Natural Phytosterols as the Nutraceuticals or Functional Agents: Insights Into Structure–Activity Relationship

Phytosterols are naturally occurring ingredients widely present in dietary sources, with over 250 types identified. Mounting studies have shown they possess various health-promoting benefits, such as lowering-cholesterol, anti-inflammatory, and anti-oxidation effects. With these advantages, they are regarded as important bioactive components that are allowed to be incorporated into foods. However, their characteristics of low absorption, rapid metabolism, and excretion significantly limit their practical efficacy. In addition, many delivery vehicles have been fabricated to solve these problems. Crucially, the structural diversity of phytosterols directly influences their physicochemical properties and bioactivities. More interestingly, their unique structures endow them with intrinsic capabilities, including self-assembly, emulsification, gelation, and liposome stabilization, enabling their use not only as nutraceuticals but also as functional agents. These properties also pave the way for the development of carrier-free delivery systems for phytosterols, potentially addressing their application issues. Despite this potential, these functional attributes remain underemphasized. Herein, we comprehensively introduced the structures, dietary and biosynthetic sources, absorption, metabolism, distribution, and bioactivities of phytosterols, with a particular focus on the structure-activity relationship. Furthermore, we highlighted their emerging roles as wall materials, emulsifiers, gelators, and liposome stabilizers, noting that they can position as both nutraceuticals and functional agents to expand their applications.