1. Introduction
Rheumatic disorders are among the most prevalent chronic diseases of the musculoskeletal system and connective tissue [
1]. The most common inflammatory rheumatic disease, rheumatoid arthritis (RA), is a debilitating, chronic autoimmune disorder affecting approximately 1% of the world population [
2,
3]. RA is considered a heterogeneous and complex disease where both genetic and environmental risk factors contribute to disease [
4]. While several genetic factors have been associated with the risk of RA, among them
major histocompatibility complex (MHC) class I alleles, in particular DR4 [
5] that recognizes the DRB1*04 gene products, the genetic associations alone are insufficient for explaining disease onset, and additional environmental triggers have been proposed.
One important environmental trigger appears to be the diet. Notably, in the last decades, an increase in the incidence of autoimmune and allergic diseases has been documented in developed countries worldwide [
6], which coincides with dramatic changes in diet, including reduced fiber intake [
7]. Dietary fibers are complex carbohydrates consisting of both soluble and insoluble components. The insoluble fibers have important bulking properties, whereas the soluble forms are fermented by certain species of the gut microbiota, leading to physiologically active metabolites. Short-chain fatty acids (SCFA) are among the most abundant of these active metabolites representing a fuel source for intestinal epithelial cells [
8], shaping the gut morphology and function [
9], and serving as an energy source for certain bacterial species [
10].
Diet affects the diversity of the gut microbiota and thus its secreted metabolites [
11,
12,
13,
14]. These findings combined with recent observations in animal models and humans further highlight the importance of the gut–joint axis and make dietary interventional studies—already conducted in the last century—again up to date. Various dietary intervention studies in RA patients have been published in the last century [
15,
16,
17,
18,
19], including double-blinded, placebo-controlled clinical trials using n-3 fatty acid supplementation, short-term fasting [
20,
21], vegan, vegetarian, or Mediterranean diets [
22,
23]. Among those, vegetarian [
20] and Mediterranean [
23]-based dietary interventions showed an attenuation of disease activity in RA patients. Following a one-year vegetarian diet, Kjeldsen-Kragh et al. report on statistically significant improvements in the number of tender joints, number of swollen joints, pain, and duration of morning stiffness [
20]. Also, a strict three-month Mediterranean-based diet in RA patients ensued clinical benefits, which were shown by a significant reduction in DAS28 scores and improved life quality assessed by Health Assessment Questionnaire (HAQ) and SF-36 health surveys compared to RA control patients on standard diets [
23]. That is of particular interest, as both the vegetarian and the Mediterranean diet are especially high in fiber, and were shown to significantly increase SCFA levels [
13]. Moreover, daily probiotic supplementation of RA patients using capsules filled with 108 colony-forming units of live gut bacteria species of the genus
Lactobacillus casei for two months exert beneficial effects on arthritis by attenuating DAS28 scores and reducing pro-inflammatory cytokine levels [
24]. Once more, the inoculation of probiotic Lactobacillus strains was shown to modulate the gut microbiota and specifically increase SCFA concentrations [
25]. Taken together, one might suggest that primarily, those dietary interventions or probiotic supplementation studies that also increase SCFA levels are most effective in attenuating clinical RA symptoms.
Encouraged on recent findings about the immunomodulatory impact of SCFA [
26,
27,
28,
29,
30,
31] and reports specifically highlighting their benefits in inflammatory arthritis and bone homeostasis [
32,
33,
34,
35] in mice, we have conducted a short-term feasibility study using high-fiber bar or cereal dietary supplementation in RA patients. Tregs act as immune regulatory cells [
36] and beneficially affect bone homeostasis [
34,
37,
38,
39], and are considered to malfunction in RA [
40]. Therefore, Tregs were used as the primary end-point of our here presented feasibility study in RA patients. We hypothesized, based on own previous work [
35,
37,
38,
39] and the Treg-inducing effects of SCFA [
26,
29], that within our dietary high-fiber supplementation study, increased Treg numbers would most likely beneficially affect arthritis and related bone loss. In order to avoid changing the participating patients’ lifestyle or daily eating behaviors, our study intervention was restricted to basically eating one single high-fiber bar (or equal cereal portion) daily over a 28-day study period. First, this approach was shaped based on the widely articulated desire of RA patients for add-on nutritional treatments to support ongoing therapies disease modifying anti-rheumatic drugs. Second, this approach was based on the ambivalent attitude of patients toward the ingestion of pharmaceutical-made nutritional supplementation. Moreover, a nutritional approach would theoretically allow us to equally and optimally fuel all known SCFA-producing pathways [
16] contrasting pharmaceutical forms of direct SCFA supplementation, and enables personalized adaptations with further dietary supplements that may be needed.
