The Food Energy/Protein Ratio Regulates the Rat Urea Cycle but Not Total Nitrogen Losses

Abstract

Nitrogen balance studies have shown that a portion of the N ingested but not excreted is not accounted for. We compared several diets (standard, high-fat, high-protein, and self-selected cafeteria) to determine how diet-dependent energy sources affect nitrogen handling, i.e., the liver urea cycle. Diet components and rat homogenates were used for nitrogen, lipid, and energy analyses. Plasma urea and individual amino acids, as well as liver urea cycle enzyme activities, were determined. Despite ample differences in N intake, circulating amino acids remained practically unchanged in contrast to marked changes in plasma urea. The finding of significant correlations between circulating urea and arginine-succinate synthase and lyase activities supported their regulatory role of urea synthesis, the main N excretion pathway. The cycle operation also correlated with the food protein/energy ratio, in contraposition to total nitrogen losses and estimated balance essentially independent of dietary energy load. The different regulation mechanisms observed have potentially important nutritional consequences, hinting at nitrogen disposal mechanisms able to eliminate excess nitrogen under conditions of high availability of both energy and proteins. Their operation reduces urea synthesis to allow for a safe (albeit unknown) mechanism of N/energy excess accommodation.

Unconnected Body Accrual of Dietary Lipid and Protein in Rats Fed Diets with Different Lipid and Protein Content

“…These results confirmed, albeit indirectly, that our high-fat diet (and the high-protein diet) [45] allowed us to analyse the effects of the selected dietary macronutrient, with less interference coming from changes in the other components, thus minimising the effects of possible variations in diet composition and allowing for transversal comparisons.…”

Urea transport and hydrolysis in the rumen: A review

“…Excess NH 3 is excreted from the body after liver detoxification, and converted to the less toxic compound, urea ( Weiner et al., 2015 ; Jin et al., 2018 ), however a portion is recycled by ruminants ( Lapierre and Lobley, 2001 ). The relationship between dietary crude protein and ruminal degradable protein concentration determines the N balance, the quantities recycled to gastrointestinal tracts, and how much is used by microorganisms in different animals ( Weiner et al., 2015 ; Mutsvangwa et al., 2016 ; Oliva et al., 2019 ).…”

Effect of cassava flour on the lipidic and redox profile of Wistar rats with dyslipidemia

“…It is possible that a lower intake of proteins associated with an energy‐rich diet decreases the formation of urea in the liver, since amino acids are used for protein synthesis and are not degraded as an energy source (Matos et al, 2005; Oliva, Alemany, & Remesar, 2019). Allied to this, the hepatic damage observed in the CF and HC groups can lead to a decrease in the activity of urea cycle reducing urea synthesis (Oliva et al, 2019; Sabater et al, 2014). Creatinine plasma levels are not affected by liver function, once it is synthetized from creatine in the kidney and muscles (Nelson & Cox, 2014), which explains that we did not observe differences among the groups.…”