Doctors and dietitians like to point out that eating whatever healthy ingredient is in fashion won't help you if your overall diet is poor. That same argument goes both ways, though. If you're eating a healthy diet and getting appropriate amounts of exercise, there's no reason in the world you shouldn't enjoy an occasional treat if that's what makes you happy. Even a big mound of icing on top of your cake or cupcake isn't out of the question, especially fluffy and fat-free boiled icing. It does call for raw egg whites, though, so you have to be careful how you make it.
Using Egg Whites in Frosting
The whole point of using egg whites in your frosting is usually to get a light, fluffy, meringue-like texture. You need that for light and delicate cakes, because a heavy buttercream is just too much for an angel food cake or fluffy cupcakes. The two most common kinds of fluffy icing are old-fashioned, boiled icing, sometimes called seven-minute icing, and Italian buttercream. There's another type of frosting made with egg whites, but it's completely different. It's called royal icing, and instead of being light and frothy, it's thin, flat and hardens to a shell once it's dried. Royal icing is used mainly for decorating, like the lacework or garlands along the sides of a wedding cake, or to put a no-mess glaze on sugar cookies.
Know the Risks
For egg whites to whip, they have to remain uncooked, and that means there's a risk that you'll have live bacteria in your frosting. With eggs, that means you may be eating salmonella bacteria. It's a nasty little bug, and it'll give you nausea, vomiting, cramps, fever and all the associated misery that goes along with that. If you serve your icing to someone who's vulnerable – kids, the elderly, a friend whose immune system isn't doing its job – it can even be fatal. Salmonella bacteria crop up in dangerous concentrations in about one out of 20,000 eggs on the average. That's not a lot, but it's a risk to definitely understand.
Learn the Options
If you love one of the frostings made with egg whites but don't want to take risks with anyone's health, you have several options. One is to buy your egg whites pasteurized in a carton, from the refrigerated or frozen section of your supermarket. They've been treated at a temperature high enough to kill bacteria without cooking the whites, so you can whip them just like fresh whites. You can also opt for dried egg white powder or meringue powder, which is basically powdered egg whites with a few other ingredients added. Those are also pasteurized, and they'll whip up just fine when they're reconstituted with water. These are your best options for royal icing, which isn't cooked as part of the recipe.
Buy a Thermometer
Boiled icing and Italian buttercream actually bring your egg whites to a food-safe temperature, if they're done properly. You'll need to have a good candy thermometer, because temperature control is absolutely critical. While your mixer whips the egg whites to a froth, you'll need to bring a sugar syrup to a temperature of 250 degrees Fahrenheit in a small saucepan. Once it reaches the right temperature, you drizzle it into the egg whites with the mixer running. The hot syrup raises the temperature of the whites to a food-safe temperature, basically pasteurizing them on the fly. If you're nervous about getting it right, or if you just don't have a candy thermometer, it's best and safest to use powdered or pasteurized egg whites.