It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].
医者の不養生・Isha no fuyojyo (A physician often neglects his own health)
Doctors usually tell their patients advice about their health but often does not pay attention to their own health.
We know in our mind but our actions does not follow it.
(A physician often neglects his own health)
Isha wa [okashi tabesugiruo muchiba ni naruyo] to iuga, jibun deha amai mono ga daisukide yoku tabeteiru. korekoso isha no fuyoujyou dato omouyo.
The doctor tell us [If you eat too much sweet you get decayed teeth.] but he love sweets and often eat them. This is what we say “a physician often neglects his own health.”
We all are good at advising people but when it comes to ourselves, we tend to ignore good advice we give to others.