…a rule that Americans traditionally break shatter.

腹八分目に医者要らず
(Hara hachibunme ni isha irazu;
“At the belly’s eighth part, no need for doctors”)

Definition:

It’s better for your health to not eat until you’re stuffed, but rather to stop before your body starts telling you to stop. Eat until you’re only 80% full, and you’ll avoid a variety of disorders that might otherwise have sent you to the doctor.

Breakdown:

We begin with the noun 腹 (hara), “belly.” Next we may imagine an elided particle such as the associative no, but what actually follows is number-noun 八分 (hachibun), “eight part(s),” where traditionally each 分 is one part of ten. (Hence 80%, above.) The following 目 (me) is acting as an ordinal number marker – that is, like “th” in English. The particle に (ni) marks this noun phrase as the metaphorical point at which the following clause applies:

This begins with the noun 医者 (isha), “doctor,” followed and modified by the verb 要る (iru), “to need,” in imperfective form and taking the negative suffix ず (zu) in conclusive form.

Notes:

The 目 is actually entirely disposable; about half of my sources do without; there’s a similar split between whether 分 is read as bun or bu. More rare is a variant that replaces 分目 with 合 (gou), which can also refer to one-tenth of a total amount. The final irazu may be written in all kana, as いらず. Another variant is more direct about the health impact, saying 病なし (yamai nashi, “no disease”) instead of mentioning doctors at all. In some cases the whole phrase may be reduced to just 腹八分.

This saying is attributed to Edo-era botanist and philosopher Kaibara Ekiken (貝原益軒), in his 1713 CE treatise 『養生訓』 (Youjoukun), apparently translated as The Book of Life-nourishing Principles.

Example sentence:

「腹八分の法則を守っているから大丈夫だと豪語していたうちの叔父は、確か比較的に健康だったけど、いくら腹八分って言ってもさすがに毎日吸ってたら、肺ガンで死んでも当然だよな」

(“Hara hachibu no housoku wo mamotteiru kara daijoubu da to gougo shiteita uchi no oji wa, tashika hikakuteki ni kenkou datta kedo, ikura hara hachibu tte itte mo sasuga ni mainichi suttetara, haigan de shinde mo touzen da yo na.”)

[“My uncle boasted that he was in good shape because he obeyed the rule of an eight-tenths-full belly, and he probably was relatively healthy. But it’s hardly a surprise, no matter how healthy his diet, that someone who smoked every day would die of lung cancer.”]