Ten-week high fat and high sugar diets in mice alter gut-brain axis cytokines in a sex-dependent manner

Abstract

Diets high in fat and sugar induce inflammation throughout the body, particularly along the gut-brain axis; however, the way these changes in immune signaling mediate one another remains unknown. We investigated cytokine changes in the brain and colon following prolonged high fat or sugar diet in female and male adult C57BL/6 mice. Ten weeks of high fat diet increased levels of TNFα, IL-1β, IL-6, IFNγ, and IL-10 in the female hippocampus and altered cytokines in the frontal cortex of both sexes. High sugar diet increased hippocampal cytokines and decreased cytokines in the diencephalon and frontal cortex. In the colon, high fat diet changed cytokine expression in both sexes, while high sugar diet only increased TNFα in males. Causal mediation analysis confirmed that colon IL-10 and IL-6 mediate high fat diet-induced neuroimmune changes in the female hippocampus and male frontal cortex. Additionally, high fat diet increased food consumption and weight gain in both sexes, while high sugar diet decreased male weight gain. These findings reveal a novel causal link between gut and brain inflammation specific to prolonged consumption of high fat, not high sugar, diet. Importantly, this work includes females which have been under-represented in diet research, and demonstrates that diet-induced neuroinflammation varies by brain region between sexes. Furthermore, our data suggest female brains are more vulnerable than males to inflammatory changes following excessive fat and sugar consumption, which may help explain the increased risk of inflammation-associated psychiatric conditions in women who eat a Western Diet rich in both dietary components.

Keywords: colon cytokines; food consumption; gut-brain axis; high fat diet; high sugar diet; neuroinflammation.

Figure 1.. Food consumption and growth over ten weeks of diet.

Estimated food consumed per animal during the A) high fat diet study and B) high sugar diet study; increasing body weight during the C) high fat diet study and D) high sugar diet study. N=2 cages/group, cage total Kcal divided by 4 mice/cage for food consumption data; n=6–8/group for body weight. Data are represented as mean ± SEM; repeated measures 2-way ANOVA. Female (solid line), male (dotted line), high fat diet (blue), high sugar diet (green), control diet (black), first significant diet by week interaction (red arrow).

Figure 2.. High fat diet changes expression of cytokines in the colon in both sexes, while high sugar diet has a negligible effect.

Expression of TNFα, IL-1β, IL-6, IFNγ, IL-10 in females and males after ten weeks of A) high fat diet or B) high sugar diet. High fat diet (blue), high sugar diet (green), control diet (grey). TNF: tumor necrosis factor, IL: interleukin, IFN: interferon. Data are represented as mean ± SEM; 2-way MANOVA (diet by sex) then univariate ANOVA for individual cytokines; *p<0.05

Figure 3.. High fat diet increases hippocampal cytokines in females.

Expression of TNFα, IL-1β, IL-6, IFNγ, IL-10 in females and males after ten weeks of high fat diet in the A) diencephalon, B) hippocampus, C) frontal cortex. High fat diet (blue), control diet (grey). TNF: tumor necrosis factor, IL: interleukin, IFN: interferon, ND: not detectable. Data are represented as mean ± SEM; 2-way MANOVA (diet by sex) then univariate ANOVA for individual cytokines; *p<0.05, ^p<0.1

Figure 4.. High sugar diet changes cytokines similarly in females and males throughout the brain.

Expression of TNFα, IL-1β, IL-6, IFNγ, IL-10 in females and males after ten weeks of high sugar diet in the A) diencephalon, B) hippocampus, C) frontal cortex. High sugar diet (green), control diet (grey). TNF: tumor necrosis factor, IL: interleukin, IFN: interferon, ND: not detectable. Data are represented as mean ± SEM; 2-way MANOVA (diet by sex) then univariate ANOVA for individual cytokines; *p<0.05, ^p<0.1

Figure 5.. Colon cytokines correlate strongly with brain cytokines after high fat diet.

Heatmap correlation plots between colon cytokine expression (rows) and cytokine levels in the diencephalon, hippocampus, and frontal cortex (columns) after ten weeks of high fat diet in A) females and B) males or after ten weeks of high sugar diet in C) females and D) males. Cytokines in the colon and brain for each diet/sex were correlated using Pearson’s r correlation coefficients.

Figure 6.. Causal mediation analyses reveal that colon IL-6 and IL-10 mediate high fat diet-induced changes in neuroinflammation.

In females, A) colon IL-6 negatively mediates hippocampal IL-1β expression, and colon IL-10 negatively mediates B) IFNγ and C) IL-1β. In males, colon IL-10 positively mediates TNFα levels in the frontal cortex. Effects of high fat diet on brain cytokines indicate the total effect followed by the direct effect in parentheses. Causal mediation analysis; *p<0.05, **p<0.01

Figure 7.. Summary of cytokine changes following ten-week high fat or high sugar diet.

Significant increases (↑) and decreases (↓) in cytokine expression in the colon, diencephalon (D), hippocampus (H), and frontal cortex (FC) are presented for females (♀, purple) and males (♂, red) for the high fat (blue, left) and high sugar (green, right) diet studies. Increases (↑) and decreases (↓) in food intake and weight gain are presented for each study, and colon cytokine mediators of high fat diet on neuroinflammation are included for females and males.