The plant to produce their fruits, in this case, the nuts, firstly have to carry out photosynthesis in their leaves, by the intake of water through its roots, and the intake of Carbon dioxide through the chloroplast of their leaves. After this process, the plant will produce glucose to develop its other functions, such as its growth in leaves and flowers, which eventually will form, after pollination, the formation of the nuts.
The nuts have inside carbohydrates, or glucose, which the plant produced by photosynthesis. These carbohydrates, consumed by a squirrel will eventually be used to perform different functions in their body, and, indeed, the formation of proteins like the ones found in muscle cells. The reason why it's said is that the carbon atom from CO2 is used to form the muscle cell.