Diet and Mind - Food for Thought

A calm and composed mind is the Mind dominated by the Sattva Guna, the state in which knowledge and insights are gained.

Constitution of mind can be changed gradually by providing mind with the wholesome food so that sattva leads and other two gunas take the backseat.

The Upanishads say:

The food we eat is transformed in three different ways: the grossest becomes excrement, the middle part is transformed into flesh, and the subtlest part goes to form the mind. (Ibid., VI. 5.1.)

Further:

Just as in the churning of curds, the subtlest part rises up and is transformed into butter, so when food is eaten, the subtlest part rises up and is transformed into mind. (Ibid. VI.6.1-2.)

As the mind consists of food, naturally the teaching follows:

When the food is pure, the mind becomes pure. When the mind becomes pure, memory becomes firm. And when a man is in possession of firm memory, all the bonds that tie him down to the world are released. (Ibid., VII. 26.2.)

Now, it is important to mention here that food not only means something we eat but everything that our senses consume.

To mould the constitution of mind in favourable ways, a shift to satvika food is needed because all other foods (rajasika or tamasika) causes aversion, attachment or disquieten in mind.  

In the Bhagavad Gita, Lord Krishna mentions the three-fold division of food as

आयु:सत्त्वबलारोग्यसुखप्रीतिविवर्धना: |
रस्या: स्निग्धा: स्थिरा हृद्या आहारा: सात्त्विकप्रिया: || 8||

āyuḥ-sattva-balārogya-sukha-prīti-vivardhanāḥ
rasyāḥ snigdhāḥ sthirā hṛidyā āhārāḥ sāttvika-priyāḥ

The food which augments longevity, energy, strength, health, cheerfulness and appetite, which are tasty with a right amount of fat and oil, nourishing and agreeable, are liked by the satvika.

कट्वम्ललवणात्युष्णतीक्ष्णरूक्षविदाहिन: |
आहारा राजसस्येष्टा दु:खशोकामयप्रदा: || 9||

kaṭv-amla-lavaṇāty-uṣhṇa- tīkṣhṇa-rūkṣha-vidāhinaḥ
āhārā rājasasyeṣhṭā duḥkha-śhokāmaya-pradāḥ

The foods that are bitter, sour, saline, excessively hot, pungent, dry and burning are liked by the rajasika and are productive of pain, grief and disease.

यातयामं गतरसं पूति पर्युषितं च यत् |
उच्छिष्टमपि चामेध्यं भोजनं तामसप्रियम् || 10||

yāta-yāmaṁ gata-rasaṁ pūti paryuṣhitaṁ cha yat
uchchhiṣhṭam api chāmedhyaṁ bhojanaṁ tāmasa-priyam

That which is stale and tasteless, stinking, putrid and impure is the food liked by the tamasika. (XVII. 8-10)