The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
40. On account of (the passage showing) respect, there is non-omission (of the prânâgnihotra) (even when the eating of food is omitted).
We read in the Khândogya under the heading of the Vaisvânara-vidyâ, 'Therefore the first food which comes is in the place of Homa. And he who offers that first oblation should offer it to Prâna, saying Svâhâ' (Kh., Up. V, 19, i). The text thereupon enjoins five oblations, and later on applies to them the term 'Agnihotra;' 'He who thus knowing this offers the agnihotra,' and 'As hungry children here on earth sit round their mother, so do all beings sit round the agnihotra' (V, 24, 2; 4).
Here the doubt arises whether the agnihotra offered to the prânas is to be omitted when the eating itself is omitted or not.--As, according to the clause, 'The first food which comes,' &c., the oblation is connected with the coming of food, and as the coming of food subserves the eating, the agnihotra offered to the prânas is omitted when the eating is omitted.--Against this conclusion the Sûtra (embodying the pûrvapaksha) declares, 'It is not omitted.--Why?--'On account of the respect.' This means: In their version of the Vaisvânara-vidyâ the Gâbâlas read as follows: 'He (i.e. the host) is to eat before his guests; for (if he would make them eat first) it would be as if he without having himself offered the agnihotra offered that of another person.' This passage, which objects to the priority of the eating on the part of the guests and establishes priority on the part of the host, thereby intimates respect for the agnihotra offered to the prânas. For as it does not allow the omission of priority it will allow all the less the omission of that which is characterised by priority, viz. the agnihotra offered to the prânas.--But (as mentioned above) the connexion--established by the Khândogya-passage--of the oblation with the coining of food--which subserves the eating--establishes the omission of the oblation
in the case of the eating being omitted!--Not so, the pûrvapakshin replies. The purpose of that passage is to enjoin some particular material (to be offered). For the fundamental agnihotra certain materials, such as milk and so on, are exclusively prescribed. Now, as through the term 'agnihotra' (which the text applies to the offering to the prânas) all the particulars belonging to the fundamental agnihotra are already established for the secondary agnihotra also (viz. the oblation made to the prânas), just as in the case of the ayana of the Kundapâyins 1; the clause, 'the first food which comes,' &c., is meant to enjoin, for the prânâgnihotra, some particular secondary matter, viz. the circumstance of food constituting the material of the oblation 2. Hence, considering the Mîmâmsâ principle that the omission of a secondary matter does not involve the omission of the principal matter, we conclude that even in the case of the omission of eating, the agnihotra offered to the prânas has to be performed by means of water or some other not altogether unsuitable material, according to the Mîmâmsâ principle that in the absence of the prescribed material some other suitable material may be substituted.
To this pûrvapaksha the next Sûtra replies.
250:1 For one of the great sacrifices lasting a whole year--called the ayana of the Kundapâyins--the texts enjoin the offering of the 'agnihotra' during a full month (cp. e.g. "Tândya Mahâbrâhmana XXV, 4). Now from the term 'agnihotra' we conclude that all the details of the ordinary agnihotra are valid for the agnihotra of the ayana also.
250:2 Whereby the materials offered in the ordinary agnihotra are superseded.