Lined up in colorful containers on grocery store shelves and available in flavors ranging from blueberry-cheesecake to key lime and mocha, yogurt hardly looks like an ancient food. But this healthy favorite actually dates back to 2000 B.C., when nomadic people storing milk in goatskin bags for transport opened them to find custard fermented by natural wild bacteria.

Today’s yogurt making is a bit more sophisticated. Lactobacillus and other beneficial bacteria get mixed into heated milk, where they eat up the natural sugars (lactose) and excrete lactic acid. This causes the mixture to solidify and take on yogurt’s characteristic tang and nutritional qualities. Here are five good reasons to add a few cartons to your shopping cart.

Yogurt is nutritious. You know it has calcium, but its overall profile is even better than you may think: A mere 1-cup serving contains 30 percent to 40 percent of your daily calcium needs. It’s also an excellent source of potassium, phosphorous, magnesium, zinc, and the B vitamins riboflavin, niacin, vitamin B6, and vitamin B12, plus about 9 grams of protein—a big nutritional lineup for such a small container.

Yogurt digests easily, even if you’re lactose-intolerant. That’s because yogurt’s bacteria turns lactose into two more easily digested sugars, glucose and galactose. “In fact, [lactose-sensitive people] can usually eat pretty much as much [yogurt] as they want to without reacting to the lactose,” says Denis Savaiano, PhD, dean of the College of Consumer and Family Sciences at Purdue University. That’s important, he says, because those who have trouble digesting lactose often limit or eliminate dairy foods from their diets, greatly decreasing their calcium intake and setting themselves up for higher risk of osteoporosis and bone fractures later in life (Nutritional Reviews, 2003, vol. 61, no. 6).

Yogurt helps you sleep. Like milk and turkey, yogurt contains the amino acid l-tryptophan, a precursor to melatonin, which helps you snooze. “l-tryptophan works particularly well when combined with a carbohydrate,” says Shawn Talbott, PhD, former professor of nutritional biochemistry at the University of Utah. “A cup of yogurt with some crackers or an apple can have a naturally hypnotic effect for those having trouble falling asleep.”

Yogurt fights infectious bacteria. Most women (and many men) know about eating yogurt to combat yeast infections. But there’s more: Yogurt can also help prevent the colonization of more harmful bacteria, such as E. coli and Helicobacter pylori, a microorganism associated with gastric cancer, ulcers, and duodenal ulcer disease. This makes yogurt good preventive medicine, says Talbott. “If you ate a cup of yogurt and then ate an E. coli–infected hamburger or came into contact with Helicobacter pylori, the yogurt would help keep the bad bacteria from setting up shop in your gut.”

Yogurt helps prevent allergies. One of the newest areas of research on fermented foods focuses on their ability to calm the immune system’s response to allergens. The most promising results to date come from a team of Finnish researchers who gathered 159 pregnant women whose unborn children were hereditarily predisposed toward developing certain allergic reactions, including eczema. Each day, half of the women received doses of Lactobacillus rhamnosus, a probiotic often found in yogurt; the other half received a placebo. Four years after birth, the risk of eczema was cut nearly in half for children who received lactobacillus in utero (The Lancet, 2001, vol. 357, no. 9262; 2003, vol. 361, no. 9372).

Zey Ustunol, PhD, an associate professor in the department of food science and human nutrition at Michigan State University, says that her own and others’ research on yogurt’s immune-system effects shows great promise. “Probiotic yogurts appear to improve the body’s ability to clear toxins and pathogens from the system,” she says. She also notes that yogurt’s bacteria may balance the immune system and keep it from over- or under-reacting to threats.

Best yogurts to buy
These days most yogurts contain “live and active cultures”—the probiotics that aid in digestion. But check labels for these as well as other ingredients; many yogurts contain a lot of added sugar or more fat than you may want. If you’re watching your carb or fat intake, buy plain, nonfat yogurt and mix in your own fruit or juice. Given all of yogurt’s health benefits, you might end up passing on the small cartons and going straight for the big buckets. At least they’re easier to store than a goatskin bag.

San Francisco–based writer Joyce Slaton is a frequent contributor to Delicious Living. Lemon and maple are her favorite yogurt flavors.