Benefits
Metabolic support
The main human trial cited here ran 16 weeks in obese premenopausal women and tested Xanthigen, a different brand's combination of fucoxanthin and pomegranate seed oil. Body weight, body fat and triglycerides improved, but those results belong to the combination product, not to fucoxanthin on its own and not to FucoVital. The one fucoxanthin specific finding in that paper was that intakes above 2.4 mg per day raised resting energy expenditure. A second trial used Mi136, a different microalgae extract product, so it also does not show what FucoVital does.
Liver fat: studied only in women with diagnosed fatty liver disease
The only liver data cited comes from that same 16 week Xanthigen trial, in women who had already been diagnosed with non-alcoholic fatty liver disease. In that group, liver fat and liver enzymes improved and weight fell by about 5.5 kg. That was a different brand's combination of fucoxanthin and pomegranate seed oil, not FucoVital, and it was studied in a disease population. Nothing here shows a liver benefit in healthy people. Fatty liver disease is a medical condition; if you have it, work with your doctor rather than treating it with a supplement.
Antioxidant defense
Fucoxanthin is a carotenoid, and the 2020 review cited here describes antioxidant activity in laboratory work. Antioxidant status was not measured as an outcome in either human study on this page, so this remains a laboratory finding rather than a demonstrated effect in people.
Mechanism of action
Thermogenic signaling
In laboratory and animal research summarized by the 2020 review cited here, fucoxanthin increased UCP1, a protein involved in burning fat for heat, in fat tissue. This has not been confirmed in people.
Antioxidant action
In test tube studies, fucoxanthin's carotenoid structure scavenges free radicals. Whether that translates into any antioxidant benefit in people was not tested in the studies cited here.