The Lord says that he alone is present in the earth and sustains the earth from falling and is the power behind it. He also said in the earlier slokas that he is the one who nourishes the plants and is the soma or juices which sustain the plants. Such he said that he is sustainer of the life on the earth .He further explains about how he sustains human life on earth.
15.14 – Aham vaisvanarah bhutva praninaam deham asritaha
Prana apana samayuktah pacami annam chaturvidham
Meaning of words:
Aham – Myself
Vaisvanara – the fire in the stomach
Bhutva— taking form of
Praninaam -- of the living beings
Deham – the body
Asritaha – residing
Prana – The vital force of breathing, that goes upwards
Apana – The vital force that goes downwards and seated in the excretion
Pacami – digest
Annam – food
Chaturvidham – four types
Meaning of sloka
I am the fire or vaisvanara in the stomach of the living beings and digest the food taken and thus sustain the body in association with the vital forces of prana apana ,etc.
The fire in the man which digests the food is called vaisvanara. Vaisvanara can be further divided into prana,apana,udana,vyana and samana. These are the vital forces of the body. Apana is the force working downwards and is seated at the excretory system. Prana is the force which moves upwards and is seated in the heart. So being the vaisvanara or the fire in the body it digests the food. So Lord says that the fire in the stomach is brahman, or sustainer of life. As we all know that food is the life giver. Without food and water mind and body do not work. One cannot stay without food and water. Energy derived from the food gets distributed to all parts of the body. Hence it is life sustainer.
Vaisvanara digests food of 4 types i.e food that is eaten by masticating, swallowing, sucking and licking. The eaten food is the soma , the eater is the vaisvanara . One who looks upon all that there is as being these two, fire and Soma, is not affected by the impurity of food.