Immunomodulatory dietary polysaccharides: a systematic review of the literature

Polysaccharide-rich fungi and plants have been employed for centuries by cultures around the world for their dietary and medicinal benefits [1‐5]. Often thought to merely support normal bowel function and blood glucose and lipid levels [6‐8], certain polysaccharides have attracted growing scientific interest for their ability to exert marked effects on immune system function, inflammation and cancers [9‐11]. Many of these chemically and structurally diverse, non- to poorly-digestible polysaccharides have been shown to beneficially affect one or more targeted cellular functions in vitro [11‐16], but much of the in vivo literature consists of studies in which polysaccharides were injected [1, 2]. For clinicians and scientists interested in immunologic effects following dietary intake, the value of such studies is uncertain. Polysaccharides that elicit effects in vitro or by injection may be ineffective or have different effects when taken orally [17]. We thus decided to conduct a systematic review to evaluate the specific immunologic effects of dietary polysaccharide products on rodents and human subjects.

A total of 62 rodent publications (Tables 1, 2 and 3) and 15 human publications (Table 4) were deemed appropriate for inclusion in this review. Available structural and compositional information for these immunomodulatory polysaccharides are provided in Table 5 and safety information is provided in Table 6. The majority of animal studies explored models in which animals were injected or implanted with cancer cells or tumors, were healthy, or were exposed to carcinogens. Other studies investigated immunodeficient, exercise-stressed, aged animals, or animals exposed to inflammatory agents, viruses, bacterial pathogens, pathogenic protozoa, radiation or mutagens. Human studies assessed immunomodulatory effects in healthy subjects, or patients with cancers, seasonal allergic rhinitis or aphthous stomatitis. Because of the limited number of human studies, we included some promising open-label controlled trials. Human study durations ranged from four days to seven years; daily doses ranging from 100-5,400 mg were reported to be well-tolerated.

A number of studies in healthy human adults demonstrated immune stimulating effects of oral polysaccharides. Arabinogalactans from Larix occidentalis (Western larch) were shown in RCTs to increase lymphocyte proliferation and the number of CD8+ lymphocytes [18] and to increase the IgG subtype response to pneumococcal vaccination [19]. A furanose extract from Panax quiquefolium (North American ginseng) was shown in an RCT of healthy older adults to decrease the incidence of acute respiratory illness and symptom duration [20]. Finally, an RCT of healthy adults consuming Undaria pinnatifida (wakame) fucoidans found both immune stimulating and suppressing effects, including increased stromal-derived factor-1, IFN-g, CD34+ cells and CXCR4-expressing CD34+ cells and decreased blood leukocytes and lymphocytes [21].

Studies in healthy animals showed a number of immune stimulating effects of various glucan products from Agaricus subrufescens (A. blazei) (aqueous extracts [22], aqueous extracts with standardized β-glucans [23], α-1,6 and α-1,4 glucans [24], and whole plant powders [25]); Lentinula edodes (shiitake) (lentinan [26] and β-glucans [27]); Saccharomyces cerevisiae (β-1,3-glucans [27, 28]); Laminaria digitata (laminarin [29]); Sclerotium rofsii (glucan phosphate [29]); Sclerotinia sclerotiorum (SSG [30]); and Phellinus linteus (powder [31] and aqueous, alcohol-precipitated extract [32]). A furanose extract from P. quiquefolium and pectins from Buplerum falcatum and Malus (apple) spp. have also been shown to enhance immune function in healthy young animals [33‐35]. Cyamopsis tetragonolobus galactomannan (guar gum) or highly methoxylated pectin feeding exerted numerous stimulating effects on antibody production in older animals [36].

Evidence for the effectiveness of oral polysaccharides against infection and immune challenges has been mainly demonstrated in animals. Immune stimulating effects have been shown in resting and exercise-stressed animals with thioglycollate, clodronate, or HSV-1 injections fed Avena (oat) spp. soluble glucans [37‐41]; animals injected with or fed E. vermiformis and fed Avena spp. particulate glucans [42, 43]; animals with E. coli injections fed L. digitata glucans (laminarin) [44]; animals with HSV injections fed U. pinnatifida fucoidans [45]; animals with Staphylococcus aureus or Candida albicans injections fed S. cerevisiae glucans (scleroglucan) [29]; and animals with fecal solution injections fed an aqueous extract of A. subrufescens (A. blazei Murrill) [46].

