Intermittent Fasting: Not So Fast

I’m sure that at least a few of you have heard or read about the latest trend in weight loss called “intermittent fasting.” The very word “fasting” is probably less than appealing, as it pretty much means you don’t eat or drink anything (except perhaps water) for a specified amount of time. Starvation is not exactly recommended among health professionals. But intermittent fasting is different. Is it something you should try?

What is intermittent fasting, anyway?
Intermittent fasting has been the talk of the town, so to speak, thanks to two recent books to hit the market: The Fast Diet by Dr. Michael Mosley and Mimi Spencer, and The Overnight Diet by Caroline Apovian, MD. Intermittent fasting essentially means that you skip a meal or severely restrict calories on certain days of the week with the intention of losing weight, controlling blood glucose, and/or decreasing heart disease risk. But on the other days of the week, you can pretty much eat what you want (within reason, of course). For many people, this concept sounds appealing. Limiting calories for a couple days a week doesn’t sound that bad if you can eat what you want the rest of the time.


The Fast Diet, also called the The 5:2 Diet has you eat between 500 and 600 calories (women get 500 calories, men get 600 calories) for two days out of the week, spread over two meals of about 250 to 300 calories. These fast days should not be right in a row, and your food choices ideally should be more plant-based and emphasize protein. The premise is that after several hours of fasting, the body burns up its carbohydrate stores and shifts to burning fat for fuel. Many claim that intermittent fasting also helps to blunt appetite.

The Overnight Diet emphasizes getting enough sleep; a lack of sleep can disrupt metabolism, making it hard to lose weight. Sufficient sleep, according to the author, will reduce hunger pangs. The diet part of this involves drinking homemade smoothies once a week, and eating a low-calorie, high-protein diet the remaining six days of the week.

Does intermittent fasting work?
In many ways, these two diets sound like just another fad to come around the bend. And maybe they are. However, there actually is some credible science behind fasting. Restricting calories in the diets of animals appears to increase their lifespan, for example. Recently, a team of researchers in the UK looked at the various approaches to intermittent fasting, with a focus on how they might help (or hinder) those with Type 2 diabetes and obesity. They found that intermittent fasting is as effective as or even more effective than simply cutting calories to lose weight.

They also found that intermittent fasting has other health benefits, including reducing inflammation, lowering blood pressure, lowering heart rate, lowering cholesterol, and reducing insulin resistance. This unique approach may even help prevent the development of Type 2 diabetes. Followers of the diet also believe that intermittent fasting can prolong your life and prevent Alzheimer disease.

Is intermittent fasting for you?
Sound appealing? Before you jump on the intermittent fasting bandwagon, realize that not a whole lot of research has been done in this area, at least with humans. The few studies thus far look promising: In one study, fasting was shown to lower blood pressure and cholesterol, and on “regular eating” days, the subjects did not overindulge, which has been a concern. But again, we still don’t know a whole lot about intermittent fasting.

Wondering what you might eat? A sample 500-calorie menu from The Fasting Diet is steel-cut oatmeal with ½ cup blueberries for breakfast, and then chicken stir-fry made with 5 ounces of chicken and some vegetables, along with a tangerine for dinner. That’s it. Would that hold you?

Could intermittent fasting be harmful? Well, that depends. If you take insulin or sulfonylurea drugs to control your blood glucose, intermittent fasting can considerably raise the risk of low blood glucose unless you make appropriate adjustments. This way of eating is not suitable for pregnant women, people under the age of 20, people who are underweight, and people who have an eating disorder. It may also not be good for people who take certain types of medicines, such as beta-blockers.

Another question to ask yourself: Can you stick with this way of eating? It may sound simple to just eat 500 calories for two days a week, but 500 calories isn’t all that much. On those days, you’re likely to feel tired, grumpy, anxious, and irritable. You also may not sleep well and your breath might not exactly smell like a bed of roses.

But, if you’d like to give intermittent fasting a try, talk it over first with your health-care provider and decide together if it’s something that could work for you. And if you have tried this, feel free to share your experience!

Want to learn more about diabetes and fasting? Read “Can Fasting Help Diabetes?” and “Fasting During Ramadan.”

  • Steve Parker, M.D.

    I’d be a little worried about hypoglycemia if I took a thiazolidinedione (pioglitazone, rosiglitazone) or bromocriptine. But certainly the risk is much greater with insulin and sulfonyureas, as you say.


  • jim snell

    Amy: none of this makes much sense to me. I need to keep my liver out of the mess and the way I do that is timely and regular eating of sufficient calories,

    This article makes no sense for me. Best wishes and special thanks for all the excellent articles and eating issues.

