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!

  • 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.

  • 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 !

  • truthbetold

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

  • 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)


  • 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.