The key thing for your viewers to understand is that when they see
intermittent fasting, intermittent fasting is an eating pattern.
It's not a diet.
Oftentimes, in kind of the lay press and so on, intermittent
fasting will be lumped in as a diet.
But it's not a diet.
It's an eating pattern.
A diet is what you eat and how much you eat.
Intermittent fasting is an eating pattern that includes intermittent periods of not
eating, sufficient to deplete the glucose stores in the liver and cause a switch
to the use of fat from your fat cells and the ketones produced from those fat cells.
Right.
So if a person eats breakfast, lunch, and dinner, and doesn't get much
exercise, every time they eat they're replenishing the glucose stores in
the liver and they may never tap in to the fat stores and, therefore, their
ketone levels will remain low because the metabolic switch hasn't occurred.
It typically takes at least 10 hours to deplete the glucose in the liver.
So if a person eats breakfast, lunch, and dinner, and then has a snack
around 8:00 or 9:00, you know, they may get up and eat breakfast and have
not depleted the energy in their liver and have not switched to using fat.
So, with intermittent fasting, the approaches that have been used in
experimental studies, both in animals and in humans, are as follows:
One, is called daily time-restricted eating, where the time window that
one eats is compressed into, say, a six to eight-hour time period.
So, that means the person would be fasting for 16 to 18 hours,
which is sufficient time for this metabolic switch to have occurred.
And scientists think, based on a lot of data, that this metabolic switching
is important for health benefits of intermittent fasting, but also maybe
even of exercise in some instances.
So, the daily time-restricted eating is one approach that can
result in daily metabolic switching, daily elevation in ketones.
So, for example, if a person skips breakfast, eats all their
food between noon and 6 pm.
And if they were to measure their ketones, they'll find that in the
morning, they'll start to be elevated.
Okay.
If they wake up and go for a run in the morning, they're already at or in, the
metabolic switch has already occurred.
So, they can actually enhance the effect of the fasting in
terms of elevation of ketones.
And we think, in terms of beneficial effects on the brain, the cardiovascular
system, perhaps even physical performance, which is an area that is being studied now
with intermittent fasting, there's very strong evidence that compared to three
meals a day plus snacks, intermittent fasting is beneficial for the heart,
and the brain, glucose regulation.
But it's not completely clear yet whether it's beneficial for athletic performance.
There's a lot of interest in that.
Okay.
So, let's get back to intermittent fasting eating patterns.
Another intermittent fasting eating pattern is one that's now
called 5:2 intermittent fasting.
And this is where the individual, 2 days a week, they'll only eat
one moderate sized meal of, say, 600 calories in those 2 days, then
the other 5 days they eat normally.
And so, in that case, 5:2 intermittent fasting, the person will have the
metabolic switch occurring two days a week, but not the other five.
In fact, this 5:2 intermittent fasting, in a sense, kind of triggered the
popularization of intermittent fasting.
And I'll just take a few minutes to kind of give a historical perspective.
We'd done a lot of work in the 1990s and early 2000s showing
that intermittent fasting was beneficial for the brains of animals.
And we can talk about that, some details on that.
And then, we'd also publish some work on intermittent fasting reducing
resting heart rate and blood pressure and having anti-inflammatory effects.
And then, I was approached by a number of clinical investigators.
One was Jim Johnson, who worked with asthma patients.
And, in 2007, we published a small study where we found that...
These were overweight asthma patients, and we put them on a really
rigorous regimen where every other day, they only ate 400 calories.
So, that's not something that can be maintained as a lifestyle
because it's very hard to maintain your body weight with that.
But in these overweight asthma patients, over two months, it had profound
beneficial effects in improving their symptoms, their air flow in their lungs.
My lab, we measured indicators of oxidative stress and inflammation in
the blood, which went down not right away but between two and four weeks
of initiating that every-other-day metabolic switching regimen.
Okay. So, we published that study.
Then, I was approached by Michelle Harvie, who was in England, and
works with women at risk for breast cancer because they're overweight
and they also have a family history.
And she came to my lab, she'd seen our work in animals.
And there's also some work in animals suggesting that fasting can be
beneficial in suppressing cancer growth.
So, anyway, Michelle and I designed a study where we took these 100 women,
and we randomly assigned them to either what's now called 5:2 intermittent
fasting or we had a control group where we had them eat breakfast, lunch and
dinner, but each meal had 25% fewer calories than they normally take in.
And because we had done a calculation that the long-term calorie intake would
be similar in the two groups, the group eating 600 calories two days a week
versus the group eating three meals every day but reducing their calorie intake.
Over six months, both groups of women lost about 8% of their initial body weight.
And both groups had improvements in glucose regulation and
other health indicators.
But the women on 5:2 intermittent fasting had a greater improvement in insulin
sensitivity and lost more belly fat compared to the group that was counting
calories, if you will, every meal.