According to first law of thermodynamics the universe's energy is constant, so every thing taken from the nature should be given back to the nature, we humans eat food and energy is produced by catabolism, so how one giving it back to nature again?

One should not differentiate between 'you' and 'nature' here. All living beings are intrinsically a part of nature. And the energy rotation is constantly ensured between the surrounding environment and the animals. There's no chance that this rotation is hindered anyway, the moment this will happen, it will surely disturb the ecological balance. For instance, we humans eat, generate energy thus and we further do farming to produce the grains again, it is this energy of us which is utilised to grow grains, thereby this is the return instalment of energy back to the nature. Also the other stuff we do has an impact on nature, the surrounding somehow. Also, the interpretation of the 'total energy being constant' should not be that 'every unit of energy animals gain is returned'; instead, a part of the energy that is in the nature is now converted into our muscular energy, which will again convert in some other form, like mechanical or anything else. This the balance is always ensured, however it is improper to account for each and every part of the 'muscular energy' humans have which will be oscillated back to the 'nature'.

well, i really don't know about thermodynamics . But as far as giving back to nature is concerned , i think it happens when we die as our body comes back to earth again in form of ashes or may be decomposed body (where the burial is done as after death ceremony).