Circadian Clocks, Food Intake, and Metabolism - Progress in Molecular Biology and Translational Science - page 106

are other modulators of themaster clock. Circadian disturbances (e.g., desynchronization
induced by shift work or chronic jet lag) are frequently associated with metabolic
dysfunctions (chronobesity) and vice versa. Pharmacological tools and natural synchro-
nizers (i.e., light and mealtime) can be useful as chronotherapeutic treatments to limit
the occurrence of metabolic risk factors.
1. INTRODUCTION
Energy metabolism, food intake, and circadian clocks are tightly inter-
connected. By providing energy substrates to the organism, feeding is essen-
tial for maintaining energy homeostasis. Most often, this does not occur
randomly at any time of the astronomical day, but takes place periodically
during a certain temporal niche (e.g., daytime or nighttime), depending
on whether the species is diurnal or nocturnal, respectively. The daily period
of feeding and food foraging also coincides with the period of wakefulness,
exercise, high metabolic activity, and anabolism. Conversely, the daily
period of fasting corresponds to sleep, low metabolic activity, and catabo-
lism. At a cellular level, glucose availability is maintained with a quite narrow
margin of variations throughout 24 h, despite the daily rhythm of food
ingestion reported above. The two main sources of energy stores include
carbohydrate (i.e., glycogen synthesized in the liver and muscle) and lipid
(i.e., triacylglycerols synthesized in the white adipose tissue). During the
12-h period of activity/feeding, glucose supply comes mostly from dietary
carbohydrate supply, as well as from glycogen for short-term needs (e.g.,
exercise). By contrast, during the 12-h period of sleep and fasting (glycogen-
olysis and lipolysis), energy used to cover basal energy expenditure comes
from energy substrates stored in anticipation during the previous period
of feeding (concomitant with glycogenesis and lipogenesis).
The 24-h temporal segregation of physiology and behavior is controlled
by the circadian system. This timing system is actually comprised of a net-
work of endogenous circadian clocks that generate, via their local or distrib-
uted outputs, an internal rhythmicity close to 24 h. At the top of the
circadian system is a master clock located in the suprachiasmatic nuclei
(SCN) of the hypothalamus. Most circadian rhythms in behavior (e.g.,
sleep-wake cycle) and physiology (e.g., hormonal rhythms, like pineal mel-
atonin, or adrenal glucocorticoids) are controlled by this hypothalamic
structure. 1-3 Besides, almost all peripheral tissues, such as liver, muscle,