It is a well-known fact that you cannot live without oxygen in the air for 3 minutes; without water for more than 3 days and without food for more than 3 weeks.

The Vedic references

Chandogya Upanishad explains about not taking food but continuing to take water.

6.7.1: षोडशकलः सोम्य पुरुषः पञ्चदशाहानि माशीः काममपः पिबापोमयः प्राणो नपिबतो विच्छेत्स्यत इति:

ṣoḍaśakalaḥ somya puruṣaḥ pañcadaśāhāni māśīḥ kāmamapaḥ pibāpomayaḥ prāṇo napibato vicchetsyata iti||

O Somya, a person has sixteen parts, [and all your sixteen parts are intact]. Do not eat anything for fifteen days, but drink as much water as you like. Life is dependent on water. If you do not drink water, you will lose your life.

The mind consists of food, the prana consists of water and speech consists of fire.

6.7.2: स ह पञ्चदशाहानि नशाथ हैनमुपससाद किं ब्रवीमि भो इत्यृचः सोम्य यजूंषि सामानीति स होवाच न वै मा प्रतिभान्ति भो इति:

sa ha pañcadaśāhāni naśātha hainamupasasāda kiṃ bravīmi bho ityṛcaḥ somya yajūṃṣi sāmānīti sa hovāca na vai mā pratibhānti bho iti||

Śvetaketu did not eat anything for 15 days. After that he came to his father and said, ‘O Father, what shall I recite?’ His father said, ‘Recite the Ṛk, Yajuḥ, and Sāma mantras.’ Śvetaketu replied, ‘I can’t recall any of them, sir’.

The above references clearly show that if you do not take food for 15 days you will lose power to think and speak. But since you are taking liquids (water), you will still be living as prana is made out of water.

Katha Upanishad explains about the water (1.1.7, 1.1.8. 1.1.9)

When Nachiketa fasts for three days and night without food and water at the place of Yama; when Yama returned on the fourth day, even he was disturbed. Indirectly, this gives the message that not offering water to a guest is considered a type of Brahma Hatya. It is consistent with the message that without water you cannot live for more than 3 days.

This is one reason as to why water is the first thing to be offered to a guest.

वैश्वानरः प्रविशत्यतिथिर्ब्राह्मणो गृहान् । तस्यैताँ शान्तिं हर वैवस्वतोदकम् ॥ ७ ॥

vaiśvānaraḥ praviśatyatithirbrāhmaṇo gṛhān ।  tasyaitām̐ śāntiṃ hara vaivasvatodakam ॥ 7 ॥

Like fire, a Brahman guest enters houses; men give this to quiet him. Vaivasvaia! fetch water.

आशाप्रतीक्शे संगतँ सूनृतां चेष्टापूर्ते पुत्रपशूँश्च सर्वान् ।  एतद्वृङ्क्ते पुरुषस्याल्पमेधसो यस्यानश्नन्वसति ब्राह्मणो गृहे ॥ ८ ॥

āśāpratīkśe saṃgatam̐ sūnṛtāṃ ceṣṭāpūrte putrapaśūm̐śca sarvān ।  etadvṛṅkte puruṣasyālpamedhaso yasyānaśnanvasati brāhmaṇo gṛhe ॥ 8 ॥

Hope and expectation, company with good men, true and pleasant discourse, sacrifices, acts of pious liberality, children and cattle, all these are destroyed in the case of the ignorant man, in whose house a Brâhman guest fasting stays (8).

तिस्रो रात्रीर्यदवात्सीर्गृहे मेऽनश्नन्ब्रह्मन्नतिथिर्नमस्यः ।  नमस्तेऽस्तु ब्रह्मन्स्वस्ति मेऽस्तु तस्मात्प्रति त्रीन्वरान्वृणीष्व ॥ ९ ॥

tisro rātrīryadavātsīrgṛhe menaśnanbrahmannatithirnamasyaḥ | namastestu brahmansvasti mestu tasmātprati trīnvarānvṛṇīṣva || 9 ||

As you have lived here. Oh Brâhman, a venerable guest in my house for three nights fasting, be my prostration to you, Oh Brâhman, may good befall me. Therefore, ask three boons in return (9).

Dr KK Aggarwal

Padma Shri Awardee

President Elect Confederation of Medical Associations in Asia and Oceania (CMAAO)

Group Editor-in-Chief IJCP Publications

President Heart Care Foundation of India

Past National President IMA