Miranda E. Kelly

High-fructose corn syrup (HFCS) consumption is a risk factor for obesity and diabetes, yet the underlying mechanisms, especially at the specific organ level, are incompletely understood. Catabolism of dietary fructose primarily occurs in the small intestine and liver, with fructose breakdown in the liver being pathological, while small intestinal fructose clearance protects the liver. Here, we report that inhibition of fructose catabolism specifically in the murine small intestine unexpectedly mitigates fructose-induced obesity and insulin resistance. Such phenotypes are attributed to decreased dietary fat absorption by the shortening of ileal lacteals. Fecal transplantation experiments revealed that the microbiome altered by blunted host intestinal fructose catabolism decreases ileal macrophages essential for lacteal growth. Thus, altered intestinal lacteal architecture likely contributes to the synergistic effects of high fat and sugar on metabolic disorders. It may also be relevant to the clinical evidence that pharmacologic suppression of fructose catabolism mitigates diet-induced obesity.

Excessive consumption of the simple sugar fructose, which induces excessive hepatic lipogenesis and gut dysbiosis, is a risk factor for cardiometabolic diseases. Here we show in male mice that the gut microbiome, when adapted to dietary fibre inulin, catabolizes dietary fructose and mitigates or reverses insulin resistance, hepatic steatosis and fibrosis. Specifically, inulin supplementation, without affecting the host’s small intestinal fructose catabolism, promotes the small intestinal microbiome to break down incoming fructose, thereby decreasing hepatic lipogenesis and fructose spillover to the colonic microbiome. Inulin also activates hepatic de novo serine synthesis and cystine uptake, augmenting glutathione production and protecting the liver from fructose-induced lipid peroxidation. These multi-modal effects of inulin are transmittable by the gut microbiome, where Bacteroides acidifaciens acts as a key player. Thus, the gut microbiome, adapted to use inulin (a fructose polymer), efficiently catabolizes dietary monomeric fructose, thereby protecting the host. These findings provide a mechanism for how fibre can facilitate the gut microbiome to mitigate the host’s exposure to harmful nutrients and disease progression.

Brown adipose tissue (BAT) engages futile fatty acid synthesis–oxidation cycling, the purpose of which has remained elusive. Here, we show that ATP-citrate lyase (ACLY), which generates acetyl-CoA for fatty acid synthesis, promotes thermogenesis by mitigating metabolic stress. Without ACLY, BAT overloads the tricarboxylic acid cycle, activates the integrated stress response (ISR) and suppresses thermogenesis. ACLY’s role in preventing BAT stress becomes critical when mice are weaned onto a carbohydrate-plentiful diet, while removing dietary carbohydrates prevents stress induction in ACLY-deficient BAT. ACLY loss also upregulates fatty acid synthase (Fasn); yet while ISR activation is not caused by impaired fatty acid synthesis per se, deleting Fasn and Acly unlocks an alternative metabolic programme that overcomes tricarboxylic acid cycle overload, prevents ISR activation and rescues thermogenesis. Overall, we uncover a previously unappreciated role for ACLY in mitigating mitochondrial stress that links dietary carbohydrates to uncoupling protein 1-dependent thermogenesis and provides fundamental insight into the fatty acid synthesis–oxidation paradox in BAT.