  • Sue Dean

    I was wondering if anyone knows if eating more than the 500-600 calories would still work. I think I would be violent if I only ate that little, but because I’m in the process of losing weight gradually, eating 1,000 calories on those “fasting” days would still be less without being so low that I feel cranky. Any thoughts?

    Thanks for the info/ideas.

  • JohnC

    I remember doing a three day ‘fast’ over 50 years ago… partially because I ran out of money and food. The first day was not very nice, but I noticed by the second day my body (and mind) decided we must be starting a famine period. As long as I drank water it wasn’t really a problem. I did appreciate food when I resumed my normal eating pattern though.

    It hasn’t been that long that we have had access to food all of the time. Perhaps our bodies are programmed to do without. Certainly in some places that still is the norm. And perhaps some of us end up as type 2 diabetics because we have ‘thrifty genes’… which of course would ensure over survival in the times of very little. The lean didn’t make it while the plump type 2s had reserves.

    There have been studies to show that when the body isn’t always busy processing food, it turns to healing modes it doesn’t always get a chance to do.

    Of course if you are on any diabetic meds (or type 1) you might want to think twice before you try it out.

    • antiguajohn

      There is an old saying,”hunger sharpens the mind”.

      It’s an evolutionary development, no food to make glucose and the body switches to breaking down fat to make ketones and the brain actually functions better on ketones, after all remember that full meal feeling and you want to sleep, so when any animal is hungry and running on ketones they are smarter and faster allowing then to catch prey.

      An added advantage is that it causes your immune system to switch to repairing the DNA of cells instead replacing them,this has the advantage of fewer DNA transcription errors which can lead to cancer and other diseases.


  • Ferne

    I quit reading books. No one agrees and they all have different solutions. I would never do intermittent fasting when the program I follow now works.

  • Pat

    I have type 2 on metaforming 2xday 500ea for 4 hrs My a1c was 7.1 for few years and went to 7.3 My morning blood sugars were creeping up some times up to 18 after a meal I keep low carbs limited no pasta potatoes take only stevia .my last visit to the dr he increased my metaformin to 1500 mg 5003x per day
    Instead I read dr mercola intermittent fasting and give it a try I try to stop ,around 6-7 pm and eat my regular breakfast at 11-12 A month after my sugars in the morning6.8 to 7.6 and after every meal went from 14-18 to 9.9 -7.6
    I don’t agree with the food piramyd I prob be on insulin if I ate like the diabetic clinic recomended (I live in Canada) instead I followed a naturopath more healthy diet
    So far this is working out and definitely lower my sugars I just got my blood test and dr didn’t call me
    So I will find out and post later if my a1c went down
    I am less hungry and crave sweets a lot less

  • Adam

    I have had Type 1 diabetes for 28 years, and I wear a continuous glucose monitor (CGM) and insulin pump.

    I’ve noticed that when I eat a higher fat diet, or when I don’t exercise, my body does not react to insulin well, similar to what happens in Type 2 diabetics.

    I tried intermittent fasting and what I noticed is that in the days after fasting, my metabolism is supercharged. My body’s uptake of insulin is significantly more efficient, even when eating fatty foods. Foods that should have sent my blood sugars lingering for hours into the 200+ range barely budge me. (Remember, I have the CGM which plots out my blood sugars every five minutes, 24 hours a day. So I have hard data to back up my observations.)

    If you’re type 1, you have to be very careful about hypoglycemia. But I’m now able to “ride” my insulin pump basal levels to counter the effect of fasting so that I do not dip into hypoglycemia.

    This has been a real eye-opening experiment for me.

    • Rudhi Wijanarko

      wow that’s rellay impressive.. thanks for sharing !!
      I have diabetes type 2 and have 30 pounds to loose, and is looking for intermittent fasting to achieve it, hoping to prevent my diabetes getting worse.
      Now learning from your experience (type 1 — wow), that’s really boost my motivation.
      Thanks Adam !

    • Diana

      How about exercise? I am type 1, and I’ve been doing research on intermittent fasting because my bf is doing it. I am scared he’ll get sick just because I am so used to doing a balanced diet to maintain my diabetes and glycemic levels as directed by my doctor. Because of this, I am very hesitant about my bf doing this diet. so… I want to start the diet a long with him to see if it’s safe, but I do like to work out. What kind of exercise do you do with this diet? and what snacks to you take, being type1?

  • truthbetold

    Sometimes not eating or fasting can cause higher blood sugar levels.

