Gnetin-C is a naturally occurring stilbene derived from the seeds of Gnetum gnemon L., an edible plant native to Southeast Asia that is called melinjo. Although the biological properties and safety of G. gnemon extract, which contains nearly 3% Gnetin-C, have been confirmed in various human studies, whether or not pure Gnetin-C is safe for humans is unclear at present. We conducted a randomized, double-blind, placebo-controlled trial. Healthy subjects were randomly divided into two groups. The interventional group (n = 6) was given Gnetin-C, and the control group (n = 6) was provided a placebo, for 14 days. Lipid profiles, biomarkers of oxidative stress and circulating blood cells were assessed before and after the intervention. All subjects completed the study, with no side effects reported across the study duration. Gnetin-C supplementation demonstrated a statistically significant increase in the absolute number of circulating natural killer (NK) cells expressing the activating receptors NKG2D and NKp46. NK cells derived from subjects who received Gnetin-C for two weeks showed higher cytotoxicity against K562 target cells than those before receiving Gnetin-C. In addition, Gnetin-C also resulted in a significant decrease in the absolute neutrophil count in the blood compared with the placebo. Furthermore, Gnetin-C significantly reduced the levels of uric acid, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, total adiponectin, and high-molecular-weight adiponectin. These data indicate that Gnetin-C has biological effects of enhancing the NK activity on circulating human immune cells. The immunomodulatory effects are consistent with a putative improvement in cancer immunosurveillance via the upregulation of the NKG2D receptor. The study was registered with UMIN-CTR, number 000030364, on 12 December 2017.

Background: Ascophyllan, a sulfated polysaccharide extracted from brown seaweed, has shown immunomodulatory and antioxidant effects in preclinical studies, yet human clinical evidence remains scarce. This randomized, double-blind, placebo-controlled pilot trial evaluated the safety and exploratory biological effects of daily ascophyllan supplementation in healthy adults. Methods: Twelve participants were randomized to receive either ascophyllan (n = 6) or placebo (n = 6) for 28 days. Safety was monitored through adverse event reporting and repeated laboratory assessments, including hematology, biochemistry, and inflammatory markers. Immune cell populations were analyzed via serial flow cytometry, serum total antioxidant capacity was measured at multiple time points, and gut microbiome composition was profiled using 16S rRNA gene sequencing. All analyses were exploratory in nature. Results: Ascophyllan supplementation proved well tolerated, with no adverse events observed and stable hematologic, renal, and biochemical parameters throughout the study. Exploratory longitudinal analyses suggested directional modulation of NK-cell-associated phenotypes during ascophyllan supplementation, including directional changes in CD57⁺, NKp46⁺, and NKG2D⁺ NK-cell phenotypes; however, group × time interaction analyses did not remain statistically significant after correction for multiple comparisons. Serum antioxidant capacity showed inter-individual variability with a directional but non-significant increase in the ascophyllan group at intermediate time points. Exploratory microbiome analyses suggested modest directional compositional differences involving members of the Bacteroidaceae and Bifidobacteriaceae families; however, no taxon remained statistically significant after correction for multiple comparisons. Conclusions: These preliminary findings indicate that ascophyllan is safe and well tolerated in healthy adults and may be associated with modulation of innate immune phenotypes, subtle microbiome compositional differences, and directional changes in antioxidant capacity. Larger, adequately powered clinical trials are warranted to confirm these observations and further investigate potential biological and clinical effects.