A Unified Theory of Satiety in Ruminants: The Role of Ruminal dCO2
Satiety in ruminants is a complex phenomenon that has been attributed to various factors. Two main theories have emerged: the physical distention of the rumen and the hepatic oxidation of nutrients (Allen, 1996; Allen et al., 2009).
The physical distention theory suggests that the expansion of the rumen with digesta triggers satiety signals (Allen, 1996; Grovum and Mossadeq, 2002). This theory aligns with the observation that highly fermentable diets, which lead to rapid rumen distention, also induce satiety more quickly than slow-fermentable diets.
The hepatic oxidation theory proposes that satiety is signaled by the production of ATP in the liver during nutrient metabolism (Grovum and Bignell, 1988; Allen and Bradford, 2012; Maldini and Allen, 2018). ATP production is closely linked to CO2 production, and evidence suggests that CO2 can act as a satiety signal in both the peripheral nervous system (carotid bodies) and the central nervous system (Annison and White, 1961; Leng and Annison, 1963; ).
Interestingly, both theories can be unified under a single framework that emphasizes the role of ruminal dCO2. High ruminal dCO2 concentrations can lead to control of hunger and satiety through two mechanisms:
Hunger signal: Ruminal dCO2 level might increase ghrelin concentrations and stimulate appetite, leading to feelings of hunger. This effect has been observed in both rats and humans (Eweis et al. in 2017).
Rumen distention: Elevated dCO2 levels can stimulate ruminal baroreceptors, triggering satiety signals (Grovum and Mossadeq, 2002).
Liver signaling: Ruminal dCO2 can be absorbed into the blood and transported to the liver. There, it can stimulate ATP production, leading to satiety signals in the CNS (Strominger and Brobeck, 1953; Whitelaw et al., 1972; Morton et al., 2006; Cummins et al., 2020).
The theoretical underpinnings of the outlined processes.
Hunger: When the ruminal fermentation increases dissolved CO2 levels, it might signal nutrient availability to the central nervous system (CNS). This might be triggered by ghlerin secretion by fat cells. The increase in intracellular bicarbonate (HCO3-) in epithelial cells, likely caused by the combined CO2 and water absorption through aquaporins, might be responsible for this effect. Intracellular HCO3- is known to stimulate AMPK activity, potentially signaling fat cells to secrete ghrelin and trigger hunger (Cummins et al., 2020).
Satiety: CO2 has traditionally been viewed as a cellular waste product. However, evidence suggests that mitochondrial activity is significantly affected by high CO2 levels (hypercapnia). Most of the reactions involving removal of carboxyl groups (decarboxylation) occur within the mitochondria (TCA cycle). Increased glucose production can quickly acidify the mitochondria due to significant CO2 production (Phelan et al., 2019). This first sign on increased metabolic activity might appears as a rise in blood CO2 levels, which should stimulate the carotid bodies (Cummins et al., 2014). This, in turn, sends feedback signals to the CNS to increase breathing and heart rate, respiratory acidosis, while also promoting leptin release, the satiety hormone (Mortin et al., 2006). Alternatively, a decrease in intracellular HCO3- might negatively impact ACC or AMPK activity, leading to reduced ghrelin acylation due to a difrect effect on complex fatty acid formation and lower GOat activation (Muller et al, 2015).
Distention signal: There's a link between cytosolic acidification by CO2 and increased intracellular calcium levels, which is a characteristic of hypocalcemia, a metabolic disease in ruminants around calving. I'll address this issue in more detail later (Phelar et al., 2021). What is important to notice is that the increase in intracellular calcium due to CO2 acidification should trigger ruminal baroreceptors explaining the relationship between dCO2 and fillness.
This unified theory suggests that ruminal dCO2 plays a central role in regulating satiety in ruminants. By monitoring ruminal dCO2 concentrations, we may be able to develop strategies to modulate feed intake and improve animal welfare.
The ATR-IR sensor, a non-invasive technology, has emerged as a promising tool for monitoring ruminal dCO2. This sensor can be used to provide real-time data on dCO2 levels, allowing for continuous assessment of satiety and nutrient intake in ruminants.
In conclusion, the role of ruminal dCO2 in satiety control offers a unified perspective on this complex physiological phenomenon. Monitoring dCO2 concentrations using ATR-IR sensors holds promise for optimizing ruminant health and productivity.
