I've been careful about my eating habits lately, so I have had much better bowel movements.
I have been eating healthier food these days which has led to more regular bowel movements.
この場合、次のような言い方ができると思います。
ーI've been careful about my eating habits lately, so I have had much better bowel movements.
「最近、食習慣に気を付けているからかなり快便だ」
to have better bowel movements で「便通がよくなる」と言えます。
ーI have been eating healthier food these days which has led to more regular bowel movements.
「最近、健康に良い食べ物を食べているので、快便になった」
ーregular bowel movements「規則正しい便通」
逆に便秘になったことを言うなら、次のように言えます。
ーI haven't been eating that well lately, so I've been constipated.
「最近きちんと食べてないから便秘になった」
to be constipated で「便秘になる」
ご参考まで！
カナダ 