4. Discussion
Most dietary fibers are fermented by gut bacteria, and thus give rise to the microbial metabolites SCFA. The three main SCFA detected in our bodies are acetate, propionate, and butyrate. A decreased intake of dietary fibers and increased intake of fat and sugar in our food, which is typical for a Western lifestyle, were shown to contribute to intestinal microbial dysbiosis by the depletion of specific bacterial taxa of the gut microbiota [
48]. Microbial dysbiosis in the gut was shown to promote severe immunological dysfunctions [
49] that in turn may contribute to the immune dysbalance observed in RA [
36]. In this context, it was proposed that inflammatory diseases may, at least in part, be affected by dietary fiber supplementation, arguing for attempts to overcome the ‘‘fiber gap’’ through selective fiber-rich adjustments in our diet [
50].
We have previously shown that a high-fiber diet or direct SCFA supplementation can attenuate experimental arthritis and inhibit bone loss [
33,
35]. At the same time, microbial dysbiosis has been associated with arthritis, pointing toward a gut–joint axis in the onset of arthritis. For example, as early as the 1970s, it was shown that intestinal infections frequently preceded the occurrence of clinical arthritis [
51]. Later, Kohashi et al. demonstrated in 1979 that germ-free (GF) rats develop a particularly severe form of RA in an adjuvant-induced arthritis model [
52]. Other studies have shown the effects of both Gram-positive or Gram-negative intestinal bacteria on arthritis, further suggesting that the gut microbiota has a significant influence on the development of RA [
52,
53]. Additional data showed that the intestines of germ-free HLA-B27 rats or B10.BR (h-2 (k)) mice—these animal models are predisposed to develop spontaneous arthritis—following selective repopulation with different bacterial strains were even more prone to develop arthritis [
54,
55]. These data were recently complemented by showing that the onset and severity of clinical arthritic symptoms in genetically susceptible mice depends on the gut microbiota composition [
56,
57,
58].
In an effort to translate these observations obtained in animal models to humans, 16s rRNA sequencing methods were used in order to correlate individual bacterial strains within the gut microbiota and the onset of clinical signs in human RA patients [
59,
60]. These data showed that commensal gut bacteria belonging to the Prevotellaceae family are enriched in RA patients, especially in pre-clinical stages of RA [
61,
62,
63,
64]. However, discrepancies in the pro-arthritogenic and anti-arthritogenic effects of specific gut microbes used in animal models, together with the so-far indistinct results of human 16s rRNA microbiota sequencing, suggest that there may be no single gut bacterial strain that is promoting or inhibiting arthritis on its own. In contrast, distinct bacterial communities or their secreted bacterial metabolites may function as environmental triggers responsible for the onset or prevention of arthritis. Taken together, existing human studies strengthen the gut–joint axis by identifying a microbial dysbiosis early in human arthritis that is partially normalized during the treatment of arthritis [
65].
Although attenuating effects on RA pathology were observed in dietary intervention studies, none of them has been successfully implemented in the clinics so far. The reasons for that might be: (i) a scarcity of acceptance and persuasion of rheumatologists on this topic, (ii) unidentified underlying immunological mechanisms explaining the improvement in RA patients, or (iii) the existence of therapeutically very effective DMARDS that make dietary alternatives less prominent. However, all dietary intervention therapies require changes to the patients’ daily lifestyle and eating behaviors, and that requires commitment, endurance, persuasion, and time. Therefore, in the current study, we considered a simple dietary supplementation using high-fiber bars or cereals to close the “fiber gap”, while leaving the standard nutrition of each individual untouched.
