Diet-induced thermogenesis in a carnivore, the arctic fox, Alopex lagopus

Abstract

Carnivores consume diets low in carbohydrate and high in protein and fat. The high dietary levels of protein and fat are thought to contribute greatly to diet-induced thermogenesis (DIT), i.e. the increase in metabolic rate associated with feeding. Low dietary levels of carbohydrate cause the carnivore to stress gluconeogenesis. Consequently, a well developed capacity for gluconeogenesis may be an important adaptation in the carnivorous arctic fox (Alopex lagopus), and may participate in DIT. The objectives of this study of the arctic fox were to (i) determine DIT associated with four diets that varied in the proportion of fat, protein, and carbohydrate, and (ii) assess glucose utilization in the fed and fasted arctic fox. Fox were fed four diets (high protein, high fat, high carbohydrate, and high protein/fat) at three levels of energy intake (sub-maintenance, near/above maintenance, and above maintenance). Pre- and postfeeding metabolic rates were measured by open circuit indirect calorimetry. The results indicate that (i) DIT contributes significantly to total heat production of the fox, but is dependent on diet type and energy intake, (ii) DIT is non-existent at sub-maintenance energy intake, regardless of dietary nutrients, and (iii) the high fat diet is associated with the highest prefeeding and postfeeding metabolic rate at sub-maintenance energy intake, although DIT is non-existent at all levels of energy intake. For the assessment of glucose turnover, four arctic fox were fed, over a long term, a low carbohydrate, high protein/fat diet. Fed and fasted fox were injected intravenously with radiolabeled glucose, and their blood assayed over time for disappearance of the labeled glucose. The results indicate that glucose metabolism, i.e. total entry rate and irreversible loss, is high compared to other animals, and may support the high blood glucose concentrations of the arctic fox, but does not participate in DIT.