Relationships between lipid availability and protein utilization during prolonged fasting

Abstract

Mammals and birds adapt to prolonged fasting by mobilizing fat stores and minimizing protein loss. This strategy ends with an increase in protein utilization associated with behavioural changes promoting food foraging. Using the Zucker rat as a model, we have investigated the effect of severe obesity on this pattern of protein loss during long-term fasting. Two interactions between the initial adiposity and protein utilization were found. First, protein conservation was more effective in obese than in lean rats: fatty rats had a three times lower daily nitrogen excretion and proportion of energy expenditure deriving from proteins, and a lower daily protein loss in various muscles. This phase of protein sparing is moreover nine times longer in the fatty rats. Second, obese animals did not show the late increase in nitrogen excretion that occurred in their lean littermates. Total body protein loss during starvation was larger in fatty rats (57% versus 29%) and, accordingly, total protein loss was greater in their muscles. At the end of the experiment, lean and obese rats had lost 98% and 82%, respectively, of their initial lipid reserves, and fatty rats still had an obese body composition. These results support the hypothesis that in severely obese humans and animals a lethal cumulative protein loss is reached long before the exhaustion of fat stores, while the phase of protein conservation is still continuing. In contrast, in lean rats, survival of fasting seems to depend on the availability of lipid fuels.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormone and metabolite changes associated with extended breeding fasts in male northern elephant seals (Mirounga angustirostris)

“…Regulators of muscle protein turnover include insulin, glucagon, glucocorticoids, thyroid hormones, GH and IGF-1 (Goldberg et al, 1980;Koea et al, 1992). Although low insulin levels may be required for net proteolysis, previous studies have not suggested a role for insulin in regulating net proteolysis during extended fasting (Goodman et al, 1980;Cherel et al, 1992). Similarly, IGF-1 and GH affect protein synthesis and increases in these hormones could reflect a progressive shift in muscle sparing.…”

“…Extended fasting is characterized by mobilization and use of lipid for energy metabolism and reduced mobilization and use of body proteins stores for gluconeogenesis and metabolism (e.g. Cherel et al, 1992). For many species extended fasts are associated with periods of reduced body temperature and activity levels (e.g.…”

“…Cherel et al, 1992). Low insulin-to-glucagon ratios are thought to be the important regulators of increased lipolytic, ketogenic and gluconeogenic process during extended fasting (Cahill, 1978).…”

“…The contribution of hormonal regulation to protein sparing during extended fasting and the degree to which potential secondary mediators contribute to the metabolic action of GH during fasting has not been completely investigated (Cahill, 1979: Cherel et al, 1992Norrelund 2005). Under non-fasting conditions, GH stimulates hepatic release of insulin-like growth factor (IGF-1) which supports protein anabolism.…”

Obesity research in the next decade

“…2 But fat mass is extensive and capable of sustaining life for weeks to months to years in the severely obese in the absence of food intake. 3 This has led to the thrifty gene hypothesis, 4 a concept not accepted by all. 5 The regulation of weight could be the defense of protein, carbohydrate and/or fat.…”