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

7 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 Fasting and 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

  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.



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

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