170cm,51kg.too thin for a man?

well ideally he should be having 70kg approx....if he's healthy and not facin any other illness then i dont think there's no need to worry...if he's feelin ill then he should start eating more of food containing protiens and carbohydrates...takin fat rich food can invite trouble...that doesnt mean he cant take fat rich food at all..he can but not too frequently...and one thing you have to keep checkin is his haemoglobin level..should make sure he's not anaemic..

I don't think so.I am178/61. I am perfectly happy with it. In fact I am I don't have to worry about Obesity or exercise for a while.

of course too thin ,,get more sport,eat alittle more.maybe helpful.