Lipids ingested from a meal are metabolized in the host body, used as energy sources and biomembranes, and also play a role as a signaling substance. Lipids are also metabolized by intestinal bacteria that are in symbiotic relationship with the host, and it becomes clear that fatty acid metabolites affect the fatty acid composition of the host. Furthermore, fatty acid metabolites, such as abundantly occurring linoleic acid metabolites, by intestinal bacteria have been reported to have various physiological activities. Thus, it has been suggested that fatty acids, which have been thought of as nutrients, are metabolized by hosts and intestinal bacteria, thereby may exhibit various physiological activities. Development of pharmaceuticals and functional foods using these gut microbial fatty acid metabolites and development of probiotics that produce functional fatty acid metabolites in the intestinal tract are expected. We are currently investigating practical functional food application of 10-hydroxycis-12-octadecenoic acid (HYA), a first compound metabolized from linoleic acid by intestinal bacteria. In this paper, we introduce HYA’s functions and efforts toward practical application.

In the past decades, many studies are reported that gut microbiota have not only to form the community in the gut but also to have a pivotal role in the human health. Especially, the alteration of gut microbiota (dysbiosis) have risk factors for human diseases, therefore, the gut microbiota has become a focus for research at the interaction between diet (key factor) and metabolic health. To date, accumulated extensive body of scientific knowledge is exhibited to the gut microbiota as a mediator of dietary impact on the host metabolism. This review will discuss the recent studies in underlying mechanisms whereby the gut microbiota modulates the effects of diet and shapes the host metabolism either towards or away from obesity and related metabolic conditions. In addition, to discuss the recent studies in relation to gut microbial metabolite of dietary lipids and host energy regulation, and evaluate for potential as novel therapeutic targets for diet-induced obesity and related metabolic disorders.

Although gut microbiota play an important role in regulating host energy metabolism, the underlying molecular mechanisms are largely unknown. Compounds produced by gut microbiota, such as short-chain fatty acids might be related to the regulation of host energy metabolism. Brown adipose tissue has a high thermogenic capacity and plays an important role in maintaining body temperature in cold environments. Importantly, the thermogenic activity of brown adipose tissue caused by brown adipocytes or beige adipocytes is inversely correlated with body fat contents in adult humans. Therefore, brown adipose tissue is an attractive target for the management of obesity. Recently, the relationships between brown adipose tissue function and gut microbiota have been reported, and compounds produced by gut microbiota from diet could regulate brown adipose tissue activity. In this review, we describe our current knowledge of the metabolites derived from diet by gut microbiota which may regulate brown adipose tissue function and possible molecular mechanisms underlying them.

ω6-polyunsaturated fatty acid (PUFA) metabolites represented by arachidonic acid promote inflammation, while, ω3-PUFAs represented by eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are known to exhibit anti-inflammatory effects. However, the mechanisms of anti-inflammatory effects by ω3-PUFA have been unknown for a long time. Recent progress of lipid analytical techniques allow the identification of ω3-PUFA-derived lipid mediators such as resolvins, protectins, and maresins with pro-resolution and anti-inflammatory activities. We recently identified 17,18-epoxyeicosatetraenoic acid (17,18-EpETE) as a new class of EPA metabolites, which has anti-allergic and antiinflammatory effect. The anti-inflammatory effects were mediated by inhibiting neutrophil migration in G protein-coupled receptor (GPR) 40-dependent manner. Accumulating evidence has revealed that ω3-PUFAs affect the intestinal bacteria, and consequently regulate immune systems. In this review, we introduce the development and control of allergic and inflammatory diseases via changes in lipid metabolites and intestinal bacteria initiated by dietary oils.