Morning
exercise has very different effects on metabolism than
the same workout later in the day, according to an ambitious new animal study
of exercise timing. The study, which involved healthy lab mice jogging on tiny
treadmills, mapped hundreds of disparities in the numbers and activities of
molecules and genes throughout the rodents’ bodies, depending on whether they
ran first thing in the morning or deeper in the evening.
اضافة اعلان
Many of these changes related to fat burning and other aspects
of the animals’ metabolisms. Over time, such changes could substantially
influence their disease risks and well-being. And though the study featured
rodents, its findings likely have relevance for any of us who wonder if it is
better to work out before work, or if we might get as much — or more — health
benefit from after-hours exercise.
As anyone with a body knows, our internal operations and those
of almost all living creatures follow a well-orchestrated and pervasive 24-hour
circadian rhythm. Recent studies in animals and people show that almost every
cell in our bodies contains a version of a molecular clock that coordinates
with a broader, full-body timing system to direct most biological operations.
Thanks to these internal clocks, our body temperature, blood sugar, blood pressure,
hunger, heart rate, hormone levels, sleepiness, cell division, energy
expenditure and many other processes surge and slow in repeated patterns
throughout the day.
These internal rhythms, while predictable, are also malleable.
Our internal clocks can recalibrate themselves, research shows, based on
complex cues from inside and outside of us. In particular, they respond to
light and dark but are likewise affected by our sleep habits and when we eat.
Recent research suggests that the time of day that we exercise
also tunes our internal clocks. In past studies in mice, running at different
hours affected the animals’ body temperatures, cardiac function and energy
expenditure throughout the day and altered the activity of genes related to
circadian rhythm and aging.
Results in people have been inconsistent, though. In a small
2019 study of men who joined an exercise program to lose weight, for instance,
those who worked out in the morning shed more pounds than those exercising
later in the day, even though everyone completed the same exercise routine. But
in a 2020 study, men at high risk for Type 2 diabetes who began exercising
three times a week developed better insulin sensitivity and blood-sugar control
if they worked out in the afternoon than in the morning. Those results echoed
similar findings from 2019, in which men with Type 2 diabetes who worked out
intensely first thing in the morning showed unexpected and undesirable spikes
in their blood-sugar levels following the exercise, while the same workouts in
the afternoon improved their blood-sugar control.
Few of these studies ventured deep beneath the surface, though,
to look into the molecular changes driving the health and circadian outcomes,
which might help to explain some of the discrepancies from one study to the
next. Those experiments that did examine exercise’s effects on a microscopic
level, usually in mice, tended to concentrate on a single tissue, such as blood
or muscle. But scientists who study physical activity, metabolism and
chronobiology suspected the impacts of exercise timing would extend to many
other parts of the body and involve intricate interplay between multiple cells
and organs.
So, for the new study, published this month as the cover article
in Cell Metabolism, an international consortium of researchers decided to try
quantifying almost every molecular change related to metabolism that occurs
during exercise at different times of day. Using healthy, male mice, they had
some jog moderately on wheels for an hour early in the day and others run the
same amount in the evening. An additional group of mice sat on locked wheels
for an hour during these same times and served as a sedentary control group.
Beginning about an hour after the workouts, researchers took
repeated samples from each animal’s muscle, liver, heart, hypothalamus, white
fat, brown fat and blood and used sophisticated machinery to identify and
enumerate almost every molecule in those tissues related to energy usage. They
also checked markers of activity from genes related to metabolism. Then they
tabulated totals between the tissues and between the groups of mice.
Interesting patterns emerged. Since mice are nocturnal, they
wake and grow active in the evening and prepare to sleep in the morning, a
schedule opposite of ours (unless we are vampires or teenagers). When the mice
jogged at the start of their active time — equivalent to morning for us — the
researchers counted hundreds of molecules that increased or dropped in number
after the exercise, and that differed from levels seen in mice running closer
to their bedtimes or not exercising at all.
Furthermore, some of these changes occurred almost identically
in different parts of the body, suggesting to the researchers that various
organs and tissues were, in effect, communicating with one another. The
rodents’ muscles and livers, for instance, shared many molecular changes when
the animals ran in their morning, but fewer when they jogged soon before bed.
“It was quite remarkable” to see how extensively exercise timing
affected the levels and activities of so many molecules throughout the animals’
bodies, said Juleen Zierath, a professor of clinical integrative physiology at
the Karolinska Institute in Stockholm, Sweden, and executive director of the
Novo Nordisk Foundation Center for Basic Metabolic Research at the University
of Copenhagen, who oversaw the new study.
Overall, the differences in molecular profiles between morning
workouts (in mouse terms) and those later in their days tended to signal
greater reliance on fat than blood sugar to fuel the early exercise. The
opposite occurred when the mice ran in their evening. If those patterns held
true in people, it might suggest morning exercise contributes more to fat loss,
whereas late-day workouts might be better for blood-sugar control.
But mice are not people, and we do not know yet if the molecular
patterns hold true in us. The study’s researchers are working on a comparable
experiment involving people, Zierath said.
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