This is a picture of a typical "muto" (or zero sugar, 無糖) black coffee - Pokka brand Biz Time Cafe Black. On such products there is a listing for torui (sugars, 糖類) separate from the obligatory carbohydrate (炭水化物) category. What exactly are the criteria for sugars?
This is a link to an informative guide which can be found on a Kanagawa Government webpage, but is also referenced and linked to by the Ministry of Health, Labor and Welfare. Sugars are any mono- or disaccarides (excluding alcohols). It is that simple, but it is certainly not perfect. By this definition, products might be fructose-free but still contain sugars - milk for example.
