63:: The science behind fasting as women (& why it is different than men!)

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Let's dive into the truth

and science about fasting and

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intermittent fasting for women.

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Welcome back to Wellness of the Pod.

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I'm your host, Dr.

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Kelsey Vic, a board certified orthopedic

doctor of physical therapy and a

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pelvic floor physical therapist.

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And I've been having this conversation

a lot lately with a lot of friends,

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men honestly, too, that are in our

age range where they're asking, why

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aren't you fasting for your workouts?

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I fast for my workouts.

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Why don't you, as a woman, there's

so much research behind fasting for

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your workouts in order to get the body

composition goals, the toning goals,

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the aesthetic goals that you want.

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Why aren't you doing that as a woman?

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And recent research has shown that women

handle fasting differently than men.

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And so I wanted to cover that

today and dive into the science.

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And why fasting might not necessarily

be the route that we as women wanna

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take based on our hormonal physiology

and just how our neurons and different

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signaling molecules within our system

respond to fasting and lower energy,

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availability, We've chatted about fasting

before as part of other episodes with

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exercise physiologists and dieticians,

so I will link those episodes below

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if you wanna dive in more with someone

who is actually doing the research

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on this and the research that I will

be pulling from for this episode.

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But we chatted with two

experts on the topic,

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Two very prominent figures in this space.

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Dr.

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Katie Hirsch, and then also Mel Silver.

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So I will link those episodes below in

addition to the breakdown for the baddies

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episodes where we dive in a little bit

deeper, a little bit of a book club

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episode where we chat about fasting

and all of the topics that we covered

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in those episodes with those experts.

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So a little background on fasting.

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Fasting has historically been recommended

across the board, and I've heard of

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it, recommended for anything from

like cell cleansing and detoxing

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each of your cells to helping with

body composition goals and aesthetic.

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And exercise.

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There was a big push in the bodybuilding

world for fasted cardio workouts and

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fast cardio workouts for men and women.

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Based on, a little bit older research

that has only been done on men.

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So today we will dive into fasting for

women specifically, and some of the

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differences in how our bodies handle

those lower energy reserves when we

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are in a fasted state compared to

how men handle the same exact state.

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So if your goals revolve around

body composition, toning, aesthetic,

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cycle regularity, fertility, hormonal

regulation, cortisol regulation,

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and cortisol balancing, then this is

definitely an episode that I think

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you'll be interested in because

fasting covers all of those different

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topics within our body and it really

affects each of those systems.

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In our bodies as women.

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So let's start with how fasting affects

our hormonal balance, our fertility and

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our cycle regularity or irregularity.

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In general, our body has a plethora

of feedback loops where our body is

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in this cycle until something cues it

to stop or produce more of something.

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And our hormonal regulation

and nutrition is no different.

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So we have what we call the hypothalamic

pituitary gonadal axis, which is basically

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a fancy term for the hormonal reproductive

feedback loop within our bodies.

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We have the hypothalamus, which

is the, top of the system when

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it comes to hormonal release.

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So the hypothalamus has.

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Signaling hormones that they release

in order to signal more hormones

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to be released from the pituitary

gland that signal more hormones to be

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released from the reproductive organs.

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So the hypothalamic pituitary gonadal

axis is just a fancy name for hormones

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the signaling involved to release

the reproductive hormones that help

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with our fertility and our cycles.

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I think of it as like an intense game

of telephone, or I picture a chocolate

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fountain or a champagne fountain

where there's just a cascading effect

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where signaling molecules signal

other molecules that signal different

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hormones that help to regulate our

cycles or irregulate our cycles.

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So we have nutrient sensitive neurons

in our hypothalamus called kisspeptin

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neurons, and women have more of

these kisspeptin neurons, and they

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are also more sensitive than men.

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So when we are in a fasted state.

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Our bodies, our KIS neurons, sense that

we are in this lower energy availability

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state, and they will cue the response

on a larger scale and quicker than

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might be queued in a male system.

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So these kisspeptin neurons respond

to nutrient availability, and they

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are the gatekeeper for how our

hormonal response and our cycles

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and our fertility is regulated.

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They control a lot of other things too,

but speaking specifically in regards

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to hormonal balance and hormonal

imbalance, right now these kisspeptin

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neurons are like the gatekeeper.

