Metabolic and Blood Pressure Effects of Walnut Supplementation in a Mouse Model of the Metabolic Syndrome

Abstract

There is extensive evidence that walnut consumption is protective against cardiovascular disease and diabetes in the healthy population, but the beneficial effects of walnut consumption in individuals with the metabolic syndrome (MetS) remain uncertain. We compared a range of cardio-metabolic traits and related tissue gene expression associated with 21 weeks of dietary walnut supplementation in a mouse model of MetS (MetS-Tg) and wild-type (WT) mice (n = 10 per genotype per diet, equal males and females). Compared to standard diet, walnuts did not significantly alter food consumption or body weight trajectory of either MetS-Tg or WT mice. In MetS-Tg mice, walnuts were associated with reductions in oral glucose area under the curve (gAUC, standard diet 1455 ± 54, walnut 1146 ± 91, p = 0.006) and mean arterial blood pressure (MAP, standard diet 100.6 ± 1.9, walnut 73.2 ± 1.8 mmHg, p < 0.001), with neutral effects on gAUC and MAP in WT mice. However, in MetS-Tg mice, walnuts were also associated with trends for higher plasma cholesterol (standard diet 4.73 ± 0.18, walnut 7.03 ± 1.99 mmol/L, p = 0.140) and triglyceride levels (standard diet 2.4 ± 0.5, walnut 5.4 ± 1.6 mmol/L, p = 0.061), despite lowering cholesterol and having no effect on triglycerides in WT mice. Moreover, in both MetS-Tg and WT mice, walnuts were associated with significantly increased liver expression of genes associated with metabolism (Fabp1, Insr), cell stress (Atf6, Ddit3, Eif2ak3), fibrosis (Hgf, Sp1, Timp1) and inflammation (Tnf, Ptpn22, Pparg). In conclusion, dietary walnuts were associated with modest favourable effects in WT mice, but a combination of beneficial and adverse effects in MetS-Tg mice, and up-regulation of hepatic pro-fibrotic and pro-inflammatory genes in both mouse strains.

Keywords: cholesterol; gene expression; glucose tolerance; metabolic syndrome; walnuts.

Figure 1

The time course body weight trajectories (mean ± sem) of wild-type (WT) mice (A) and MetS-Tg mice (B) over the 21 weeks of rodent standard diet (open symbols) or walnut supplemented diet (closed symbols), demonstrating that walnuts were not associated with additional weight gain.

Figure 2

(A) Relative levels of gene expression in liver for genes associated with fibrosis (Hgf, Sp1, Timp1), as medians (Interquartile Ranges) with WT control expression set to 1, * p < 0.05, ** p < 0.005. (B) Representative sections of liver tissue stained with Masson Trichrome, indicating a gradient of interstitial fibrosis (blue), lowest in WT Control diet, and highest in MetS-Tg Walnut diet (10× magnification).

Figure 2

(A) Relative levels of gene expression in liver for genes associated with fibrosis (Hgf, Sp1, Timp1), as medians (Interquartile Ranges) with WT control expression set to 1, * p < 0.05, ** p < 0.005. (B) Representative sections of liver tissue stained with Masson Trichrome, indicating a gradient of interstitial fibrosis (blue), lowest in WT Control diet, and highest in MetS-Tg Walnut diet (10× magnification).

Figure 3

Relative levels of gene expression in liver for genes associated with metabolism (Fabp1, Insr, upper panel), cell stress (Atf6, Ddit3, Eif2ak3, central panel) and inflammation (Tnf, Ptpn22, Pparg, lowest panel), shown as medians (Interquartile Ranges) with WT control expression set to 1, * p < 0.05, ** p < 0.005.

Figure 3

Relative levels of gene expression in liver for genes associated with metabolism (Fabp1, Insr, upper panel), cell stress (Atf6, Ddit3, Eif2ak3, central panel) and inflammation (Tnf, Ptpn22, Pparg, lowest panel), shown as medians (Interquartile Ranges) with WT control expression set to 1, * p < 0.05, ** p < 0.005.

Figure 4

Relative levels of gene expression in kidney tissue for genes associated with fibrosis (Serpine1, Col2a1), shown as medians (Interquartile Ranges) with WT control expression set to 1, * p < 0.05, ** p < 0.005.