This study aims to analyze the peach palm by-product (shells) bioconversion by culinary-medicinal mushroom Lentinula edodes to obtain a food ingredient for dietary supplementation containing high contents of dietary fiber, protein, and β-glucans. The β-glucans production by L. edodes mycelium was optimized through a solid-state fermentation, checking the influence of the heart of palm shells and supplements (rice bran, manioc flour, and sorghum flour) through an experimental mixing plan. The cultivation treatment that presented the highest tendency for β-glucans production was analyzed by the centesimal composition and in vivo biological activity. Treatments 4 (with shells, rice bran, and manioc flour) and 6 (with shells, sorghum flour, and manioc flour) presented the highest β-glucans content. A flour was obtained with high dietary fiber and protein content, and low lipids and carbohydrates content, and low caloric value. The in vivo biological activity demonstrated high protein quality and promoted a lower elevation of the glycemic curve. Thus, technology for the transformation of peach palm shells into a food ingredient was made feasible. It could generate a gluten-free and lactose-free dietary supplement that is both nutritive and bioactive, enhancing human health and well-being as well as environmental sustainability.