intermittent fasting is incredibly popular. people report weight loss, improved glucose metabolism,
lower cholesterol, the works. but are those results just a consequence of eating less calories overall
or is there a specific benefit to fasting? in other words, if i just eat a little bit less every day
is that just as good as fasting or does fasting itself deliver some special metabolic benefit? many
of the podcasts and popular books out there will promise all kinds of specific benefits to fasting
they'll tell you that there is something special about it, something evolutionary, the wonderful
benefits of autophagy, and therefore that we should all fast to get those benefits and if we don't
we're leaving gains on the table. now, we made a whole video on intermittent fasting a while back
and in preparation for that i went over all the existing literature at the time, this was like two
years ago, and what i found was that in studies that match for calories, intermittent fasting
didn't seem any better than just eating a little bit less calories daily without fasting. so who's
right after all? what's the truth? well, recently a new study came out trying to settle precisely this
question, and it was designed in a very smart way, first they watched the volunteers for four weeks
and recorded everything they ate, just their normal baseline diet. then they took some of the
volunteers and they asked them to start fasting and they used what's called alternate day fasting
or ADF, basically you fast one day, you don't eat anything, you can drink water or non-caloric drinks
next day you can eat whatever you want, the day after that you fast again and so on. but on the
eating days they asked them to eat exactly double their normal baseline intake, that way they're
fasting 50% of the time, every other day, so it's substantial but there's no overall caloric deficit
they're still eating the same total number of calories they were eating before so if fasting
delivers some special benefit aside from calories we should see it in this group but if it's all
about calories there should be no change. and what they found was that this group didn't lose
any weight, didn't lose any fat mass and their metabolic parameters also stayed the same.
alright but eating twice as much every other day is pretty intense. that's a lot of stuffing yourself
people fasting probably will compensate a bit on the eating days but maybe not that much? so
they did another experiment, they asked another group to start fasting, ADF, alternate day fasting
every other day, but they asked them to eat 150% of normal on the eating days, so a bit more than
normal but not enough to completely compensate for the fasting so they're still getting less calories
than at baseline, they're eating 75% of normal baseline intake, right? zero on the
fasting days, 150% on the eating days, averaged out, 75% right? so they're cutting back 25% of calories
which is pretty substantial. and they compared this group to another group that started eating 25%
less calories every day without fasting. other than, you know, sleep. so both groups eating 25% less
calories than at baseline but one is fasting, one is not, if fasting is something special we should
see an advantage of the fasting group, if it's all about calories they should look about the same. and
they found that both groups lost some weight, not surprising, they're both eating less than before,
and the amount was about the same, no significant difference. so again, at face value suggesting that
it's mostly about the calories, not fasting per se. but then they looked beyond just weight loss
because nobody wants to lose weight, what we want to lose is fat, extra fat mass. so the group eating
a little bit less every day without fasting, almost all the weight they lost was fat mass,
which is good, but the intermittent fasting group, only half the weight they lost was fat
the other half was lean mass, so this result indicated that in these conditions the fasting
protocol actually inhibited fat loss. as the researchers state,
alternate day fasting less effectively reduces body fat than matched energy restriction, without
evidence of fasting-specific effects on metabolic regulation or cardiovascular health. now, why would
they lose less fat if they're eating the same total amount of calories? well, they found that the
fasted group that was on 75% of calories had less physical activity overall. this isn't exercise, it's
just general physical activity like moving around or general body movement. that got reduced in that
group. the investigators speculate that there's a subconscious process reducing spontaneous movement
in those conditions of fasting coupled with some caloric restriction and weight loss. so that's not
an absolute limitation of fasting, we could just make sure to include more physical activity, like
for example exercise more or even just move around more. in their own words, the main practical message
for individuals to consider if planning to use alternate day fasting is to consciously insert
