Smoothies sound like a healthy snack or meal replacement, and they can be -- if you go about it the right way. But because some smoothies contain more sugar than fresh and frozen fruits, yogurt, and skim (or non-dairy) milk, they can easily turn from diet-friendly to diet-busting as fast as you can say chunky monkey.
From one shop, a 20-ounce cherry smoothie with bananas and papaya juice, for example, has fewer than 300 calories, while a 20-ounce peanut butter plus chocolate smoothie nearly tops 700 calories. The best smoothies are those with no more than 17 calories per ounce (which means you're looking at 340 calories for a 20-ounce smoothie) and no less than 4 grams of fiber per serving. If you're having a meal-replacement smoothie, aim for at least 5 grams of protein and 5 grams of fiber per serving [source: Magee].