I eat a very healthy diet; I almost never eat restaurant or processed foods and I eat lots of fruit, vegetables, and whole grains. Yesterday was a fairly typical food day for me; the only added sugar I ate came from half a loaf of my homemade bread. I use 2 tablespoons a loaf, so I would have eaten 1 tablespoon, or 12 grams of added sugar, which is well under the WHO's new suggested limit.
Back in the 80's, fats were demonized. Food manufacturers responded by making 'low fat' foods with lots of added sugars. Now it is the added sugars that are taking the blame for obesity. It is true that sugary foods with few micronutrients (vitamins, minerals, amino acids) do bad things to your body in excess, just like fatty foods with few micronutrients.
We are seeing more and more recommendations to cut sugar consumption. This is probably good advice for most people, but it is important to keep a sense of perspective.Nutrition advice tends to go through fads, and these fads rarely have strong evidence behind them. We cannot do proper random trials on the macronutrient (fat, sugar, protein) mix of human diets for ethical reasons, and there is rarely even any good observational data that you can base strong conclusions on. So people are left to guess based on animal testing and metabolic models.
But I also ate three apples and a pound of strawberries, which according to Wolfram Alpha contains 79 grams of sugar. So theoretically I am way over the limit, but I find it highly unlikely that eating three apples and a pound of strawberries for lunch is a bad idea.
Biologically, there is no difference between added sugar and sugar in fruit. Fructose is fructose. The fruit gives me lots of vitamins and fiber, of course, but that is the only thing that makes it better.
I believe that the only two nutrition rules that really matter are to eat as many calories as you burn, and make sure you have enough vitamins, minerals, and amino acids. The more you exercise, the easier it is to follow these two key rules, and of course exercise is very good anyway.
Rules like ‘keep sugar under 5%’ are probably just fads. Beware any nutrition advice that makes absolute claims or blames a particular food item or macronutrient for all of your problems.