Hypercarnivory and the brain: protein requirements of cats reconsidered

Abstract

The domestic hypercarnivores cat and mink have a higher protein requirement than other domestic mammals. This has been attributed to adaptation to a hypercarnivorous diet and subsequent loss of the ability to downregulate amino acid catabolism. A quantitative analysis of brain glucose requirements reveals that in cats on their natural diet, a significant proportion of protein must be diverted into gluconeogenesis to supply the brain. According to the model presented here, the high protein requirement of the domestic cat is the result of routing of amino acids into gluconeogenesis to supply the needs of the brain and other glucose-requiring tissues, resulting in oxidation of amino acid in excess of the rate predicted for a non-hypercarnivorous mammal of the same size. Thus, cats and other small hypercarnivores do not have a high protein requirement per se, but a high endogenous glucose demand that is met by obligatory amino acid-based gluconeogenesis. It is predicted that for hypercarnivorous mammals with the same degree of encephalisation, endogenous nitrogen losses increase with decreasing metabolic mass as a result of the allometric relationships of brain mass and brain metabolic rate with body mass, possibly imposing a lower limit for body mass in hypercarnivorous mammals.

Milk Composition in the Weddell SealLeptonychotes weddellii: Evidence for a Functional Role of Milk Carbohydrates in Pinnipeds

“…Precociality is also associated with a higher brain metabolic rate and substrate requirements approaching those of adult animals (Erecinska et al 2004). Unlike RBCs, the brain oxidizes almost all glucose it consumes to CO 2 and water (Sokoloff et al 1977) and so creates an absolute and continual metabolic demand for carbohydrate (Eisert 2011). Assuming a neonatal brain mass of 393 g for newborn and of 561 g for adult Weddell seals (Elsner and Gooden 1983;Eisert 2011;R.…”

“…Most lactating Weddell seals appear to fast for at least 2 wk postpartum (Eisert et al 2005), and provision of milk carbohydrate places a nontrivial demand on maternal reserves in addition to that required for the brain and other tissues. A fasting Weddell seal producing about 3.54 kg milk (Tedman and Green 1987) containing 1.1% sugar (average for 0-14 dpp; this study) would have to supply the mammary glands with at least 39 g glucose d Ϫ1 for milk sugar synthesis, equivalent to a daily loss of 350 g of body fat (if generated from glycerol; Streja et al 1977) or of 285 g of lean body mass (if generated from protein; Jungas et al 1992;Eisert 2011). Because the metabolic constraints of fasting discourage the use of carbohydrate for nonessential purposes, the fact that Weddell seals produce the highest-carbohydrate milk early in lactation, while they are fasting, suggests that milk sugar in the Weddell seal is functionally significant.…”

“…Weddell seals consume a diet of marine fish and invertebrates (Dearborn 1965;Casaux et al 1997Casaux et al , 2006Casaux et al , 2009Lake et al 2003) that is expected to contain very low levels of available carbohydrate (≤0.5% by mass) in the form of glycogen (MacPherson 1932;Oehlenschläger 1991;Rosa et al 2005). Like other specialized mammalian carnivores that subsist exclusively on animal prey ("hypercarnivores"), adult seals have to meet their metabolic glucose demand by synthesizing glucose from amino acids and glycerol (Eisert 2011). During fasting, body tissues are catabolized to provide the energy and substrates required to maintain metabolic activity; in fasting lactation, breakdown of maternal tissues must also provide all precursors for milk synthesis (e.g., fatty acids, glycerol, amino acids, calcium, glucose ;Oftedal 2000).…”

“…Glucose is the metabolic substrate in shortest supply during fasting because mammals do not have significant body stores of carbohydrate and yet they require a constant supply of glucose for the brain and other glucosedependent tissues, including the gravid uterus in pregnancy and the mammary glands during lactation (Eisert 2011). During fasting, body protein and fat are catabolized to provide amino acids and glycerol as precursors for gluconeogenesis (Cahill 2006).…”

Geometric analysis of macronutrient selection in the adult domestic cat, Felis catus

“…The carbohydrate ceiling explains many of the intake patterns seen in both dry and wet diet experiments and suggests that cats may only be able to process ingested carbohydrate up to a certain level. Cats have a number of sensory and metabolic adaptations that reflect their expected low carbohydrate intake (Eisert, 2011), including the absence of a functional sweet taste receptor (Li et al, 2005), low rates of intestinal glucose uptake (Buddington et al, 1991), a lack of salivary amylase and reduced activities (compared with dogs, for example) of pancreatic amylase and intestinal disaccharidases (Meyer and Kienzle, 1991). The reduced enzymatic capacity for digesting carbohydrate may mean that high carbohydrate intake could have untoward effects on cats, with any carbohydrate escaping digestion .…”