Laudator Temporis Acti

Theopompus, History of Philip, book 5, quoted by Athenaeus 4.157 e (tr. C.B. Gulick):

Too much eating, as well as meat-eating, destroys the reasoning faculties and makes souls more sluggish, and fills them besides with irascibility, hardness, and awkwardness.

τὸ γὰρ ἐσθίειν πολλὰ καὶ κρεαφαγεῖν τοὺς μὲν λογισμοὺς ἐξαιρεῖ καὶ τὰς ψυχὰς ποιεῖται βραδυτέρας, ὀργῆς δὲ καὶ σκληρότητος καὶ πολλῆς σκαιότητος ἐμπίπλησι.

Theodor Kock, ed., Comicorum Atticorum Fragmenta, III.2 (Leipzig: B.G. Teubner, 1888), p. 613, no. 1234 (author unknown; my translation):

A fat belly does not produce a subtle mind.

παχεῖα γαστὴρ λεπτόν οὐ τίκτει νόον.

Cicero, On the Nature of the Gods 2.16.43 (tr. H. Rackham):

Moreover the substance employed as food is also believed to have some influence on mental acuteness.

quin etiam cibo quo utare interesse aliquid ad mentis aciem putant.

Cicero, Tusculan Disputations 5.35.100 (tr. J.E. King):

Why so? because we cannot make proper use of our minds when our stomachs are filled with meat and drink.

quid, quod ne mente quidem recte uti possumus multo cibo et potione completi.

Horace, Satires 2.2.77-79 (tr. H. Rushton Fairclough):

Nay more, clogged with yesterday's excess, the body drags down with itself the mind as well, and fastens to earth a fragment of the divine spirit.

                                   quin corpus onustum
hesternis vitiis animum quoque praegravat una
atque adfigit humo divinae particulam aurae.

Musonius Rufus, fragment 18 a, p. 95 Hense (tr. Cora E. Lutz):

On the other hand he showed that meat was a less civilized kind of food and more appropriate for wild animals. He held that it was a heavy food and an obstacle to thinking and reasoning, since the exhalations arising from it being turbid darkened the soul. For this reason also the people who make larger use of it seem slower in intellect.

τὴν μέντοι κρεώδη τροφὴν θηριωδεστέραν ἀπέφηνε καὶ τοῖς ἀγρίοις ζῴοις προσφορωτέραν. εἶναι δὲ ταύτην ἔλεγε καὶ βαρυτέραν καὶ τῷ νοεῖν τι καὶ φρονεῖν ἐμπόδιον· τὴν γὰρ ἀναθυμίασιν τὴν ἀπ´ αὐτῆς θολωδεστέραν οὖσαν ἐπισκοτεῖν τῇ ψυχῇ· παρὸ καὶ βραδυτέρους φαίνεσθαι τὴν διάνοιαν τοὺς πλείονι ταύτῃ χρωμένους.

Pliny the Elder, Natural History 11.79.200 (tr. H. Rackham):

On this account species in which the distance from the belly is longer are greedier for food; moreover those with a very fat abdomen are less clever.

idcirco magis avidi ciborum quibus ab alvo longius spatium; item minus sollertes quibus obesissimus venter.

Seneca, Letters to Lucilius 15.3 (tr. Richard M. Gummere):

Second, their keen edge is dulled by heavy eating.

deinde copia ciborum subtilitas impeditur.

Galen, To Thrasybulus 37 (5.878 Kühn; tr. P.N. Singer):

The reality, though, is that wakefulness and intelligent thought, not sleep, are conducive to sharpness of wit; and it is an almost universally approved proverb—because it happens to be perfectly true—that a fat stomach does not make a fine mind.

καὶ μὴν ἐγρήγορσις μᾶλλον καὶ φροντὶς οὐκ ἀμαθὴς ἢ ὕπνος ὀξὺν τὸν νοῦν ἀπεργάζονται καὶ τοῦτο πρὸς ἁπάντων σχεδὸν ἀνθρώπων ᾄδεται, διότι πάντων ἐστὶν ἀληθέστατον, ὡς γαστὴρ ἡ παχεῖα τὸν νοῦν οὐ τίκτει τὸν λεπτόν.

Tertullian, On Fasting 6.1 (tr. Sydney Thelwall):

Nature herself will plainly tell with what qualities she is ever wont to find us endowed when she sets us, before taking food and drink, with our saliva still in a virgin state, to the transaction of matters, by the sense especially whereby things divine are handled; whether (it be not) with a mind much more vigorous, with a heart much more alive, than when that whole habitation of our interior man, stuffed with meats, inundated with wines, fermenting for the purpose of excremental secretion, is already being turned into a premeditatory of privies, (a premeditatory) where, plainly, nothing is so proximately supersequent as the savouring of lasciviousness.

ipsa natura enuntiabit, quales nos ante pabulum et potum in virgine adhuc saliva exhibere consuerit rebus dumtaxat sensu agendis, quo divina tractantur, si multo pollentioris mentis, si multo vivacioris cordis, quam cum totum illud domicilium interioris hominis escis stipatum, vinis inundatum, decoquendis iam stercoribus exaestuans praemeditatorium efficitur latrinarum, in quo plane nihil tam in proximo supersit quam ad lasciviam sapere.

Jerome, Letters 43.2.1 (tr. F.A. Wright):

I say nothing of the heavy meals which crush such mental faculties as we possess.

praetermitto prandia, quibus onerata mens premitur.

Jerome, Letters 52.11.4 (tr. F.A. Wright):

The Greeks have a pretty proverb which perhaps in our language loses some of its force: "A fat paunch never breeds fine thoughts."

pulchre dicitur apud Graecos, sed nescio utrum apud nos aeque resonet: "pinguis venter non gignit sensum tenuem."

I owe most of these references to Walter C. Summers, Select Letters of Seneca. Edited with Introductions and Explanatory Notes (1910; rpt. London: Macmillan and Co., Limited, 1952), p. 177; Renzo Tosi, Dictionnaire des sentences latines et grecques, tr. Rebecca Lenoir (Grenoble: Jérôme Millon, 2010), #1401, pp. 1035-1036; and Andrew Cain, Jerome and the Monastic Clergy. A Commentary on Letter 52 to Nepotian, with Introduction, Text, and Translation (Leiden: Brill, 2013), p. 231.