The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
21. The flesh, &c., originates from earth, according to the scriptural statement; and (so also) in the case of the two other (elements).
From tripartite earth when assimilated by man there are produced as its effects flesh, &c., according to scripture. For the text says, 'Food (earth) when eaten becomes threefold; its grossest portion becomes feces, its middle portion flesh, its subtlest portion mind.' The meaning is that the tripartite earth is eaten in the shape of food such as rice, barley, &c.; that its grossest parts are discharged in the form of feces, that its middle parts nourish the flesh of the body, and its subtlest parts feed the mind. Analogously we have to learn from the text the effects of the two other elements, viz. fire and water; viz. that urine, blood, and breath are the effects of water; bone, marrow, and speech those of fire.--Here now an objection is raised. If all material things are tripartite (i.e. contain parts of the three elements alike)--according to the indifferent statement, 'He made each of these tripartite'--for what reason then has there been made the distinction of names, 'this is fire, this is water, this is earth?' And, again, why is it said that among the elements of the human body, flesh, &c., is the effect of the eaten earth only; blood, &c, the effect of the water drunk; bone, &c., the effect of the fire eaten?--To this objection the next Sûtra replies.