Intermittent Fasting… Does It Work?

intermittent fasting

The big three areas of healthy living are diet, exercise, and sleep.  One, currently popular dietary technique that integrates all 3 of these areas is intermittent fasting.  So what is intermittent fasting and what are the benefits?  This is a program where you restrict your calories based on time.  Fasting also does not mean total lack of food, it can just mean a reduction of food/calories taken in as in the amount or type of food (eg. only high fiber, whole foods before lunch).  Also water and coffee are typically not counted in most fasting programs.

Ways Of Intermittent Fasting

There are almost unlimited ways to do this, but here are the most common ways:

  1. Alternate day fast. Total fast from food 2-3 days a wk or every other day.
  2. 5:2. Eat normally 5 days a week, but cut down calories to 500 (for women) or 600 (for men) 2 days a week. Typically these are set days such as Tuesday and Thursday, but it can be tailored to each individual need.
  3. Time restricted eating or feeding. The best example of this is 16:8 which is where you eat, or feed, for only 8 hour a day and fast for the other 16 hours.

There is also data that shows that even fasting 5 days a month leads to health benefits.  That would be about 1 day week!  Also many people will build up to 16 hours fasting not just start there.  An example would be to start off fasting for 12 hours and feeding 12 hours a day for several weeks and then decreasing the feeding time by 2 hours.  For the 5:2 plan you could cut down to 1500 calories on your fasting days for several weeks then to 1000 calories, and then finally to the target goal of 500-600 depending on gender.

What Ever Doesn’t Kill You…

So we know what intermittent fasting is, but why would we want to do it?  Low level stressors activate our cellular survival systems priming them for when we need them the most. There are 2 examples that help to explain this concept.  One is the old saying that ‘whatever doesn’t kill you will make you stronger’.  That’s basically true.  If you can survive a stressor, you’ll know what to expect and be better prepared mentally and physically for it next time.  The other great example is exercise.  Exercise is a low level stressor that allows us to tolerate and withstand a similar stressor the next time, getting stronger as a result.

David Sinclair, PhD who is a longevity researcher and expert at Harvard, and author of the book Lifespan states that there are 4 major things that impact longevity: 1) calorie restriction is the most robust, 2) then food quality, 3) then exercise, and finally 4) cold exposure.  As you can see 3 of the 4 factors are low level stressors.  Also he doesn’t do intermittent fasting, he just typically skips lunch as a method of calorie restriction.

Intermittent fasting induces a host of cellular changes.  It increases cell survival, it enhances metabolic pathways (better glucose and cholesterol maintenance), increase mitochondrial function and energy production, and increases the production of brain derived neurotrophic factor (BDNF) which improves brain function.  The other stimuli that induces the production of BDNF is exercise.  For a more thorough review of that research consider reading the book Spark by Dr. John Ratey.

The concept of intermittent and low level stress is important.  If you do not allow a time of recovery then there will be limited or no benefit.  Think about fasting all the time.  Do you know what that’s called?  It’s called starvation.  Think about cold exposure.  Intermittent cold exposure activates brown adipose tissue which burns calories to keep you warmer.  Too much cold exposure leads to frost bite and tissue damage. Now to exercise.  When you exercise you break down muscle and stress joints and ligaments.  Almost all the gains come during recovery when your bodies DNA are in recovery mode and can transcribe proteins that lead to not only muscle recovery, but to muscle growth.

Finally, lets explore chronic emotional stress.  We have all seen, or even experienced the destruction that goes along with chronic stress from a terrible job, family illness, money worries, etc.  Those constant (not intermittent) stressor lead to the breakdown of the human mind, body, and spirit.  While low level stress typically aids in focus and concentration.

A Scientific Study Confirms The Benefits

The most recent and most interesting study of intermittent fasting was recently published in January 2020 by Wilkinson et al in Cell Metabolism.  They studied a group of people who had metabolic syndrome and who were already on blood pressure and cholesterol medications and had them eat for only 10 hours a day (or fast for 14 hours a day).  The subjects could pick whatever times of the day to eat during as long as it was continuous.  In 3 months the subjects lost weight, lost fat, decreased their waist circumference, lowered their blood pressures, lowered their bad cholesterol levels (LDL), lowered their hemoglobin A1c levels (a measure of diabetic control), and slightly increased their sleep length.  What was most impressive about this study is that they did not alter the quality of their diets-they were asked to keep eating the same things they typically ate.  They also were not started on any type of new exercise program. The only difference is when they ate!

Intermittent Fasting Is A Method Of Calorie Restriction… So Is Sleep.

