What Does Fasting Do To Insulin?

By August 13, 2017Blog

POST SUMMARY: What this means for you…

Restricting eating to a specific window of time (e.g. skipping breakfast), is a strategic way to lower insulin and improve insulin sensitivity.

A common thread among many dietary plans is compressing eating times. On the one hand, some plans narrow periods to two to three meals per day, with substantial time gaps in between (i.e. time-restricted feeding). Variation include other strategies that suggest eating normally for a few days, then avoiding food entirely for a few days (i.e. intermittent fasting). On the other hand, some diet plans encourage eating several meals throughout the day (i.e, “grazing”; 6-8 small meals per day).

Because elevated insulin is one of the most, if not the most, relevant factor in developing insulin resistance, a highly rational strategy is to follow a dietary plan that incorporates periods of time throughout the day wherein insulin is low. This philosophy immediately suggests that frequent eating is less effective than less frequent eating—indeed, three meals per day is better than six [1]—but are fewer than three meals best of all? Maybe.

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Fasting’s Effectiveness Partially Depends on How It’s Done

Time-restricted feeding and intermittent fasting strategically include periods of deliberate food avoidance. The evidence regarding its efficacy in improving insulin sensitivity is valid, though it partially depends on how it’s done. Two studies used this idea by having study subjects eat normally one day (i.e. unrestricted) and essentially fast the entire second day (i.e., alternate-day fasting), repeated seven times over a two-week period and found conflicting results—one reporting an improvement in insulin sensitivity [2], while the other observed no benefit [3]. An alternative strategy, wherein the person confines eating to a specific window of time each day (e.g., eating breakfast and dinner only [4], or lunch and dinner only [5]) yielded robust improvements in insulin sensitivity.

A critical distinction must be made between thoughtful food restriction and starvation. Whereas fasting and time-restricted eating is a deliberate restriction of food daily (i.e. time-restricted eating) or for longer (i.e. intermittent fasting), each is a scheme that involves eating fully for some period of time, with deliberate restriction for only certain periods—part of a day each day or an entire day every so often. There is ideally no calorie counting with either strategy—simply avoiding food certain times and eating normally (until satiated) other times.

A Very Important, Fine Line

It may seem like splitting hairs, but there’s an important, yet fine line between eating to keep insulin low vs. starving your body. Taken to an extreme, it’s possible fasting can do more harm than good. There is no definite time past which fasting becomes harmful; so much depends on the constitution of the person fasting, how they define “fasting” (e.g. what are they drinking, how are they supplementing, etc.), and how they’re compensating for not eating essential minerals (e.g. magnesium). Importantly, early studies into prolonged fasting found a potentially lethal consequence can develop after fasting ends, termed “refeeding syndrome” [6]. And, if not done smartly, prolonged fasting becomes actual starvation, which can paradoxically cause the body to become insulin resistant [7].

The Macronutrient Mix Also Affects Success

The elephant in the room with studies exploring time-restricted eating or intermittent fasting is the macronutrient content of the diet. (To my knowledge, these studies have yet to be performed.) Altering the macronutrients of the diet (e.g., carbohydrates vs. fats) may yield disparate responses not only with how well the individual can maintain the diet, but also whether it’s healthy. Whether it’s feasible and sustainable are affected predominantly by the amount of calories consumed; if a person fasts ~18 hours each day (e.g., eating from noon to 6 PM), whether this is healthy will depend on what they eat when they do eat. By consuming more of one macronutrient over another, the person may ensure they’re providing sufficient energy to the body, despite eating less total amount (i.e., volume) of food, which means increased satiety [8].

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Making Fasting Easier

Energy (e.g., calories) from a meal conveys greater satiety than bulk—if your cells are fed, they don’t care whether there’s something in your stomach. Thus, meals that provide energy without spiking insulin (i.e. dietary fat) not only provide energy to the body, but also prevent the hunger that comes with increasing insulin [9], which typically makes fasting much easier.


