational tone. Google autophagy. Do your own research. All I ever see here is “calories in, calories out” “move more/eat less.” If that was the cure was that easy we wouldn’t have an epidemic.

  11. Avatar sinnerman says:

    Dr. Linsenmeyer is using old data for her warning. Long term caloric restriction or dieting, can and often does slow the metabolism. Fasting doesn’t.

  12. Avatar Michael Gerity says:

    So, if you take a look at the American Journal of Clinical Nutrition study, which is the only one this article relies upon for it’s conclusion, the “fasting” tested was, oddly, 5 d without energy restriction and 2 d with 75% energy deficit. I’m not sure why they chose that approach to IF. I’ve read a lot about the topic, and every IF approach I’ve ever seen required keeping caloric intake down to as close to zero as possible. (In fact, the very definition of the word fast is “to abstain from food.”) Most of the benefits alleged by the IF supporters stem from the things your body does in response to zero calories. As such, the decision by the researchers in this study to use a “fast” that was only a reduction to 25% normal intake was, to be blunt, bizarre–somewhat akin to testing whether or not daily vitamins work, but only giving the subjects 1/4 of the pill.

    While there are admittedly a lot of variations to IF, basically NONE of them calls for partial caloric restriction.
    I would, therefore, love to see a similar study that actually applies fasting in one (or more) of the ways that IF proponents actually propose, which all involve zero or nearly zero caloric intake during the fasting periods.

    • Avatar John Morgan says:

      Actually, the very popular “Fast Diet” calls for the very caloric restrictions of 5/2 fasting. I have used this program to get past a very long plateau and lose additional weight. As mentioned in some comments, it has helped me break some poor eating habits and is much easier than the constant calorie counting of many weight reduction programs.

      BTW, I am not surprised that conventional nutritionists still stick with the calorie bank concept; it’s an easy and false concept. By this thinking, if you only eat 1,500 calories of donuts a day, an active adult male should loose weight as quickly as one eating healthy choices. (I exaggerate to make my point). Look at what most dieticians come up for meals in hospitals.

  13. Avatar Maria Tcherni-Buzzeo says:

    Jodi, huge thanks for this excellent article! I see that a lot of people who commented are firm believers in Dr. Fang’s approach and criticize you for not being supportive enough of this cult-like following. But the reality is: Dr. Fang’s theory, as attractive as it is, is a theory. If it works for some people, wonderful! But anecdotes and individual cases cannot replace careful scientific studies to test the theory (for example, what if out of 100 people who tried it, IF worked for 10 and this is who we are hearing from?). So it is very valuable that Jodi’s article summarizes the relevant research findings. After all, for any theory, we would want to know if it is supported by research or not.

    • Avatar Thales Nemo says:

      Comment is clearly unlettered, why your misuse of the word theory when you meant hypothesis!

      Dr JASON FUNG has a plethora of data and IF is neither a fad nor a religious following !

      The RELIGIOUS DIET is a vegan diet with ABSOLUTELY no science behind it and founded upon RELIGION!

  14. Avatar robertroepke says:

    One very important benefit of IF I did not see mentioned is autophagy. This may be more important than weight loss.

  15. Avatar allanholtz says:

    I am an experiment of one. With that in mind, for the past 5 years (I am 69 and retired) I eat out 3-4 times a week. When I do, I eat one meal that day. It is a combination of vegetables, fruit, lean meats and 1-2 days a week desserts (yes plural). I start that meal at (depending on location) at anywhere from 10:30 AM to 3:00 PM. It then takes me about 1.5 hours to finish. I slowly eat a lot. On the other days I eat 2 meals a day at home. The first one is a blend of oatmeal, Udos oil, berries, grapes, eggs, no fat cottage cheese, nuts and soy or rice or almond milk. I start that meal from 10:00 AM – noon. Then at about 7:00 PM I have a large meal of lots of vegetables of a huge variety, some lean meat and fruit. Some is processed with sugar, i.e. homemade butternut squash pie filling. I likely will have a piece of homemade zucchini bread (flour and more sugar). When I have one meal a day and am done eating by noon that day, my resting heart rate out of bed the next morning (measured at 5:20 AM) is typically 41-43 bpm. I am an endurance athlete. I have run more than 1500 miles per year for the last 26 years. Some years I ran over 3000 miles per year. Hence my low resting heart rate. But when I eat at home and finish eating about 8:00 PM, my resting heart rate the next morning is usually 45-48 bpm. This (a lower resting heart rate) means my metabolism is lower the longer I have gone since eating. I currently participate in a floor exercise class at a local fitness center from 6:00-7:00 AM Monday through Friday and then go for a 5 mile treadmill walk or run immediately after. At my age now it is more walking than running. On Saturdays I go for a 3-4 hour treadmill effort and a 1-2 hour treadmill effort on Sunday. My height is 5’8″ and my weight is a stable 158-165 pounds depending on salt consumption which is a function of high salt eating out or low salt eating at home. My morning out-of-bed weight is higher after eating out and lower after eating at home. My electrical conductivity % body fat measurement shows lower body fat and higher water level the morning after eating out and higher body fat and lower water level after eating at home. In contrast my side caliper body fat measurement shows lower body fat (less skin fold) the morning after eating at home and higher body fat the morning after eating out. This is just the opposite of the electrical impedance method of measuring body fat. My blood pressure is 110 plus or minus 10 over 65 plus or minus 5. The higher my weight is, the higher the value in that range. I enjoy eating a lot of a lot of foods. Exercise is good! Your results may vary.

  16. Avatar Magz2004 says:

    Such a poorly researched article that totally misses the point. I suggest Jodi buys a copy of the fast 800 and reads the research references in the back!

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