This Sutta and Vinaya study discusses controversies regarding working out and keeping the precept not to eat after solar noon.
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Note: the distinction between milk fat and milk protein appears to be the reason why milk is not allowed after noon, but butter is. Also note that whey protein powder is derived from milk.
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Relevant Sutta and Vinaya Passages:
Majjhima Nikāya (MN)
MN 2 – Sabbāsava Sutta (All the Effluents) — Thanissaro Bhikkhu
https://www.dhammatalks.org/suttas/MN/MN2.html
Pātimokkha Rules (Vinaya)
Pācittiya 37 (Pc 37) — Food at the wrong time (BMC discussion & full rule context)
https://www.dhammatalks.org/vinaya/bmc/Section0019.html#37
Nissaggiya Pācittiya 23 (NP 23) — The five tonics (7-day limit)
Index: https://www.dhammatalks.org/vinaya/bmc/Section0030.html
Medicine chapter note: https://www.dhammatalks.org/vinaya/bmc/Section0044.html
Mahāvagga (Vinaya)
Mv VI (Bhesajjakkhandhaka) — Allowance of the five products of a cow
https://www.dhammatalks.org/vinaya/Mv/MvVI.html (see “The allowance of the Five Products of the Cow”)
Cullavagga / Khandhaka (Vinaya)
Senāsanakkhandhaka — Assigning lodgings (Dabba Mallaputta)
https://suttacentral.net/pli-tv-kd16
Saṅghādisesa (Vinaya)
Saṅghādisesa 8 — Background with lodging-assignment formula