Intermittent Fasting and Cortisol


Intermittent Fasting and Cortisol

Intermittent Fasting and Cortisol: This “article” started as a response to a Facebook thread I was tagged in yesterday, where people were warning the original poster about exercising in the morning on an empty stomach due to it raising cortisol.

If you want the low-down on Intermittent Fasting and a customizable fat loss plan for men and women, check out my book here: Feast.Fast.Fit. – Train Your Body to Torch Fat, Build Muscle, and Never Diet Again

Let’s examine the relationship between intermittent fasting and cortisol real quick.

Exercise also raises cortisol, so should we not exercise? Dieting will lead to more cortisol, as will low carbohydrate diets, coffee, etc. – do we warn people about that? We like cortisol during exercise as its release leads to mobilization of fatty acids from fat stores thus providing us with energy. After a workout where glycogen is depleted, cortisol is raised to spare glucose and then when you eat post-workout, it will offset this.

Cortisol is NOT bad – it’s just a hormone. It is released in a pulsatile fashion and its release in the morning is part of what helps us get moving. CHRONIC high levels of cortisol can certainly be a problem and negatively impact the immune system, but acute responses are normal.  Please stop vilifying hormones like cortisol and insulin simply because you read a headline and don’t understand all of their functions in the body.

Transient increases during a short time of fasting or exercise or fasted exercise or a stressful situation, are not too much for our body to handle. If such a minor stressor (fasted exercise) was so detrimental, we would not have evolved to where we are today. As a matter of fact, one of the main benefits of intermittent fasting is its ability to boost the resiliency of cells in response to “stress.”

Cortisol and fasting are often pitted against each other in certain fitness circles. Those who preach, “Beware of cortisol increases in fasting,” are typically using cherry picked studies that don’t accurately depict the discussion at hand or simply ignore the entire picture.

For example, some of the common studies for those in this camp include one where the participants fasted for 5 straight days and cortisol increased. Well of course, why wouldn’t it? Your body is trying to spare glucose. This leads to clickbait-esque thinking…fasting = more cortisol and cortisol = bad therefore fasting = bad.

Next they might use some, not all (because they hurt their point), studies done on Ramadan. Here’s one thing to remember, they cannot drink water during their fast so they are severely dehydrated and they get one meal per day. And guess what? Dehydration also leads to increased cortisol – so is this an accurate way to judge fasting? Is your guru explaining all of this when they bash  fasting?

Those of us who use intermittent fasting would never recommend that. Once again, we cannot compare these scenarios to waking up and doing some cardio on an empty stomach, which has benefits in my opinion –

Fasted Cardio For Fat Loss


Here’s one, “testosterone/cortisol ratio values did not change significantly” Effects of Ramadan fasting on physical performance and metabolic, hormonal, and inflammatory parameters in middle-distance runners” (2009) —- These participants again with limited water intake and one meal per day for one month.

Here’s another, The proinflammatory cytokines IL-1β, IL-6, and tumor necrosis factor α; systolic and diastolic blood pressures; body weight; and body fat percentage were significantly lower (P < .05) during Ramadan as compared with before Ramadan or after the cessation of Ramadan fasting. These results indicate that RIF attenuates inflammatory status of the body by suppressing proinflammatory cytokine expression and decreasing body fat and circulating levels of leukocytes. (2012)

But are we really ONLY focused on one hormone? Does anything else matter?

What about – Alternate-day fasting trials of 3 to 12 weeks in duration appear to be effective at reducing body weight (≈3%–7%), body fat (≈3–5.5 kg), total cholesterol (≈10%–21%), and triglycerides (≈14%–42%) in normal-weight, overweight, and obese humans. Whole-day fasting trials lasting 12 to 24 weeks also reduce body weight (≈3%–9%) and body fat, and favorably improve blood lipids (≈5%–20% reduction in total cholesterol and ≈17%–50% reduction in triglycerides).” (2015)

Let’s use another Ramadan study done on well trained men, what did they find? Ramadan fasting was associated with a reduction of body mass and body fat. Lipolysis might have occurred because of increased plasma triglycerides and HDL cholesterol concentrations.” (2008)

These studies, if read closely, show pretty clearly that short-term fasts or fasting will not negatively effect average cortisol levels.

 My advice?

  • Stop relying on your facebook friends for information.
  • Stop vilifying single hormones, foods, exercises, etc. The human body is complex.
  • I will take improved blood glucose, lower LDL, higher HDL, lower body fat etc. for slight acute increases in cortisol.
  • Read my series on intermittent fasting and think for yourself . There are over 60 references.

    The Ultimate Guide To Intermittent Fasting I – Feast vs. Famine

18 Responses to “Intermittent Fasting and Cortisol”

    • Fred Duncan

      Hi Ro, Thanks for reading. In my book that just came out, Feast.Fast.Fit., I have a chapter on women and Fasting. I also provide a diet for women based on activity and body weight. It may be of interest to you.

