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wo teams of researchers analyzing
different aspects of exercise in mice found that the time of day might affect
the productivity of physical activity.
Research in mice shows that time of
day affects the effectiveness of exercise, but applying these results to humans
is complicated.
Scientists already know that the
circadian rhythm interacts with our metabolism. A person's circadian rhythm
includes physical, mental, and behavioral changes that follow a cycle of 24
hours.
These behavioral patterns develop in
response to light and darkness and relate to the circadian clock, which follows
the solar time. Circadian rhythms are present in most living things.
Two teams of researchers decided to
explore how the time of day can affect the body's response to exercise.
Gad Asher, who works in the
Department of Biomolecular Sciences at the Weizmann Institute of Science in
Rehovot, Israel, is senior author of the first study, while Paolo Sassone-Corsi
of the Center for Epigenetics and Metabolism at the University of California
(UC), Irvine, is senior author of the second.
"It's quite well known that
almost every aspect of our physiology and
metabolism is dictated by the circadian clock," notes Asher.
"Previous studies from our lab
have suggested that at least 50% of our metabolism is circadian, and 50% of the
metabolites in our body oscillate based on the circadian cycle. It makes sense
that exercise would be one of the things that's impacted," says
Sassone-Corsi.
Examining mice's response to exercise
The two studies confirm that the
circadian rhythm plays an essential role in the way that the body responds to
physical movement. Although each team investigated a different component of
exercise, the two studies complement each other.
Both teams explored the link between
the time of day and exercise performance in mice. These creatures are
nocturnal, so to make the results relatable to humans, the researchers had to
focus on the active and resting phases of the mice rather than the time on the
clock.
In the first study, the results of
which feature in Cell Metabolism, Asher and
team compared the exercise performance of mice at different times of the day by
putting them in treadmills during their active phase. The mice performed better
in the later stages of this phase, meaning that the "mouse evening"
was a better time for them to exercise.
In the mouse evening, levels of a
compound called 5-aminoimidazole-4-carboxamide ribonucleotide (ZMP) were
higher. ZMP is necessary for metabolism because it activates metabolic pathways
that lead to the breakdown of glucose and fatty acids.
This breakdown relies on the
activation of AMPK, a master cellular metabolic regulator. The study findings
suggest that ZMP may play a part in increasing exercise capacity in the
evening.
"Interestingly,
ZMP is an endogenous analog of AICAR (aminoimidazole carboxamide riboside), a
compound that some athletes use for doping," says Asher.
The researchers built on their
findings by analyzing exercise performance in 12 humans. Using oxygen
consumption as a measure of exercise efficiency, they concluded that the
participants also had better exercise performance in the evening than in the
morning.
Studying how exercise changes muscle
Sassone-Corsi and team also
evaluated the performance of mice on treadmills, but they focused on the
changes that exercise produced in the mice's muscle tissue. Their results also
appear in Cell Metabolism.
In taking this approach, they were
able to investigate further the process that leads to glucose breakdown and
lipid oxidation (fat-burning).
The findings showed that exercise
activates a protein called hypoxia-inducible factor 1-alpha (HIF-1α) in
different ways at different times of the day. HIF-1α responds to changes in
oxygen levels in the body tissue by stimulating certain genes.
"It makes sense that HIF-1α
would be important here, but until now, we didn't know that its levels
fluctuate based on the time of day," says Sassone-Corsi.
Based on
their findings, the researchers concluded that exercise has a more beneficial
effect on the metabolism at the beginning of the mice's active phase than
toward the end. Translating this to human time, the effect was most positive in
the late morning.
However, it is important to keep in
mind that both studies used mice and that translating the findings to humans
may be complicated because behavioral patterns vary greatly from person to
person.
"You may be a morning person,
or you may be a night person, and those things have to be taken into
account," concludes Sassone-Corsi
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