# Phytosterols in Seaweeds: An Overview on Biosynthesis to Biomedical Applications
## 4 Health Benefits and Biomedical Applications of Phytosterols  
Phytosterols are renowned for their cholesterol-lowering activity, which potentially reduces the intestinal absorption of cholesterol by up to 30-50%. One of the major mechanisms driving the hypocholesterolemic activity of phytosterols is reported to be related to the competition of phytosterols with structurally similar mammalian cholesterol in the intestinal lumen, which, in turn, reduces the quantity of cholesterol accessible for intestinal absorption. Several other mechanisms underlying the hypocholesterolemic activity of phytosterols have also been previously reported, which include (i) alterations in the expression level of genes encoding sterol-carrying proteins, such as the ATP-binding cassette transporters ABCG5 and 8, that increase the discharge of cholesterol from enterocytes into the intestinal lumen or Niemann-Pick C1-like 1 protein that decrease the transfer of cholesterol to enterocyte; (ii) a reduction in the esterification of cholesterol in enterocyte; and (iii) an increase in the level of cholesterol elimination from the body via transintestial cholesterol excretion [58]. For instance, the intragastric administration of sitosterol/ (25 mg) and cholesterol (25 mg) encompassing an emulsified lipid meal was identified to interfere with the absorption and micellar solubility of cholesterol in rats [59]. Additionally, phytosterols are also reported to act as anticonvulsants, antidepressants, hepatoprotectants, angiotensin-converting enzyme inhibitors, and regulators of cholesterol homeostasis [14]. For example, the _S. fusiforme_-derived fucosterol was found to exhibit antidepressant activities in mice by curbing the immobility time for 30 min in tail suspension test and forced swimming test. The pathogenesis of depression is mainly hypothesized to be related to the reduction in the levels of monoamine neurotransmitters such as dopamine, serotonin and noradrenaline. Interestingly, fucosterol was found to elevate the levels of these monoamine neurotransmitters, which affirmed its antidepressant activity [60]. In another investigation, oral administration of 30 mg/kg of fucosterol from _Pelvetia siliaquasa_ was found to exhibit inhibitory activity against streptozotocinocin-triggered diabetes in rats by significantly reducing serum glucose level and the accumulation of sorbitol in lenses. Further, oral administration of 300 mg/kg of fucosterol was identified as reducing blood glucose concentration and glycogen degradation in epinephrine-induced diabetic rats [61]. Altogether, the myriad health-promoting effects of phytosterols open the door for the development of new drugs for treating several chronic diseases and the fabrication of phytosterols-enriched functional foods that could be used as a part of a healthy routine. Nevertheless, the health benefitsof phytosterols are not only limited to cholesterol-lowering potential, but also extend to anti-cancer, antioxidant, anti-inflammatory, anti-adipogenic, anti-obesity, anti-diabetic, anti-Alzheimer, and anti-atherosclerosis activities. The different biological properties and biomedical applications of phytosterols are provided in Figure 3.