Mikkipedia | I Was Wrong About Why Carbs at Dinner Help You Sleep

Episode Summary

Mikki has said many times — on this podcast, on other podcasts, and repeatedly on Instagram — that eating carbohydrate at dinner helps calm the nervous system, and that this is why people who feel wired at night sleep better when they add some back in. In this episode she corrects herself. Not on the advice, which she still gives and still sees work, but on the mechanism she has been attaching to it: insulin clearing branched-chain amino acids out of the bloodstream, tryptophan winning the competition at the blood-brain barrier, and the resulting serotonin and melatonin sedating you. Every step in that chain is real biochemistry, which is exactly why it sounds right. It just does not happen at the doses found in any meal she would actually recommend.

From there she works through what is more likely going on. She covers the head-to-head trial that put carbohydrate, two proteins and a placebo up against each other before bed and found nothing; why the pre-sleep protein literature is muscle research rather than sleep research; the difference between an energy deficit and low energy availability; and the body of work on cognitive dietary restraint — the mental effort of not eating, which carries measurable physiological consequences independent of how much is actually eaten. She is careful throughout about how messy cortisol data is, and about which parts of it she thinks still stand. The practical conclusion is unchanged but better founded: if you are wired and under-eating relative to your training, it is probably the calories, and carbohydrate is usually the easiest thing to add back.

Key Topics

Why the mechanism matters even when the advice is fine — Mikki's argument is that people do not just follow advice, they extend it using the mechanism you gave them, so an explanation that isn't real can cause problems in situations you never had in mind.
The tryptophan pathway, and where it breaks — the effect requires carbohydrate high enough and protein low enough for tryptophan to win the competition at the blood-brain barrier. Add meaningful protein and it disappears, and the melatonin generated downstream is nowhere near a dose that influences sleep.
Lower carbohydrate was associated with more slow-wave sleep, not less — a meta-analysis of polysomnography studies found lower carbohydrate intake linked to more slow-wave sleep, less REM and a shorter time to fall asleep. That is the opposite direction to what Mikki had been saying.
The head-to-head trial in elite female athletes — 40 g of alpha-lactalbumin, 40 g of casein, 40 g of carbohydrate or a non-caloric placebo, taken 30 minutes before bed for three nights each. No difference in total sleep time, sleep onset latency, subjective sleep quality, resting heart rate or heart rate variability, and no difference in overnight glucose on CGM.
Pre-sleep protein is muscle research, not sleep research — protein before bed does do what it says for the overnight muscle protein synthetic response. That is a separate question from whether it sedates you, and when sleep is the outcome tested directly it is no better than carbohydrate or nothing.
Why it still works in clinic: it is the calories — the null trial was run in adequately-fed people. Mikki's clinical population is in a deficit and often training hard, so adding food at dinner adds total energy to the day. Carbohydrate is simply the lever she usually reaches for, because protein is normally already handled in a fat-loss meal.
Energy deficit is not the same thing as low energy availability — two women in the same deficit can be in very different positions depending on training volume. Mikki cites the 30 kcal/kg fat-free mass threshold and is explicit that it is not a number everyone agrees on.
Cognitive dietary restraint has physiological consequences — over two years in 123 healthy young women, higher restraint was associated with more cycles showing subclinical ovulatory disturbances (56% versus 34%), and physical activity, general stress, BMI and energy intake did not explain the difference. Related work found higher 24-hour urinary cortisol in high-restraint women, replicated in postmenopausal women aged 49 to 75.
A one-off cortisol test tells you very little — within-person variability is large, the cortisol awakening response has test-retest reliability too poor for individual comparisons, and the research Mikki is drawing on compared groups using 24-hour urinary collections rather than single spot samples. Noise that swamps one person averages out across sixty or a hundred.
What to actually do — if you are wired, under-eating and training, add the calories back. Put a small portion of carbohydrate on a plate whose protein is already covered; if protein is the thing that is low, add protein instead, particularly through midlife. And if fear of evening carbohydrate is the real barrier, add it back and watch what happens — evidence builds confidence.

Chapters

00:00 Where I got the mechanism wrong
02:59 Benton, dose and the sleep studies
04:49 Carbs, casein or placebo before bed
07:12 Why it still works in clinic
08:56 Energy deficit, training load and cortisol
11:39 Fear of carbs and dietary restraint
14:19 How reliable is cortisol testing
16:09 What to actually do

Guest / Resources

This is a solo episode. No guest.