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So they control the release of

gonadotropin releasing hormone, which

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goads, again, reproductive organs,

gonadotropin releasing hormone.

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This is a hormone released

from the hypothalamus.

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That again, is that telephone game

that signals another release from the

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pituitary gland, luteinizing hormone,

and follicle stimulating hormone.

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So this gonadotropin releasing hormone

is released from the hypothalamus and

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helps to signal the hormonal release

from the pituitary gland of luteinizing

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hormone and follicle stimulating hormone.

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And if you've listened to our cycle

episodes, you know that there's

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a sort of rollercoaster ride that

happens between these two hormones

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during our menstrual cycle.

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So fasting causes a decline in these

kisspeptin neurons, which affects

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gonadotropin releasing hormone release,

and then affects the regularity

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of our cycle because luteinizing

hormone and follicle stimulating

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hormone will be affected as well.

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So this cascading effect, this

telephone game, all starts

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with our bodies sensing a fasted

state, which cues those kisspeptin

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neurons, which cues less release of

gonadotropin releasing hormone in that

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hypothalamus at top of the pathway,

which then affects luteinizing hormone

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and follicle stimulating hormone

release from that pituitary gland.

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Again, that middle guy for that hormonal

cascade, which in turn affects our cycles.

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So let's switch to

metabolism a little bit.

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And this is where women actually thrive

because we are metabolically superior,

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or metabolically more efficient

or more flexible in how we utilize

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certain fuel sources within our body.

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We are able to switch between

different fuel sources and take

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from different fuel sources more

efficiently than a man system is.

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So historically, I guess what we

utilize for our fuel sources has

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typically followed this pathway.

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We first utilize blood glucose,

then we utilize stored glycogen

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within our liver and our muscles,

which is the stored form of glucose.

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So glycogen, glucose, you can

think of them as the same thing.

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Glycogen to just takes a little bit to

get to the point where we can use it

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as energy, as glucose, and it is the

stored form of glucose in the liver.

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And the muscles.

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So we first tap into the blood

glucose levels, 'cause that's

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usually the most available and

the most readily available for us.

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Then we tap into liver and

muscle glycogen stores, and then

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we tap into free fatty acids.

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That's historically what we've believed

as how a body taps into different fuel

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sources for energy, whether that's for

exercise or just like day-to-day tasks.

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However, it's been shown that women

specifically have this flexibility within

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their metabolic system where we tap

into that blood glucose first, but then

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we actually transition to those free

fatty acids a lot quicker than a manez.

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They have to go through that process

where we can go straight from blood

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glucose, utilizing blood glucose

as our fuel to free fatty acids.

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This is because we have a lot

more oxidative fibers within

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our skeletal muscle tissues.

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And so that is one of the theories

that has been brought up to why

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we tap into those free fatty

acids quicker than Amand does.

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So one of the goals with fasting could

be, Hey, let's decrease our glucose

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stores initially so that we tap into

those free fatty acid quicker, or, that's

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historically been one of the theories.

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And for a man that might actually make

sense because if we can limit the amount

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of glucose that they have to utilize at

the beginning of that pathway, they might

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tap into those free fatty acid storage

is early on in their workout, where for

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women, we don't actually have to do that.

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We kind of skip that step.

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We skip the benefit of fasting

because, we utilize that glucose

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within the bloodstream first before

we tap into those free fatty acids.

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And a lot of people think, oh, let's tap

into those free fatty acids as fuel as

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soon as we can so that we're burning fat.

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But this is not necessarily

like a one-to-one ratio.

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Yes, it does help that we can tap

into those free fatty acids, but

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it's a lot more complex than that.

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But that is one of the reasons fasting

has been promoted is because if we can.

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Decrease the circulating glucose

levels or stored glucose levels,

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then we might be able to tap into

those free fatty acids sooner.

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Whereas women, we have that flexibility to

go, okay, utilize all that blood glucose

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first that circulating quick energy first

before we tap into those free fatty acids.

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But we don't necessarily have

to go through the entire process

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that we've historically thought.

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That men go through, but maybe a

woman does not necessarily go through

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So let's chat fasting and

how it affects our cortisol.

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And cortisol is always presented as the

bad guy and we've done episodes on it,

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calling it the misunderstood mean girls.

