summarize "Zone 2 output is highly variable, depending on one’s fitness. In San Millán
and Brooks’s study, the professional cyclists produced about three hundred
watts of power in zone 2, while the sedentary, metabolically unhealthy
subjects could generate only about one hundred watts at the same relative
level of intensity. That’s a huge difference. If we express this output in terms
of watts per kilogram of body weight, the difference becomes even more
stark: The seventy-kilogram cyclists put out more than four watts per
kilogram of body weight, while the one-hundred-plus kilogram sedentary
subjects could only manage about one watt per kilogram.
This pronounced difference comes back to the fact that the unhealthy
subjects’ mitochondria—their engine(s)—were much less efficient than those
of the athletes, so they very quickly switched over from aerobic respiration,
burning fat and glucose in the mitochondria with oxygen, to the much less
efficient glycolysis, an energy-producing pathway that consumes only glucose
and produces loads of lactate (similar to the way cancer cells produce energy,
via the Warburg effect). Once we start producing energy this way, lactate
accumulates and our effort quickly becomes unsustainable. There are other
(fortunately rare) genetic diseases that target the mitochondria and produce
far more severe sequelae, but in terms of mass-acquired chronic conditions,
type 2 diabetes does a real number on the mitochondria, and San Millán’s
data very elegantly demonstrate the disability that it creates.
Even when we are at rest, our lactate levels tell us much about our
metabolic health. People with obesity or other metabolic problems will tend
to have much higher resting lactate levels, a clear sign that their mitochondria
are not functioning optimally, because they are already working too hard just
to maintain baseline energy levels. This means that they are relying almost
totally on glucose (or glycogen) for all their energy needs—and that they are
totally unable to access their fat stores. It seems unjust, but the people who
most need to burn their fat, the people with the most of it, are unable to
unlock virtually any of that fat to use as energy, while the lean, well-trained
professional athletes are able to do so easily because they possess greater
metabolic flexibility (and healthier mitochondria).
[*2]
—
Mitochondrial health becomes especially important as we grow older, because
one of the most significant hallmarks of aging is a decline in the number and
quality of our mitochondria. But the decline is not necessarily a one-way
street. Mitochondria are incredibly plastic, and when we do aerobic exercise,
it stimulates the creation of many new and more efficient mitochondria
through a process called mitochondrial biogenesis, while eliminating ones that
have become dysfunctional via a recycling process called mitophagy (which is
like autophagy, touched on in chapter 5, but for mitochondria). A person who
exercises frequently in zone 2 is improving their mitochondria with every run,
swim, or bike ride. But if you don’t use them, you lose them.
This is another reason why zone 2 is such a powerful mediator of
metabolic health and glucose homeostasis. Muscle is the largest glycogen
storage sink in the body, and as we create more mitochondria, we greatly
increase our capacity for disposing of that stored fuel, rather than having it
end up as fat or remaining in our plasma. Chronic blood glucose elevations
damage organs from our heart to our brain to our kidneys and nearly
everything in between—even contributing to erectile dysfunction in men.
Studies have found that while we are exercising, our overall glucose uptake
increases as much as one-hundred-fold compared to when we are at rest.
What’s interesting is that this glucose uptake occurs via multiple pathways.
There is the usual, insulin-signaled way that we’re familiar with, but exercise
also activates other pathways, including one called non-insulin-mediated
glucose uptake, or NIMGU, where glucose is transported directly across the
cell membrane without insulin being involved at all.
This in turn explains why exercise, especially in zone 2, can be so
effective in managing both type 1 and type 2 diabetes: It enables the body to
essentially bypass insulin resistance in the muscles to draw down blood
glucose levels. I have one patient with type 1 diabetes, meaning he produces
zero insulin, who keeps his glucose in check almost entirely by walking
briskly for six to ten miles every day, and sometimes more. As he walks, his
muscle cells are vacuuming glucose out of his bloodstream via NIMGU. He
still needs to inject himself with insulin, but only a tiny fraction of the amount
that he would otherwise require.
One other plus of zone 2 is that it is very easy to do, even for someone
who has been sedentary. For some people, a brisk walk might get them into
zone 2; for those in better condition, zone 2 means walking uphill. There are
many different ways to do it: you can ride a stationary bicycle at the gym, or
walk or jog or run around the track at the local high school, or swim some
laps in the pool. The key is to find an activity that fits into your lifestyle, that
you enjoy doing, and that enables you to work at a steady pace that meets the
zone 2 test: You’re able to talk in full sentences, but just barely.
How much zone 2 training you need depends on who you are. Someone
who is just being introduced to this type of training will derive enormous
benefit from even two 30-minute sessions per week to start with. Based on
multiple discussions with San Millán and other exercise physiologists, it
seems that about three hours per week of zone 2, or four 45-minute sessions,
is the minimum required for most people to derive a benefit and make
improvements, once you get over the initial hump of trying it for the first
time. (People who are training for major endurance events, such as running a
marathon, obviously need to do more than this.) I am so persuaded of the
benefits of zone 2 that it has become a cornerstone of my training plan. Four
times a week, I will spend about an hour riding my stationary bike at my zone
2 threshold.
One way to track your progression in zone 2 is to measure your output in
watts at this level of intensity. (Many stationary bikes can measure your
wattage as you ride.) You take your average wattage output for a zone 2
session and divide it by your weight to get your watts per kilogram, which is
the number we care about. So if you weigh 60 kilos (about 132 pounds) and
can generate 125 watts in zone 2, that works out to a bit more than 2
watts/kg, which is about what one would expect from a reasonably fit person.
These are rough benchmarks, but someone who is very fit will be able to
produce 3 watts/kg, while professional cyclists put out 4 watts/kg and up. It’s
not the number that matters, but how much you are improving over time. (If
you’re a runner or a walker, the same principle applies: As you improve, your
zone 2 pace will get faster.)
Zone 2 can be a bit boring on its own, so I typically use the time to listen
to podcasts or audiobooks, or just think about issues that I’m working on—a
side benefit of zone 2 is that it also helps with cognition, by increasing
cerebral blood flow and by stimulating the production of BDNF, brainderived neurotrophic factor, which we touched on earlier. This is another
reason why zone 2 is such an important part of our Alzheimer’s disease
prevention program.
I think of zone 2 as akin to building a foundation for a house. Most
people will never see it, but it is nevertheless important work that helps
support virtually everything else we do, in our exercise regimen and in our
lives.
"