Pineapple is a very intriguing item as far as low carb is concerned. Raw, its carb count - quite reasonable - but in juice form, it is extremely high in sugar!
If you eat pineapple in a 1/4 cup serving, it is only 4.3g. Compare that to banana that is 21.2g, or raisins which are 31g. Pineapple is extremely healthy compared to those!
On the other hand, consider a mere 8oz glass of pineapple juice. You're downing a full 34g of carbs if you drink that! If you drank orange juice instead, you'd only get 26g of carbs. Pineapple juice is very, very sweet.
If you can stick with raw pineapple, you get some good health benefits. Pineapple has far more than your daily RDA of manganese. It's also got nearly half of your daily RDA of Vitamin C. Just eat it in moderation!