What happens if i put sugar in tea? Is it fine to put anything sweet to a tea?

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My cousin told me not to put sugar in tea but it taste like garden if w/out it. I'm really wondering what happens if you put sugar in tea. I heard about diabetes. Now what I do is eating a piece of chocolate while I drink the tea so it will be easier for me to drink it but I know it makes no difference if I use sugar, when someone saw me holding a sachet of white sugar while I hold a cup of tea, "hey sugar?" they'll ask me but when I was once sitting eating a piece of chocolate while I drink the tea, no one reacted. What do you think guys?