Researchers and work referred to in the episode

David Benton and colleagues, Swansea University — work evaluating the putative mechanisms linking carbohydrate to sleep, and a meta-analysis of polysomnography studies finding lower carbohydrate intake associated with more slow-wave sleep, less REM sleep and shorter sleep onset latency.
Andrew Huberman — Mikki recalls hearing him make the starchy-carbohydrate-and-serotonin case for sleep. She refers to this from memory; no specific episode is identified.
Zach Bitter — mentioned as a low-carbohydrate proponent who reported that higher carbohydrate intake worsened his sleep. An individual account, not a study.
Luc van Loon's group — including Jorn Trommelen and Tim Snijders — the pre-sleep protein and overnight muscle protein synthesis research. Mikki's point is that this body of work is about muscle, not sleep.
Anne Loucks — the work establishing that luteinising hormone pulsatility is disrupted below roughly 30 kcal per kg of fat-free mass per day, after subtracting the energy cost of exercise. Mikki notes in the episode that this threshold is not universally agreed.
Yuko Nakamura and colleagues — a meta-analysis separating the severity of energy restriction, finding that fasting raises cortisol strongly while low-calorie and very-low-calorie diets do not, and that the effect fades over several weeks.
Jennifer Bedford and colleagues — a two-year study of 123 healthy young women, finding higher cognitive dietary restraint associated with more cycles showing subclinical ovulatory disturbances (56% versus 34%), with physical activity, general stress, BMI and energy intake failing to explain the difference.
Judy McLean and colleagues — 24-hour urinary cortisol excretion compared in premenopausal women with high versus low dietary restraint scores, matched for age, BMI and cycle length.
Candice Rideout and colleagues — the same finding in postmenopausal women aged 49 to 75, who did not differ in age, BMI, energy intake, perceived stress or exercise.
James Abelson and colleagues — a study of 140 people providing nine saliva samples a day over six days alongside five hormonal challenge tests, examining how much of the diurnal salivary cortisol curve actually reflects underlying regulatory biology. It supports a focus on the diurnal decline and raises questions about the biological meaning of other parts of the curve, including the cortisol awakening response.

Studies described without attribution in the episode

The randomised, double-blind crossover trial of pre-sleep alpha-lactalbumin, casein, carbohydrate and placebo in elite female athletes, and its continuous glucose monitoring companion.
A mass spectrometry study sampling 20 people six times a day across five days, reporting within-subject cortisol variation of roughly 30 to 57% depending on time of day.
A meta-analysis of the test-retest reliability of the cortisol awakening response.

Terms mentioned

Branched-chain amino acids · tryptophan · serotonin · melatonin · polysomnography · slow-wave sleep · REM · sleep onset latency · heart rate variability · alpha-lactalbumin · casein · continuous glucose monitoring · muscle protein synthesis · muscle glycogen · low energy availability · HPA axis · cognitive dietary restraint · subclinical ovulatory disturbances · cortisol awakening response · 24-hour urinary cortisol · DUTCH test

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Creators and Guests

Host

Mikki Williden

What is Mikkipedia?

Mikkipedia is an exploration in all things health, well being, fitness, food and nutrition. I sit down with scientists, doctors, professors, practitioners and people who have a wealth of experience and have a conversation that takes a deep dive into their area of expertise. I love translating science into a language that people understand, so while some of the conversations will be pretty in-depth, you will come away with some practical tips that can be instigated into your everyday life. I hope you enjoy the show!

Hey everyone, it's Mikki here. You're listening to Mini-Mikkipedia on a Monday, and if you've ever heard me say that carbohydrate at night with dinner helps calm the nervous system and therefore you're able to sleep better, I'm wrong.

I'm not wrong on the clinical outcome of eating carbs at dinner, but I am wrong on the mechanism.

And I have said this several times on this podcast, on other podcasts, I've written it probably at least like 30 times on Instagram over the last year.

When someone tells me that they are wired at night and I say, hey, have some carbohydrate at dinner, it calms the nervous system down.

I suppose it's a shorthand way of saying what I say.

I never really sat down and sort of had a look at the science behind it.

It just matched what I was seeing and it made for a pretty good explanation.

And it wasn't just me saying it. There are loads of people out there.

And I'm not saying that it's wrong to say it, but I do think that the mechanism I've been implying, it just doesn't hold up.

So you could argue that mechanism doesn't really matter that much because the advice attached to it is fine.

And whilst I don't 100% trust it.

Disagree with that, I still don't think that that's right, because when you give someone a mechanism, they don't just follow your advice.

They may extend it and apply it to situations that you weren't even thinking about.

And a mechanism that isn't real may, best case scenario, just be completely useless, but worst case could create harm.

Not in this situation, probably for most people, but in other situations.

