Diabetes is one of the current leading causes of death. The diabetic population has been increasing at a stable rate since 2000 and is predicted to reach 4.4% worldwide in 2030 [
118]. Diabetes is associated with a wide variety of complications. Some severe complications include retinopathy, nephropathy, neuropathy, coronary heart disease, hypertension, peripheral vascular disease and amputations [
119]. Dietary modification plays an essential role in diabetes management, typically type 2 diabetes [
120,
121,
122]. For example, the diet incorporated with cereal fiber was effective in prevention of diabetes [
123,
124] and a high intake of cereal fiber for diabetic patients also improved their health conditions [
125].
Consumption of fiber like polysaccharides has been frequently indicated to protect against the incidence of diabetes [
126,
127,
128,
129]. In contrast, diabetic patients usually have a lower dietary fiber intake [
130,
131]. Diabetic patients have changed gut microbiota compared with non-diabetic people [
71,
132]. There are some investigations about polysaccharides in diabetes therapies. Polysaccharides can affect the progression of diabetes via modifying gut barrier and microbiota homeostasis. A western diet combined with resistant starch was supplied to germ-free mice or mice containing microbiota. The insulin sensitivity was improved in resistant starch-fed normal mice, and the insulin levels were also improved in resistant starch-fed germ-free mice. Gene expressions of adipose tissue macrophage markers and cecal concentrations of several bile acids were reduced in both germ-free and normal mice [
133]. According to Zhang et al. [
134], in the inulin-treated diabetic rat groups, the abundance of probiotic
Lactobacillus,
Lachnospiraceae,
Phascolarctobacterium and
Bacteroides which produced SCFAs significantly increased, while the abundance of lipopolysaccharide-producing
Desulfovibrio reduced. Exopolysaccharides separated from the fermentation liquor of
Hypsizigus marmoreus ameliorated the histopathological alterations in the kidney of streptozocin-induced diabetic mice. Additionally, an increase in superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase, total antioxidant capacity, and albumin, and a decrease in the contents of malondialdehyde, lipid peroxide and levels of serum urea nitrogen and creatinine were observed [
135]. Additionally, the ratio of
Firmicutes/
Bacteroidetes and the richness of
Ruminococcaceae and
Lactobacilli increased to achieve antidiabetogenic effect. Liu et al. [
136] fed type 2 diabetes rats with
Cordyceps sinensis polysaccharides for 4 weeks. The insulin sensitivity index was increased, the levels of fasting blood glucose and fasting insulin were reduced, and the number of apoptotic cells and the expressions of both homologous protein and c-Jun were decreased in the diabetic rats.
Cordyceps cicadae crude polysaccharides decreased blood glucose of diabetic rats, total cholesterols, triglycerides, low-density lipoprotein, malondialdehyde, urea, creatinine, alanine transaminase, aspartate aminotransferase and alkaline phosphate, and increased body weight, high-density lipoprotein, SOD and GPx [
137]. According to Tang et al. [
138], six fractions of polysaccharides derived from different parts (whole plants, roots and leaves) of
Anoectochilus roxburghii and
Anoectochilus formosanus were fed to streptozocin-induced diabetic mice and the body weight, blood glucose, glycogen, insulin, total cholesterols, triglycerides, low-density lipoprotein, high-density lipoprotein, malondiadehyde, and antioxidant enzyme activities in the liver and kidney of mice were tested. They found that all polysaccharides had antidiabetic activities, and root polysaccharides performed better than leaf polysaccharides in the antidiabetic activities.
Gastrointestinal symptoms are common in diabetic patients with possible disordered neuroendocrine functions [
139]. Many undigested polysaccharides can be excreted whereas a portion can be fermented by gut bacteria. The ‘in and out’ process allows these polysaccharides to have opportunities to carry part of gut bacteria, dead cell debris as well as toxins to be removed together. Similarly, polysaccharides can also reduce nutrient absorption evidenced by increased fecal output when incorporating dietary fiber into diet [
140]. Soluble dietary fiber has been applied as a treatment for slowing transit constipation by regulating intestinal microecology. Clinical improvement and remission of constipated patients were observed, and the patients felt satisfied with improved gastrointestinal quality-of-life index with continuous consumption of soluble dietary fiber for 4 weeks [
141].
There are researches demonstrating the potential beneficial effects of dietary fiber in the chronic kidney disease (CKD) population by reducing the serum urea and creatinine levels [
142]. Gum arabic was supplemented to CKD patients at 10–40 g/day, and significantly decreased serum sodium level and C-reactive protein level, which was effective to alleviate these patients’ morbidity and mortality [
143]. Oral administration of fucoidan derivatives from
Laminaria japonica significantly decreased the serum urea nitrogen and serum creatinine levels of CKD rats, ameliorating the histopathological symptom of renal tubules, interstitium and mesangial areas via substituting the electronegative element of the glomerular cells and suppressing mesangial cell proliferation [
144]. In addition, two sulfated polysaccharides of low molecular weight fucoidan and high uronic acid fucoidan deprived from
Laminaria japonica Aresch showed the same effect on CKD rats. Both reduced the peroxidative and renal damage and ameliorated CKD [
145].