High-protein diet selectively reduces fat mass and improves glucose tolerance in Western-type diet-induced obese rats

Abstract

Obesity is an increasing health problem. Because drug treatments are limited, diets remain popular. High-protein diets (HPD) reduce body weight (BW), although the mechanisms are unclear. We investigated physiological mechanisms altered by switching diet induced obesity (DIO) rats from Western-type diet (WTD) to HPD. Male rats were fed standard (SD) or WTD (45% calories from fat). After developing DIO (50% of rats), they were switched to SD (15% calories from protein) or HPD (52% calories from protein) for up to 4 weeks. Food intake (FI), BW, body composition, glucose tolerance, insulin sensitivity, and intestinal hormone plasma levels were monitored. Rats fed WTD showed an increased FI and had a 25% greater BW gain after 9 wk compared with SD (P < 0.05). Diet-induced obese rats switched from WTD to HPD reduced daily FI by 30% on day 1, which lasted to day 9 (-9%) and decreased BW during the 2-wk period compared with SD/SD (P < 0.05). During these 2 wk, WTD/HPD rats lost 72% more fat mass than WTD/SD (P < 0.05), whereas lean mass was unaltered. WTD/HPD rats had lower blood glucose than WTD/SD at 30 min postglucose gavage (P < 0.05). The increase of pancreatic polypeptide and peptide YY during the 2-h dark-phase feeding was higher in WTD/HPD compared with WTD/SD (P < 0.05). These data indicate that HPD reduces BW in WTD rats, which may be related to decreased FI and the selective reduction of fat mass accompanied by improved glucose tolerance, suggesting relevant benefits of HPD in the treatment of obesity.

Keywords: diet-induced obesity; food intake; gut hormones; high-protein diet.

Figures

Fig. 1.

Food intake under conditions of Western-type diet feeding. Rats were fed either a standard or a Western-type diet (those selected as DIO responders) for 9 wk, and food intake was monitored daily and expressed as intake in kilocalories per week (A) or adjusted for body weight in kilocalories per 300 g body wt per week (B). Data are expressed as means ± SE of 20 rats/group. **P < 0.01, and ***P < 0.001 vs. standard diet.

Fig. 2.

Body weight gain under conditions of Western-type diet feeding. Rats were fed either a standard or a Western-type diet (those selected as DIO responders) for 9 wk, and body weight was monitored daily and expressed as cumulative weekly body weight change. Data are expressed as means ± SE of 20 rats/group. ***P < 0.001 vs. standard diet.

Fig. 3.

Food intake after switching to standard diet or high-protein diet. After development of diet-induced obesity on a Western-type diet or feeding of standard diet, rats were fed a standard or high-protein diet resulting in four experimental groups: standard diet/standard diet, standard diet/high-protein diet, Western-type diet/standard diet, and Western-type diet/high-protein diet. Food intake was assessed daily for a period of 14 days and expressed as intake in kilocalories per day (A) or adjusted for body weight in kilocalories per 300 g body wt per day (B). Data are expressed as means ± SE of 8–10 rats/group. *P < 0.05, **P < 0.01 and ***P < 0.001 vs. standard diet/high-protein diet. +P < 0.05, ++P < 0.01 and +++P < 0.001 vs. Western-type diet/standard diet. #P < 0.05, ##P < 0.01, and ###P < 0.001 vs. Western-type diet/high-protein diet.

Fig. 4.

Body weight change after switching to standard diet or high-protein diet. After development of diet-induced obesity on a Western-type diet or feeding of standard diet, rats were fed a standard or high-protein diet resulting in four experimental groups: standard diet/standard diet, standard diet/high-protein diet, Western-type diet/standard diet, and Western-type diet/high-protein diet. Body weight was assessed daily for a period of 14 days and expressed as daily body weight change. Data are expressed as means ± SE of 8–10 rats/group. *P < 0.05, **P < 0.01 and ***P < 0.001 vs. standard diet/standard diet; +P < 0.05 and +++P < 0.001 vs. standard diet/high-protein diet; #P < 0.05, ##P < 0.01 and ###P < 0.001 vs. Western-type diet/standard diet.

Fig. 5.

Body weight and body composition before and after switching to standard diet or high-protein diet. After development of diet-induced obesity on Western-type diet or feeding of standard diet for 9 wk (A), rats were fed a standard or high-protein diet, resulting in four experimental groups: standard diet/standard diet, standard diet/high-protein diet, Western-type diet/standard diet, and Western-type diet/high-protein diet. Body composition was assessed in conscious lightly restrained (∼1 min) rats directly before switching diets and after the 14-day measurement period on day 15 using quantitative nuclear magnetic resonance analysis. Changes of total body weight (B), fat mass (C and D), lean mass (E), and water (F) were assessed. Data are expressed as means ± SE of 8–10 rats/group. **P < 0.01 and ***P < 0.001 vs. standard diet/standard diet; +P < 0.05 and +++P < 0.001 vs. standard diet/high-protein diet; #P < 0.05 vs. Western-type diet/standard diet.

Fig. 6.

Glucose tolerance and insulin sensitivity after switching to standard diet or high-protein diet. For assessment of glucose tolerance, rats were food-deprived overnight with free access to water and received an orogastric gavage with 2 g·kg−1·4 ml−1 glucose in ddH2O. Blood was obtained by tail prick before and at 15, 30, 60, and 120 min after gavage, and blood glucose was assessed using standard glucose test strips (A). In the insulin sensitivity test, rats were fasted overnight and received an intraperitoneal injection of 1 IU·kg−1·ml−1 insulin. The experiment started at 9 AM, and blood glucose was assessed using standard glucose test strips before and at 15, 30, 60, 120, 180, and 240 min postinjection (B). Data are expressed as means ± SE of 5 or 6 rats/group. *P < 0.05 and **P < 0.01 vs. standard diet/standard diet. #P < 0.05 vs. standard diet/high-protein diet.

Fig. 7.

Changes in intestinal hormones after 2-h dark phase feeding compared with the preprandial light phase. After development of diet-induced obesity on Western-type diet or feeding of standard diet, rats were fed a standard or high-protein diet for 32 days, resulting in four experimental groups: standard diet/standard diet (SD/SD), standard diet/high-protein diet (SD/HPD), Western-type diet/standard diet (WTD/SD), and Western-type diet/high-protein diet (WTD/HPD). The four groups fed ad libitum with standard or high-protein diet, respectively, were euthanized by decapitation to obtain trunk blood between 3 and 4 PM during the light phase. The remaining rats were euthanized after a 2-h dark phase feeding period, trunk blood was collected, and the plasma levels of amylin (A), gastric inhibitory polypeptide (GIP; B), insulin (C), leptin (D), pancreatic polypeptide (PP; E), and peptide YY (PYY; F) were assessed. Data are expressed as mean ± SE of 5 rats/group. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. same diet group during light phase; #P < 0.05 and ##P < 0.01 vs. standard diet/standard diet group during same photoperiod.