Motor organs, sensory organs, Prana, (the life force) are all formed by the food we consume.
When Ghee, oil or Tejas in the food gets digested, it is divided into three parts: The “crude” part makes bone, the middle part makes bone marrow and the subtle part makes the Karmendriyas (vak).
When the non-fat part of the food is digested, it too gets divided into three parts: The “crude” part makes the feces; the “middle part” Manas and the “subtle” part Jnanendriyas.
When the liquid part of the food is digested, it also gets divided into three parts. The “crude” part gets converted into urine, the “middle” part into blood and “subtle” into Prana.
This explains the process of fasting. A person can live without air for 3 minutes, without water for 3 days and without food for 3 weeks.
The first effect of fasting unto death is cessation of speech, then hearing and then the prana and the Tejas.
The above is true in non-Samadhi state. In the state of Samadhi, your metabolism may be so slow that you may live longer without food.
Even today many Jain Munis live without food for months.
annamaśitaṃtredhāvidhīyatetasyayaḥsthaviṣṭhodhātustatpurīṣaṃbhavatiyomadhyamastanmāṃsaṃyoṇiṣṭhastanmanaḥ || 6.5.1 ||
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.
āpaḥpītāstredhāvidhīyantetāsāṃyaḥsthaviṣṭhodhātustanmūtraṃbhavatiyomadhyamastallohitaṃyoṇiṣṭhaḥsaprāṇaḥ || 6.5.2 ||
When we drink water, it becomes divided in three parts. The grossest part of it becomes urine; that which is less gross becomes blood; and the finest part becomes prāṇa, the vital force.
tejośitaṃtredhāvidhīyatetasyayaḥsthaviṣṭhodhātustadasthibhavatiyomadhyamaḥsamajjāyoṇiṣṭhaḥsāvāk || 6.5.3 ||
When we eat fire [i.e., butter, oil, etc.], it divides itself into three parts. The grossest part of it becomes bone; that which is less gross becomes marrow; and the subtlest part becomes speech.
annamayaṃhisomyamanaḥāpomayaḥprāṇastejomayīvāgitibhūyaevamābhagavānvijñāpayatvititathāsomyetihovāca || 6.5.4 |||| itipañcamaḥkhaṇḍaḥ ||
- ‘O Somya, the mind is nourished by food, prāṇa by water, and speech by fire.’ [Śvetaketu then said,] ‘Sir, will you please explain this to me again?’ ‘Yes, Somya, I will explain again,’ replied his father.