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SB 4.23.5

kanda-mūla-phalāhāraḥ
 śuṣka-parṇāśanaḥ kvacit
ab-bhakṣaḥ katicit pakṣān
 vāyu-bhakṣas tataḥ param
Synonyms: 
kanda — trunk; mūla — roots; phala — fruits; āhāraḥ — eating; śuṣka — dry; parṇa — leaves; aśanaḥ — eating; kvacit — sometimes; ap-bhakṣaḥ — drinking water; katicit — for several; pakṣān — fortnights; vāyu — the air; bhakṣaḥ — breathing; tataḥ param — thereafter.
Translation: 
In the tapo-vana, Mahārāja Pṛthu sometimes ate the trunks and roots of trees, sometimes he ate fruit and dried leaves, and for some weeks he drank only water. Finally he lived simply by breathing air.
Purport: 

In Bhagavad-gītā, yogīs are advised to go to a secluded place in the forest and live alone in a sanctified spot there. By Pṛthu Mahārāja’s behavior we can understand that when he went to the forest he did not eat any cooked food sent from the city by some devotees or disciples. As soon as one takes a vow to live in the forest, he must simply eat roots, tree trunks, fruits, dried leaves or whatever nature provides in that way. Pṛthu Mahārāja strictly adopted these principles for living in the forest, and sometimes he ate nothing but dried leaves and drank nothing but a little water. Sometimes he lived on nothing but air, and sometimes he ate some fruit from the trees. In this way he lived in the forest and underwent severe austerity, especially in regards to eating. In other words, overeating is not at all recommended for one who wants to progress in spiritual life. Śrī Rūpa Gosvāmī also warns that too much eating and too much endeavor (atyāhāraḥ prayāsaś ca) are against the principles by which one can advance in spiritual life.

It is also notable that, according to Vedic injunction, to live in the forest is to live in the mode of complete goodness, whereas to live in the city is to live in the mode of passion, and to live in a brothel or drinking house is to live in the mode of ignorance. However, to live in a temple is to live in Vaikuṇṭha, which is transcendental to all the modes of material nature. This Kṛṣṇa consciousness movement affords one the opportunity to live in the temple of the Lord, which is as good as Vaikuṇṭha. Consequently, a Kṛṣṇa conscious person does not need to go to the forest and artificially try to imitate Mahārāja Pṛthu or the great sages and munis who used to live in the forest.

Śrīla Rūpa Gosvāmī, after retiring from his minister’s seat in the government, went to Vṛndāvana and lived beneath a tree, like Mahārāja Pṛthu. Since then, many people have gone to Vṛndāvana to imitate Rūpa Gosvāmī’s behavior. Instead of advancing in spiritual life, many have fallen into material habits and even in Vṛndāvana have become victims of illicit sex, gambling and intoxication. The Kṛṣṇa consciousness movement has been introduced in the Western countries, but it is not possible for Westerners to go to the forest and practice the severe austerities which were ideally practiced by Pṛthu Mahārāja or Rūpa Gosvāmī. However, Westerners or anyone else can follow in the footsteps of Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura by living in a temple, which is transcendental to residence in a forest, and to vow to accept kṛṣṇa-prasāda and nothing else, follow the regulative principles and chant sixteen rounds daily of the Hare Kṛṣṇa mantra. In this way, one’s spiritual life will never be disturbed.