Very difficult,
Sadly nutrition research is not yet very conclusive on what is good and what is bad for our health.
(It's very difficult to have correct experiments as the volunteers never have the same physiology)
And our nutriments needs may be very specific to one's genetics and activity level.
Sugar is fuel for the brain and the body, it is not necessary bad, but abusing it is bad. Maybe sugar (eg pasta) is bad when mixed with other ingredients and healthy with others. It is extremely difficult to reach a good consensus even if you read the litterature.
And usually (micro)nutriments have trade-offs : good for somethings bad for others.
Look at the different kind of diet, they are all based on different researches and can be extremely different while all are using 'valid' research data.
The main idea behind them is usually to have 'good' proportions between macronutrients (each diet define a different proportion) while keeping the number of kcal in the range you want to reach your objectives.
For a first rule of thumb that I would use: abusing in any direction is bad. Sugar is ok in 'small' quantities, fat, etc also.
However reality is way more messy :)
The thing is that sugar is harmful is kinda counter-intuitive: it's tastes good and we all want it. We get overweight because of fat but sugar is not fat. And most importantly, the scientists were covering it up: blaming fat and protein rather than sugar.
I did not believe sugar was the problem until randomly decided to have a try while I was trying to lose some weight. Tried to eat less walk/run around 8km per day for around a year, it helped a little. I did manage to lose more or less 10kg. Unfortunately, problems with the project and had to work over time and thus stopped running, eat lots of junk food for roughly a month, during which time drunk lots of Coke and when it was over, I found my self picked up 15kg. Then I decided to try something new, stopped drinking Coke and eat less carbs in diets by eating more meat and vegs. And the result was much better than I thought: I lost almost 20kg in less than 3 months and the exercise I took actually dropped a little bit.
Refined sugar is fine when used in part to fuel physical activity. Though it's a complete disaster when consumed in excess when sedentary.
140 calories for a can of Coke is pretty modest; I just ate a third of a lemon meringue containing 500 calories - most supermarket food is calorific muck, cheap to manufacture yet tasty enough.
This is absolutely contrary to the evidence. Sucrose is rare in plants in nature (save in tiny quantities.) We aren't evolved to absorb it gradually, it just floods in to the bloodstream; it behaves very differently than either of it's components, or fructose or other vegetable and fruit sugars. It's so evolutionarily weird that even bacteria and fungi generally aren't able to eat it. Therefore, if you're on a FODMAP diet you are allowed to drink Mexican Coca-Cola (has sucrose) but not American Coca-Cola (has liquid invert sugar.) Sucrose won't cause SIBO yeast and bacteria growth because yeast and bacteria haven't evolved to cope with it either.
Tests on athletes (some now decades old) show even moderate sucrose consumption reduces athletic performance and health.
Very hard. I tried and failed for the good part of the last 15 years despite mounting evidence (and resurfacing burried one). Dr. Lustig has some good talks online that were revelatory for me at the time of "my transition".
It's kind of with everything else... people are addicted (I certainly still am, after decades of sugar abuse). Then there's a whole culture around that addiction and our natural craving for easy calories (fuck evolution, really).
As with all of those issues (religion, ideology, conspiracy theories) you need to "nudge" people in the right direction so they can "discover" the issues themselves. Facts don't help nearly as much as one might hope and a lot of it is correlation obfuscated by wishful thinking and genetic variability ("but my cousin Joe always drinks a coke a day and still isn't fat!").
Ever tried to convice a smoker of non-smoking? An anti-vaxxer of his narccisistic ignorance? It's always the same pattern.
