Few foods elicit such strong opinions as milk and dairy products. People don't simply like milk, or avoid milk, they preach about its virtues or its evils. The cow's-milk-is-for-calves folk warn that dairy is behind nearly all our major diseases, from cancer and diabetes to heart disease and even osteoporosis. Meanwhile, the other side would have you believe that dairy is essential, that your bones will crumble if you don't drink milk, and that it will prevent hypertension and even promote weight loss.

Milk used to be considered a perfect food, and nearly everyone agrees it's very nutritious and by far the leading source of calcium in the diet. Thus, last year's new federal dietary guidelines specified three cups of low-fat or non-fat milk or other dairy products every day (two cups for kids age two to eight)or else calcium-fortified foods and beverages. This caused an uproar among anti-milk groups, and even some mainstream nutrition experts suspected that the strong influence of the dairy industry played a role.

There has been lots of research about dairy in recent years, so it's time for an update. Here are the main issues.

Good or bad for your heart?

If you consume lots of whole milk and cheese, you're likely to see your blood cholesterol levels rise. That's true, however, of any foods rich in saturated fat. But more and more dairy products these days are nonfat or low-fat, and thus do not raise cholesterol levels. In fact, there's some evidence that certain substances in milk may help lower cholesterol.

Milk opponents often cite a few studies that indicted milk (sometimes even nonfat milk) as a cause of heart disease. But these studies are not convincing, and many others find no increased coronary risk or even show reduced risk. Moreover, since dairy is rich in calcium, potassium, and magnesium, it can help reduce the risk of hypertension, and thus it is an important part of the DASH diet.

A cancer connection?

Dairy opponents often claim that dairy products increase the risk of breast, prostate, and ovarian cancer. Some studies have linked high intakes of calcium and/or dairy to increased risk. But others have found no connection, and even a reduced risk. In 2004, for instance, a review of 46 studies on dairy consumption and breast cancer found no clear link negative or positiveto breast cancer. There is, however, fairly good evidence that dairy products help reduce the risk of colon cancer. One complication: dairy products contain many nutrients, some of which may decrease the risk of certain cancers (such as vitamin D), while other may increase it (such as fat or milk sugar).

Diabetes

A decade ago a group of anti-milk physicians, including the late Dr. Benjamin Spock, shocked parents by claiming that dairy products increase the risk of Type 1 (previously called juvenile) diabetes. This has never been proven, though breast-fed children do seem to have a lower risk of the disease. No reputable authorities have proposed that children avoid milk and dairy products. As for adults, dairy may reduce the risk of Type 2 diabetes. Last year, for instance, Harvard researchers linked increased dairy (especially low-fat) with a lower risk in men.

Your Bone Health

You may have thought that this at least was certain that getting calcium from dairy products throughout your life is the key to keeping bones strong and thus preventing osteoporosis. In fact, the calcium/bone story is very complicated. Some studies have found that consuming dairy or calcium leads to greater bone density, but others have not. So many factors affect bone health from genetics, physical activity, and body size to age at menopause and a wide array of nutrients that it's hard to tease out dairy's effect. Consuming adequate calcium is important, especially from childhood through early adulthood. However, once an older adult has weak bones, increasing calcium intake by itself has minimal effect.

Some anti-milk groups claim that dairy products actually increase the risk of osteoporosis. They point to the fact that in countries such as China and India, where dairy products are rarely consumed and calcium comes primarily from green vegetables, the rate of osteoporosis and fractures is much lower than in the U.S., where dairy consumption is high. But it's not possible to blame these national differences in bone health solely on dairy intake, since genetic, cultural, and lifestyle factors, as well as other dietary ones, undoubtedly also come into play.

One possible problem with dairy products is that they are rich in protein and a high protein intake slightly increases calcium excretion in urine and may reduce bone density. However, adequate protein helps keep bones strong, and the high levels of calcium in dairy products may more than offset the small adverse effect their protein has on bones. In addition, milk is almost always fortified with vitamin D, which is as important for bone health as calcium, according to recent research.

In the grand scheme of factors affecting bone health, dairy and calcium intake after early adulthood is likely to play a small beneficial role. Don't believe claims that dairy products hurt your bones.

Weight Loss

The latest argument about dairy is whether it helps people lose weight, or at least prevents weight gain. The dairy industry has advertised this virtue, based largely on studies it has funded. However, not all studies have linked dairy or calcium to weight loss. Even the positive studies found only a small benefit over long periods. The key to weight control is, as always, consuming fewer calories than you burn. Calcium by itself will not make you lose weight. But if you are trying to lose weight, don't drop dairy foods from your daily fare. Just choose low-fat or nonfat products. They may help a little.

Lactose Intolerance

Many people are anti-milk because they think milk is anti-them that is, it causes bloating and diarrhea, symptoms of lactose intolerance. Most Asian and African adults, as well as Native Americans, Latinos, and Ashkenazi Jews, have trouble digesting milk because they have lower levels of lactase (the intestinal enzyme that digests milk sugar, or lactose) than people of northern European descent. Still, many studies have shown that lactose intolerance is less prevalent than is commonly believed. Also, even those who truly are lactose-intolerant are often able to digest a cup or two of milk a day, if consumed at meals, with few if any symptoms. Lactose intolerance is not an all-or-nothing scenario, but a matter of degree. In one study, black Americans who were on the DASH diet were able to eat three servings of dairy products daily without any adverse effects.

The bottom line

Nonfat and low-fat dairy products are good foods, but like other good foods, you don't have to consume them if you don't like them or they don't like you. While for most people a balanced diet featuring all major food groups is the best way to get the nutrients they need, a diet lacking a food group can be healthy too, though it may take a little extra planning. For instance, vegetarians, including those who avoid dairy, have a lower risk for many chronic diseases.

For most people, dairy is an easy way to get calcium and other important vitamins and minerals. Some green vegetables (such as collards and broccoli), salmon or sardines (with the bones), soybeans, and almonds are also fair-to-good sources of calcium, though most of us don't eat lots of these foods. Calcium-fortified foods, such as orange juice, soy milk, and breakfast cereals, are good options. And calcium supplements can easily make up for calcium missing from your diet. The 2005 federal guidelines strongly emphasized dairy largely because its potassium helps prevent hypertension. But, again, you can get potassium from other foods, especially produce.

If your diet is good (lots of fruits, vegetables, and whole grains, plus small portions of lean protein), consuming little or no dairy won't be a problem, as long as you make sure you get adequate vitamin D, calcium, and potassium from other sources. If you are over 50 and don't drink milk, you almost certainly need to take vitamin D supplements to get 400 IU a day (that's the amount in most multivitamin/mineral pills). If you are over 70, even if you do drink lots of milk, you'll need supplemental D to reach the 800 to 1,000 IU a day.