## 1 Introduction  
Marine algae, containing bioactive compounds not present in terrestrial plants and beneficial to human health, are a source of phytochemical active compounds (such as polysaccharides, sterols, carotenoids and tocopherols) with antiviral, antimicrobial, antifungal, anticoagulant and antioxidant properties. Sulphated polysaccharides are widespread in marine algae, especially in brown seaweeds. The biological activities of sulphated polysaccharides include anticoagulant, antioxidant, antiviral, anticancer and immunomodulating activities [1; 2; 3; 4]. Alginic acid has been reported to be capable of decreasing cholesterol levels, exerting an antihypertensive effect, preventing absorption of toxic products and protecting the surface membranes of the stomach and intestine [5]. Phltorotannins, polymers of phltoroglucinol, which are present in brown algae, are valuable antioxidant compounds [6]. Algae are exposed to light and high concentrations of oxygen, leading them to produce oxidative agents, but algae also generate compounds that protect against oxidation [7]. Among the compounds present in algae, antioxidants have probably attracted the most interest. Brown algae possess potential antioxidant activity, and different in vitro studies have demonstrated that _Sargassum_ spp. has compounds conferring it potent antioxidant properties [8; 9; 10; 11]. Although scarce, the in vivo antioxidant activity of _Sargassum_ spp. extracts have been explored. The consumption of _Sargassum_ polycystum extracts lowers the risk of hyperglycaemia, dyslipidemia and atherogenesis in mammalian models of type 2 diabetes [12; 13]. _Sargassum muticum_, an invasive brown algae on the Atlantic coasts, represents an important environmental problem and can also affect recreational activities in coastal areas. Eradication trials were unsuccessful, and periodic removal of specimens has proven effective in preventing further spread. Therefore, valorization of the removed biomass has been proposed. Hydrothermal processing offered higher biomass solubilization yields than other intensification methods [14] and conforms a suitable strategy for the extraction and depolymerization of the fucoidan fraction [15; 16]. Further processing with ultrafiltration membranes allowed the concentration in bioactives and the elimination of minerals in the concentrated product [17]. Some fractions could be especially enriched in bioactives [18]. Reactive oxygen species (ROS) cause damage to biomolecules and participate in the development of many disorders and diseases. ROS may be highly damaging, as they can attack biological macromolecules (namely lipids, proteins and DNA), induce oxidation and cause membrane damage, enzyme inactivation and DNA damage [19]. Oxidative stress is considered to play a pivotal role in the pathogenesis of aging and several degenerative diseases such as atherosclerosis, cardiovascular disease, type 2 diabetes and cancer [20]. ROS scavenging or detoxifying mechanisms involve enzymatic and non-enzymatic antioxidants, either those produced in the body (i.e., endogenous antioxidants) [21] or those supplied in the diet [22]. Antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) act by catalyzing the decomposition of oxidants and free radicals and exerting protective effects against cellular damage [23]. Methods of increasing antioxidant capacity have been suggested as being potentially useful for preventing or delaying the adverse actions of ROS. In this study, the in vivo antioxidant properties of _Sargassum muticum_ hydrothermal extracts were evaluated by examining the effects on the antioxidant enzyme activity in growing male Sprague-Dawley rats fed with a diet supplemented with these extracts. The activity of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) in liver and in red blood cells were determined and the total amounts of lipid peroxidation products in liver were assayed.