Intermittent Fasting Raises Cortisol. So Does Learning Something New.

BOTTOM LINE

Intermittent fasting raises cortisol… and so 🤷🏻‍♀️?

Exercise, cold or heat exposure, public speaking, poor sleep, cognitive challenge and Mauritian traffic also raise cortisol – no one’s having a social media drama about those.

A temporary cortisol increase does not equate to hormonal dysregulation.

The more useful question to ask is whether the stressor is:

• manageable
• recoverable
• appropriate for you

Because that’s how hormesis works: a moderate challenge can make the system more resilient. Too little challenge? Nothing changes. Too much? The wheels fall off a bit.

A temporary cortisol increase does not equate to hormonal dysregulation.

Current evidence does not support the loud social media claims that intermittent fasting universally “destroys female hormones.”

Current evidence also does not support fasting as magical, mandatory or appropriate for everyone.

What matters are the variables within the context:

• fasting duration
• training load
• calorie and protein intake
• sleep
• existing stress levels
• metabolic health
• life stage

All of these influence whether fasting becomes adaptive … or just another thing your body has to deal with.

Some people do brilliantly with intermittent fasting.
Others feel awful.

Both can be true.

THE HOW TO

If you want to experiment with intermittent fasting, in my opinion, short and steady wins the race, because it is not such a shock to the system, but it does depend on your goal:

Start simply

• Stop snacking between your meals.
• No night-time grazing.
• The time frame from your supper to your breakfast counts as a fast if you don’t eat anything within that time. It’s the easiest fasting time frame to start with as you are hopefully sleeping during most of it.
• Once you have the overnight haul mastered, you can add on a few hours on either side.

However, if you are that “cold turkey” kinda person, you could just jump to the 24-hour liquid (as in water, black coffee or teas and electrolytes) fast, followed by a feeding day. Once you have these under wraps, you can extend your fasting time if your body responds well to it.

 In either scenario, you do need to ensure that you always:

• Break your fast with a nutritionally sound meal (McDonald’s is not it).
• Ensure you are consuming sufficient proteins when you are eating.
• When you eat, ensure your calorie intake meets your needs.
• Do some resistance training to support lean muscle mass – yes, even when you are fasting and it is totally do-able.
• Don’t stack a fast on top of poor sleep, extreme training, chronic stress or illness.

And pay attention to feedback from your body with respect to:

• energy
• mood
• recovery
• training performance
• sleep
• menstrual changes

The goal is not to become “good at fasting.”
The goal is to determine whether your body is adapting well to it.

THE SCIENCE BIT

What Intermittent Fasting Actually Means

Intermittent fasting is an eating pattern — not a diet  — that alternates periods of eating with periods of not eating.

Researchers generally divide intermittent fasting into:

• Time-Restricted Eating (TRE): eating within a daily time window
• Alternate-Day Fasting (ADF): alternating fasting and feeding days
• Prolonged fasting: usually 24 to 48 hours

The distinction becomes important when discussing cortisol, stress physiology or menopause, because a 12-hour, overnight fast is physiologically very different from repeated 36-hour fasts.

Potential Benefits of Intermittent Fasting

The benefits of intermittent fasting include:

• Lowered blood glucose and insulin
• Improved metabolic flexibility
• Glycogen depletion and greater fat utilisation
• Autophagy
• Improved mental and physical performance
• Improved immunity
• Hormesis
• Suppression of inflammation
• Lowered body weight
• Improved blood pressure
• Lowers triglycerides

Intermittent Fasting as a Hormetic Stressor

One of the more interesting arguments for intermittent fasting is that it may function as a hormetic stressor.

Hormesis is essentially the biological version of Kelly Clarkson’s song “What doesn’t kill you makes you stronger” … provided you recover properly afterwards.

It’s a biological phenomenon in which a low or moderate dose of a stressor produces a beneficial, adaptive response.

The same principle can be applied to intermittent fasting.

Cortisol Isn’t One-Dimensional

Cortisol is one of the body’s normal stress-response hormones, so yes, intermittent fasting does temporarily raise cortisol, but our brain cells don’t necessarily interpret all cortisol increases in the same way.

