**3. Discussion**

Postprandial hyperglycemia is a key factor in the formation and development of DM, and abnormally elevated intestinal SGLT1 activity is the main cause in DM patients with postprandial hyperglycemia. As SGLT1 is crucial for intestinal glucose absorption, important strategies in the prevention and treatment of hyperglycemia include exploring compounds with significantly inhibitory effects on SGLT1 activity [53]. In our previous studies, we found that fucoidan with a type II structure exhibited significant inhibition of α-glucosidase activity in vitro, rather than fucoidan with a type I structure [31]. In addition, AnF reduced α-glucosidase activity to about 30%, whereas the application of FvF resulted in a 90% decrease in α-glucosidase activity relative to that of the control. And we also elucidated that FvF increased glucose consumption and relieved insulin resistance via ROS-mediated JNK and Akt signaling pathways by using HepG2 cell line. In vivo, FvF could regulate lipid metabolism to attenuate metabolic syndrome [30]. For clarifying the mechanism of inhibiting glucose absorption of type II structure fucoidan, another type II fucoidan, AnF with lower α-glucosidase inhibition was used to rule out other factors. Furthermore, it is encouraging to develop potential antihyperglycemic polysaccharide compounds from natural resources. Thus, we assessed the effects of AnF (type II structure), LjF (type I structure), and KcF (type I structure) on postprandial blood glucose levels, and found that only AnF could effectively alleviate postprandial hyperglycemia. The underlying mechanism might be that only AnF and FvF, with type II structures, exhibited effective binding affinity to SGLT1 via SPR, which further indicated that fucoidans with type II structures could reduce postprandial hyperglycemia by suppression of SGLT1 activity.

SGLT1 is pivotal for the absorption of glucose in the intestinal tract [54]. The inhibitory activities of fucoidans on SGLT1 activity were evaluated by in vitro (Caco-2 monolayer model), semi-in vivo (everted gu<sup>t</sup> sac model), and in vivo (OGTT in Kunming mice) assays, which demonstrated that only AnF could significantly decrease the transport of glucose by inhibiting the activity of SGLT1, while LjF and KcF could not. It has been verified that the bioactivity of fucoidans depends highly on their structural properties [21]. Both LjF and KcF had no marked effects on glucose transport, which indicated that MW and fucose content may not play a pivotal role in this ability. In addition, the various effects of AnF and LjF on glucose transport indicated that the type of fucosidic linkages may play a more crucial role. Combined with the result that FvF with a type II structure could conspicuously reduce the transport of glucose, these results indicate the importance of the type of glycosidic linkages in inhibiting glucose transport.

It was also reported that type II fucoidans could be a promising α-glucosidase inhibitor to reduce blood glucose levels [31]. Thus, OGTT was used to evaluate the effects of fucoidans on SGLT1 activity in vivo, avoiding the influence of α-glucosidase, and the absorption of glucose in the intestinal tract can only be realized through the transport of the SGLT1 protein. OGTT results in Kunming mice and db/db mice further confirmed the effect of AnF on decreasing postprandial blood glucose, improving glucose tolerance and insulin sensitivity. In addition, AnF with a type II structure effectively increased the GLP-1 levels, which further confirmed the inhibition of AnF on SGLT1 [51,52]. Moreover, fucoidans cannot be digested by gastric enzymes in the gastrointestinal tract and exhibit extremely low bioavailability after oral administration [43], which indicated that fucoidans, with type II structures, can play an effective and lasting role in inhibiting SGLT1 activity in vivo. Additionally, it has been reported that the in vivo effect of fucoidan on blood coagulation was not obvious, probably due to its low intestinal absorption [55]. However, it is necessary to assess the cytotoxicity and blood-thinning properties (e.g., activated partial thromboplastin time (APTT)) of various fucoidans for developing as potential nutraceuticals or dietary supplements for treating the hyperglycemia. All in all, we will pay more attention to the toxicity of various fucoidans in future studies.
