OPTION A: My kidney function has significantly declined, and I’m currently recently, and am refraining from certain foods. OPTION B: My kidney function has significantly declined in recent years, and refrain from certain foods. OPTION A: My kidney function has significantly declined recently, and am refraining from certain foods. OPTION B: My kidney function has significantly declined in recent years, and refrain from certain foods.

SUGGESTION: Add a time marker such as "in recent years" or "recently" to make this more concrete and vivid. 提案: 「Recently」または「in recent years」などのタイムマーカーを追加して、これをより具体的で生き生きとしたものにします。

My kidney function has significantly declined, and I’m currently refraining from certain foods. My kidney function has significantly declined, and I’m currently refraining from certain foods.

"Currently" implies the continuous tense, so you should use "refraining".

For the past 14 years, I’ve hardly been able to exercise due to the aftereffects of a rear-end collision. For the past 14 years, I’ve hardly been able to exercise due to the aftereffects of a rear-end collision.

"Aftereffects" is okay, but I might suggest using a hyphen: "after-effects". I think it's easier to read this way, but it's ultimately up to preference.

I believe my severe muscle loss is also related to thea decline of kidney function. I believe my severe muscle loss is also related to a decline of kidney function.

I believe my severe muscle lossloss in muscle function is also related to the decline of kidney function. I believe my severe loss in muscle function is also related to the decline of kidney function.

"Severe muscle loss" would refer to the NUMBER of muscles in your body decreasing. However, you were probably referring to the strength of your muscles.