Remember that intermittent fasting is a method of calorie restriction.  The people in this study ate about 10% less food than they did before the study.  If you stuff the same amount of food in during a shorter time interval its very unlikely that you will get any of the benefits mentioned above!

What is the easiest way to fast?  Sleep!  Think about it.  Even when you sleep you’re are burning calories-our resting metabolic rate.  The more you sleep, the longer you are fasting, and the more calories you are burning! Just based on that concept alone we should all want to sleep more, but do we?  About 1/3 of Americans do not get enough sleep.  A good sleep duration is 6-9 hours. Less, and more, are associated with increased cardiovascular events and death.

Some feel that the longer they stay up, the more calories they will burn, but what studies have found is that the longer you stay up the more calories you take in!  When you stay up you tend to snack, which leads to more calorie intake (not restriction).  Also you eat more the next day.  Not just because your will power is used up from the night before, but because poor sleep activates your reward centers and tired people often seek out more fatty, comfort type foods.  This is a triple whammy: poor sleep, eating more calories, and eating worse quality foods.  You’ll probably be too tired to exercise as well.

Shoot for 6 to 9 hours of sleep total, including naps.  Sleep has a U shaped response curve.  Too little and too much sleep are not good for you. Find your sleep sweet spot.

Now finally to exercise.  How does intermittent fasting relate to exercise?  The first thing is that you can’t outrun a bad diet.  It will take about 30-45 minutes on a treadmill to work off a can of Coke.  We just don’t have that type of time.

The Metabolic Factor

In a recent study they measured the metabolic rates of contestants from the TV show ‘The Biggest Loser’ they found that as they lost weight their metabolic rate dropped tremendously.  Of the contestants they followed every one of them, except the one that underwent gastric bypass surgery regained all their weight.  One person had cut back his diet to 800 calories a day and was still gaining weight!  It’s hard to imagine a more frustrating and depressing situation than that.

Why did that happen.  At the ranch their job was to lose weight. They had a professional chef cook them a healthy 1800 calorie a day diet. They had personal trainers help them exercise for hours a day.  To me that sounds like the ultimate vacation!  As they lost weight and mass they didn’t need to use as much energy to move around and run their bodies (their resting metabolic rate dropped).  Also they had lost mostly fat, but didn’t build excess muscle which is metabolically more active than fat.  Finally, they moved back home from the ranch and back to real life with jobs, stress, prepping their own meals, and exercising on their own.

When you lose weight your body also tries to maintain it’s weight as much as possible-the process is called homeostasis.  As you reduce your calories your body will invoke every mechanism it can to save those calories for a rainy day.  The same thing happens with exercise.

What do you call running on a treadmill in the fat burning zone for 30 min everyday? ‘The road to nowhere!’  We all say that we want to lose weight, but to be more specific we want to lose fat, and less frequently acknowledged we want to build muscle.  Our glycogen stores are typically sufficient to get us to 90-120 mins of aerobic exercise. That means that we will have very little fat burning until that time.  Exercise is great, but to really make a dent into our fat stores simple cardio is not the best way.

Another problem is that energy/calories burned does NOT happen in a linear relationship with exercise time.  An extremely well done study showed that calorie burn would plateau the longer you exercised. Similar to what the body does when it faces dieting.  It tries to save every calorie it can.  This has been called the ‘constrained’ energy theory.  So what happens in exercise is that you actually do come face to face with the law of diminishing returns.

So is there a way that we can more easily tap into fat without running for 2 hours a day?  Yes there is and no it’s not the keto diet.

The Problem With Keto

I know many have had success with the keto diet to lose weight, but there are some significant issues that you should be aware of, and honest about, as there is an easier way to become ketotic.

The keto diet as you know tries to get you into a state of ketosis so that you burn more fat and lose more weight. To do that you have to eat a diet high in fat and very low in carbs and low in proteins as well.  The main reason people lose weight on the keto diet early on is that for every molecule of carbohydrate absorbed by your GI tract it take 4 molecules of water to transport it across our gut wall.  There is a big water weight loss at the start.  Even more importantly is that with the SAD diet (standard American diet) 50-60% is highly processed carbs and getting rid of them will lead to weight loss regardless of if you are ketotic or not.

That brings us the main problem with the keto diet.  Most people on the diet never bother to check if they are ketotic.  That’s a good thing as most would find that they are not.  If you are not achieving the stated goal of your plan then why would you continue to do it?  If you really believe in the ketosis hypothesis then there is another method that more reliably induces a state of ketosis.  That method is intermittent fasting.

