As a trainer and our studio's Nutrition Consultant, I get this question a LOT. Like, a lot a lot.
The simplest answer to this question is - YES. It is definitely okay to eat fruit. Fruits are loaded with vitamins, fiber, enzymes and wonderful disease-fighting things called antioxidants.
But fruit is also full of fructose. Fructose is sugar. It's a carbohydrate and, therefore, used by the body for high intensity energy. [Need a reminder of what this means? Read here.] If you are not a person who is expending high energy very often, you don't need a ton of carbohydrates in your diet. That's why, in general, if you want to be a lean person, you should engage in smart high-intensity exercise and keep your carbs in check accordingly.
Fruit it a lovely thing, but it makes sense to not go to town on fruit all day everyday. Digestive system upset is only one reason! Below, I've outlined a few tips for having fruit in your lean physique life:
1) Eat the skin
This is important, folks. If your fruit has an edible skin, you should eat it. Citrus, bananas and melons, okay to compost those peels and rinds. But apples? Peaches? The skins of those fruits provide fabulous fiber, which helps control the rate at which the fruit's fructose is absorbed into your bloodstream. The same goes for the pulpy membranes in oranges and grapefruits, and the little tiny edible seeds inside raspberries, blueberries and strawberries. "Juicing" a fruit merely isolates the sugar water from the rest of the food, which makes that fructose digestion lightning fast (and not great unless you're immediately post-workout).
2) Fresh or frozen fruit, not dried
Sure, dried fruit has a lot of fiber in it (hello, prunes!), which is a good thing, but it's also missing almost all of its water. Therefore, the VOLUME of the food is much less, and the "fullness" effect much harder to reach. The number of raisins you're likely to eat in the amount of time you would eat a whole apple could be 5x the amount of sugar. A true serving of raisins is about half the size of a golf ball. Can you see why it would be easy to overdo it?
3) Some fruits are more "sugary" than others
It's true - some fruits have more fructose in them per serving than others. You can see what fruits are "low on the Glycemic Index (like blueberries) or "high" on the Glycemic Index (pineapple, grapes, bananas and mangoes are some examples), but that doesn't mean those fruits are bad. It just means that you should eat those fruits fresh or frozen (again, not dried) and keep it to a serving.
4) Consider fruit a carb
Speaking of a "serving", you should consider fruit in your diet to be a carbohydrate. Just as you wouldn't necessarily have bread, rice, pasta or potatoes at every meal and snack if you're trying to lean up, you wouldn't do the same with fruit. Keep your serving size of fresh fruit to 1/2 cup twice a day and you are plenty fine.
5) Carrots count
Okay, they aren't technically fruits, but from a nutritional standpoint you should consider them as such. Carrots are similar to most fruits in fructose per volume, so treat them like you would treat a fruit.