[Rhonda]: I think people may be confused by this time-restricted feeding, which is essentially,
you know, feeding within our active hours.
[Sachin] Yeah.
[Rhonda]: The daylight hours, and intermittent fasting.
So there's obviously some overlap between the two because if you're, let's say, you're
feeding within a 12-hour period.
So you wake up, you have your first sip of coffee, that starts your clock.
All right.
That's it.
So, 8:00, then you better stop eating by 8 p.m.
Right?
That's for 12 hours.
[Satchin]: Yeah, yeah.
[Rhonda]: Then from 8 p.m. all the way till 8 a.m. the next morning, you're fasting, right?
[Satchin]: Yeah, yeah.
[Rhonda]: You're not getting any energy.
[Satchin]: Yeah.
[Rhonda]: So, so in some ways you're getting a lot of...there are some overlap between
this time-restricted feeding and intermittent fasting, for example, which has been shown
to increase ketone bodies like beta-hydroxybutarate, which I know you've also shown restricted
feeding does increase that as well.
It takes around, I think, 10 to 12 hours...
[Satchin]: Yeah.
[Rhonda]: ...for your liver glycogen to deplete and fatty acids get immobilized, they go to
the liver, you start to make beta-hydroxybutyrate and other ketone bodies which then get transported
to other tissues and are used for energy in the brain, or they act as signaling molecules.
[Satchin]: Yeah.
[Rhonda]: Which Eric Verdin at UCSF published.
There's lots of...oh, have...can you, first of all, differentiate for people, like, the
difference between intermittent fasting and time-restricted feeding?
Like, what are the main differences, and maybe what some of the similarities are?
[Satchin]: Well, both of these depend on this idea, as the commonality is this prolonged
period of fasting.
When I say prolonged, that's usually longer than six to eight hours because that's how
long it takes for glycogen to deplete or maybe the fatty oxidation to begin so that we begin
to use some of the fat.
And you also pointed out ketone bodies and beta-hydroxybutyrate, those are also produced
maybe after 8 to 10 hours.
So the bottom line is this, that is when we eat we have a type of physiology where we're
using glucose and we're driving some bodily function.
And at the same time, we may be also damaging some cellular components because of all the
reactive oxygen species that we generate during eating, during metabolizing all of this.
So all of these have to be repaired, and for some reason, we do not know why, the repair
mechanism happens only during the period of fasting.
And during this period of fasting, we switch to a different kind of metabolism.
Just like you said, our primary energy source is not the readily available glucose from
food anymore.
It has to come from different sources.
In some cases it can come from a little bit of protein, that's gluconeogenesis or from
fat oxidation or, just like you said, ketone bodies.
So these things, this physiology, the fasting physiology, we actually know...we are just
seeing the tip of the iceberg of fasting physiology.
We're just learning about a very few molecules.
We don't know what happens to lot of signaling molecules, how the mitochondria actually repair
during fasting.
Is it actually necessary, to why some repairs happen only during fasting.
Why can't they happen when we're eating?
So all of these questions are out there but what is common between this intermittent fasting
and time-restricted feeding is this fasting physiology that we're beginning to understand.
The reason why we coined and use the word time-restricted feeding is we are not restricting
calories, at least in experimental animals.
So in that way the intermittent fasting came from calorie restriction, and every other
day feeding that had a serious component of caloric restriction that many people thought
is difficult to achieve.
So that's why we stayed away from the word caloric restriction or fasting, and we used
the word feeding because people thought, people may have a positive attitude towards it.
Other than that, I think the idea of fasting is ingrained in evolution.
Just like in circadian rhythm, the animals have access to food only during their awake
time which can be less than 12 hours, and also for diurnal animals, which are hunter-gatherers,
the only time, actually, they have to hunt is twilight time because if you ever go to
Savannah, or any of the African countries where there is still wild animals, or if you
go to even a zoo, then you know that animals are not active in the middle of the day.
They're mostly active during morning and evening.
So in nature, animals actually have only two chances to eat, and the rest of the time they're
fasting.
So this fasting physiology is a very natural response to repair and rejuvenate, and in
time-restricted feeding, we're kind of exploiting, or we're kind of bringing back that primordial
physiology that's ingrained in our genome, that our genome has to respond to that fasting
on a daily basis.
At the same time it syncs with another aspect of the genome; that is it helps us stay awake
for 10 to 12 hours and to reduce our energy level and go into a sleep or less active state
for the rest of the day.
So in that way, it brings back the primordial rhythms in our physiology, metabolism, repair
and rejuvenation, whereas intermittent fasting actually helped us to learn various basis
for this fasting physiology.
[Rhonda]: That makes sense.