Fucoxanthin has a unique structure including an unusual allenic bond and 5, 6-monoepoxide in its molecule. We found that abdominal white adipose tissue (WAT) weights of rats and mice fed fucoxanthin were significantly lower than those fed a control diet. The daily intake of fucoxanthin in mice also caused a significant reductions of body weight. Clear signals of uncoupling protein 1 (UCP1) and its mRNA were detected by Western and Northern blot analyses in abdominal WAT in mice fed fucoxanthin, although there is little expression of UCP1 in WAT in mice fed a control diet. UCP1 expression in WAT by fucoxanthin intake leads to oxidation of fatty acids and heat production in WAT mitochondria. Substrate oxidation can directly reduce WAT in animals. Fucoxanthin intake also significantly reduced blood glucose and plasma insulin. Furthermore, feeding fucoxanthin significantly increased the level of hepatic docosahexaenoic acid (DHA), a most important n-3 functional polyunsaturated fatty acid in biological systems. These multi-functionalities of fucoxanthin indicate that it is an important bioactive carotenoid that is beneficial for the prevention of the metabolicsyndrome.