The aim and primary end-point of our here presented feasibility study in RA patients was to examine the effect of short-term high-fiber dietary supplementation on Treg numbers. We hypothesized that dietary supplementation with fiber can strengthen regulatory immune elements, thereby helping to control and even suppress disease progression in RA, and in consequence inhibit bone loss and the impairment of function. Tregs, which act as immune regulatory cells [
36] and beneficially affect bone homeostasis [
34,
37,
38,
39] are considered to malfunction in RA [
40]. Therefore, Treg-supporting effects, as observed in our dietary high-fiber supplementation study, are the most likely to beneficially affect arthritis and related bone loss. Alongside the increased Treg numbers observed in our feasibility study, RA patients also showed beneficial Th1/Th17 cell ratios, lowered citrullinated vimentin peptide 18 and serum IgA concentrations, and improved parameters for quality of life, as shown by the HAQ and SF36 questionnaire outcomes. It was shown previously that RA patients exhibit a decreased Th1/Th17 ratio compared to healthy individuals [
66,
67]. Interestingly, dietary high-fiber supplementation, as shown here in our feasibility study, skewed the ratio in the opposite direction, initiating anti-arthritic processes. While the impact of IgG autoantibodies on the development of RA has been intensively studied, the role of IgA autoantibodies is still less well defined, but potentially more relevant for understanding intestinal function [
68]. Synovial biopsies of RA patients revealed that up to 38% of plasma cells produced IgA [
69], and subjects at risk for developing RA showed an increased percentage of IgA-producing peripheral blood plasmablasts [
70]. These data suggest that reducing serum IgA levels in RA patients may represent another health-promoting effect observed in our feasibility study. In addition, citrullinated vimentin was shown to be highly abundant in RA synovial tissues [
71], and among the possible 43 putative citrullinated arginine residues, the citrullinated vimentin 58-GRVYATRSSAVR-69 (p18) peptide had one of the highest reactivities with RA sera [
72]. An epitope spreading of RA-related immunity against citrullinated peptides has been observed in the evolution of RA [
73,
74], while data from our here presented study suggest that some of these changes may indeed be at least partially reversible, as shown for reduced p18 concentrations, and that dietary fiber supplementation is a key driver of this reversal.
Calprotectin is a 24-kDa dimer of calcium-binding proteins S100A8 and S100A9. The measurement of fecal calprotectin has emerged as one of the most useful tools for quantifying intestinal inflammation [
75], and zonulin controls intestinal permeability and disintegrates intestinal tight junctions [
76,
77]. Short-term dietary high-fiber supplementation clearly supported gut homeostasis by reducing the intestinal inflammation marker, calprotectin [
72] and the intestinal barrier marker zonulin [
69] in our feasibility study in RA patients. Zonulin expression has been recently shown to be augmented in autoimmune conditions associated with tight junction dysfunction, including celiac disease (CD) and Type 1 diabetes (T1D) [
78,
79]. Interestingly, both animal studies [
80] and human trials [
81] using the zonulin synthetic peptide inhibitor AT1001 (Larazotide acetate) established that zonulin is integrally involved in the pathogenesis of autoimmune diseases. Therefore, decreased zonulin levels by dietary high-fiber supplementation might be a valuable additional beneficial therapeutic effect for RA patients. Following our high-fiber supplementation period, we also observed a tendency (
p = 0.0558) toward higher eosinophil numbers. This is of potential further interest, especially with respect to a recent study highlighting a beneficial effect of eosinophils in animal models of arthritis [
82], whereas the translational proof in RA patients is still missing to date.
As a limitation of this first short-term feasibility study, RA patients were recruited independently of their current treatment, not showing substantial inflammatory activity at baseline due to stable effective therapies. In addition, no control group was included. Hence, future follow-up studies should include new-onset RA and active RA patients, taking into account different DMARD therapies as well as longer, up to three-month dietary supplementation periods using a randomized cross-over design. A further limitation is that easy-to-use high-fiber supplementation bars or cereals yet need to be improved for stability and optimal daily dosage. On the other hand, the here presented data from our first high-fiber dietary supplementation feasibility study in RA patients clearly showed the potential of nutritional medicine in supporting ongoing standard therapies.
In summary, in this short-term high-fiber diet-based dietary supplementation feasibility study, the physical functioning and quality of life in RA patients was significantly improved, correlating with improved markers of intestinal homeostasis, thereby establishing a stage of immune tolerance by increased Treg numbers in patients with RA. However, these findings should be regarded as preliminary and interpreted with caution, since baseline versus follow-up comparisons in a single-arm study do not necessarily prove a causal treatment effect.