    • Jeffrey Yaakov Richman

      no it does not…explain the physiology of that,,you can’t

      • Dimi

        Type 2 diabetic here (recent) and I agree with what this person said. My blood sugar is high. It might have to do with exercise (cortisol raises blood sugar), or with coffee or maybe not all diabetics are equal. My diet is fantastic (strictly low carb) and I am slim with a six pack. Go figure. I think that the people who don’t have a positive experience from intermittent fasting should be more vocal because all you hear are the success stories.

        • Ben Wagenmaker

          Truthbetold, I agree partially. My blood glucose is often highest after 12-16 hours of fasting. However, I attribute this to gastroparesis. Never diagnosed officially, but I suspect it, and am planning to discuss with my doctor. Dr. Robert Berstein notes that high fasting blood sugars are the most common sign of gastroparesis. With this condition, food might remain in your stomach for several hours before being released into the small intestine, at which point your blood glucose will spike, although you may have been fasting for several hours. I find that I have much better success with controlling my blood glucose during an extended fast of 24 hours or more.

        • Ben Wagenmaker

          I also agree. I do intermittent fasting regularly, and extended fasts occasionally. My highest blood glucose readings are often in a fasted state, and especially after a fasted workout. However, I’m not concerned, since I know that my liver is just converting stored glycogen to glucose: essentially using endogenous glucose instead of exogenous (from what I eat).

  • r Negoro

    ….. It has been scientifically tested and it works.

  • antiguajohn

    Type 2 diabetic, one and a half years ago I started a fasting diet, no food from late Sunday night till Friday afternoon.

    So far I have lost 150 pounds and my insulin requirements are one fifth of what they were when I started and my health and energy is much better.

    By Sunday evening I look forward to the week of fasting, Monday is like I have a food hangover, Tuesday till Friday I have fantastic energy and clarity of mind.

    A life without some treats and joys is not good, so I take holidays and special days off, I test my blood sugars 3 to six times a day and use Humalin R insulin to prevent highs, my last A1c results were 4.8 percent.

    I have another 80 pounds to loose and I am having a great time doing it.

    When I arrive at my goal I will reduce the fasting days until I start to gain a few pounds, then I can add and subtract days to keep my weight in a target range, no calorie counting or restricted foods for me.

    One serious side effect, it may kill a multi-billion dollar diet industry.

    And that may be a good thing, one of the first diet efficacy studies in the early 1900’s showed only 10% maintained that loss for a year, 2% after 2 years and after 120 years it’s not improved much after all if there was an effective diet we would all be slim, which would be good for the dieter but a disaster for the diet industry.

    Scientia Non Domus,
    (Knowledge has No Home)


    • Natalie Fernandez

      you dont eat anything for 5 days?

      • antiguajohn

        I drink water or soda water with some fresh squeezed lime.

        Once you get into it it’s real easy to do.

        On most diets they ask you to make a list of food you like and then tell you, “never eat that again and if you find new food that tastes, spit it out’.

        On weekends I can eat anything I like, though I must admit healthy food started tasting good after a 5 and 1/2 day fast.

        After all anyone with a modicum of willpower can wait for the weekend to eat something they crave.

        On the other hand telling people to wait months and months to have a tiny amount of their favorite foods is a recipe for failure.

        Scientia Non Domus,
        (Knowledge has No Home)


  • Paul Hackney

    There is an absolute ton of scientific and medical studies on intermittent fasting. To start with, go see Dr. Jason Fung on youtube. If you stay on your BG pills and or insulin as a type 2 diabetic you will continue to stay fat and sick. It is highly unlikely that your health care professional has even looked at the studies Dr. Jason Fung refers to. Your doctor is operating off of the best guesses about type 2 diabetes from years ago. Dr. Fung even says that up until five years ago he was treating type 2 diabetics incorrectly as well. Watch this video by Dr. Fung

    • bkungl

      I saw came across Dr Fung’s video quite by accident and it was very interesting and made sense. He says that we need to treat the insulin and not the sugar. He, also, verified what I have often thought that once you are on medication it will only increase and increase. He has a high success rate with his program which cost $200 CDN for a year! You can, also, do it long distance via skype. I developed diabetes last year (runs in the family) and haven’t been on medication so far as I’m trying to control it via diet but I’m going to sign up for his program before my doctor puts me on medication.

  • Ben Wagenmaker

    Type II diabetic here, just diagnosed this year on Jan. 29th. Immediately started intermittent fasting, stopped eating/drinking excess sugar and drastically reduced my meat content. After 5 weeks, I’m down 21 pounds, and haven’t felt this good in years. I’m less tired, more alert, no more acid reflux, no more gout attacks, and my blood pressure is down. My eating window is 6 hours, and I do so at least twice a week. I’d be more than happy to continue doing this for the long haul.

