— Martha Northcutt, Windham, New Hampshire
A: You are definitely changing the nutritional value. If you pour off just the whey—the fluid at the top of the carton—you'll lose a small amount of calcium. However, if you strain low-fat yogurt for several hours using cheesecloth or a dish towel, about half the calcium will go down the drain.
On the other hand, the thicker, creamier result of straining yogurt, which is sometimes referred to as Greek yogurt, is higher in protein than the standard stuff, providing roughly 20 grams versus 13 for an 8-ounce serving. It's also lower in carbohydrates because some of the milk sugar, or lactose, is lost.
You're wise to use nonfat yogurt: Removing the whey has the effect of increasing the yogurt's density, and usually when you do that to a food, you boost the concentration of fat and calories. Strain the full-fat version and you'll end up with yogurt that is more than twice as fatty and caloric as regular yogurt. Fat-free Greek yogurt has about the same amount of calories as standard varieties.