| Two experiments were conducted to investigate effects of dietary protein levels on white adipose tissue content in Sprague-Dawley (SD) rats, and to understand the mechanisms responsible for the changes in energy metabolism.(1) The objective of the first experiment was to determine effects of dietary protein levels on adipose tissue content in growing rats. Thirty male SD (30d of age) were randomly allotted into one of3groups (n=10) fed purified isocaloric diets containing10%casein (low-level protein, LP),20%casein (medium-level protein, MP), or40%casein (high-level protein, HP) for80days. Pair-feeding per kg body weight was performed by offering the amount of food taken by HP rats. On d1, d30and d80day of different dietary protein treatment, the energy expenditure measurements were conducted. At the end of the study period, the animals were sacrificed by cervical dislocation. Blood and tissue samples were collected rapidly, snap-frozeing in liquid nitrogen, and stored at-80℃for future analysis. The tissues and organs were used to analyze the free amino acids profiles and the expressions of proteins related to the energy metabolism. Results indicated that there was no significant difference in body weight between MP and HP group, but the body weight of LP rats was lower (P<0.01) than that of the other two groups. Rats in LP and HP groups had less white adipose tissue (P<0.01) than MP rats. On d30and d80of treatment, HP diet resulted in a higher respiratory exchange ratio (P<0.01) in young rats; on d80of treatment, LP rats produced more CO2(P<0.01), consumed more O2per kg body weight (P<0.01), had a higher resting metabolic rate(P<0.01), and generated more heat (P<0.01) than the rats of other two groups. In the liver of rats, higher dietary protein level lower the expressions of phospho-AMP-activated protein kinase a (p-AMPKa. P<0.01), hormone-sensitive lipase (HSL, P<0.01), p-HSL(P<0.01), sterol regulatory element binding protein (SREBP-1, P<0.01), p-SREBP-1(P<0.01) and cytochrome c (P<0.01), as well as the ratio of p-AMPKa to AMPKa (P<0.01). Compared with the MP group, LP and HP diets decreased the expression of AMPKa (P<0.01) in the liver. Dietary protein level changed the profiles of free amino acids in tissues and organs of rats. Those changed amino acids may play roles in the fat-reducing effect of high-protein diet, and can be selected as biomarkers for the further studies.(2) The objective of the second experiment was to study effects of HP diet on adipose tissue content in diet-induced obese (DIO) rats. Forty eight male SD rats (10weeks old) were randomly allotted into two groups (n=20/group), fed normal (NF) or high fat (HF) diet for12weeks to induce obesity. Basing on the body weight,12rats from each group were randomly allotted into two subgroups (n=6), fed either the MP or HP diet for10weeks. Pair-feeding was performed by offering the amount of food taken by the rats in the HF-HP group. On d70of different dietary protein treatment, the energy expenditure measurements were conducted. At the end of the study period, rats were sacrificed by CO2asphyxiation. Blood and tissue samples were collected rapidly, snap-frozen in liquid nitrogen and stored at-80℃for future analysis. The tissues and organs were used to analyze the free amino acids profiles and the expressions of proteins related to energy and protein metabolism. Results showed that HP diet reduced major white adipose tissue mass (P<0.05) in rats; DIO rats fed a HP diet had a lower RER (P<0.01), lower protein levels for p-AMPKa and a lower ratio of p-AMPKa to AMPKa in the white adipose tissue, and a lower ratio of p-HSL to HSL in the liver; Dietary protein level changed the profiles of free amino acids in tissues and organs of rats. Those changed amino acids may play roles in the fat-reducing effect of high-protein diet, and can be selected as biomarkers for the further studies. In conclusion, under the similar dietary energy intake, high dietary protein also can reduce fat in rats. This effect may due to changed energy metabolism induced by high protein diet, and the changed free amino acids may participate in this regulation. |
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