And so to be complete about it, I thought that I would do a dive into it.

So, the potential mechanism behind what I was describing, you eat carbohydrate at night with dinner, insulin goes up, insulin clears the branched chain amino acids out of the bloodstream and into muscle.

Tryptophan, another amino acid, is then left with less competition at the blood brain barrier, so more of it gets to the brain and tryptophan converts to serotonin and some serotonin becomes melatonin.

So you feel better.

Feel calm and you sleep.

And at every step of the way that I've just described, that's a real biochemical step, which is why intuitively you would think that that would make sense.

And if I say I'm not the only one saying it, like I remember listening to a Huberman podcast about it too.

Listening to Huberman talk about how starchy carbohydrate helps promote sleep by boosting serotonin and that many people on low-carb diets struggle with sleep due to insufficiency.

And if I say I'm not the only one saying it, like I remember listening to a Huberman podcast about how starchy carbohydrate helps promote sleep by boosting serotonin and that many people on low-carb diets struggle with sleep due to insufficiency.

And it sort of is in line with the idea that fasting and low-carb states induce alertness while carbohydrate-rich foods enhance calmness and sleepiness.

So, you know, all of this is sort of interlinked.

However, the problem is in the dose.

David Benton and his group at Swansea went through this properly, actually.

And their conclusion came out different from what I was anticipating.

The tryptophan effect only happens when carbohydrate intake is high enough and protein intake is low enough that tryptophan wins the competition.

So no normal meal that I would advocate really would do that.

If you add any meaningful protein to a meal, the effect will disappear.

And the amount of melatonin you generate downstream is nowhere near a dose that would influence sleep.

I mean, if you're sitting down to a delicious meal that Dad and I used to have when I was a teenager,

that was, you know, pasta with a tomato-based sauce, basically just coating the pasta, nothing more,

and then garlic bread on the side, you'd probably be fine because there was zero protein.

Well, pasta and bread have some protein, but you get the drift.

A whole lot more carbohydrate than there is protein.

Nowadays, I'm not telling many people to do that.

So the whole mechanism behind it doesn't actually stand up under that sort of scientific scrutiny.

And in fact, Benton's team also ran a meta-analysis.

of polysomography studies, which are sleep studies, looking at the brain during sleep.

Lower carbohydrate intake was associated with more slow-wave sleep,

less rapid eye movement, and a shorter time to fall asleep.

Not only is that the opposite direction to what I was telling you,

but I will say that others have said something similar.

Not many people, but Zach Bitter is one that stands out to me as being a proponent for a low-carb diet.

He noted that a higher carbohydrate intake impacted negatively,

on his sleep, and this was reversed when he became low-carbohydrate back in the day.

So that's super interesting.

There have been other studies looking at carbohydrate around sort of bedtime.

And here's one in younger people.

21 elite female athletes was randomized, double-blinded.

It was a crossover study, and they had four different conditions.

Three nights in each.

They provided the participants with either 40 grams of alpha-lactoalbumin,

which is the whey protein.

The whey fraction highest in tryptophan, often one that is sold for sleep specifically.

You had 40 grams of casein, that slow-release protein that's often recommended.

40 grams of carbohydrate, or a non-caloric placebo.

And these were taken two hours after the last meal, about 30 minutes before bed.

And sleep and recovery were tracked objectively.

And they found, well, absolutely nothing.

There was no difference in total sleep time,

no difference in sleep onset latency,

no difference in subjective sleep quality,

no difference in resting heart rate,

and no difference in heart rate variability.

The same group also put continuous glucose monitors on the participants

and found no difference in overnight glucose between the protein, the carbohydrate, and the placebo.

So that's super interesting, eh?

A head-to-head. Carbohydrate before bed did not beat protein,

and neither of them beat nothing at all.

Although clinically what I will also say is I hear other practitioners swear

by giving pre-sleep protein to help with sleep.

So clinically speaking as well, we've got carbohydrate before sleep appears to help,

and protein before sleep appears to help.

And I've got a reason for why this might be the case, but I'll get to that in a minute.

But what I will actually say though is the pre-sleep protein literature,

since we're talking about that in the previous study,

that is super interesting and all of that work,

Luke Van Luke's group, Tromelin, Snyder's, it's muscle protein synthesis research.

So that research from those groups was never about sleep,

and protein before bed absolutely does what it says on the overnight muscle protein synthetic response.

So I'm certainly not arguing that it's absolutely useless,

but if we're thinking about sleep, if we look at a study particularly testing that theory,

it's no different to carbohydrate and no different to having nothing.

So this gets me to why do we keep seeing that it works?