    • Fred Duncan

      Jim, The study you provided was done on diabetics, who already suffer from blood sugar regulation difficulties. Of course, those with diabetes and those without will need their own plan. We wouldn’t use a study on those with a medical condition to determine how a non-diabetic population would eat. Here’s a study done on healthy men that showed improvements in blood glucose and insulin action.

      “Effect of Intermittent Fasting and Refeeding on Insulin Action in Healthy Men.” National Center for Biotechnology Information. U.S. National Library of Medicine, Dec. 2009. Web.

      Here are two more studies done on diabetics if that’s what you’re interested in: – Effects of different diets on type 2 diabetes – Intermittent fasting improves those with type 2 diabetes

      • Robert More

        Neither of these article appear to confirm benefits of intermittent fasting and type 2 diabetes. One points to an article that has one source being “sage publications” and the comment is a vague comment and even states it is a “tantalizing notion”. The other article is about low fat, low carb or Mediterranean diet effects. So you responded to a link to an actual scientific study with unrelated studies, and publications with poor sourcing. I also want to point out… if intermittent fasting is commonly used for weight loss and obese ppl are also quite frequently type 2 diabetic does this basically mean intermittent fasting is just a way to improve things in ppl who are already healthy?

        • Fred Duncan

          Robert, Did you read the “actual scientific study” you point out as credible? Let’s start there.

          – 22 patients (WAY too small to make any sort of claim from)
          – TWO DAY test (not nearly long enough to make any claim at all)
          – No mention of the food given at any meals or if calories were controlled for both days (pretty important piece to know)
          – If they matched calories, lunch and dinner on the breakfast skipping days would be much larger. This would account the larger glycemic response, no?
          – They felt so strongly about their poorly designed study that they say “breakfast COULD BE a successful strategy for reduction in post-prandial hyperglycemia.” And as I said earlier, if the two meals were much larger of course PPHG would be higher!
          – Notice that they left everything else out there? Not statistically significant.
          – Also, what does this haphazard study have to do with the original article at all?

          Why did you skip over this study? (“Effect of Intermittent Fasting and Refeeding on Insulin Action in Healthy Men.”) Why didn’t you address that the once credible “scientific study” was posted with the writer saying “but skipping breakfast can cause bad things to happen with your blood sugar” in which he used a shoddy study on diabetics to make a sweeping generalization about no breakfast in all individuals?

          The first article was originally done by British Journal of Diabetes and Vascular Disease and then published by SAGE. So actually that’s not the source.

          The second source I write directly next to it, “EFFECTS OF DIFFERENT DIETS ON TYPE 2 DIABETES.” I don’t even mention fasting there.

          I didn’t claim that intermittent fasting is commonly used for weight loss. It can be, sure, but you just made that claim not me. I have personally used intermittent fasting with Type-2 diabetics and had their symptoms improve. Now, before you get all upset, yes this is anecdotal. It could be a myriad of things I recommend in that plan (exercise, calorie control, carbohydrate control, increased protein, fasting, etc.) So once again, I am not saying this is the end all be all treatment but as I’ve stated in all of my articles on fasting and my book, IT IS A TOOL. This tool happens to offer some unique benefits and can help you understand your relationship with food.

          Lastly, the article was not about fasting and diabetics. I posted two responses to a question so a reader can read more. I make no claims in my response nor do I state that it be the end all be all. Clearly this is an area that we are still studying.

          That was fun though, thanks.

  1. Vikki Cooper

    Hi Fred.
    I am currently trying to conceive. I typically IF for 5 days a week and feel great. This is usually 19/ and maybe one day at 24 (OMAD).

    Because of all the mixed responses with increased cortisol/ stress levels etc being bad for us, i am confused as to either carry on IF or not. I was wondering if ‘when trying to conceive’ IF may actually hinder this?

    I am not overweight, my BMI is around 21%. I actively workout 5 days a week which is HIIT & weights, however I have now reduced the intensity of the workouts to avoid (again) any extra stress on the body.

    I feel better when working out and fasting. Thus controls my food intake (I have a huge appetite & when not fasting find I can eat all day long and feel dreadful, tired, bloated etc)

    Any opinion would be most appreciated

    • Fred Duncan

      Hi Vicki, I am glad that you enjoy and have had success with intermittent fasting! As I mention in my book, if your cortisol levels are high, the first thing you likely need to address is sleep and stress management. Are you levels high currently? Also, as I lay out in my book, you don’t need to fast every single day, you can blend it in weekly if this is a cause for concern.

      Dieting in general (restricting energy intake) or low levels of body fat, will also make conception more difficult, so it’s not really the fasting per se. Energy balance provides a strong signal to our body. If you are in a caloric deficit for long periods of time, many “less important” functions in the body are turned off so that we can shift nutrients to more vital functions. Reproduction is an example of this. Women competing in figure/bodybuilding shows will get to a point in prep where they no longer get their menstrual cycle as they don’t have the energy availability to sustain this process at the moment. Of course, I would recommend that you discuss these things with your doctor (hopefully they are open minded) and if you are having difficulties, get some blood work ordered.

