I don’t have sources for you, but I’ll try to summarize the gist as I understand it.
Sugar is fine in moderation from natural sources provided you don’t have a medical condition such as diabetes. By natural I mean like fruit or milk. By moderation I mean a single serving. Those natural sources will almost always come with other nutrients and fiber that you won’t get from juice or processed foods
Excessive sugar will cause you to generate more insulin and over time desensitize your body to it. Sugar also triggers reward pathways in the brain which can cause addiction and dependency issues. Sugar is also the metabolically cheapest energy and your body will always process it first, which means excess calories from fat or protein are more likely to be stored instead of burned, especially since your body tends to need switch “modes” of calorie burning (carbs then fats then proteins). This can, however, be a good thing if you are engaging in intense physical activity.
The more processed the sugar generally the higher it’s bioavailability and glycemic index which ultimately means the stronger it’s effects on the body.
Tl;dr a little fruit here and there is good for the average person and the occasional glass of milk can be nutritious, but juice, desserts, and processed foods should be avoided as they will muck with your metabolism and act like a drug on your body.
Robert lustig, a professor and neuro-endocrinologist that specialises in obesity (mainly in youth, I think) has an answer to all your questions.
Great stuff. Teaches you exactly how much we know. We know enough to know it’s a chronic health hazard that’s slowly killing more and more of the world population.
The good:
Fine in small amounts (darker varieties are better with molasses), efficient/effective energy source during athletic exercise, and less bad than most artificial sweeteners.
The bad:
Depresses the immune system, feeds pathogenic bacteria and yeasts, and pretty bad/damaging in the presence of any insulin resistance. In excess, associated with various other problems related to the other major organs (liver, gallbladder, etc)
If health is a priority, it’s simply better to avoid it,