And I'm not imagining it.

And if you've been told to do it and you've noted that it's helped,

then you're not imagining that either.

And I put it down to a couple of things, if you like.

And the case study that's useful enough to sort of use,

and this is what I see often, is that I will have an active woman

who is improving body composition,

in a calorie deficit,

therefore she's not eating as much as she otherwise would,

she may be training quite a lot,

and she's sleeping very poorly.

She's waking at two in the morning.

So I tell her to put some potato back into dinner.

And within a fortnight or two weeks,

she tells me she's sleeping through and she feels way less wired.

So there you go.

Doesn't that just prove that it calms the nervous system?

The thing is though, is that whilst it helps improve cleanliness,

clinically with the people that I see,

the study that I was referring to before,

where it appeared to do absolutely nothing,

that was in a group of people who were adequately fed.

And they're already eating enough calories.

So any additional calories at dinner or after dinner

isn't going to make much difference.

In clinic and in my fat loss groups,

I'm talking to a calorie restricted group.

So I think that's where the difference lies.

She just eats more.

There is more total energy in that day.

And I've chosen to use carbohydrate.

And the thing that actually drives that wired,

under-slept, high-strung presentation

is that she's now got more calories,

particularly if she's training.

And I do want to mention that as well, right?

Because there's an energy deficit

and there's low energy availability.

And they are actually two different things.

They're often conflated.

So, and Luke's work established

that below 30 kilocalories per kilogram of fat-free mass,

after you subtract the cost of exercise,

this is when the reproductive axis starts getting suppressed.

And above it, it's largely fine.

So a woman can be in a modest deficit

and actually be completely fine,

you know, fairly active and completely fine.

And another woman in the exact same deficit,

eating the same food,

but she may be training double the amount of the other person,

she can be well enough.

She can be well under that threshold

for low energy availability.

And to be fair with that 30 kilocalories per kilogram

of fat-free mass,

that isn't an established number across the board

that everyone agrees with,

but it does give you an indication.

Like when you do dive a little bit deeper into it,

a deficit from a training perspective,

not a food perspective,

it can really impact the HPA axis.

And so that deficit plus the training load

could well be the issue.

And there is something about cortisol as well.

And I will just note that the cortisol data is pretty messy.

And I've talked about this before.

People talk about elevated cortisol

and higher cortisol, et cetera.

So anything I say about cortisol,

do take with a grain of salt

because measuring it is pretty difficult

and there's a lot of critique around it.

However, I will say that, you know,

there has been a meta-analysis by Nakamura

who separated the severity

of a deficit and found that if you fast,

then cortisol rises quite strongly

and that low calorie and even very low calorie diets

don't particularly raise it significantly.

And in fact, the effect fades over several weeks.

So, you know, a sensible deficit

that with someone who does in fact have fat to lose

probably isn't going to wreck you particularly.

But if you're noticing that wide effect

and you add something to your plan,

and typically I say carbohydrate,

it is going to make that meal more complete

because protein is often covered in the meals

that I advocate for fat loss.

And, you know, if you look around the board,

most people are suggesting a higher protein diet with fiber

and moderating your carbohydrate.

So if you're wired,

it makes sense to add in more carbohydrate

rather than up the protein.

Of course, if you do add in protein,

that's likely to have a similar impact

on your ability to sleep,

which really sort of speaks to the fact

of the difference in calories there,

which for the most part actually isn't a lot.

The other thing I think is worth noting

is that, you know, food stops being a threat,

if you like.

So a lot of people, particularly dieting women,

are afraid to add carbohydrate in,

well, one, in general because carbs make you fat,

and two, because we shouldn't eat carbs at night.

I mean, these are the two perpetuating myths

that are out there around carbohydrate.

Now, I'm not suggesting that carbohydrate

can be eaten freely with no considerations at all.

I mean, I think you probably know that as well.

But if you have a fear around eating carbohydrate,

then that is just going to heighten that sort of anxiety,

and you sort of need evidence

that you're not going to be negatively impacted

if you include it in your evening meal.

And there is, of course, a body of literature

around cognitive dietary restraint,

which this is not about how much you eat,

how much you actually eat.

It's that ongoing mental effort of trying not to eat,

the monitoring, the negotiating,

that low-grade vigilance.

And Bedford did a study, actually.

The research group followed 123 healthy young women

for two years and looked at something functional

rather than just a hormone.

So the woman with the higher restraint

had more menstrual cycles

with subclinical ovulatory disturbances,

56% of cycles versus 34%.

And physical activity, general stress,

BMI and energy intake

did not explain the difference.

