"keep in good health" 是动介短语，结构为 "keep + in + 名词短语（good health）"，表示“使（自己/他人）处于…状态”。其用法需注意以下几点：
主语多为“人”，表示“某人保持健康”。
I try to keep in good health by going to the gym three times a week.
我通过每周去三次健身房来保持健康。
My grandma keeps in good health by eating organic vegetables every day.
我奶奶每天吃有机蔬菜来保持健康。
常用 by doing sth（通过做某事）或 with sth（用某物）引出保持健康的方法：
She keeps in good health by getting enough sleep (≈ through adequate sleep).
她通过充足的睡眠保持健康。
You can keep in good health with a balanced diet and regular exercise.
你可以通过均衡饮食和规律运动保持健康。
与 "stay healthy" / "keep healthy" 同义（但结构不同）：
"keep in good health" 后接名词短语（good health）；
"keep healthy" / "stay healthy" 后接形容词（healthy）。
例：He keeps healthy = He keeps in good health.
一般疑问句：Do you keep in good health? 你保持健康吗？
否定句：He doesn’t keep in good health because he smokes a lot.
他因为抽烟太多，身体不好。
日常对话：
A: How do you keep in good health? 你怎么保持健康？
B: I walk my dog every evening and avoid junk food. 我每天晚上遛狗，不吃垃圾食品。
建议/提醒：
Doctors always advise us to keep in good health by drinking more water.
医生总是建议我们多喝水保持健康。
"keep in good health" 是表达“保持健康”的正式/常用短语，侧重“主动维持状态”，需结合方式状语说明具体方法，适用于日常交流、健康建议等场景。