1. Out of the trigunas:

Sattva - enhances knowledge
Rajas - enhances activity
Tamas - enhances dullness, vice. https://twitter.com/mmpandit/status/1346071596210003968

2. Mantra sadhana is about getting the grace of the mantra devata. Devatas bless the 'knower'. Mantra sadhana therefore is proportional to the increase in jnana of the devata in ourself.

3. So anyone pursuing mantras has but no choice to increase Sattva in himself. Choice of environment, time and indeed food matters a lot.

4. Alcohol greatly increases Tamo-guna. No brainer that it is prohibited. Meat increases rajo-guna. Hence forbidden during sadhana for others and forbidden always for V1s.

5. Am not an expert on meat varieties but likely red-meat is closer to alcohol in terms of Tamasic content. Hence prohibited (even from that angle). Of course primary reason remains divinity of go-mata.