Obesity is a chronic inflammatory disease, but the relationship between inflammation and obesity is not fully understood. High energy density (HED) diets contribute to obesity by triggering microbiota dysbiosis, increased body fat, and metabolic syndrome. Adipose tissue, once considered inert, is now recognized as an active endocrine organ. In obesity, proinflammatory adipokines activate macrophages, and free fatty acids (FFAs) activate toll-like receptor (TLR) 4, generating proinflammatory signals. Previous work showed that 4 weeks of HED diet triggers microglia activation in the NTS. Another obesity comorbidity is non-alcoholic fatty liver disease (NAFLD), where increased TG production outpaces apolipoprotein production, leading to hepatic lipidosis. This study aimed to determine the timeline of systemic changes (increased body fat, microbiota dysbiosis, serum cytokine changes, NAFLD, and microglia activation) induced by long-term HED diet consumption.

The introduction thoroughly reviews the existing literature on obesity, inflammation, and the role of the gut microbiome and liver in the development of obesity-related complications. Studies cited establish the link between HED diets and microbiota dysbiosis, the inflammatory role of adipose tissue and macrophages, the impact of FFAs and TLR4 activation, and the prevalence of NAFLD in obese individuals. The authors' previous findings on the effects of short-term HED diets on microglia activation are also referenced, setting the stage for the current study's investigation of long-term effects.

Male Sprague-Dawley rats (n=15) were acclimated for one week, then fed a low energy density (LED) diet for two weeks, followed by a high energy density (HED) diet for 26 weeks. A control group (n=9) remained on the LED diet for the entire 26 weeks. Food intake, body weight, and body composition were measured twice a week using a minispec LF 110 BCA Analyzer. Blood and fecal samples were collected at baseline and at weeks 1, 4, 8, and 26 post-HED diet introduction. Serum was analyzed for insulin, leptin, and inflammatory cytokines using ELISA. Fecal samples underwent 16S rRNA gene sequencing to assess gut microbiota composition. After perfusion, hindbrains and livers were harvested for analysis. Microglia activation in the NTS was assessed via Iba-1 immunostaining, and hepatic lipidosis was evaluated using H&E and Oil-Red-O staining. Data analysis was performed using GraphPad Prism 7, with two-tailed t-tests or ANOVA as appropriate.

The HED diet significantly increased body weight and body fat mass, even with similar caloric intake compared to the control group after week one. Microbiota dysbiosis, characterized by decreased bacterial diversity and altered abundance of specific taxa (increased Firmicutes, decreased Bacteroidetes), was observed within one week of initiating the HED diet. Significant microglia activation in the intermediate NTS was evident after four weeks and was more pronounced after 26 weeks. Marked hepatic lipidosis was observed after four weeks on the HED diet, with no significant progression observed by 26 weeks. The long-term HED diet also resulted in significant changes in the serum cytokine profile, including increased levels of certain pro-inflammatory markers (e.g., FGFβ, MCP-1, MIP-1α, TGFβ) and decreased levels of others (e.g., IFNγ, IL-5).

The results support the hypothesis that long-term HED diet consumption leads to a cascade of events starting with rapid microbiota dysbiosis. This gut dysbiosis may trigger inflammation and subsequent metabolic changes leading to increased fat accumulation in both the liver and adipose tissue. Microglia activation in the NTS indicates a neuroinflammatory response to the metabolic changes triggered by the HED diet. The changes in serum cytokine levels reflect the systemic inflammatory state associated with long-term HED diet consumption. The study highlights the complex interplay between diet, gut microbiota, liver function, and neuroinflammation in the development of obesity.

This study demonstrates the long-term effects of a high-energy-density diet, revealing a temporal sequence of events starting with gut dysbiosis and progressing to hepatic steatosis, neuroinflammation, and systemic inflammation. Future research could focus on the specific mechanisms by which microbiota dysbiosis contributes to hepatic steatosis and neuroinflammation, and on potential therapeutic interventions targeting the gut-liver-brain axis.

The study used only male Sprague-Dawley rats, limiting the generalizability of the findings to other species and sexes. The specific mechanisms underlying the observed changes in the gut microbiome and cytokine profiles were not fully elucidated. Future studies should investigate potential sex differences and explore the specific molecular pathways involved in the development of these obesity-related complications.