It was the higher restraint that explained the difference

between that disturbed ovulatory cycle.

And so that is, you know,

a pretty real physiological consequence

attached to how much mental effort

a woman is putting into not eating,

independent of how much she's actually eating.

And 123 people is by no means

a small study in nutrition.

And again, there is other studies

such as Judy McLean's group.

They actually looked at 24-hour urinary cortisol

in women with high versus low restraint scores.

And the high restraint women had higher cortisol,

same age, same BMI, same energy intake,

same food, different cortisol.

That was replicated again by Rideout

in postmenopausal women aged 49 to 75,

which may, you, if you're listening to this,

you may well be falling

into that category.

So while I will still be mindful

of what that cortisol data can or cannot tell us,

the fact that that dietary restraint

is the only difference found

in the difference in cortisol output

between these two groups,

I just think that's worth noting.

What I will just say around that cortisol thing,

because I've talked about it before,

but I will just remind you,

it is pretty difficult to measure.

Within-person variability is enormous.

One study using mass spectrometry

sampled 20 people six times a day across five days

and found that within subject variation

of roughly 30 to 57%,

depending on time of day.

So that's pretty big, right?

And reliability of the individual measures are worse.

So a meta-analysis found that cortisol awakening response,

which is one of the most commonly sold to you,

and look, it's one that I've suggested,

you know, some clients run as part of a Dutch test as well.

It has a test-retest reliability too poor

to use for individual differences at all.

And this study is pretty sobering.

So Elberson's group took 140 people,

collected nine saliva samples a day for six days,

then put them through five actual hormonal challenge tests

to see what the saliva curves were tracking,

and only the afternoon decline

mapped onto real regulatory biology.

So morning levels and the awakening response

largely did not.

What I will just say is that a one-off cortisol test

really will tell you nothing about yourself.

I don't think that that necessarily sinks the studies

that I've just described though.

So the studies compared groups of women

and mostly used 24-hour urinary collections

rather than single-spot samples.

So that's worth noting.

Noise that swamps one person's measurement

averages out across a group of 60 or 100.

So you do get that root level data,

which is quite useful for looking at directions and trends,

but it does mean you can't take your own cortisol result

and conclude anything from it,

particularly not one point in time.

So all of that said,

what does that really mean

in terms of what I might tell a client

or what you should do particularly, et cetera?

So I guess the first thing is,

is that, you know,

macronutrients at bedtime

aren't really going to have a sedative effect.

The head-to-head trial says the macronutrient

in that last thing before bed

is not the liver,

it's not the carbohydrate,

and it isn't the protein.

And if you are in an energy deficit,

then it's probably just the calories.

If you're training hard

and then you're eating like you're not training,

that's probably the problem.

So that's point number two.

And it will show up as sleep

long before it shows up in the scales.

So a small portion of carbohydrate to a plate

that is already taken care of with regards to protein

makes sense in terms of adding back in those calories.

And that will also do service

to your muscle glycogen stores as well.

Three, if your protein is low

and you're in a calorie deficit,

then probably you'll need to add some protein in

for muscle retention,

which matters obviously through midlife,

because it is a calorie deficit.

And if you have a low protein intake overall,

then muscle loss will be accelerated.

So if your protein isn't accounted for,

I'd probably choose protein to put in

rather than carbohydrate.

And, you know,

if you have this sort of longstanding fear

of carbohydrate at night

because you're worried that it will make you fat

because that is what you've heard on the internet,

and I still see this like on the daily on Facebook,

then I would just say, you know,

evidence builds confidence.

Add back that carbohydrate

and just note how you feel

and note how you sleep.

This is particularly true

if you're in a calorie deficit.

And I'd be very surprised if it had a negative impact

and for anything it could be helpful.

So that's sort of the take home of it

is that the wired but tired feeling that you feel,

particularly in a calorie deficit when you're training,

is largely due to the calorie deficit itself

and not an absence of any particular nutrient.

But if carbohydrate is the thing that we've been restricting

and quite often it is

because we look at those energy nutrients,

then pop the carbs in in the evening meal.

And that will, I've seen it happen,

help you wind up,

wind down and probably sleep better, right?

And if for whatever reason you're restricting protein,

make that the move that you make.

Add in some protein.

So hopefully that clears things up.

Not sure if I did,

but thanks for listening as always.

You can find me over on Instagram,

ThreadsNX,

at Mikki Williden,

Facebook,

at Mikki Williden Nutrition,

or head to my website,

mikkiwilliden.com

and book a one-on-one call with me.

All right, guys.

You have the best week.

See you later.

Thank you.

See you later.

Bye.