opportunities for physical activity. so, should be possible to work around it. we'll come back
to this in a second and to that issue of the loss of lean mass, but this was the gist of the study, it
was a very flashy study, got a lot of press, but we always want to be cautious with isolated studies,
maybe there was something about the conditions that make it an outlier, so we always want to ask,
is this reflective of all the available evidence? with regard to intermittent fasting not providing
a special benefit beyond calories, yes. for example, this meta-analysis looked at 11 trials comparing
intermittent fasting to continuous energy restriction. so basically just eating a little
bit less every day but not fasting per se. and it found no apparent superiority of a dietary regimen
so intermittent fasting was fine, there was weight loss but it was identical to eating a little bit
less every day. another meta-analysis looked at six trials and it found that intermittent fasting was
comparable to continuous energy restriction for short-term weight loss in overweight and obese
adults, and duration of the studies was 3 to 12 months. and this review of the field concludes: the
efficacy of intermittent fasting appears broadly equivalent to standard daily energy restriction
and the results for other metabolic parameters, like changes in glucose metabolism or blood lipids,
also follow this trend, because they usually track with overall caloric intake and weight loss. now,
here's an interesting tidbit: on occasion i've seen isolated studies shared online as evidence
that intermittent fasting does provide a specific independent benefit. for example this randomized
controlled trial from 2019. they had one group on intermittent fasting and another continuously
eating less and they did try to match for calories, and they report that the intermittent fasting
group showed greater reductions in weight, fat mass, cholesterol, etc. so it sounds really impressive, and
i've even seen this study mentioned in reviews as an outlier, as an example of a study where
intermittent fasting did deliver some benefits above and beyond just calories, but when i dug into
the details, when i read it carefully, it does show the estimated caloric intake and the intermittent
fasting group did end up eating significantly less calories, so this study actually turns out
not to be an exception, it's actually totally aligned with the idea that caloric intake explains
the differences, so just a little aside that this is very common, there's a study that seems to be
an exception and clashes with everything and maybe everything we thought we knew was wrong
and then you look into the details and not only is there no contradiction but the odd study actually
ends up perfectly aligning with everything we know. very common. and there are some other studies like
that in the literature and some meta-analyses that show better results on intermittent fasting
when calories are not matched or when the caloric intake is not reported. one example is the matador
trial, which had a really interesting design and found more weight loss on intermittent fasting but
unfortunately they don't report actual caloric intake so it's hard to draw a firm conclusion. okay,
what about that loss of lean mass on intermittent fasting, is that a concern? the first thing to say
is that lean mass isn't necessarily muscle. it can be, but it could also be water or glycogen. most
of these studies don't really differentiate. the second thing is that the loss of lean mass didn't
reach statistical significance in the recent study we just looked at, there was a lot of variation so
it didn't pass statistical tests. but there are some meta-analyses, like this one for example,
that do find a significant reduction in lean mass on alternate day fasting. but i've also
seen several others that don't show a reduction in lean mass, so it's not consistent, it depends
on the specific conditions, so this is something to bear in mind for people doing intermittent fasting,
especially the longer forms like 24 hours, like in ADF, or even longer. now, there are some potential
strategies to get around it. because that's what we do here, we bring solutions to the table. so one
solution to retain muscle mass while fasting is resistance training, basically weight lifting or
just challenging your muscles somehow. in fact a systematic review indicates this might work
fasting paired with resistance training generally maintained lean body mass. so
that's good news. another thing that may help is making sure that we get adequate protein intake
fasting can mean less calories which can mean less protein sometimes, so we want to make sure
we get enough. and yet another strategy is to have some food on the off days, instead of being
100% fast, having a couple hundred calories on those days and making it a protein rich food to give
your muscles that anabolic signal. so what's the bottom line here? does intermittent fasting work?