Additional controlled human and animal studies have shown anti-inflammatory and anti-allergy effects of some polysaccharide products. In an RCT of adults with seasonal allergic rhinitis, S. cerevisiae β-1,3;1-6 glucans decreased IL-4, IL-5 and percent eosinophils, and increased IL-12 in nasal fluid [47], while a placebo-controlled study of patients with recurrent aphthous stomatitis (canker sores) consuming β-1,3;1-6 glucans found increased lymphocyte proliferation and decreased Ulcer Severity Scores [48].

Animal models of inflammatory bowel disease have shown anti-inflammatory effects of Cladosiphon okamuranus Tokida fucoidans [49], Cyamopsis tetragonolobus galactomannans [50], Malus spp. pectins [51], and mixed polysaccharide supplements [52]. Animals challenged with ovalbumin have demonstrated anti-inflammatory/allergy effects of A. subrufescens aqueous extracts [22], an aqueous extract of Ganoderma tsugae [53], and Pyrus pyrifolia pectins [54]. Anti-inflammatory effects have also been seen in animals with cotton pellet implantations fed a Pholiota nameko heteroglycan (PNPS-1) [55].

Trametes versicolor glucans have demonstrated anti-cancer effects in humans. In two RCTs and five controlled trials, PSK from T. versicolor mycelia increased survival of advanced stage gastric, colon and colorectal cancer patients [56‐62] with one study showing increased immune parameters (including blood NK cell activity, leukocyte cytotoxicity, proportion of helper cells and lymphocyte suppressor cells) [62]. An RCT of advanced stage lung cancer patients consuming PSP from T. versicolor fruit bodies found increased IgG and IgM antibodies and total leukocyte and neutrophil counts, along with a decrease in the number of patients withdrawing from the study due to disease progression [63]. An RCT of ovarian or endometrial cancer patients consuming A. subrufescens glucans showed increased NK cell activity and fewer chemotherapy side effects [64].

In numerous animal models of cancer, a wide range of polysaccharides have shown anti-tumorogenic effects. Glucan products sourced from A. subrufescens demonstrating anti-cancer activities in animal models include an aqueous extract [65], an aqueous, acid-treated extract [66], and an aqueous extract with standardized levels of β-glucans [23]. Anti-cancer effects have been reported following intake of aqueous extracts of G. lucidum [67‐69]; the powder and D fraction of G. frondosa [70‐72]; Hordeum vulgare β-glucans [73‐76]; Laminaria angustata powder [77]; Lentinula edodes products (powders [70, 78, 79], SME [80], β-glucans [27], and lentinan [81, 82]); Pleurotus ostreatus powder [70], Saccharomyces cerevisiae particulate β-1,3;1,6 and β-1,3glucans[27, 73]; and a glucan from Sclerotinia sclerotiorum (SSG) [30, 83]. A glucomannan from L. edodes (KS-2) improved survival of animals with cancer cell injections [84]; apple and citrus pectins have exerted anti-cancer effects, including decreased tumor incidence [85‐90]. Finally, heteroglycans from Lycium barbarum (LBP_3p), Lentinus lepidus (PG101) and A. subrufescens (ATOM) demonstrated a number of immune stimulating effects in animal cancer models [91‐93]. Interestingly, only one animal study has been performed using glucans from T. versicolor (PSP): animals with cancer cell implantations showed decreased tumor growth and vascular density [94].

Most polysaccharide products appear to be safe, based on NOAEL, acute and/or chronic toxicity testing in rodents (Table 6). As would be expected, powders, extracts and products that have not been fully characterized pose the most concerns. Other than for aloe vera gel, which was shown in a small human trial to increase the plasma bioavailability of vitamins C and E [95], the impact of polysaccharide intake on the absorption of nutrients and medications is not known. While one rat toxicity study raised concerns when guar gum comprised 15% of the daily diet [96], the product was safe in humans studies when 18-39.6 g/day was consumed for up to a year (Table 4). Product contamination may explain three case reports of hepatotoxicity and/or death following intake of an A. subrufescens aqueous extract [97]. Seven animal studies reporting positive immunologic effects of A. subrufescens extracts in healthy animals or animals with cancers found no evidence of toxicity (Tables 1 and 2). In humans, six weeks of A. subrufescens glucans intake was safe for cancer patients, and four months of 3 g/day intake by 24 healthy adults and 24 adults with liver disease reported no evidence of toxicity (Table 4). Another case report associated liver toxicity with G. lucidum intake, but the elderly subject also took an unidentified product a month previous to her admission for testing [98]. Three animal studies reported immunologic benefits and no adverse effects following intake of G. lucidum aqueous extracts; in one study intake was 5% of the diet for 5 months (Table 1). While adverse effects were also reported in a study in which 10 adults consumed 4 g/day L. edodes powder for 10 weeks [99], immunologic animal studies reported no ill effects of either L. edodes powder (5 studies, up to 5% of the diet up to nine months) or extract (7 studies, up to 40 days intake) (Tables 1 and 3). Finally, while intake of 319 mg/kg of an aqueous extract of P. ostreatus by mice for 1 month caused hemorrhages in multiple tissues [100], there was no reported toxicity when mice consumed the mushroom powder as 5% of their diet for nine months (Table 3). While ≥1 gram/day of T. versicolor glucan products were safely consumed by cancer patients for up to 10 years, the long-term effects of ingestion of the other polysaccharide products discussed in this review is also not known.

