A process for preparing a composition based on a synthetic polymer and on a starchy component, comprising: • the introduction, into a reactor containing a molten synthetic polymer (component 1) subjected to shear forces and heated to a temperature between 60°C and 260°C, of a nonplasticized starchy component (component 2) and of a plasticizer therefor (component 3), and • blending of the mixture obtained at a temperature of between 60°C and 260°C, with a shear force and for a time that are sufficient to obtain plasticization of the starchy component (component 2) with the plasticizer (component 3) and homogeneous mixing of the synthetic polymer and of the plasticized starchy component wherein: the synthetic polymer is chosen from the group comprising polyethylenes and polypropylenes, which are optionally functionalized, polyamides, thermoplastic polyurethanes, styrene-ethylene-butylene-styrene (SEBS) copolymers, amorphous poly(ethylene terephthalates) (PETG) and mixtures thereof; the plasticizer is chosen from glycerol, polyglycerols, isosorbide, sorbitans, sorbitol, mannitol, and hydrogenated glucose syrups, and any mixture of these products; the starchy material is a native starch of cereals, tubers, or legumes, and mixtures of such starches; the starchy component is incorporated into the synthetic polymer in a proportion of from 20% to 400% by dry weight relative to the dry weight of the synthetic polymer present in the composition; the plasticizer is used in a proportion of from 10 to 150 parts by weight per 100 parts by weight of starchy component; the plasticizer is introduced into the reactor and incorporated into the synthetic polymer before the introduction of the nonplasticized starchy component. • the introduction, into a reactor containing a molten synthetic polymer (component 1) subjected to shear forces and heated to a temperature between 60°C and 260°C, of a nonplasticized starchy component (component 2) and of a plasticizer therefor (component 3), and • blending of the mixture obtained at a temperature of between 60°C and 260°C, with a shear force and for a time that are sufficient to obtain plasticization of the starchy component (component 2) with the plasticizer (component 3) and homogeneous mixing of the synthetic polymer and of the plasticized starchy component wherein: the synthetic polymer is chosen from the group comprising polyethylenes and polypropylenes, which are optionally functionalized, polyamides, thermoplastic polyurethanes, styrene-ethylene-butylene-styrene (SEBS) copolymers, amorphous poly(ethylene terephthalates) (PETG) and mixtures thereof; the plasticizer is chosen from glycerol, polyglycerols, isosorbide, sorbitans, sorbitol, mannitol, and hydrogenated glucose syrups, and any mixture of these products; the starchy material is a native starch of cereals, tubers, or legumes, and mixtures of such starches; the starchy component is incorporated into the synthetic polymer in a proportion of from 20% to 400% by dry weight relative to the dry weight of the synthetic polymer present in the composition; the plasticizer is used in a proportion of from 10 to 150 parts by weight per 100 parts by weight of starchy component; the plasticizer is introduced into the reactor and incorporated into the synthetic polymer before the introduction of the nonplasticized starchy component. the synthetic polymer is chosen from the group comprising polyethylenes and polypropylenes, which are optionally functionalized, polyamides, thermoplastic polyurethanes, styrene-ethylene-butylene-styrene (SEBS) copolymers, amorphous poly(ethylene terephthalates) (PETG) and mixtures thereof; the plasticizer is chosen from glycerol, polyglycerols, isosorbide, sorbitans, sorbitol, mannitol, and hydrogenated glucose syrups, and any mixture of these products; the starchy material is a native starch of cereals, tubers, or legumes, and mixtures of such starches; the starchy component is incorporated into the synthetic polymer in a proportion of from 20% to 400% by dry weight relative to the dry weight of the synthetic polymer present in the composition; the plasticizer is used in a proportion of from 10 to 150 parts by weight per 100 parts by weight of starchy component; the plasticizer is introduced into the reactor and incorporated into the synthetic polymer before the introduction of the nonplasticized starchy component.