  1. Hutchison AT, Heilbronn LK: Metabolic impacts of altering meal frequency and timing – Does when we eat matter? Biochimie, 2016, 124:187-197.
  2. Halberg N, Henriksen M, Soderhamn N, Stallknecht B, Ploug T, Schjerling P, Dela F: Effect of intermittent fasting and refeeding on insulin action in healthy men. Journal of applied physiology 2005, 99:2128-2136.
  3. Soeters MR, Lammers NM, Dubbelhuis PF, Ackermans M, Jonkers-Schuitema CF, Fliers E, Sauerwein HP, Aerts JM, Serlie MJ: Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism. The American journal of clinical nutrition 2009, 90:1244-1251.
  4. Zakaria A: Ramadan-Like Fasting Reduces Carbonyl Stress and Improves Glycemic Control in Insulin Treated Type 2 Diabetes Mellitus Patients. LIfe Science Journal 2013, 10:384-390.
  5. Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, Cuzick J, Jebb SA, Martin B, Cutler RG, et al: The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond) 2011, 35:714-727.
  6. Mehanna HM, Moledina J, Travis J: Refeeding syndrome: what it is, and how to prevent and treat it. BMJ 2008, 336:1495-1498.
  7. Koffler M, Kisch ES: Starvation diet and very-low-calorie diets may induce insulin resistance and overt diabetes mellitus. J Diabetes Complications 1996, 10:109-112.
  8. de Graaf C, Hulshof T, Weststrate JA, Jas P: Short-term effects of different amounts of protein, fats, and carbohydrates on satiety. The American journal of clinical nutrition 1992, 55:33-38.
  9. Rodin J, Wack J, Ferrannini E, DeFronzo RA: Effect of insulin and glucose on feeding behavior. Metabolism: clinical and experimental 1985, 34:826-831.

About Benjamin Bikman, Ph.D. – Ben earned his Ph.D. in Bioenergetics and was a postdoctoral fellow with the Duke-National University of Singapore in metabolic disorders. Currently, his professional focus as a scientist and professor (Brigham Young University) is to better understand chronic modern-day diseases, with special emphasis on the origins and consequences of obesity and diabetes. He frequently publishes his research in peer-reviewed journals and presents at international science meetings.

The information on this website is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.

top image credit:
James Harris


  • Sherene Kershner says:

    Thank you for this post, Dr. Bikman. I am new to exploring IF and am doing all the research I can to find out the best approach if you’re using IF to restore insulin sensitivity as well as lose weight. Do you have any information about how long it takes to restore insulin sensitivity to very resistant cells? I was pre-diabetic before I found a low-carb lifestyle that really helped me lose weight. However, after losing 125 lbs, I started to regain it quickly and I think the problem is that I’m somehow still in fat-storage mode. My recent lab work showed I have reactive hypoglycemia and very high insulin levels. Mind you, this is after prolonged low-carb dieting. I suspect I need something even deeper – i.e. extended fasting – to give my cells enough of a break from insulin to truly heal. I am working with my doctor on supplementing for electrolytes and basic minerals. The question we haven’t been able to answer is how long does it really take for resistant cells to wake up? While I realize the answer will be very individual, I’d love to know if you have found any general indications about this in your research or studies?

    • Ben Bikman says:

      Hi Sherene. I’m deeply impressed at your 125 lbs weight loss–good for you. Do you have any reason why you began to regain quickly? Did you change your diet? Reactive hypoglycemia is common with hyperinsulinemia–the excess insulin drives blood glucose below ideal levels.

      I’m afraid I can’t tell you how long it takes to become insulin sensitive–it’s just too variable. There are studies that show remarkable improvement in insulin resistance in just 24 h of carbohydrate restriction (where insulin-treated type 2 diabetes need to half the insulin dose). In addition to diet, resistance exercise is good at improving insulin sensitivity due to increasing muscle mass (muscle is the largest insulin-sensitive tissue).

      Regarding fasting, it can certainly be an effective way to improve insulin sensitivity, particularly if your physician is on board with you. You do need to be mindful of minerals and micronutrients, as you mention. Again, how quickly this can help is dependent on myriad factors.

