A concept of composite materials reinforced by branchingmicro or nanotubes optimized for both heat transfer andstrength of the material is presented. Numerous examples ofreinforcement by branched fibers in cells, tissues and organs ofplants and animals are studied. It is shown orientation of thefibers according to principals of the stress tensor at givenexternal load is the main principle of optimal reinforcement innature. The measurement data obtained on venations of theplant leaves revealed clear dependencies between thediameters, lengths and branching angles that correspond todelivery of the plant sap to live cells of the leaf with minimalenergy expenses. The mathematical problem on geometry ofasymmetrical loaded branched fibers experienced minimalmaximal stress is solved. Heat propagation in the fibers isdescribed by generalized Guyer-Krumhansl equation. It isshown the optimality for the heat propagation, fluid deliveryand structural reinforcement are based on the same relationsbetween the diameters, lengths and branching angles. Theprinciple of optimal reinforcement is proposed for technicalconstructions, advanced composite materials and MEMSdevices.