Not always true. When I acquired pre diabetes the GP referred me to a specialist nurse that sent me to a couple of free courses on eating healthy, and exercise ( I do loads of physical work, so no needs for gyms or such, but I needed to stop eating sugar. After doing so I lost over 10 kg and a few centimeters off the waist in a year ).
I learned a lot from the eating healthy course, such as certain foods like white bread, pasta etc are converted to sugars after consumption, so not just sugar in sweets and desserts that has to be eliminated. Makes for a boring diet, but better than losing a leg to diabetes like my mother did.