  2. Alvaro Lopez

    Can mundane stress (work, life problematic situations, etc.) during fasting break your fasting because of cortisol and the release of glycogen and then high levels of insulin???
    2nd question: If you are fasting and wake up in the morning, because of the morning cortisol spike, would that break your fasting?? this guys says it would break your fasting, but i am skeptical; reference below:

    • Fred Duncan

      Hello Alvaro, The release of cortisol does not break a fast. A fast is strictly abstaining from ingested calories thus leading to the downstream effects associated with low nutrient availability. Cortisol is released in an effort to “cope” with this low calorie environment and while it raises blood sugar a little, it also plays a role in the release of hormone sensitive lipase which facilitates the release of fatty acids and glycerol from fat stores. So yes, cortisol provides some “energy” by tapping into storage, but it isn’t adding exogenous calories, only allowing us to utilize what is already stored. This also doesn’t address the metabolic flexibility of the person fasting. Some people have much better met flex and can switch over to fat utilization easier than others (thus controlling blood glucose levels more efficiently), so that’s why fasting for a lean person is more effective than fasting for an overweight person.

      Let’s use exercise as an example. Exercise raises cortisol. Exercise also improves insulin sensitivity even though cortisol secretion will dump a little glucose back into the blood stream. Does this make the insulin sensitive benefits of exercise null? Absolutely not.

  3. Ren

    Hi Fred,
    I have been told i am suffering from adrenal fatigue and have way too much cortisol. Is fasting something that could get me back on track?
    I fasted for years before having my kids and it was amazing for weight management and also energy level.

    • Fred Duncan

      Hi Ren, Thanks for visiting my site. The term “adrenal fatigue” has come under serious scrutiny recently as we no longer consider it a disease. That doesn’t mean that you won’t or aren’t having symptoms that match this older term. If you are suffering from serious levels of fatigue and your controllable lifestyle factors are already in place (sleep, exercise, nutrition), you might want to get further examination. However, the most common reason is not managing stress appropriately and this is exacerbated by lack of quality sleep, limited exercise, and poor eating habits. Fasting can’t save you from those issues. I’d focus on sleep first as I think this is one of the most overlooked aspects of our health. Are you covering these bases?

      • Ren

        Im definitely not getting enough sleep and then the carry on effect is definitely struggling to get motivated to exercise. Thanks for that. Ill try to find a way to sleep more.

  4. Daniel

    Hi Fred, what is your view on intermittent fasting when having a stressful job, or generally being someone who tends toward worry (ie lifelong high trait neuroticism)? I find that IF suits me lifestyle-wise, but it has been suggested to me that fasting and increased cortisol, particularly in the morning may be contributing to anxiety etc. Would be great to hear your thoughts..

    • Fred Duncan


      As a business owner, I’ve encountered some serious stress at times and I haven’t noticed fasting exacerbating this (though we are all unique individuals). If you have read my book, you would see that my diets do not include a fast every single day, it follows more of an ebb and flow approach. However, I’m not familiar with research on cortisol and anxiety (though I’m not saying it doesn’t exist either). I think this is something you would need to monitor and see how it impacts you. The majority of people I work with feel more energized, alert, and notice improved mood ONCE they have adapted to fasting.

      Personally, I believe the cortisol and fasting issues are over-hyped. I wrote a 4 part series on Intermittent Fasting on my website. I went through almost 80 studies for this article and fasting raising cortisol to unsafe levels seems unjustified given the research. Check it out if you have time.

  5. Kati

    Hi Fred.
    I started IF 6 months ago and I feel great. I do 16/8. I train regularly 5 times a week and I tend to have high cortisol levels. I storage fat on my stomach and recently the past 3 weeks I started to do cardio while fasting and another work out by the end of the day because I want to burn that fat on my stomach. I am already in a good shape but I can’t never have that flat tummy and I didn’t see any difference in fact it is almost increasing so I am wondering if cardio while fasting it is actually not good idea for me. I also tend to have middle stress job.


    • Fred Duncan

      Hi Kati, Thanks for reading. Sounds like you’ve had some success implementing fasting which is great. However, you’re having a very common problem, here’s what I typically see.

      First, it’s important that you understand that exercise itself has a weak effect on weight loss (I wrote about this on my site). This doesn’t mean don’t do it, because it has tremendous benefits, but it’s a piece of the weight loss puzzle. When most people add more exercise, they tend to eat more – this is called a compensatory mechanism. Your body is used to roughly “x” amount of energy/calories per day. If you go out and burn an additional 200 calories, your body will drive you to make this back up. Whether that’s by moving less to conserve energy or sending some additional hunger signals to get more energy. So what’s likely happening is you are eating more on these days whether you believe it or not. You cannot gain weight without an excess of calories and in order to lose weight you must be in a deficit of energy. I would suggest monitoring your calories and macronutrients for a few weeks and see how much that changes thing. You can find a basic primer for this on my site or it’s laid out a bit more clearly in my book Feast.Fast.Fit.



  1.  Intermittent Fasting : Feast vs. Famine - Fred Duncan

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