The integral role of gut microbiota in the physiological regulation of host energy metabolism has attracted considerable attention. A number of studies have shown that obesity and metabolic disorders are associated with profound changes in gut microbiota 
. However, mechanistic insights are lacking, and whether microbiota dysfunction plays a causal role in the pathogenesis of metabolic diseases is unclear. In particular, how microbiota-derived metabolites, such as SCFAs, interact with host nutrient sensing pathways to modulate energy metabolism is poorly understood. Although SCFAs have been postulated to regulate gut hormone secretion, in vivo evidence was scant, and the downstream signaling pathway was not characterized. In this study, we systematically examined the effects of each major SCFA naturally present in the colon–butyrate, propionate, and acetate–on energy metabolism and gut hormones. We found that all three SCFAs protected against diet-induced obesity, with butyrate and propionate being more effective than acetate. Butyrate and propionate regulate body weight at least partially by inhibiting food intake, consistent with their stimulatory effects on anorexigenic gut hormones. In contrast, acetate inhibited weight gain independent of food intake suppression and had no acute effect on gut hormones. Our finding on the hypophagic effect of butyrate differs from a previous report, which concluded butyrate supplementation led to hyperphagia 
. As Gao et al. presented weekly food intake normalized to body weight, which was already significantly lower in butyrate-fed mice after the first week of diet switch, the acute hypophagic effect of butyrate may have been masked in that study. However, the current data cannot rule out potential impact on food intake due to altered palatability by butyrate and propionate supplementation. Although mice fed SCFA-supplemented diets did not display overt signs of malaise, additional studies will be required to formally address this possibility. In addition to effects on food intake, changes in energy expenditure likely also contribute to body weight regulation by SCFAs. Butyrate-treated mice showed an increased capacity for cold-induced adaptive thermogenesis 
. Systemic administration of propionate acutely increases heart rate 
. And acetate-treated rats were reported to have increased oxygen consumption 
. Future studies will be needed to determine the contributions of these mechanisms to energy homeostasis in the chronic setting.
SCFA levels in the gut lumen were reported to reach high mM levels in the colon of human and pigs, but are much lower in jejunum and ileum 
. In the current study, the doses of SCFAs chosen were in the high mM range, likely achieving supraphysiological levels in vivo, especially in the proximal intestine. Thus, the effects on gut hormones may not reflect physiological action of SCFAs, but suggest potential benefit of pharmacological SCFA treatment. Orally administered SCFAs can likely reach the proximal small intestine within 10 minutes–the early time point chosen for the acute studies–as gastric emptying of a liquid bolus in mice is very rapid and can exceed 80% within 15 minutes 
. However, it’s unlikely that the oral SCFA bolus would reach the distal small intestine and colon within 10 minutes. Therefore, the effects on GLP-1 and GIP may reflect direct stimulation of enteroendocrine cells in the proximal small intestine, where both L cells and K cells can be found. On the other hand, L cells that express PYY are only present in the colon and distal small intestine, suggesting an indirect mode of action of butyrate on this gut hormone.
Butyrate- and propionate-dependent inhibition of food intake and weight gain was intact in Ffar3
knockout mice, implicating other endogenous mediators in these effects. It's worth noting that Ffar3
knockouts on control HFD showed modest hyperphagia but had normal body weight and adiposity. This is consistent with a previous report showing accelerated intestinal transit and increased fecal energy excretion in an independent Ffar3
knockout line 
, suggesting FFAR3 is required for normal gut motility and nutrient absorption. The mechanisms responsible for increasing food intake and body weight normalization in the absence of FFAR3 are unknown. SCFAs were shown to regulate leptin secretion from the adipose tissue. However, the role of FFAR3 in mediating these effects remains controversial 
. We showed that Ffar3
knockouts maintain normal plasma leptin levels, suggesting that leptin has no major role in normalizing energy homeostasis in Ffar3
knockout mice were recently reported to have reduced resting heart rate and sympathetic activity 
. However, this effect is expected to reduce energy expenditure and cannot explain the reduced feed efficiency of Ffar3
Despite an intact anorectic response to butyrate, Ffar3
knockouts showed an attenuation of butyrate-stimulated GLP-1 release. Fasting GLP-1 levels were normal in Ffar3
knockouts. These data suggest FFAR3 plays a role in nutrient sensing in L cells but is not required for basal GLP-1 release. Conversely, Ffar3
knockouts showed largely normal GIP stimulation by butyrate and propionate and an increased sensitivity to acute ghrelin suppression by butyrate. One potential explanation is that alterations in gastrointestinal motility in Ffar3
knockouts may differentially affect delivery of orally administered SCFAs to enteroendocrine cell types along the proximodistal axis of the gut, contributing to the different sensitivities of various gut hormones to FFAR3 deficiency. Alternatively, FFAR3 may act as the primary butyrate sensor in L cells, while FFAR2 or additional SCFA sensors may play a more important role in other enteroendocrine cell types. Although butyrate displays poor agonism against FFAR2 in vitro, the local level present in the gut lumen after an oral administration may be sufficient to activate FFAR2 in the intestinal epithelium. In contrast to the adipose tissue, where FFAR2 expression tended to be reduced in FFAR3-deficient mice 
, we found FFAR2 mRNA expression to be normal in intestinal mucosa in Ffar3
knockouts (data not shown). However, we cannot rule out the possibility that, in the absence of FFAR3, FFAR2 expression may be selectively up- or down-regulated in enteroendocrine cells, which only account for <5% of the intestinal epithelium. A recent report showed Ffar2
knockout mice on HFD are protected from diet-induced obesity, most likely due to increases in energy expenditure and fecal energy output 
. Future studies will be needed to determine the effects of SCFAs on body weight regulation and gut hormones in Ffar2
Butyrate is a well-known histone deacetylase inhibitor and affects gene transcription 
. Butyrate was also shown to regulate autophagy in colonocytes by acting as an energy source 
. However, these mechanisms are unlikely to impact gut hormone release acutely on the scale of minutes. The niacin receptor GPR109A can also be activated by butyrate in the mM range and is expressed on the apical surface of colonic epithelium 
. Its role in the regulation of gut hormone secretion and body weight remains to be determined.
In summary, the present findings demonstrate butyrate and propionate regulate gut hormone release, suppress food intake, and protect against diet-induced obesity. We also show that FFAR3 is required for maximal GLP-1 induction by butyrate, but is dispensable for butyrate- and propionate-dependent effects on body weight and GIP stimulation. As enteroendocrine nutrient-sensing and the incretin axis are subjects of intense interest in drug discovery for metabolic disorders 
, future studies to determine the signaling mechanisms responsible for SCFAs' beneficial effects may have a major impact on the development of novel therapies for diabetes and obesity.