Benefits
Immune defenses during cold season
In 199 adults who said they caught frequent colds, 4.5 g/day of the branded form for 12 weeks did not quite reach its main target for how often cold episodes occurred (p = 0.055), though fewer people had at least one doctor-confirmed episode and the per-protocol analysis did reach significance. Symptoms at the start of an episode were rated more severe. This was the branded ingredient, not generic larch fiber.
Antibody response after vaccination
Two small trials of the branded form looked at antibody levels after vaccination. One raised the pneumococcal antibody response above placebo for 2 of 7 strains; the other raised tetanus antibodies at 1.5 g/day but not at 4.5 g/day, and neither dose changed influenza antibodies. Antibody levels are a lab marker, not a count of infections avoided.
Prebiotic fiber fermented in the colon
Larch arabinogalactan resists digestion in the small intestine and passes to the colon, where gut bacteria ferment it, which is the basis for its use as a soluble prebiotic fiber. Human studies have mostly measured changes in the types and balance of gut bacteria rather than a direct effect on how the gut feels or works.
Shifts in gut bacteria composition
In a 6-week crossover trial of the branded form at 15 g/day, the ratio of Firmicutes to Bacteroidetes bacteria in stool fell and Bifidobacterium tended to rise, while overall bacterial diversity decreased. Bowel habits, stool consistency and digestive comfort did not change. This measured bacterial makeup, not a gut health outcome.
Gut bacteria and fecal ammonia in healthy adults
In a 6-week study in 20 healthy adults, 15 or 30 g/day of non-branded larch fiber raised total fecal anaerobes and Lactobacillus and lowered fecal ammonia, but did not change transit time, bowel frequency, stool weight, fecal pH or short-chain fatty acids. The fiber was well tolerated and easily stirred into a drink.
Mechanism of action
Fermented by bacteria in the colon
As a non-digestible soluble fiber, larch arabinogalactan is not absorbed in the small intestine and reaches the colon, where resident bacteria ferment it. Human studies report shifts in the balance of gut bacteria; effects on the gut barrier have not been tested in people.
Natural killer cell activity in laboratory studies
In cultured human blood cells, larch arabinogalactan increased natural killer cell killing of target cells, an effect that depended on the signaling protein interferon gamma. Cells from roughly a third of donors did not respond. These findings come from cell-culture work, not from people.
A proposed gut-immune link
Reviewers have suggested that any immune effect of larch arabinogalactan may work indirectly, through the fiber's effect on gut bacteria or through immune tissue in the gut wall. This is a hypothesis to explain the trial findings; the mechanism has not been established in people.
Clinical trials
Randomized, double-blind, placebo-controlled trial of the branded ResistAid at 4.5 g/day or placebo for 12 weeks in adults who reported about three colds in 6 months (Riede et al. 2013, Curr Med Res Opin)
199 healthy adults prone to colds (101 arabinogalactan, 98 placebo).
The main outcome, incidence of cold episodes, was lower on arabinogalactan but narrowly missed significance in the full analysis set (p = 0.055); it was significant in the per-protocol set (p = 0.040). Fewer participants had a cold (p = 0.038). Symptom severity at the start of an episode was rated higher on arabinogalactan (p = 0.028). Tolerability matched placebo.
Randomized, double-blind, placebo-controlled pilot of the branded ResistAid at 4.5 g/day for 72 days with a 23-valent pneumococcal vaccine on day 30, funded by the maker (Udani et al. 2010, Nutr J)
45 healthy adults not previously vaccinated against pneumococcus.
The IgG antibody response was significantly greater than placebo for 2 of 7 measured pneumococcal strains (18C and 23F); the other five strains, salivary IgA, white blood cell count, inflammatory cytokines and complement did not differ. Antibody titers are a surrogate marker, not fewer infections.
Randomized, double-blind, placebo-controlled trial of the branded ResistAid at 1.5 or 4.5 g/day for 60 days, with tetanus and influenza vaccines on day 30 (Udani 2013, J Am Coll Nutr)
75 healthy adults aged 18 to 61.
At 1.5 g/day the rise in tetanus IgG at day 60 was significantly greater than placebo (p = 0.008); at 4.5 g/day tetanus IgG rose from baseline but did not differ from placebo. Neither dose changed influenza A or B IgG or IgM. No significant adverse events. The result was mixed by dose and by vaccine.
Randomized, double-blind, placebo-controlled crossover trial of the branded ResistAid at 15 g/day versus maltodextrin for 6 weeks; two authors were employees of the maker (Chen et al. 2021, Nutrition)
30 healthy adults.
Compared with placebo, the fecal Firmicutes-to-Bacteroidetes ratio fell and Bifidobacterium tended to rise, while overall microbial diversity decreased and three minor fecal fatty acids (isovaleric, valeric and hexanoic) fell. Bowel habits, stool consistency and gastrointestinal symptoms did not change.
Two three-week periods of a commercially available Western Larch arabinogalactan at 15 or 30 g/day added to the usual diet, with no washout between them (Robinson et al. 2001, J Am Coll Nutr)
20 healthy adults (11 men, 9 women).
Total fecal anaerobes rose at 15 g (p = 0.01) and 30 g (p = 0.001), and Lactobacillus rose over the 6 weeks (p = 0.02); fecal ammonia fell at both doses. Transit time, bowel frequency, stool weight, fecal pH and short-chain fatty acids did not change, and there were no changes in blood lipids or insulin.
6-month randomized, double-blind, parallel trial of 8.4 g/day larch arabinogalactan, tamarack arabinogalactan or a rice-starch placebo added to the usual diet (Marett and Slavin 2004, J Am Diet Assoc)
54 healthy adults aged 18 to 55 (18 larch, 19 tamarack, 17 placebo).
Serum cholesterol, LDL, HDL, triglycerides, apolipoproteins, glucose and insulin were measured monthly. Levels appeared to dip at month 2, but there were no statistically significant differences between the fiber groups and placebo for any measure over 6 months.
Laboratory study of arabinogalactan from Larix occidentalis added to cultured human blood cells for 48 to 72 hours (Hauer and Anderer 1993, Cancer Immunol Immunother)
Cultured human peripheral blood cells from multiple donors; not a human trial.
Pretreatment with larch arabinogalactan enhanced natural killer cell killing of tumor cells, an effect driven by interferon gamma; blocking interferon gamma or interleukin-2 removed it. Cells from about 37% of donors did not respond. A cell-culture study, not evidence of an effect in people.