The majority of studies that qualified for inclusion in this review employed models investigating immune stimulation; fewer explored anti-inflammatory effects. Animal studies reported immune system effects in the gut, spleen, bone marrow, liver, blood, thymus, lungs, and saliva; controlled human studies reported evidence of immune stimulation in the blood, anti-inflammatory effects in nasal lavage fluid and improved survival in cancer patients. The literature is highly heterogenous and is not sufficient to support broad structure/function generalizations. For the limited number of studies that investigated well-characterized, isolated products (primarily glucan products), effects can be unequivocally attributed to polysaccharides. Such associations are certainly more tenuous when considering product powders or products obtained by extraction methods designed to isolate polysaccharides, but without complete compositional analyses.

Dietary polysaccharides are known to impact gut microbial ecology [101, 102], and advances in microbial ecology, immunology and metabolomics indicate that gut microbiota can impact host nutrition, immune modulation, resistance to pathogens, intestinal epithelial development and activity, and energy metabolism [103‐107]. Other than fucoidans, the polysaccharides discussed in this review appear to be at least partially degraded by bacterial enzymes in the human digestive tract (Table 7). Arabinogalactans, galactomannans, a glucan (laminarin), glucomannans, and mixed polysaccharide products (Ambrotose^® products) have been shown to be metabolized by human colonic bacteria. Orally ingested fucoidans, glucans and mannans (or their fragments) have been detected in numerous tissues and organs throughout the body [73, 108, 109], (Carrington Laboratories, personal communication). We know of no study that has determined the specific identity of orally-ingested polysaccharide end products in animal or human tissues.

One can only speculate upon the mechanisms by which the polysaccharides discussed in this review influence immunologic function, particularly when one considers the exceedingly complex environment of the GI tract. It is possible that fragments of polysaccharides partially hydrolyzed by gut bacteria may either bind to gut epithelia and exert localized and/or systemic immune system effects, or be absorbed into the bloodstream, with the potential to exert systemic effects. Current studies investigating the link between the bioconversion of dietary polysaccharides, their bioavailability and their downstream effects on the host metabolism and physiology are utilizing metabolomic and metagenomic approaches that can detect and track diverse microbial metabolites from immunomodulatory polysaccharides [103]. These and other innovative approaches in the field of colonic fermentation are providing novel insights into gut microbial-human mutualism [110, 111], its impact on regulating human health and disease, and the importance of dietary modulation [112‐115].

Additional RCTs of well-characterized products are needed to more completely understand the immunomodulatory effects and specific applications of oral polysaccharides. Such studies will need to better investigate the optimal timing and duration for polysaccharide ingestion. That is, should they be consumed continuously, before, at the time of, or after exposure to a pathogen or environmental insult? Only a few studies have actually investigated the impact of timing of polysaccharide intake to achieve optimal benefits. Daily feeding with some polysaccharides appears to result in tolerance (and diminished benefits); this has been demonstrated for some mushroom β-glucans [3, 26]. For those polysaccharides whose immunologic effects are dependent on their prebiotic activities, regular feeding would be presumed necessary.

The dietary polysaccharides included in this review have been shown to elicit diverse immunomodulatory effects in animal tissues, including the blood, GI tract, and spleen. In controlled human trials, polysaccharide intake stimulated the immune system in the blood of healthy adults, dampened the allergic response to a respiratory inflammatory agent, and improved survival in cancer patients. Additional RCTs of well-characterized products are needed to more completely understand the immunomodulatory effects and specific applications of oral polysaccharides