A concept of composite materials reinforced by branching micro or nanotubes optimized for both heat transfer and strength of the material is presented. Numerous examples of reinforcement by branched fibers in cells, tissues and organs of plants and animals are studied. It is shown orientation of the fibers according to principals of the stress tensor at given external load is the main principle of optimal reinforcement in nature. The measurement data obtained on venations of the plant leaves revealed clear dependencies between the diameters, lengths and branching angles that correspond to delivery of the plant sap to live cells of the leaf with minimal energy expenses. The mathematical problem on geometry of asymmetrical loaded branched fibers experienced minimal maximal stress is solved. Heat propagation in the fibers is described by generalized Guyer-Krumhansl equation. It is shown the optimality for the heat propagation, fluid delivery and structural reinforcement are based on the same relations between the diameters, lengths and branching angles. The principle of optimal reinforcement is proposed for technical constructions, advanced composite materials and MEMS devices.