So why is keto so popular?  Like all diet fads, its new, trendy, and cool.  To make money each new plan has to be different to distinguish themselves from the competition.  It’s hard to sell your program if you focus on what’s similar to other plans.  “Eat less carbs” is different from Atkin’s or paleo where you eat more meat, which would be too similar to keto.  Focusing on less carbs and becoming ketoic sells better.

So does intermittent fasting deliver on enhanced fat burning?  It does.  With or without exercise it leads to more fat loss.  Also when you exercise in a fasted state you pull more from fat, both in cardio or resistance exercise.  Now if you are used to having fuel before you exercise this will take some time to adapt especially since carbs are the best fuel for all types of exercise.  All of the world’s best endurance athletes use carbs to fuel themselves. Take Eliud Kipchoge the world marathon record holder and the first and only man to run 26.2 miles in under 2 hours.  On his runs he drinks a high carb drink that is more concentrated than the typical sports drink.  Also if you want to build muscle, which is more metabolically active than fat, then you’ll want to eat protein, which is also limited in the keto diet.

Concept Of EPOC

To be complete we need to mention the concept of EPOC. Excess post-exercise oxygen consumption. Basically this is the ‘Holy Grail’ of exercise-the ‘after burn’.  Once you stop exercising your metabolic rate starts going down back to your resting levels.  Can you keep it up?  There are two ways that we will mention, but not go into detail here.

The first is HIIT-high intensity interval training. 4-5 rounds of HIIT (about 2.5 minutes of hard work in a 20-25 minute exercise session) burns more calories, fat, and increased your metabolic rate more than 45 minutes of steady state running.  The other method is resistance training. Weight, rubber bands, or body weight exercises increase your metabolic rate for about 24-48 hours after you stop.

What About Hormones?

Finally hunger hormones.  Not only does your body slow down your metabolism and try and save fat as you diet and exercise, it also produces hormones that make your brain feel hungry.  Even after 1 year of stable weight loss people still had elevated levels of hormones like ghrelin which stimulate your brain to make you want to eat!  Fighting against being hormonally and biochemically hungry can take up all of your willpower.

In Conclusion

Interestingly enough intermittent fasting does 2 things to help in this regard.  One it shrinks your stomach so it’s easier to get that full feeling which signals your brain to stop eating.  Also over time fasting increasing leptin production which is an anti-hunger hormone which tells our brain that it does not need to eat!

In summary, intermittent fasting is simple, easy to do, adaptable to your lifestyle.  It helps you lose weight, but more importantly helps you lose fat while helping lower your blood pressure, blood glucose levels, and cholesterol levels.  It primes your epigenetic survival systems to help you live longer.  Also it’s only program that can be used with ANY diet program out there and where sleeping counts as a bonus!  Experiment with different fasting programs and see what works for you.

Harvey Hahn, MD, FACC

Dr. Hahn graduated from Loma Linda University in 1994. He is currently the director of the Cardiovascular Fellowship Training Program at the Kettering Medical Center in Kettering Ohio.

  1. Enjoyed reading this article when I read that my body mass was that of 109 year old woman I have to do something I am 76 years old so I know I have not one foot on a banana peel but both so I would like to save a few months of life. I have being a week of IF and my appetite has improve all just needed some encouragement to continue this fasting perhaps perhaps for a few more months of life. Thanks again Josie LeNoir

  2. I need guidance/coaching for intermittent fasting along the way. Instead of doing this on my own and making mistakes along the way is there a reputable group/organization that I can sign up for that will answer my questions as they come up? For example, how should I get started? Also, I exercise quite regularly and my calorie needs change dramatically each day. How do I know if I’m eating more than I should which defeats the benefits of fasting? I have used a calorie counting app in the past which I used to keep track of calories in and calories out. Should I use the app along with a fasting regiment and set my calorie intake to a fixed number that is adjusted based on exercise calories out and food calories in? For example, if I set my daily calorie intake to 1500 calories a day and eat 1800 calories and burn 300 my net allowed caloric intake remaining for that day is 0.

  3. This was an amazing article. Packed full of relevant, researched data. My big takeaway, which was really a confirmation was the sleeping part. The later I’m up, the more I travel to the kitchen for unhealthy food, the worse I sleep and in the morning I want to ravish processed carbs. I noticed that and tested it by doing a 10 day green smoothie challenge. I don’t eat (tea and water) before 11:00 AM and don’t eat any food or smoothie after 8:00 PM. That gives me two hours before bedtime at 10:00. When I do this, I feel amazing and I’m losing weight. Once I get off the 10 days, which “breaks the back” of my sugar cravings, I’ll continue the water and tea before 11:00 and do lower carb whole foods. What has helped me out so much here is to understand the real value of IF and to continue to prioritize it. Thanks much.

