By Lisa Drayer

CNN

(CNN) -- Fruit juice is not as nutritious as fresh fruit, but it can be a healthy part of your diet, if it's consumed in small portions.

A glass of fresh orange or grapefruit juice with breakfast isn't just refreshing. It also delivers a healthy dose of vitamin C and potassium, which can be especially helpful if you tend to forgo fruit. Some store-bought juices are fortified with bone-building calcium, too.

But even when it contains only naturally occurring fructose from whole fruit and no added sugars to boost sweetness, fruit juice is still a concentrated source of sugar and calories, which can be problematic for those watching their weight or blood sugar.

For example, one 8-ounce cup of fresh orange juice has 21 grams of sugar and 112 calories. By comparison, one medium orange has 12 grams of sugar and only 62 calories.

Similarly, a cup of cranberry juice has 28 grams of sugar and 110 calories, but a cup of whole cranberries has only 4 grams of sugar and 46 calories. The counts for grape juice are even higher, with 36 grams of sugar and 140 calories per 8-ounce cup.

Fruit juice also lacks the fiber found in whole fruit, which means we not only miss getting the health benefits of fiber -- which include its ability to lower cholesterol and help us feel full -- we experience a more rapid rise in blood sugar after consuming juice, since fiber slows the entry of sugar into the bloodstream.

When purchasing juice, look for brands containing 100% fruit juice with no sugar added. According to US dietary guidelines, juices may be partially fruit juice, but only the proportion that is 100% fruit juice counts toward your daily fruit intake. (For example, 1 cup of juice that is 50% juice counts as ½ cup of fruit juice.)

Technically, only 100% juice can be called "juice." Juice "drinks" may be a lower-calorie version of juice and contain artificial sweeteners. For example, Mott's light apple juice drink contains only 42% juice, but it also has less than half the calories and sugar as the brand's 100% apple juice. You can opt for the pure juice version and consume a 4-ounce serving for almost the same nutritional value.

According to the guidelines, sweetened juice products that are primarily composed of water with added sugars fall under the category of sugar-sweetened beverages.

One important note: If you consume grapefruit juice, keep in mind that it can negatively interact with some medications, such as statin drugs. If you have any concerns, check with your doctor to see whether you can safely consume the juice.

Bottom line? If you enjoy juice, go for the real thing, and limit yourself to one 8-ounce glass per day -- or 4 ounces if you're limiting calories or sugars.

Lisa Drayer is a nutritionist, author and health journalist.