L-Serine Supplementation Blunts Fasting-Induced Weight Regain by Increasing Brown Fat Thermogenesis

Abstract

Weight regain after fasting, often exceeding the pre-fasting weight, is a common phenomenon and big problem for the treatment of obesity. Thus, novel interventions maintaining reduced body weight are critically important to prevent metabolic disease. Here we investigate the metabolic effects of dietary L-serine supplementation, known to modulate various organ functions. C57BL/6N-Rj male mice were supplemented with or without 1% L-serine in their drinking water and fed with a chow or high-fat diet. Mice were fed either ad libitum or subjected to repeated overnight fasting. Body weight, body composition, glucose tolerance and energy metabolism were assessed. This was combined with a detailed analysis of the liver and adipose tissues, including the use of primary brown adipocytes to study mitochondrial respiration and protein expression. We find that L-serine supplementation has little impact on systemic metabolism in ad libitum-fed mice. Conversely, L-serine supplementation blunted fasting-induced body weight regain, especially in diet-induced obese mice. This reduction in body weight regain is likely due to the increased energy expenditure, based on elevated brown adipose tissue activity. Thus, L-serine supplementation during and after weight-loss could reduce weight regain and thereby help tackle one of the major problems of current obesity therapies.

Keywords: BAT; L-serine; body weight; energy expenditure; fasting; obesity; weight regain.

Figure 1

L-serine supplementation has no major effects on metabolism in ad libitum-fed mice. (a) Body weight of chow diet (CD) and high-fat diet (HFD) fed mice supplemented with or without L-serine for 8 weeks, starting at 4 weeks of age (n = 8). (b) Body composition (fat and lean mass) of CD-fed and HFD-fed mice with and without L-serine supplementation assessed after 8 weeks (n = 8). (c) Randomly fed blood glucose levels of CD-fed and HFD-fed mice with and without L-serine supplementation after 3 weeks (n= 8). (d) Randomly fed insulin levels of CD-fed and HFD-fed mice with and without L-serine supplementation after 3 weeks. (e) Glucose Tolerance Test (GTT) and baseline subtracted area under the curve (AUC) of CD -fed and HFD-fed mice with and without L-serine supplementation at week 6 (n = 8). (f) Representative H&E stainings of perigonadal fat (PGF), SCF (subcutaneous fat) and liver of random CD-fed and HFD-fed mice with or without L-serine supplementation after 8 weeks. Scale bars represent 100 µM. Data are shown as mean ± SEM, one or two-way ANOVA with Tukey’s post-test. * p < 0.05.

Figure 2

L-serine supplementation blunts fasting-induced body weight regain. (a) Body weight increase (%) of mice a fed chow diet (CD) or high-fat diet (HFD) with or without L-serine supplementation and repeated overnight fasting for 8 weeks starting at 8 weeks of age (n = 6). (b) Body composition (fat and lean mass) of mice shown in (a) after 8 weeks (n = 6). (c) Weight of perigonadal fat (PGF) and subcutaneous fat (SCF) of mice in (a) after week 8 (n = 6). (d) Representative H&E stainings of PGF and SCF from mice shown in (a) after 8 weeks. Scale bars show 100 µM. (e) Glucose Tolerance Test (GTT) of CD-fed and HFD-fed mice with and without L-serine supplementation at week 6 (n = 6). Data are shown as mean ± SEM, one or two-way ANOVA with Tukey’s post-hoc test. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001.

Figure 3

L-serine supplementation alters food intake and respiratory exchange ratio (RER), but not white adipose tissue (WAT) lipolysis. (a) Experimental setup for indirect calorimetry measurements. The figure was generated using Biorender.com. (b) Average food intake of random fed and fasted chow diet (CD) and high-fat diet (HFD)-fed mice with or without L-serine supplementation at week 7 (CD) or 8 (HFD) (CD random fed: n = 5; CD fasted, CD+L-ser random fed, CD+L-ser fasted: n = 6; HFD random fed: n = 4; HFD fasted, HFD+L-ser random fed, HFD+L-ser fasted: (n = 6). (c) Mean respiratory exchange ratio (RER) of random fed and fasted CD and HFD fed mice with or without L-serine supplementation at weeks 7 (CD) or 8 (HFD) (CD random fed: n = 5; CD fasted, CD+L-ser random fed, CD+L-ser fasted: n = 6; HFD random fed: n = 4; HFD fasted, HFD+L-ser random fed, HFD+L-ser fasted: n = 6). (d) Quantifications of Western blots for phosphorylated HSL (Ser563, Ser660, Ser565) and total HSL of PGF and SCF (n = 6) of fasted CD-fed and HFD-fed mice with or without L-serine supplementation. (e) Ex vivo lipolysis assay of PGF (n = 3). Data are shown as mean ± SEM, one or two-way ANOVA with Tukey’s post-hoc test. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.001.

Figure 4

L-serine supplementation alters energy expenditure and potentiates mitochondrial respiration. (a) Energy expenditure after overnight fasting in relation to body weight of random fed and fasted CD (chow diet) and HFD (high-fat diet)-fed mice with or without L-serine supplementation at weeks 7 (CD) or 8 (HFD) (CD random fed: n = 5; CD fasted, CD+L-ser random fed, CD+L-ser fasted: n = 6; HFD random fed: n = 4; HFD fasted, HFD+L-ser random fed, HFD+L-ser fasted: (n = 6). (b) Representative H&E stainings of brown adipose tissue (BAT) from random fed and fasted CD-fed and HFD-fed mice with or without L-serine supplementation after 8 weeks starting at 4 (random fed) and 8 (fasted) weeks of age. Scale bars show 100 µM. (c) Quantification of uncoupled protein 1 (UCP1) protein content normalized to beta actin from BAT of random fed and fasted CD-fed and HFD-fed mice with or without L-serine supplementation (n = 6). (d) Oxygen consumption rate (OCR) of differentiated primary brown adipocytes pre-treated or not with 400 µM L-serine and acutely exposed (injected) to 400 µM L-serine and/or 1 µM isoproterenol (n = 3 replicates from 2 mice). Statistical analysis in panel a is described in Section 2.10. Data are shown as mean ± SEM, one or two-way ANOVA with Tukey’s post-hoc test. * p < 0.05, ** p < 0.01,*** p < 0.001.