SB 11.7.45

tejasvī tapasā dīpto
 durdharṣodara-bhājanaḥ
sarva-bhakṣyo ’pi yuktātmā
 nādatte malam agni-vat
Synonyms: 
tejasvī — brilliantly luminous; tapasā — by his austerity; dīptaḥ — glowing; durdharṣa — unshakable; udara-bhājanaḥ — eating only that needed by his stomach; sarva — everything; bhakṣyaḥ — eating; api — even though; yukta-ātmā — one who is fixed in spiritual life; na ādatte — does not assume; malam — contamination; agni-vat — like the fire.
Translation: 
Saintly persons become powerful by execution of austerities. Their consciousness is unshakable because they do not try to enjoy anything within the material world. Such naturally liberated sages accept foodstuffs that are offered to them by destiny, and if by chance they happen to eat contaminated food, they are not affected, just like fire, which burns up contaminated substances that are offered to it.
Purport: 

The word udara-bhājana indicates that a saintly person eats only to keep body and soul together and not for sense gratification. One should eat palatable foodstuffs to maintain one’s mind in a cheerful mood; however, one should not eat luxuriously, because this will cause sex desire and laziness. A saintly person is always a perfect gentleman and is never greedy or lusty. Although māyā tries to defeat him by offering different material allurements, ultimately these attractive material features are themselves defeated by the spiritual power of a saintly person. Thus, one should never disrespect a spiritually advanced personality but should worship him reverentially. To carelessly approach a Kṛṣṇa conscious personality is just like carelessly approaching fire, which immediately burns if not handled properly. The Lord does not excuse mistreatment of a pure devotee.