By Carrie Decker, ND

Founder and Medical Director, Blessed Thistle, Eugene, OR

Dr. Decker reports no financial relationships relevant to this field of study.

SUMMARY POINTS

  • Twice daily supplementation of 1010 colony-forming units of Lactobacillus reuteri SD5865 for a period of four weeks significantly increased secretion of insulin, C-peptide, and GLP-1 and GLP-2 in non-diabetic individuals.
  • Maximal GLP-1 and GLP-2 secretion (measured via oral glucose tolerance tests) increased in the subcategory of non-diabetic lean individuals, but not in non-diabetic obese individuals.
  • No change was found in peripheral or hepatic insulin sensitivity, body mass index, muscle and hepatic fat content, blood glucose levels during oral glucose tolerance tests, or levels of circulating cytokines.

SYNOPSIS: In this prospective, double-blind, randomized trial, the probiotic strain Lactobacillus reuteri SD5865 at a dosage of 1010 colony-forming units or placebo was provided to non-diabetic individuals twice daily for a period of four weeks to investigate the effect on various parameters associated with blood sugar handling, including secretion of insulin, C-peptide, and glucagon-like peptides-1 and -2.

SOURCE: Simon MC, Strassburger K, Nowotny B, et al. Intake of Lactobacillus reuteri improves incretin and insulin secretion in glucose-tolerant humans: A proof of concept. Diabetes Care 2015;38:1827-1834.

Probiotics have been studied as potential therapies for patients with cardiovascular disease,1 digestive disorders,2 allergies,3 and metabolic syndrome.4 Human and animal studies involving interventions to alter the gut microbiota, including prebiotics, probiotics, or fecal transplantation, have shown conflicting effects on blood sugar metabolism.5,6,7 Various lactobacillus strains have been studied for their effect on parameters such as body weight and insulin sensitivity. This proof-of-concept study investigated the mechanisms by which probiotics may affect parameters associated with glucose metabolism and inflammation in healthy individuals. Lactobacillus reuteri was selected for the current study as it has been demonstrated to be both safe and have strong probiotic activity compared with other lactobacillus strains.8

In this prospective, double-blind, randomized trial, non-diabetic individuals (11 lean people with body mass index [BMI] of 23.6 ± 1.7 kg/m2 and 10 obese people with BMI of 35.5 ± 4.9 kg/m2) were randomized to treatment with L. reuteri SD5865 at a dosage of 1010 colony-forming units or placebo twice daily for a period of four weeks. Inclusion criteria were 40-65 years of age, non-smoking, and absence of gastrointestinal disease. Also, individuals with known chronic disease, pregnancy, cancer, or treated with antibiotics within three months prior to the study were excluded. All individuals participating in the study were required to abstain from intake of other probiotic food products during the study period with no other changes to eating habits. See Table 1 for parameters assessed.

Table 1: Parameters Assessed Prior to the Study and at the End of 4-week Intervention

  • Anthropometric measurements
  • Oral glucose tolerance tests
    • During which the response of glucagon-like peptides (GLP)-1, GLP-2, insulin, glucagon, and C-peptide were assessed
  • Isoglycemic clamp tests
  • Euglycemic-hyperinsulinemic clamp tests
  • Liver and muscle fat measurements
  • Blood cell counts
  • Fecal samples
  • Fasting blood glucose
  • Fasting free fatty acids
  • High-sensitivity C-reactive protein
  • Hemoglobin A1c
  • Lipid oxidation rates
  • Lipopolysaccharide
  • Interleukin-1ra
  • Tumor necrosis factor-α
  • Interleukin-8
  • Monocyte chemoattractant protein-1
  • Macrophage inflammatory protein-1β

All participants completed the study with 100% adherence, and no adverse effects, including gastrointestinal symptoms, were reported. Fecal samples obtained at the start and end of testing found that one subject (from the lean placebo group) was positive for L. reuteri both prior to and at the end of testing; however, it was not found in any other subjects prior to treatment. At the end of testing, all participants treated with L. reuteri tested positive for the presence of L. reuteri in their stool, while no participants in the placebo group tested positive. Neither treatment altered the levels of total lactobacilli, total bacterial load, or overall fecal microbiota composition with regard to relative abundance as analyzed by next-generation sequencing using α- and ß-diversity metrics.

With the daily administration of L. reuteri, glucose-stimulated glucagon-like peptides (GLP)-1 and GLP-2 release increased by 76% and 43% (P < 0.01), respectively, compared with placebo, while insulin and C-peptide secretion increased by 49% and 55% (P < 0.05), respectively, compared with placebo (no confidence intervals given). There were no significant changes in the intervention group compared to the placebo group regarding the other parameters assessed.

