Brown seaweeds are a rich source of phlorotannins, a characteristic class of phenolic compounds which are unique to seaweeds of this type and can comprise 5 to 15% of the dried weight⁽ Reference Koivikko, Eranen and Loponen ^{1 )}. Unlike other classes of polyphenols, there is a lack of knowledge regarding phlorotannins and their bioactivity, particularly after their metabolism by the gut microbiota.

We investigated the effects of a phlorotannin-rich Food Grade Extract (FGE) from the brown seaweed Ascophyllum nodosum and its High Molecular Weight (HMW) fraction and Low Molecular Weight (LMW) fraction on colon cancer cells after being subject to in vitro gastointestinal digestion (GID)⁽ Reference McDougall, Dobson and Smith ^{2 )} and colonic fermentation (CF)⁽ Reference Tzounis, Vulevic and Kuhnle ^{3 )}.

HT-29 cells were exposed to the GID and CF extracts and we tested the ability of the three extracts to reduce H₂O₂ induced cellular DNA damage (24 h, COMET assay) and inhibit cancer cell growth (48 h, SRB assay). The statistical analysis was conducted by one-way ANOVA followed by a Bonferroni post-hoc t-test.

As shown in Fig. 1, a low level of DNA damage (% of DNA in tail) was measured in the controls whereas in cells challenged with H₂O₂, the DNA damage was significantly increased (P < 0.05). The pre-treatment of cells with GID extracts did not significantly reduce DNA damage compared to H₂O₂ alone, however the CF extract HMW was significantly effective (P < 0.05), whereas the CF extracts LMW and FGE did not induce any significant protective effect. Fig. 2 shows that all GID extracts significantly inhibited (P < 0.05) HT-29 cell growth (at 250 and 500 μg/ml), with HMW being the most effective. When the cells were treated with the CF extracts (Fig. 3), only HMW was able to significantly inhibit (P < 0.05) the growth of the cells (at 250 and 500 μg/ml).

Fig. 1. COMET assay.

Fig. 2. SRB assay.

Fig. 3. SRB assay CF.

In conclusion, the gut microbiota metabolism of the seaweed extracts increased their ability to counteract the H₂O₂ induced DNA damage, whereas it reduced the ability of the extracts to inhibit the cell growth. In both cases, HMW was the most effective, indicating that the high molecular weight phlorotannins potentially exert a stronger beneficial effect in the colon.