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So I will link all of those episodes

below 'cause I do not want you

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to think cortisol is the bad guy.

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We need cortisol, but fasting

for prolonged periods of time

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can elevate our cortisol levels.

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Fasting for almost greater than 12 hours.

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It's been shown to elevate

our cortisol levels.

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And if you think.

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Your sleep window also counts as fasting.

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We're not expected to wake up

and eat a snack during sleep,

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so that counts as fasting.

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So greater than 12 hours is actually,

it sounds like a lot, but it's

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actually not that long of a time.

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So fasting for greater than 12

hours can increase some of our

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cortisol levels within our body.

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So higher cortisol for a prolonged

period of time can lead to muscle

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loss, fatigue, and thyroid dysfunction.

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So all of those things we definitely do

not want, especially if our goals are

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energy levels, fertility, regularity,

hormonal balance, muscle building, body

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composition changes, all of the things.

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But again, it's not that cortisol's

bad, it's just that high levels

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for a prolonged period of time

is not necessarily what we want.

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We wanna maintain that cortisol balance

and that cortisol rollercoaster that

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we have throughout the day with those

natural elevations that happen in

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the morning, and then the natural

declines that happen later in the day.

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So this is all relatively recent research

in the grand scheme of things where we are

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just now starting to study the female in

comparison to the male, where we actually

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separate our study groups, male and female

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there's so many things that are different

about our bodies, how we store fat, how

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our muscle fibers are structured when we.

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Tap into different fuel

sources within our bodies.

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So we've started biasing out

different groups to know how does a

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woman's body actually handle this?

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So more research is definitely needed.

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More research on humans

is definitely needed.

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We've done a lot of animal studies,

but more is definitely needed on

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humans and I think future research is.

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Going to need to be dedicated towards

people with special conditions.

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'cause there is a study that has been

hed on PCOS, I believe it was: 2023

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or 2024, so again, relatively recent where

women with PCOS have found benefits with

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fasting when it comes to muscle building,

and then also decreasing visceral fat

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and subcutaneous fat within their system.

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So I think there needs to be

condition specific research under

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the umbrella of female research.

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So this is just something that my mind

has been gravitating towards recently

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and this is something that we have

just chatted about recently with a lot

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of family and friends, and I wanted

to cover this from this research and

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science back perspective so that you

guys are aware of the newer research

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on fasting for females specifically.

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But with all of that being said, I had

a great conversation with a friend of

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mine who is reading more on fasting and

she was talking about the intuitiveness

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of fasting and the need to be intuitive

with your own body despite the research.

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And I really loved this because yes,

there is research on female physiology

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and how our bodies tap into different

fuel sources, how we raise certain

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hormones within our system, how our body.

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It taps into different resources that

we might not wanna tap into for fuel,

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like muscle tissue earlier than a man.

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So there's all of these reasons

why someone might choose to fast

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or not fast, But fasting can

mean a lot of different things.

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It can mean complete fast.

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It can mean liquids only.

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It can mean intermittent fasting

or time restricted eating.

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It can mean eating less, but more

frequently throughout the day.

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There's so many descriptions

of fasting and they're not all.

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The same across research.

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Is it time restricted fasting,

where we're fasting for 12 hours?

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Is it littler meals throughout the day?

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Is it liquid only?

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Is it a complete 24 hour

fast or a multi-day fast?

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So there's lots of different ways

to fast, and I'm not saying that.

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No female can benefit from fasting.

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So I think there needs to be some

intuitiveness about listening to your

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own body when it comes to fasting.

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So if you found that your body

actually does benefit from fasting,

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or it feels better with fasting

for some reason or another.

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I'm not doubting you on that.

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I'm not saying that is wrong at

all, or that you are wrong for

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listening to your body in that way.

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I think there are some people who do

benefit from fasting in whatever form

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that might be, and so if you are . One

of those people that might benefit

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from fasting or think that your body

or your sleep or your fatigue or

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energy levels benefits from fasting?

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There are a few things to keep in mind, I

guess to offset maybe the negative impacts

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that fasting has on a woman's body.

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We don't want those raised cortisol

levels for a prolonged period of time.

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We don't want cycle irregularity.

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We don't want muscle loss.

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We don't want so many

things that fasting and.

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Having lower energy availability might

lead us to, and so there's a few things.