  4. I was recently diagnosed with pancreatitis. My physician has prescribed
    creon to assist with the breakdown of food. I go today for CAT Scan for my stomach and pelvis as on 8/12/21 I had an endoscopy and colonoscopy. The following day I had pain on my right side and my stomach had a bulge on the right side. Before the tests my stomach was flat, that is reason for the CT Scan. I was wondering if I decide to fast for 2 days a weak, will this effect the enzymes in any way. I want to do everything I can to heal my pancreas.

    I thank you in advance for your time and help.

    Geri Biuckley

  5. Very interested and informative. A sensible way to eat and to keep your body healthy without depriving yourself.

  6. Great article. It wasn’t until the last year or so I’ve heard the phrase of intermittent fasting. I have always felt to myself that fasting once in awhile would be a good thing to do, not that I did it. I have recently tried to find out more about doing it the right way. The internet is great and terrible in that respect. There is sooo much info out there it is extremely difficult to figure out what’s good and what’s not.
    I’ve recently turned 66. I spent many years running (with no proper training) 6k’s, 12k’s even one marathon. I spent about 20 years doing martial arts and 40+ years in construction. Through all that I figured I learned about every which way a body is not suppose to go and most of those lessons probably came the hard way. I got into zen trying and meditation and then zen body therapy figuring it would be good to learn a way to put the body back together again.
    Now we’re retired on a farm in Idaho. Lots of work to keep me busy here, but I still need to exercise. I used TRX straps hanging from the ceiling using my own body weight instead of weights. I am also able to jump rope, (a layover from my boxing days). Between arthritis and pseudo gout my running days have been long over as much as I would like to. Because of this I feel my actual activity level has dropped considerably.
    Since so much of what’s on the internet is BS is there a right and wrong way or a better way to do intermittent fasting that you can recommend?

  7. Thank you for this article,I have been at the right weight for 22 years ( I was 210 now 115) I eat no sugar or flour nothing processed.But My LDL levels have gone up consistently for the last 10 years ,it is 230 now,my HDL is 75.My doctor and I have decided to try this 16:8 eating plan and retest me in 2 weeks .I have my fingers crossed .My question is have you seen this help in other people and can you make any suggestions? Today day is my first day I exercised at 8 am and had breakfast at 9:30

  8. Great article for those of us that are not deep into the scientific community. I have been working for about 20 to 25 YEARS to get my sleep patterns, exercise efforts, and nutritional intake where they should be. I find this information wonderful to help me move forward in my efforts. I still need to lose 12 to 15 pounds. I have hit a plateau which I am familiar with. I also still have much effort left in the ‘good’ sleep area. I have a good handle on my exercise efforts but it is constant effort. Thank you for your help.

  9. Great post! As I have always been very skeptical about intermittent fasting so thank you for sharing this well balanced argument! 🙂

  10. Thank you; Your article answered the questions I had about intermittent fasting and answered the questions I had not thought about. Olympian yo-yo dieter with weight related diseases. A OP procedure 4 days ago jump started my interest in intermittent fasting. Now that I have lost weight and not regained my full appetite I want to consciously and mindfully practice your 3rd option Intermittent Fasting. Thank you again.

  11. Great article! The one thing I would like to clarify, is exercise most effective during the hours of fasting? That is how I understood it. That is my challenge. My time of exercise is most likely to occur during the hours I eat.

  12. Thank you for the information. I just started IF, and your article helped. So far, I haven’t had any problems with IF.

  13. I have been doing Intermittent fasting since 2017, And I could say it really did changed my lifestyle.

  14. Dr. Hahn,
    Thank you for this information. It is very informative. I have been doing the IF for 6 weeks and I am so impressed with the results. I am doing it to get healthier and lose some weight at the same time. So far I have reduced my weight by 18 pounds. I am so elated by the fact that it is so much easier and convenient to maintain for my busy lifestyle. The fact that I rarely feel hunger is such a plus!
    Since experiencing the COVID 19 Pandemic, and learned the impact of lives lost due to Pre-existing conditions, I have rallied some of my face Book friends to join me in a 30 days challenge. I am educating them on health and the benefits of Intermittent fasting. We are all doing well. This article will be shared with my group and will give you and the researchers props!.
    Thank you so much Dr Hahn! Your work is so appreciated!! Rosemarie A.

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