      In the end, I’m interested that LCHF stopped working and I’m even more curious what caused the rapid weight regain. This, to me, is the real issue worth pursuing.

  • Jon W says:

    Dr. Bikman
    I have been trying to overcome insulin resistance through intermittent fasting (14-18 hours) and a ketogenic diet. It has been 3 months and my insulin fasting numbers are great.
    However I have read that prolonged LCHF diet can lead to insulin resistance of the muscles, (physiological insulin resistance).
    Here is an excerpt from the Hyperlipid blog, which explains the concept better than I ever could:

    “LC eating rapidly induces insulin resistance. This is a completely and utterly normal physiological response to carbohydrate restriction. Carbohydrate restriction drops insulin levels. Low insulin levels activate hormone sensitive lipase. Fatty tissue breaks down and releases non esterified fatty acids. These are mostly taken up by muscle cells as fuel and automatically induce insulin resistance in those muscles.
    This is patently logical as muscle runs well on lipids and so glucose can be left for tissues such as brain, which really need it. Neuronal tissue varies in its use of insulin to uptake glucose but doesn’t accumulate lipid in the way muscle does, so physiological insulin resistance is not an issue for brain cells.
    However, while muscles are in “refusal mode” for glucose the least input, from food or gluconeogenesis, will rapidly spike blood glucose out of all proportion. This is fine if you stick to LC in your eating. It also means that if you take an oral glucose tolerance test you will fail and be labelled diabetic. In fact, even a single high fat meal can do this, extending insulin resistance in to the next day.
    Without insulin, muscle cells withdraw the glucose transporters and resort to burning fat.”
    Some prominent (but not necessarily right) people recommend for those on a HFLC diet, a 1-2 times per week high carb load, supposedly to prevent chronically low insulin levels, high blood glucose levels, and “physiological insulin resistance”. These people include Dr. Mercola and John Kiefer (author of Carb Nite). I have not included this carb loading in my diet, as it seems speculative to me.

    I would be interested in your take on this.
    Jon W

    • Ben says:

      Hey Jon. There is a lot to unpack here. There are genuine states of “physiological insulin resistance”–a condition where it is natural for the body to be insulin resistant. These are puberty and pregnancy. Based on the data available in humans, a LCHF does not induce insulin resistance, but rather improves fasting glucose and fasting insulin, and insulin in particular is a necessary measurement to understand insulin resistance. Whether a human can manifest with a degree of glucose intolerance from a LCHF diet is possible (though I know of no evidence to support this). I could speculate however: if a person is eating too much fat (i.e. calories are simply too high), glucose intolerance could develop.

  • Jacki Moya says:

    Now I’m confused as to what I should be doing 🙁 My lab blood sugar levels have been normal however, in testing my blood glucose at home, I’ve come to realize I’m developing insulin resistance (the 3 hour level is higher than the pre-meal number). My protocol at this time is to try to fast for 36-44 hours twice a week, with 4-5 hour eating windows at ‘feast’ days, LCHF. I’m allowing myself 1 high carb item a week (just started experimenting with this and realize this may not work/be beneficial).

    How will I know if what I’m doing is working to reverse insulin resistance and not make it worse?

    • Ben says:

      Hi Jacki. I want to make sure I understand: your recent clinic/lab visits reveal normal fasting glucose, but your own home measurements reveal a high fasting glucose? Unfortunately, using glucose as the sole marker of insulin resistance is insufficient. Is there a way for you to get your insulin itself measured at the clinic/lab?

    • jon walawitch says:

      I recently had a “fasting insulin” blood test done. The results of this test should tell you whether and to what degree you are insulin resistant. It’s affordable and available through Life Extension’s website. Cost $29.90. Here’s the link:

      The Life Extension blood tests work this way:
      -You pay them on their website for the blood test
      -They send you a requisition by email, which you then bring to any Labcorp
      -You get the results, and you can call Life Extension if you need a consult on the results.
      call 1-866-598-6746 for more information.


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