  • Saleem Bassadien

    Ben, thanks for your feedback on this. Good luck on your journey!

  • Carol

    Hi Ben,
    That’s great news are you on any medications for your type II diabetes?

    • Ben Wagenmaker

      Hi Carol, when I was diagnosed, my doc prescribed metformin and lipitor. However, I only took metformin for a few days before deciding to forego the meds in favor of fasting and low-carb diet. I’ve met with my doc twice since, and he knows what I’ve decided to do. He is ecstatic about the results I’ve seen, and he has encouraged me to continue.

  • Zen Lizards

    Martin Berkhan of is my go to guy for all things to do with Intermittent Fasting. Following his fast for 16 hours and eat for 8 is the easiest method I’ve found that I can stick with and works great for my Type II. Plus I can build muscle at the same time thanks to his advice on using BCAA’s. So this method is one that I can refer to as a “Lifestyle” and not a “diet.”

  • Martin Yago Jezek

    i do i like 4 months and i am starting to have problems with my pancraease it cause me a pain after a while if i am fasted state dont know what is that. In my opinion is cause IF lower your blood sugar lvl and it drops me a much under those lvls like hypoglycemia

  • solluna80

    I’m currently doing this but not intentionally. Recently went through a break-up after 6 years. Was diagnosed with Type 2 at the beginning of 2014. Break-ups are great if you want to lose your appetite. I didn’t eat very much, maybe some cantaloupe here and there for 5 days. Ended up in a psych facility (unrelated to not eating), so they put me on a diabetic diet and back on Metformin. Before beginning that diet, my blood sugar levels were low, but not too low. Been eating healthy every since (practically no carbs and sugar, mostly lean meats, and veggies) The unintentional fasting kickstarted healthier blood sugar levels and I’ve maintained them with medication of course. But also with averaging less than 1000 calories a day, intense water aerobics for over an hour twice a week, cardio for 20-30 minutes/daily, and strength training every other day. I do believe some type of fasting can manage the diabetes.

  • tmp1356

    I have been doing 4:3 intermittent fasting for 6 months now and I have lost 22.6 lbs. The first week I felt dizzy when standing after a fasting day, but after that first week, I no longer felt any ill affects of the diet. I do eat 3 meals on my fasting day, keeping breakfast and lunch to about 100 calories and then having 250-300 calories for dinner. This helps me to avoid stomach problems when taking my regular medicines. Some of the benefits that I have found (other than the weight loss) is that my GERD had cleared up and my painful joints are better. Being a woman (50 years old), I have also noticed that I have better bladder control. 🙂 I am halfway to my goal weight which will be me in the “normal range” for weight and BMI and I plan to continue this way of eating for life. Once I reach my goal, I will switch from 4:3 to 5:2 and may raise my diet day calories some to maintain. I find this a great way of eating, I feel so much better over all.

  • L

    May I know which specific method of fasting you used? The Leangains guide is the one I’ve most commonly heard of but I’d be open to hearing about other techniques as well.

  • Rajendra

    Please provide the update as of now Ben. My condition is very similar to yours. Recently diagnosed with Diabetes Type 2. Weight 254 lbs.

    • Ben Wagenmaker

      Hi Rajendra, My current weight is 210 pounds (down from 273), BMI of 27.0. Most recent HbA1c is 5.3%. My weight has been stable for approximately two months, and my health is great. My inflammation has decreased in the past 4 months. I’ve seen a significantly noticeable improvement in my lifelong chronic post-nasal drip; it’s much less bothersome, and even absent on some days. And rather unexpectedly, I no longer get motion sickness when I read in a moving vehicle. I attribute this to lower inflammation in my sinuses, since ear/nose/throat is interrelated, and motion sickness and equilibrium are tied to inner ear issues (which are tied indirectly to sinuses). I attribute the lower inflammation to my low-carb diet and fasting, both of which keep my blood glucose low, since perpetually high BG is tied to high inflammation levels. It follows that a lower BG will result in less inflammation in the body. Oh, and my HDL-to-triglyceride ratio indicates my cardiovascular health as “ideal.”

      I should also add that a few months ago, I altered my diet from “moderately low carb” (100g carbs/day) to a “very low carb” ketogenic diet, keeping my net carbs at 30g or less per day.

  • ruth

    was seriously considering purchasing the book, does it cover how to get started on the plan for beginners ? and does it cover the use of diabetes meds while fasting?

