Is It Safe To Not Consume Any Kind Of Sugar For A Long Period Of Time ?

Is it safe to NOT consume any kind of sugar for a long period of time? Like for year? You see my father in law believes because his Dr. told him that ALL sugar is bad. So he hasn t consumed sugar in months. Not even a grapfruit he thinks is ok sugar to consume. He thinks now that they are all unhealthy. My sister gave me an explanation opposing the Dr. views on no sugar consumption. And she also told me how sailors use to get sick when they would go on their long expeditions if their fruit and veggies rotted because it brought forth a desease to not have sugars or soemthing like that. So anyone who can be insightful by answering my question please do so. I ll appreciate it. I mean he is one strict dieter. and oldschool so if he thinks dr. know absalutley everything and can never and have never been wrong. I mean he doesnt it food to eat he ll eat just to survive which is good. no salts as much as possible and only water i think just chicken and low sugar veggies he eats. Water/chiecken/low sugar veggies. insert sarcasam now... he reminds me of someone who can become anorexic. That srict! Hope that helps you understant what im talking about and hwy i am asking this question. Thank you all!! This im adding is in regard to the know it all Debbie who seems to have a problem understanding my simpl question. You see Debbie you are wrong in assuming i have judged my father in law missy. lol I appreciate your answer in only a slight way because i feel more like you were coming after me for thinking i have judged my father in law rather than just being insightful. Sorry you took it personally. Wtch on here debbie you might not be able to handle other peoples questions and attitudes. And also i dont know why it went into this daibetes category. i did not choose it. He wasnt even borderline diabetic.