Verse 6.5.1

अन्नमशितं त्रेधा विधीयते तस्य यः स्थविष्ठो धातुस्तत्पुरीषं भवति यो मध्यमस्तन्मांसं योऽणिष्ठस्तन्मनः ॥ ६.५.१ ॥

annamaśitaṃ tredhā vidhīyate tasya yaḥ sthaviṣṭho dhātustatpurīṣaṃ bhavati yo madhyamastanmāṃsaṃ yo'ṇiṣṭhastanmanaḥ || 6.5.1 ||

1. When we eat food, it divides itself into three parts. The grossest part of it becomes excreta; that which is less gross becomes our flesh; and the finest part becomes our mind.

Word-for-word explanation:

Annam aśitam, when food is eaten; tredhā, in three ways; vidhīyate, is divided; tasya, of it; yaḥ sthaviṣṭhaḥ, that which is the grossest; dhātuḥ, part; tat purīṣam bhavati, that becomes excreta; yaḥ madhyamaḥ, that which is less gross [lit., middle]; tat māṃsam, that [becomes] flesh; yaḥ aṇiṣṭhaḥ, that which is the subtlest; tat manaḥ, that [becomes] the mind.

Commentary:

When we eat, the food nourishes the body in different ways. How? First, the grossest part of the food is rejected by the body. Then that which is less gross, a bit finer, becomes our flesh. And finally the finest part becomes the mind. According to Vedānta, the mind is material. It functions through the nervous system, which is called hitā.

How does this food get transformed into the mind? Śaṅkara says that the subtlest part of the food becomes blood and enters into the heart. From there it goes through the veins and nerves, nourishing the mind. The term mind refers here to the phenomena of our thinking, feeling, willing, etc., plus the aggregate of our sense organs—our seeing, hearing, and so forth.

Suppose you do not eat anything. What happens? Slowly you will feel that your mind is becoming more and more feeble. Your memory starts failing, and you cannot concentrate. The finest part of your system is affected. Then the body begins to shrink. You become thinner and thinner, and finally you die. Food nourishes all these. This is why Vedānta says that the mind is material.