  • casualsuede

    I do 16:8 hours fasting and over two months I’ve lost 11 pounds. It is not hard, and I do cheat a bit (I will always fast for 16 hours but sometimes eat for 10 hours, not 8 due to schedule), but my calorie intake stays steady around 2K and I work out 3 or 4 days a week (which is what I always done).

  • freelyn

    I just started intermittent fasting and a few days in I feel great. It hasn’t been hard to only eat during a window each day. It amounts to not eating breakfast and having lunch late That’s not a big deal.

  • Rohan Zalani

    This is an incorrect description of intermittent fasting.

    • Rohan, this is a totally useless comment, since you give no detals. Please delete it.

      • Brian badonde

        He is correct though…

        • How so? Explain yourself, young man!

          • Botzilla

            There are different types of IF. I use it to cut body fat for weight lifting, I eat normally for 6 hours a day and then fast for 18 only drinking water or zero calorie drinks. The fast they are describing above is an extreme version.

    • Debra

      Yes it is.

  • Ben Wagenmaker

    I’m sure I’m type 2. In Type I, your body does not produce insulin, and I have no reason to believe that is the case in my body.

    Case in point, I know of Type I athletes that have major blood spike after an intense workout, since their body releases stored glycogen, and there is no insulin to counteract. When I workout, I get a moderate spike (maybe 1 mmol/L), but nothing major, so I have no reason to believe that my pancreas is not releasing insulin.

  • My blood sugar tests are way too high and there is neuropathy, so I have been advised to add insulin to my metformin. Oog. So instead I just started IF, eating one normal meal per day, and avoiding snacks and intense carbs. I also have colon cancer, so I’m hoping this will help reduce the side effects of chemotherapy and radiation and increase their effectiveness.

    I don’t expect immediate results (except for losing weight), but I do find it easy to do. I also feel lighter than normal, which feels nice. I’m comitting to this for a month or two and I’m hoping.

  • S. Radhakrishnan

    I’d like to know what you did with the medications you were on when you started fasting. Did you continue with them while fasting? Did you stop all of them? Did you stop them gradually? If so, in what proportion? Thanks.

    • Truc Giarc

      I didn’t take any drugs.

  • Joan Galt

    I have suffered from chronic constipation for 30 years. I started eating every other day about 6 months ago and it was cured from day one. If slip up and eat a few days in a row it comes back. It also cured my acid reflux from day one. If I slip up it also comes back after a few days.

    I am doing this for the rest of my life.

  • Hi Ben, I am starting to discover this IF, what do u mean 16:8? can you explain your daily eating habit routine or in a week. thanks for being inspiration.

    • Ben Wagenmaker

      Hi Ken, 16:8 is a breakdown of a 24-hour period, specifically meaning that I fast for 16 hours and then restrict my eating to a 8-hour window, usually from noon to 8pm. So my first meal is usually lunch, which is often leftovers from the previous evening. My supper is usually a meat (steak, chicken or seafood) plus veggies with butter. If still hungry, I might have some peanuts or almonds with dark chocolate. Hope this helps!

  • orbit

    this article is kind of weak.. it makes no arguments “but can you stick with this diet? what if you get grumpy?” ie zero difference between this and any other diet
    fasting blows every other diet out of the water. try it, dont eat for 3 days, drink coffe, tea, diet cola, whatever – just no calories. the results will be quite evident. you dont have to eat every day, your ancestors sure didn’t

    • acampbell

      Hi orbit,

      Thanks for your comment. Research doesn’t back up your claim that fasting “blows every other diet out of the water.” But if you have a credible source to support that claim, please do share.

      The reality is that a lot of people are unable to fast for medical reasons; in addition, fasting can exacerbate eating disorders, compromise sleep, and lead to loss of muscle mass. Plus, many people feel weak and lethargic when fasting.

      Since there’s no one approach that works for everyone, the decision to try a fasting approach should be made in conjunction with a qualified health-care provider.

  • bold


    • acampbell

      Hi bold,

      Thanks for your comment. You’re right — slowing digestion helps to slow the movement of glucose from the gut into the blood stream. Fiber and fat help to slow digestion. However, antacids do not, and because they’re medications, it’s not advisable to use them for this purpose unless directed by a health-care provider.

  • Debra

    People have been fasting for thousands of years. This is hardly a fad. And calorie restriction is not necessary. You can consume the same amount of calories over a 8-9 hour window and then fast for 15-16 hours and still reap the same benefits.

  • Debra

    16/8 works well for me. And very easy to do.