well, it depends what we mean by work, if we mean, can it help with weight loss? absolutely. that's
been shown many times. is it going to work for every single person? no. but it can work, it's an
option on the table. now, if by work we mean a special benefit that only fasting can give you
like the marketing often implies, then the answer is probably not. the available evidence is pretty
consistent that you get the same benefits whether you eat a little bit less every day or you stop
eating for longer chunks of time. when you match for calories, they look pretty similar, so whatever
works for you. and by the way, i wish it was special, i wish it was some magic ancestral switch, out of
all the internet dietary trends i've seen this is probably my favorite. it doesn't involve anything
crazy, it doesn't involve eating any unhealthy foods, it's simple, it's free, you don't have to
pay anybody, you just stop eating, i love it. they basically found a way to get westerners to eat
a little bit less. i'm all about it. and if the evidence changes tomorrow and we find a specific
way to do it that does deliver a special benefit you can't get anywhere else and we start seeing
study after study and meta-analyses convincingly showing it, i'll make a video about it the same day
and i'll start fasting, duh. until then, it's another option on the table for people who like, it a valid
option among many. and in fact this is how it's usually described in the medical literature
intermittent fasting is a valid but not superior alternative to caloric restriction. the language is
dispassionate when they're objectively looking at the data, they're not trying to sell you anything
another way to look at it is that intermittent fasting may provide an alternative approach
for individuals who struggle with daily energy restriction. so option on the table for those
who prefer it, great. now in terms of individual variability, as we said earlier, in some studies
where calories aren't matched or intake isn't reported we sometimes do see intermittent fasting
work better, people often don't eat enough on the on days to completely compensate for the fast so
they end up being overall calorically restricted and losing weight. and that's good, that's what most
people want, but in some other studies we don't see that and fasting does not lead to any weight
loss. for example, this meta-analysis looked at eight intermittent fasting studies and found
body fat was reduced in five of those eight so it depends how we do it, just like everything else,
so if you figured out a setup that works for you, phenomenal, in fact share it below, might
help other people. yeah it's anecdotal but maybe it'll work for some others as well. it's the same
for all of these approaches. low fat, low carb, low calorie, intermittent fasting, they can all work for
some people and then there's always some other people who don't do well in each one of those
which is why it's great that we have all of these possibilities. if one thing doesn't work for you
try something else. what we have to be careful about is the marketing hype
and this sales pitch that some guy figured out the hidden secrets of weight loss
nope, he didn't. and this religious fanaticism that the thing that worked for me is special and it has
to work for everybody else and if it doesn't work for you then you're doing something wrong
sometimes people get offended because this thing that saved their life, we're saying it's just like
the others, it feels like a personal attack. but we're not taking anything away from your
experience, if it worked for you just stick with it, there's no need to extrapolate and make these
sweeping claims, that requires robust scientific evidence. now, often the hype has a nugget of truth
with intermittent fasting a lot of it is based on animal research because in mice fasting does
provide special benefits beyond calories and a lot of the internet messaging piggybacks on
that and suggests it translates to humans. a lot of this excitement around autophagy, for example,
comes straight from animal research, but it's pointless to speculate if intermittent fasting
delivers benefits through autophagy or this gene or that biochemical pathway
if the human trials don't show a specific benefit in the first place. it's all superseded by human
trials and those keep showing that when we measure caloric intake and account for it
intermittent fasting doesn't offer any independent benefits on top of cutting calories. if new data
comes tomorrow, phenomenal, but right now this is where we're at. these investigators spell it out:
the hypothesized benefits of intermittent fasting, extensively studied in animal models,
include preferential reduction of adipose mass, increased insulin sensitivity, reduced inflammation,
autophagy, apoptosis. they might all be possible, and many of these have been demonstrated in
animal models, but not in humans. so unlike the very promising data in animals, the evidence is
not sufficiently robust to suggest superiority of intermittent fasting when compared to continuous
caloric restriction on cardiovascular factors in humans. so throwing sciency words at the public
is great to sell books, it's great to sound smart, but when it comes to actionable advice,
stuff that people can implement and get results, who cares about autophagy? are you going to go
into your kitchen and activate autophagy? would it cause net harm or benefit? we have no idea, so
this is a complete distraction. we focus on things that are actionable and shown to work reproducibly
in humans. it's fine to speculate out of academic interest and talk about basic research, i mean,
nerd power all day long, i love it, but the logic leap that we should all go do something
because it worked in rats, that's not compelling, so just learn to spot that pattern, it's usually
a big red flag. these guys go even further: bluntly, the number of diet books advising how
intermittent fasting can be incorporated into our lives is several orders of magnitude greater
than the number of scientific papers examining whether intermittent fasting should be encouraged
at all. that's a perfect description of hype. all right, last thing to say is that there is
a difference between fasting and meal timing, what time of day you eat. unlike fasting, meal
timing does seem to deliver some specific benefits beyond just calories and we covered all of that
here. and here's our first video on intermittent fasting from a while back for some more background
information. take care, see you next time. all the fasting talk made me hungry