Preface: sugar is all the same. Whether it was adding in during processing or already present, the word sugar refers to a carbohydrate in its most basic form. There really is no [as far as calories, obesity and health are concerned] difference between sugar found in a strawberry or soda. Let's begin!
Once we get past the "all carbohydrates are bad" mantra, we have to start differentiating between the good and bad ones. Sugar is a type of carbohydrate characterized as simple, perhaps even bad or unhealthy depending on the source. A wide variety of foods contain sugar, and while some of them are deemed healthy, others are not. Though the sugar found in doughnuts and apples is nearly identical, there's an important distinction between foods high in added sugar and "naturally occurring" sugar.
Why is sugar bad?
Sugar is generally labeled a bad calorie. According to the National Institutes of Health, sugar increases the risk of tooth decay, obesity, type 2 diabetes, metabolic syndrome and high blood pressure. Sugar provides a quick source of energy and is therefore a great ingredient in a pre- or intra-workout snack. Beyond energy, sugar provides virtually no benefits.
Are fruits healthy if they are almost all sugar? What's the difference between sugar in fruit and junk food?
The calories contained in fruit and soda come almost entirely from sugar. Why then, are fruits considered one of the healthiest foods while sodas are universally despised? While fruit contain a wide array of nutrients such as minerals, vitamins, fiber and antioxidants, soda has nothing to offer beyond sugar (energy). Soda is made up of empty calories. While fruit provides calories in addition to important nutrients, soda, and other junk food, offer nothing.
Why is distinguishing between added sugar and natural sugar important?
Sugar is sugar. The sugar found in fruit is essentially identical to the sugar found in junk food. The difference, and reason why fruit is healthy while soda is not, is what else is found in the fruit (nutrients). While calorie content of sugar in healthy and unhealthy foods is identical, effects on overall health (and even weight management) are not.
Fruits are filled with fiber, a key ingredient which increases satiety. Junk food, such as soda, is void of any fiber. Fiber keeps you full and therefore lowers calorie intake. Think of how many sodas or doughnuts you can eat before you're full versus how many apples for the same effect. The difference here is even though an apple contains sugar, you typically do not eat five or six apples in one sitting because you get full fairly quickly. It's fairly simple to eat/drink multiple servings of doughnuts/sodas in one sitting which adds up to quite a bit of sugar and calories.
Is natural sugar healthy (or healthier than added sugar)?
Natural sugar and added sugar are the same thing. It's not whether the sugar was already in the food (as is the case with fruit) or added during processing (as with soda and junk) that makes a food healthy or not. A food's entire makeup determines its worthiness; sugar, nutrient and calorie content all play an important role.
Fruit juices are a great example of why foods don't need any added sugar to be labeled unhealthy. Fruit juice and soda contain similar amounts of sugar yet, a juice container gets away with using a "no added sugar" label. When a fruit is processed into juice, all the good stuff (fiber, vitamins and minerals) is taken out, leaving you with flavored sugar water. Fruit juice is a perfect example of how naturally occurring sugar isn't always as good as it sounds.
"Natural sugar" is a misleading term. Anyone who reads the word "natural" assumes a food is healthy. Too much "natural" sugar is just as bad as too much "added" sugar. Many "natural" sweeteners make the claim (overtly or otherwise) that they're healthier than normal sugar. These "natural" sweeteners include honey, agave nectar and brown sugar. While these sweeteners may include trace (tiny, insignificant) amounts of nutrients not found in regular white sugar, their sugar content is identical to the white stuff, and when eaten is large quantities, lead to the same problems.
The Bottom Line
The type of sugar found in a food is not what makes it healthy or unhealthy. The difference between so called "natural sugar" and added sugar isn't the actual sugar, but simply how the sugar got into the food. As far as how sugar affects health, it makes no difference where the sugar came from, only the overall quantity you eat. It's much harder (if not impossible) to suffer from a high sugar intake eating apples than it is drinking soda or fruit juice. Also, the types of food containing added sugar are generally unhealthier (sodas, doughnuts, cookies, fruit juices) than foods containing natural sugar (mainly whole fruit).
More important than a single food ingredient, is the larger picture of a lifestyle. It's difficult to label a food as unhealthy or healthy unless you take into consideration the entire choices an individual makes. Sugar is generally bad, but drinking a few sodas probably isn't detrimental to someone who makes most of the right choices.