We sampled plasma from 109 females with active AN, 108 females who had recovered from AN, and 110 healthy controls and evaluated concentrations of the microbiota metabolites SCFAs. We successfully analyzed seven SCFAs using a novel high-throughput LC-MS method and found a significantly lower concentration of butyric acid in plasma from individuals actively ill as well as recovered from AN, compared with healthy controls. Although this partially aligns with some prior results—lower fecal levels of butyrate and butyrate-producing species have previously been reported in active AN[9; 10; 22]—it has to our knowledge, not been documented to persist after recovery from AN, in neither plasma nor feces. In addition, we observed reduced plasma concentrations of isobutyric- and isovaleric acid in both AN groups compared to CTRL. Taken together the lower concentration of these three SCFA in both AN and AN-REC compared with CTRL indicates that the changed SCFA profile is not a direct effect of weight loss as has been discussed previously[26] since AN-REC per definition were weight recovered for at least one year prior to blood sampling.
Contrary to our results, isobutyrate has previously been reported to be increased in fecal samples from AN patients both at hospital admission and the end of the hospital stay when weight was partially recovered[14]. We also report a significant positive correlation between BMI overall and isobutyric concentrations. When evaluating the correlations between BMI and SCFA concentrations stratified by the three groups, we observed significant negative correlations between BMI and plasma SCFA concentrations for propionic- and succinic acid in the AN group only. This indicates a specific association between these gut microbiota metabolites in the extremely low BMI/AN condition. The lower fecal propionate and acetate previously reported in AN[9; 10] were not reflected in lower plasma concentrations in our study. On the other hand, the propionate-producing microbe Akkermansia muciniphila, a known modulator of metabolism, has been reported to be increased in fecal samples from individuals with AN, whereas reduced levels have been seen in individuals with obesity[22; 27]. It is, however, important to mention that there does not seem to be any clear correlation between fecal and plasma SCFA concentrations, likely reflective of SCFA absorption efficiency [28; 29], thus our results in plasma cannot be directly compared with previous results on fecal samples.
We also report a significantly higher concentration of succinic acid in AN with reported use of laxatives compared with those not reporting such use. This difference is seen despite the fact that we were not able to control the time since the last laxative use episode, the frequency of usage nor the type of laxative. However, laxatives have been shown in mice to cause long-term changes in gut microbiota, i.e., a new steady state in the microbiota, and extinction of key taxa [30]. Furthermore, we report significantly lower plasma isovaleric acid concentration in AN-REC with reported binge eating compared with non-binge eating AN-REC. However, no differences were identified when evaluating reported binge eating in active AN patients.
The potential causality between deviant SCFA plasma concentrations and AN cannot be elucidated with the present design. However, preclinical and clinical studies, including fecal transplantation, support the role of the gut microbiome and SCFA in weight regulation[15; 16; 19; 20]. Additional evidence for the role of SCFA in energy homeostasis[31; 32] includes their ability to stimulate secretion of food intake by inhibiting gut hormones, glucagon-like peptide-1 (GLP-1), and peptide YY (PYY)[33; 34]. Fermentable carbohydrates, as well as intraperitoneal administration of acetate, have also been reported to increase signaling in the food intake regulating centers of the hypothalamus[35; 36; 37]. Therefore, we speculate that the aberrant SCFA profile in AN may interfere with the systems regulating food intake and weight, thereby supporting underweight and negative energy balance.
Other routes by which SCFA could influence weight and food intake regulation are via hypothalamic inflammation and microglia. SCFA—in particular butyrate—here shown to be lower in AN and AN-REC plasma, are crucial for microglia maturation and activation. These brain cells appear to require continuous stimulation from the gut microbiota to remain mature[38; 39]. Microglia are supportive glial cells in the CNS that, among others, respond to brain injury and are involved in immune activation[40]. We previously reported activation of microglia selectively in the food intake regulating systems in the hypothalamus of an animal model of AN, the anx/anx mouse[41]. Similarly, hypothalamic inflammation, including activation of microglia and astroglia, has been documented in diet-induced obesity of rodents as well as in humans with obesity prior to the onset of weight gain[42; 43]. Recent data show that the gut microbiota regulates western diet-induced hypothalamic inflammation and microglia maturation via a GLP-1R-dependent mechanism in astroglia[44]. In addition, microglial activation has been documented in several additional psychiatric disorders[45; 46; 47], whereas deviations in SCFA have been documented in autism spectrum disorder[48; 49]. Thus, taken together these findings support a role for SCFA-induced hypothalamic microglia activation being involved in weight and food intake regulatory aspects of AN.
The main limitation of our study is the cross-sectional design, which precludes inferences about causality. Another limitation is the blood sampling procedure, as blood
samples were sent via overnight mail before being processed. Even if this may have influenced the absolute SCFA concentration [50], it is unlikely to have affected group differences since all samples were collected and handled in this same way. We did in fact detect plasma concentrations of butyrate higher than previously observed in any dataset analyzed in the laboratory (data not shown), while formate was somewhat lower than typically observed[51]. However, this was true also for the healthy controls, and plasma butyrate as well as formate concentrations, in similar ranges as reported here, have previously been reported[52; 53; 54; 55]. Furthermore, valeric and caproic acid were both excluded since the quality control of LC-MS analysis indicated high inter-batch variation and low run-rerun correlation in another cohort originating from our laboratory (Lavebratt et. al., unpublished), respectively, likely due to the very low concentrations of these SCFA in plasma. However, isovaleric acid was also low in plasma, and these results thus should be interpreted with caution. In addition, we required a one-year duration of recovery from AN, which is consistent with many definitions of recovery but it may not be adequate for the normalization of the microbiota[56]. It is also possible that the dietary habits of the AN-REC group still differ from individuals without any history of an eating disorder[57] in ways that could influence the gut microbiota, e.g., the amount of ingested dietary fibers and resistant starch. Future research should evaluate plasma concentrations of SCFA in AN-REC with a longer time since recovery. The optimal design would be a longitudinal study that followed individuals over the course of treatment through recovery that measured SCFA concentrations combined with dietary records to decipher the effect of nutritional recovery post AN on the gut microbiota and its metabolites. Lastly, it would also be valuable to further explore the effects of SCFA on glial cells and neurons of interest for the AN pathology, i.e., neurons involved in energy homeostasis.
In conclusion, we report significantly lower plasma concentrations of butyric-, isobutyric-, and isovaleric acid in individuals with active AN—as well as in individuals who had been recovered from AN for at least one year—compared with healthy controls. The data suggest an opportunity to test the augmentation of standard treatment with these specific metabolites as a potential route to support, enhance, and maintain weight recovery in AN[58]. Such precision approaches may be more acceptable to patients than, for example, fecal transplantations, which have been tested with some success[16; 59]. Given the documented poor outcome and absence of any effective medications for the treatment of AN, novel approaches to enhance treatment with strategies that are acceptable to patients are of considerable urgency.