Sub-group analysis of the lean and obese groups showed some differences between these groups. The lean group had higher levels of whole-body insulin sensitivity compared to the obese group prior to and after the intervention. Maximal responses of GLP-1 and GLP-2 upon glucose stimulation were significantly increased with the intervention (P < 0.02, P < 0.04) in the lean participants, but not for the obese individuals subsequent to the same intervention. Serum endotoxin levels were higher in the obese group prior to the intervention but not afterward (P < 0.019, P = 0.169). Obese participants had higher high-sensitivity C-reactive protein levels prior to and after treatment, as well as higher levels of IL-1ra, MCP-1, and TNF-α.

COMMENTARY

Modulation of the gut microbiota in animal models has been shown to contribute to the development of obesity and regulation of insulin resistance.9,10 Increased intestinal permeability and endotoxin translocation are proposed to be mechanisms by which low-grade inflammation is associated with the complications of obesity and the metabolic syndrome.11 As many studies with interventions directed at modulation of the gut microbiota fail to look at possible mechanisms by which this may alter glucose and insulin metabolism, this study further evaluates many parameters that may contribute to the effect that has been seen.

GLP-1 and GLP-2 are secreted by the enteroendocrine L-cells of the ileum from transcription of the proglucagon gene at roughly equivalent amounts in response to nutrients in the small intestine.12 GLP-1 expression is more strongly tied with blood sugar metabolism, as it acts as a antihyperglycemic agent by inducing glucose-dependent stimulation of insulin secretion as well as increasing insulin synthesis and beta-cell proliferation.13 Thus, it is not surprising to find that it should rise in combination with insulin. C-peptide is the protein cleaved from proinsulin in conjunction with insulin when insulin is released, and thus serves as a marker of pancreatic insulin secretion. C-peptide has potential therapeutic effects, as it has been shown in various studies to affect neuropathy and renal function.14,15,16,17

GLP-2 has been shown to have an effect on intestinal permeability and has been studied as a therapy for this purpose in multiple studies.18,19,20 Gastrointestinal effects of GLP-2 include regulation of gastric motility, gastric acid secretion, decreasing mucosal injury, cytokine expression, and bacterial septicemia. As diabetes, heart disease, and obesity are associated with a chronic, low-grade inflammation, the alteration of gastrointestinal permeability and related endotoxemia by increasing GLP-2 may have therapeutic benefits for these conditions as well as obvious benefits for gastrointestinal disease and infection. More recently, studies have looked at the association between obesity and intestinal permeability, finding an association between obesity, insulin resistance, and circulating zonulin levels, which is a marker of intestinal permeability.21,22

Berberine alkaloids are another natural agent that have been studied extensively for their potential therapeutic role in diabetes. Possible mechanisms by which berberine may affect insulin resistance include increasing insulin sensitization,23 modifying the gut microbiota,24 and promoting GLP-1 secretion and biosynthesis.25 It is noteworthy that in many ways the mechanisms by which berberine may affect insulin resistance significantly overlap the action of the probiotic L. reuteri shown in this study. Berberine also has anti-hyperlipidemic effects, which support its use in the prevention of the complications associated with obesity.26

Limitations of current study were that it was a small population, the study duration was short, and none of the subjects were diabetic. In healthy individuals without diabetes, in theory, there should be no changes in insulin resistance. With insulin resistance, many aspects of physiology adapt and may be less sensitive to interventions. As the effect of improving secretion of GLP-1 was not observed in the obese subgroup, there are concerns that this effect also would not be observed in obese diabetic individuals. With a study of a short duration, it is not known if the body will adapt to the intervention and no longer experience an improved metabolic response over time. It also is not known if the increases in insulin and incretin secretion would persist beyond the period of the intervention. These variables should be investigated further with subsequent studies.

As mentioned previously, Lactobacillus reuteri was selected for this study because of its strong probiotic activity compared with other lactobacillus strains and its well-studied safety.8 Other studies using probiotics to investigate their effects on the various parameters associated with diabetes have used strains such as Lactobacillus acidophilus NCFM,27 L. acidophilus La5, and Bifidobacterium lactis BB12,28 as well as various other Lactobacillus species. A recent meta-analysis concerning the effect of various Lactobacillus species on weight gain has shown mixed results that were not consistent between humans and animals.29 Hence, it would be appropriate to increase knowledge about a well-studied strain such as L. reuteri. As there were no adverse effects reported in the current study, and many other investigations show beneficial results with probiotics as an adjunctive therapy, a reasonable clinical intervention would be to recommended twice-daily intake of 1010 colony-forming units L. reuteri for reducing the risk of insulin-resistant diabetes. Of course, this should not be an isolated intervention; recommendations also should be given for dietary changes and increased exercise if these lifestyle issues have not been addressed.

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