The brahmanas and Vaisnavas know what to eat, and by their personal example they do not eat anything which is not offered first to the Supreme Personality of Godhead. They eat only prasada, or remnants of the food offered to the Lord
SB Canto 4
The kṣatriyas, vaiśyas and śūdras eat their food by virtue of the brāhmaṇas' mercy. It is the brāhmaṇas who enjoy their own property, clothe themselves with their own property and give charity with their own property.
The Supreme Personality of Godhead is worshiped with the words namo brahmaṇya-devāya, which indicate that the Supreme Lord accepts the brāhmaṇas as worshipable gods. The Supreme Lord is worshiped by everyone, yet to teach others He worships the brāhmaṇas. Everyone should follow the instructions of the brāhmaṇas, for their only business is to spread śabda-brahma, or Vedic knowledge, all over the world. Whenever there is a scarcity of brāhmaṇas to spread Vedic knowledge, chaos throughout human society results. Since brāhmaṇas and Vaiṣṇavas are direct servants of the Supreme Personality of Godhead, they do not depend on others. In actuality, everything in the world belongs to the brāhmaṇas, and out of their humility the brāhmaṇas accept charity from the kṣatriyas, or kings, and the vaiśyas, or merchants. Everything belongs to the brāhmaṇas, but the kṣatriya government and the mercantile people keep everything in custody, like bankers, and whenever the brāhmaṇas need money, the kṣatriyas and vaiśyas should supply it. It is like a savings account with money which the depositor can draw out at his will. The brāhmaṇas, being engaged in the service of the Lord, have very little time to handle the finances of the world, and therefore the riches are kept by the kṣatriyas, or the kings, who are to produce money upon the brāhmaṇas' demand. Actually the brāhmaṇas or Vaiṣṇavas do not live at others' cost; they live by spending their own money, although it appears that they are collecting this money from others. Kṣatriyas and vaiśyas have no right to give charity, for whatever they possess belongs to the brāhmaṇas. Therefore charity should be given by the kṣatriyas and vaiśyas under the instructions of the brāhmaṇas. Unfortunately at the present moment there is a scarcity of brāhmaṇas, and since the so-called kṣatriyas and vaiśyas do not carry out the orders of the brāhmaṇas, the world is in a chaotic condition.
The second line of this verse indicates that the kṣatriyas, vaiśyas and śūdras eat only by virtue of the brāhmaṇa's mercy; in other words, they should not eat anything which is forbidden by the brāhmaṇas. The brāhmaṇas and Vaiṣṇavas know what to eat, and by their personal example they do not eat anything which is not offered first to the Supreme Personality of Godhead. They eat only prasāda, or remnants of the food offered to the Lord. The kṣatriyas, vaiśyas and śūdras should eat only kṛṣṇa-prasāda, which is afforded them by the mercy of the brāhmaṇas. They cannot open slaughterhouses and eat meat, fish or eggs or drink liquor, or earn money for this purpose without authorization. In the present age, because society is not guided by brahminical instruction, the whole population is only absorbed in sinful activities. Consequently, everyone is deservedly being punished by the laws of nature. This is the situation in this age of Kali.