A short-lived stress response from fasting may trigger repair processes, whereas chronic stress, without recovery, is more likely to become harmful.

For a stressor to be considered hormetic it must be:

• Specifically relative to the individual
• Transient
• Recoverable

It’s worth noting that lots of other perfectly normal things also raise cortisol:

• Performance situations
• Difficult conversations
• Learning a new skill
• Deadlines
• Parenting toddlers and teenagers

Yet we don’t automatically interpret every cortisol increase as detrimental to our health.

De Cabo and Mattson (2019, para 10) state that “most if not all organ systems respond to intermittent fasting in ways that enable the organism to tolerate or overcome the challenge and then restore homeostasis. Repeated exposure to fasting periods results in lasting adaptive responses that confer resistance to subsequent challenges.” In other words, the body can adapt to short, manageable fasting periods rather than simply being “damaged” by them. And this is more so if intermittent fasting is habitual.

Even if intermittent fasting can be beneficial, that doesn’t mean it gets a free pass from scrutiny:

Fasting and Female Hormones

Cienfuegos (2022) and Kim (2021) note that there are still too few studies examining the effects of intermittent fasting on female hormones, and the current literature is mixed with heterogenous populations from lean healthy women to obese premenopausal women, or postmenopausal women.

And this is where things get noisy online.

Trending on social media are several voices claiming that intermittent fasting may disrupt female hormones because it increases cortisol and adds stress to an already stressed system — particularly in peri/menopausal women or hard-training female athletes.

And to be fair, this concern is not completely unfounded.

There is evidence showing that chronic low energy availability, under-fuelling and excessive training loads can negatively affect endocrine and reproductive function in women.

But an important nuance often gets lost somewhere between the podcast clips and the “3 Foods Destroying Your Hormones” reels.

A significant chunk of the discussion around fasting and reproductive signalling pathways (using jargon like kisspeptin signalling) comes primarily from animal studies involving rats and monkeys.

Beware the Source

Using rat studies to make broad claims about female reproductive hormones ignores the fact that hormones fluctuate according to circadian rhythms.  Rats are nocturnal. Most humans aren’t.

Context Matters More Than Internet Certainty

In addition, many of these concerns originally emerged from the context of the Female Athlete Triad: highly trained, under-fuelled athletes with low energy availability.

That is not automatically the same thing as a recreationally active woman finishing dinner at 7pm and eating breakfast at 9am.

On the whole, though, current evidence suggests that intermittent fasting is not universally disruptive to female hormones, although responses may differ depending on metabolic health, stress load, energy availability, and life stage.

Muscle Mass Concerns

Another common concern is whether intermittent fasting causes muscle loss.

Research to date does not consistently show significant lean muscle loss with intermittent fasting —  particularly when:

• protein intake is adequate
• resistance training is included
• energy intake remains sufficient

Can you build muscle optimally during fasting phases? That is still unclear.

Circadian Rhythm and Hormonal Signalling

At the moment, the research suggests that intermittent fasting can influence hormonal signalling and circadian-related processes in humans — but the response appears to depend heavily on:

• the fasting protocol used
• metabolic health
• gender
• life stage
• overall stress load
• and the population being studied

Which is science’s slightly frustrating way of saying:

“It depends.”

Some adaptive hormonal and circadian responses have been observed with intermittent fasting, but much of the evidence still comes from animal studies, while human findings remain mixed rather than universally consistent.

Kim’s (2021) review therefore supports neither the idea that intermittent fasting is universally beneficial nor universally harmful for hormones.

Having addressed the more serious concerns around fasting, it’s also worth noting the more common side effects like hunger, irritability, headaches or dizziness.

These can happen, particularly during the adjustment phase, but they are often:

• Temporary
• Manageable
• Improved as the body adapts

So What Can We Actually Conclude?

Yes, intermittent fasting probably does raise cortisol levels …  transiently

But a better placed drama would be to determine if the overall stress load is appropriate for YOU.

And these variables will be affected by the fluctuating context you find yourself in.

So rather ask yourself:

“Is intermittent fasting helping your body to adapt — or simply adding to an already overloaded system?”

References (with supporting statements)