RN here, it's because your body has a more difficult time accessing the sugars in fruit form. They are wrapped up in multiple types of fiber which add bulk to your digestive system and serve as kind of a shield for your body to less easily access the sugar. The juice is essentially just the sugar with all the fiber strained out, has others have said this causes a situation where you can drink the calories from like 18 oranges in 45 seconds, and your body can access it immediately and easily. This is also why whole fruit is actually a good thing for diabetics because the glycemic index is actually pretty low.

There is sugar, absolutely. And that's probably where most of the calories come from. But there is also water, cellulose (fiber), and vitamins/minerals - doesn't have much non-sugar caloric value to change that balance, but it's still important. And nobody serious is suggesting you eat only fruit, so you can get non-sugar calories from other sources and it can be balanced in the big picture.

It's kind of like an appropriate amount of dressing on a salad, the good outweighs the bad and makes you more likely to actually eat that nutrition-positive food.

Source: I'm some guy on the Internet. You can trust me.

There was some guy on telly did a test. Half the group had to eat oranges. The other half had to drink orange juice. Then swapped them over the next day. I can't remember the exact setup but i think it was like 'eat/drink as much as you want, stop when you feel full'.

Everyone was able to consume far, far more calories in juice form and probably far more sugar than they needed.

I think like even eating enough oranges for 1x300ml glass was hard for many people to do in fruit form. Basically, the rest of the orange filled them up and that's what we're better evolved for: slower digestion of a more varied mush and lots of fibre and stuff like that.

The juice is far too easy for us to eat way more than needed.

There’s also quantity. Eating an orange is healthy. Drinking a glass of orange juice is like eating six oranges after removing the fiber: not heslthy

All that sugar is bound up in fiber, making it slower to release and keeping it from spiking your blood sugar into pre-diabeetus. Grind that same fruit down (juice), destroy its fibers, and now you got diabetes in a can.

They are like 11g of augars in 100g lf berries. They are not mostly sugar

Because it takes quite a lot of effort to eat a huge amount of sugar in the form on fruits and berries. They also have some vitamins, fibers and other stuff in them too.

Fiber. Fiber helps you feel full, so it is harder to over-eat fruit in comparison with chocolate bars, gummy bears, or even fruit juice.

People have spoken a lot about how digestible the sugars are, but in terms of overall healthiness, the fibre is an important component even beyond its impact on sugar absorption. Many people do not get enough fibre in their diets.

Healthy is relative. A handful of fruit is generally fine. Eating a few pounds of grapes in a day is probably a bad choice. There's also a lot of people that conflate fruit with things that have fruit in them as about the same.

They are not mostly sugar, sugars are just a part of the nutrients. Most fruits don't even have that much sugar in them, it varies wildly though. There is also the way these sugars are intertwined with fiber, that make it much harder for these sugars to be processed in your body. So the sugars are released over a greater period of time giving your body more time to react. as opposed to refined sugars. Fruits are always healthier than candy, cookies, or soda.

There is a lot more in fruits than just sugars, there are proteins, vitamins, minerals, fibers. Which are all necessary for a healthy body. Sugars as well are necessary for your body to function.

It is practically impossible, if you're otherwise healthy, to eat too much fruit. I personally eat at the very least 3 kilos or 5 pounds of various fruits a week. within an otherwise varied (vegan) diet. I've done so for the past 10 years. I make sure to test my blood, and so far had zero issues except low vit. d. Which you can't get from fruit.

Why is it healthy? Well, we evolved next to fruits. Our ancestors always plucked and eaten them for millions of years. Just like we've done with all kinds of plants. Our gi tract is the right length, our body cant make most vitamins itself and completely functions on sugars. Fruit is part of a varied diet.

Are they? Depends how many you eat.

fiber

Sugar: THE BITTER TRUTH

https://youtu.be/dBnniua6-oM

Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology, explores the damage caused by sugary foods. He argues that fructose (too much) and fiber (not enough) appear to be cornerstones of the obesity epidemic through their effects on insulin. Recorded on 05/26/2009.

How the sugar is packaged is also important. Standard white sugar is refined to be easier to digest - less gets pooped out. Fruits and berries sugar is (mostly) fructose with fibers and other elements. In the mouth fructose tastes equally sweet but the stomach has more troubles digesting it and converting it into usable energy. So you absorb way less and poop out way more.

Because sugar isn't unhealthy in the first place, you'll die without any sugar.

Whole fruits are pretty healthy in reasonable moderation

if you gorge on 3 boxes of grapes you're still gonna have smashed through over a thousand calories

The big caveat is fruit juices which remove all the fiber that makes you feel full, particularly anything concentrated.

At that point you're getting closer to a soft drink than fruit (though you'll still at least get the vitamins)

A nutritionist will be better able to explain this but I'll give it a try :)

You're maybe overthinking the sugar part of the equation. Berries/fruits contain natural sugar that is a part of the fruit itself. Your body processes that differently since that sugar comes integrated with other nutrients (fiber, Vitamin C, antioxidants, etc.). And you typically won't want to eat say a few buckets of berries in one sitting to equal the same sugar high you get in a processed sugar, all that fiber will feel much heavier and your body is just going to tell you to slow down on its own.

The much worse types of sugars are added sugars e.g. sugars that were processed and now exist separately, then re-added into something else. Take your berry example, process all the sugar out of them so only the sugar exists, then you add those sugars to some other food you wanted to sweeten. Now it's a sugar without any integrated benefits (no fiber/vitamin C/antioxidants/etc/) - your body won't process this processed sugar the same way it used to when it existed as part of the fruit.. you're only getting the bad without anything useful. So you can gobble a whole ton more of those added sugars to get your sugar high without your body getting any indicators to hey, slow down, maybe it's time to stop eating these added sugars.

The dose makes the poison, really. It's quite hard to reach a harmful amount of sugar by just eating fruit - you're likely to get either full or bored with eating fruit before you start reaching unhealthy levels of sugar. Combine this with fruits and berries generally being a good source of dietary fiber, this makes for a good combination of attributes you want in healthy food.

Because these are naturally grown sources of sugar that is untainted by the joke of the food industry's idea of processing sugar?

1st: they are NOT healthy. You just never eat berries in such amounts as you consume sugar in confectionery things.
2nd: there are many different types of sweet substances. Some are worse like sugar, others are safer like fructose.

Please read (or listen to) How not to die. It is a great book, funny and full of cool information, it changed my life a little.