Figure 2.

Circadian functions of gastrointestinal tract play a key role in metabolic control. Light, the strongest ZT, entrains the SCN which is the master clock. The SCN sends out signals through the ANS, as well as hormonal, temperature, and behavioral outputs to synchronize peripheral tissues. However, food intake can directly synchronize the GI tract through FEOs that are independent of the SCN. Circadian rhythms in key GI functions such as nutrient digestion and absorption, barrier integrity and immunity, as well as enteroendocrine hormone secretion all feed into establishing diurnal metabolic homeostasis. ANS, autonomic nervous system; FEO, food-entrainable oscillator; GI, gastrointestinal; SCN, suprachiasmatic nuclei; ZT, zeitgeber.