Through controlled assembly of protein and polysaccharide, especially anionic soluble fibers, biopolymer particles with desirable functional properties can be created. The majority of research has focused on fabrication conditions under associative interaction (e.g., at pH < isoelectric point of protein) where they form complex coacervates. Much less attention has been given to their interactions under limited thermodynamic compatibility (e.g., at neutral or near neutral pH) where the positively charged patches of protein can form electrostatic attraction with negatively charged polysaccharides. Research has shown that unheated and heated soluble complexes between whey protein and anionic soluble fibers can be formed under limited thermodynamic compatibility. At optimum conditions, structuring of these complexes improves functional properties of whey protein, leading to enhanced texture and stability of food products. Using in vitro digestion model, recent studies have revealed that high protein beverages containing whey protein and anionic soluble fiber complexes can form intragastric gel when entering the simulated gastric environment. The transformation of liquid (e.g., beverages) to gel results in slow gastric emptying and slower protein degradation. Formation of intragastric gel also traps other ingredients including sugar, resulting in delayed sugar-release from the gel network. Potentially, intragastric gelation behavior could lead to the development of high protein food products with healthy blood sugar or high satiety claims. As the demand for high protein products continues to grow, complexes between dairy protein and soluble fibers could be utilized to improve the quality and healthfulness of high protein foods and beverages.