Remember that first milk mustache? Naomi Campbell, 1995. Followed by a herd of others--Kim Cattrall, Dr. Phil, Michael Phelps--all promoting the wholesome drink. Last fall teen actress Lindsay Lohan donned the 200th "Got Milk?" mustache, making the campaign one of the most successful in American history. The goal: to boost milk consumption, which, despite the ads, has dropped from a high of 45 gallons per person per year in 1945 to 22 gallons today.

It's not just dairy farmers and celebs who want to see your milky smile--the government does, too. Last week the departments of Health and Human Services and Agriculture released the 2005 dietary guidelines, recommending lots of fruits and veggies and more milk, from two glasses a day to three for most Americans. Many nutritionists, worried about a nationwide calcium deficit, applaud the increase, and it is good news for the $21 billion milk market. But the move has also heightened concern among some scientists, who say the powerful dairy industry overstates milk's attributes, understates possible health risks and wields too much influence over consumers. "The public has been given this idea that milk is essential for good health," says Dr. Walter Willett, of the Harvard School of Public Health (HSPH), but "where's the evidence that high amounts of dairy are beneficial?"

Just about everybody agrees that milk provides a powerful cocktail of nutrients, especially calcium, the building block for teeth and bones. But most Americans consume only half the recommended servings of dairy a day, and every year osteoporosis leads to 1.5 million fractures. Too little calcium, however, is not the sole culprit; genetics, inactivity and smoking also play a role. Harvard studies of health professionals have found that those who drank one glass or less of milk per week were at no greater risk of breaking a bone than those who had two or more glasses. And in India and Africa, where calcium intake is low, fracture rates are, too. "There are plenty of societies in the world," says HSPH's Dr. Meir Stampfer, "where people live long, healthy lives without drinking milk."

Then there's the question of whether milk makes you thinner. In a study published last year, Michael Zemel, of the University of Tennessee, found that obese adults eating a high dairy diet (three servings a day) each lost an average of 24.5 pounds over six months, compared with 14.5 pounds lost by those on a control diet. The dairy industry, which funded Zemel's research, seized on the findings, promoting weight loss in its ads. But other data are mixed. While there's no evidence that milk will increase your waistline, critics say the research is too limited to conclude that dairy will shrink it, either.

Could too much milk be unhealthy? Even though the new guidelines recommend low-fat or fat-free dairy, some nutritionists worry that Americans will still reach for the Hagen-Dazs, upping their intake of saturated fats. Then there's lactose intolerance, which makes digestion difficult for as many as 50 million Americans. Perhaps most worrisome, says Willett, are studies suggesting that a diet high in dairy could be a risk factor for prostate cancer.

The prostate data are in no way conclusive; in fact, other studies suggest that dairy protects against colon cancer. The 2005 guidelines advisory committee weighed the science and made its decision: three cups of milk a day, already recommended for teens and adults over 50, now makes sense for everyone over 9. In the end, it was the potassium in milk, a nutrient that helps reduce blood pressure, that tipped the scale. The industry couldn't be happier. Not only are dairy products safe, says Greg Miller of the National Dairy Council, they're "wholesome and great value in terms of nutrition." "Got Milk?" versus "Need Milk?" The debate is far from over.