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If your body does seem to enjoy

fasting in some sort of form, there

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are some things that are recommended

in order to help your body, combat

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some of those negative effects.

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So if you do benefit from fasting in some

way, make sure you are getting adequate

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protein intake during that feeding window.

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Protein specifically all macronutrients,

of course, but protein specifically

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is one where if you are not

getting it during your feeding

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window, your body is not able to.

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Build muscle.

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It's not able to help some of these

processes that go on within the body.

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Same thing with carbohydrates,

same thing with fat.

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So making sure you're getting

adequate macronutrients and

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micronutrients during that feeding

window is super duper important.

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Fasted exercise specifically

also messes with our appetite

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control center, let's call it.

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So if you can plan your

exercise when you are in a non.

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Fasted state, so when you're in

your feeding window, That is only

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going to help your body adapt to

that exercise stimulus better.

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If you don't respond well to eating

before exercise, even giving your body

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a little bit of a cue that you are not

in a low energy availability state, by

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feeding it with a little bit of glucose,

a little bit of protein, giving it some

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signal to say, Hey body, we are okay.

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We don't need to break down

necessities within our body

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in order to fuel this workout.

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So almost eating, even if it's like

a quarter of a banana or a little bit

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of protein before your workout to help

your body sense that it is okay, we

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can use those for fuel for a little bit

and then we can tap into some of the

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other fuel sources that isn't things

that we want, like muscle tissue.

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And then try keeping your fasted

window between 12 and 14 hours.

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Again, anything over 12 has been

shown to raise those cortisol levels.

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So if we can keep it between that

12 to 14 hours, that's a little

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bit safer of a fasting window.

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Try to eat within 30 minutes of waking

as well, and then stopping eating around

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that six to 7:00 PM timeframe too.

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Keeping that fasted window safe

rather than pushing it to those 20

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hour fasts or those 16 hour fasts.

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So I hope it was helpful to go over some

of the science regarding fasting for

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women compared to men, and then also

if your body does sense that fasting

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is good for it, understanding what you

can do to combat some of the negative

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effects that might come from fasting.

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So I hope you guys enjoyed this episode.

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I hope you guys are having the most

wonderful start to your summer.

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I know I am.

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I've been enjoying this little bit of a.

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I guess slow down with the podcast

where I'm only releasing one a week.

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And it's so funny that if you guys

remembered a few episodes back, I said

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I was gonna drop to one episode a week

during the summer because I was just

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sensing that God had put it on my heart

to, not push so hard to release two

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episodes a week, even though I really

do enjoy it, and I really do feel

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like it's good for the podcast growth.

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For some reason, it was just on my heart

that, hey, maybe this time you do one

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a week during the summer and then pick

it back up to two a week in the fall.

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And I didn't know why that was on

my heart, but it's already been

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revealed to me why that was the

case and I just feel like it was.

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A wonderful, I guess example of

actually listening and hearing those

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little whispers and then following

through with them and obeying them.

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And so I've already been reaping some

of the blessings from going back to,

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one time a week and understanding why

I might've been receiving that whisper

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and the future of the summer for me.

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So I will maybe share a little bit more.

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: 00:17:58

Once everything's finalized, but literally

the week after I decided and released that

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episode saying, Hey, I've been feeling

called to release only one episode a week.

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: 00:18:07

I'm not sure why, but I feel like

there's a bigger plan for this.

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: 00:18:10

Almost a week after I immediately knew

exactly what he was calling me to do.

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So I am very grateful.

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I just wanted to share that with you guys

as a little bit of a message of, hope and.

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Trust and patience and

listening to those whispers.

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: 00:18:24

So this episode took a turn, but

I hope you guys enjoyed the rest

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of this episode, the fasting part.

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: 00:18:30

I hope you learned a lot.

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: 00:18:31

If you've also been having this

conversation with girls in your

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life or even men in your life,

please share this episode.

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: 00:18:37

It.

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: 00:18:37

Super Duper helps the podcast to grow,

and I just appreciate it so very much.

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: 00:18:42

So thank you guys for listening.

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I hope you enjoyed this episode.

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: 00:18:45

I'll leave all of the links below to all

the episodes that I referenced in this

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: 00:18:49

podcast and I'll see you guys again on the

next episode of Wellness Books is the pod.