Too much milk?
Few things in life look as pure and simple as a glass of milk. The ingredient list on the carton is refreshingly short too. All it says is “milk,” perhaps along with some added vitamin A and vitamin D. No preservatives, no artificial colors, no high-fructose anything. Just milk.
But like many things that appear simple from the outside, there’s a lot going on beneath milk’s surface. That glass is swirling with natural cow hormones, which isn’t surprising considering the source. Milk contains sugars found nowhere else in nature, and it offers a particular blend of nutrients — including protein, calcium, magnesium and potassium — that you can’t get anywhere else.
Yet, almost 8,000 years after nomadic herders realized they could tug at the udders of slow-moving livestock, we still aren’t sure how much of the stuff we should be drinking. The USDA recommends three cups of dairy a day for all adults, but the science behind milk hasn’t been settled. “This is one of the most complicated and interesting areas of nutrition,” says Dr. Walter Willett, chairman of nutrition at the Harvard School of Public Health, “and we don’t have all of the answers.”
Many high-profile nutritionists — often working with large research grants from the dairy industry — say that milk in great quantities is an essential part of the daily diet that can help prevent osteoporosis, heart disease, cancer and other illnesses. “Anything less than three glasses a day, and you won’t get all of the nutrients that you need,” says Connie Weaver, head of food and nutrition at Purdue University in West Lafayette, Ind. Most of Weaver’s funding comes from the National Institutes of Health, but she’s also supported by the National Dairy Council.
On the other side, groups promoting animal rights and veganism — including PETA and the Physicians Committee for Responsible Medicine — say that cow’s milk is a nutritional nightmare that doesn’t belong in the human diet. “It’s gross,” says Dr. Neil Barnard, author and founder of the PCRM. “Milk is nutritionally perfect for one purpose: feeding a calf,” he says. “The idea that we should be drinking milk from a cow is just bizarre.”
Willett, one of the world’s most prominent nutrition experts, doesn’t belong to either camp. From his viewpoint, one or two cups of milk each day is a safe, reasonable and nutritious goal. “But beyond that,” he says, “the benefits are unclear, and there may be some risk.”
One or two cups? That’s not as much as the USDA recommends but more than many milk critics could possibly stomach.
Critics take aim
Of the foods that have their own tier on the pyramid, dairy products catch a lot of grief. A PETA website says that “dairy products are a health hazard” that are linked to “allergies, constipation, obesity, heart disease, cancer and other disease.” For a topper, the site says that milk is often contaminated with cow’s blood and pus.
The Physicians Committee for Responsible Medicine has always singled out milk as a particularly dangerous part of the typical western diet. The PCRM website says that saturated fats in dairy products increase the risk of heart disease. It also says that the natural hormones in milk encourage cancer of the breast, prostate and ovaries. Turning popular wisdom on its head, the organization says that dairy products won’t help prevent osteoporosis, the bone-thinning disease. The website highlights the Nurses Health Study, a 12-year examination of more than 77,000 women published in 1997 that found no link between reported dairy intake and the incidence of broken bones.
Anti-milk sentiment may have grown in recent years, if Internet chatter is any gauge. But so has the evidence suggesting that dairy products really do have some health benefits throughout the body, including the bones.
As reported in the Journal of the American College of Nutrition in 2009, at least 11 randomized intervention studies in a row have all shown that increasing the amount of dairy in the diet increases bone density. One 12-month study funded by a Greek dairy company found that women chosen to receive 1,200 milligrams of calcium and 300 IU of vitamin D from dairy products every day (which works out to about four cups of fortified milk) developed stronger bones in the hip and spine than women who stuck to their normal diets.
Recent studies are also shining some light on the link between dairy foods and cardiovascular disease, a particularly interesting part of the milk story. Although dairy foods are a major source of saturated fat in the American diet — we’re fond of our milkshakes, cheese and ice cream — there has never been any clear evidence that dairy foods are bad for the heart. In fact, they might do some good. The potassium in dairy can help lower blood pressure, and a National Institutes of Health-funded study published in the July issue of the American Journal of Clinical Nutrition found that conjugated linoleic acid — a fatty acid found in milk and beef — seems to lower the risk of a heart attack.
In another study published in the same issue, Swedish researchers checked the blood of 1,000 men and women (including 444 heart attack survivors) for certain compounds that come from milk fat. The study, which is partly funded by the National Dairy Council, found that women who had evidently consumed the most milk were about 25% less likely to have had a heart attack. Dairy seemed to protect men, too, but the effect wasn’t as strong.
To complicate matters, milk also contains natural hormones — including a hormone called insulin-like growth factor-1 or IGF-1 — that spur growth.
This is handy if you’re a mother cow trying to fatten your calf, but Willett and other experts worry that large amounts of the hormone could also help encourage cancer, especially prostate cancer and, to a lesser extent, ovarian cancer. Barnard, for his part, is beyond concerned. “If I were raising a boy, I would make sure he got his calcium from other sources,” he says. “Prostate cancer is one of the most common cancers he could face.”
Scattered studies over the years — including an Italian study published in July — have found that men who drink a lot of milk are at increased risk for prostate cancer. But the case isn’t completely closed. In a 2008 study funded by the National Dairy Council, researchers at the University of South Carolina who pooled the results of 45 previous studies covering nearly 27,000 cases of prostate cancer found no suggestion that milk or dairy products increase the risk of the disease.
Likewise, the link between dairy and ovarian cancer remains unclear. While some studies have found higher-than-expected rates of ovarian cancer in milk drinkers, others have turned up nothing. Most notably, the EPIC (European Prospective Investigation into Cancer and Nutrition) study of more than 325,000 women in 10 countries found no evidence that animal products — including eggs, meat and milk — raised the risk of ovarian cancer. The study was funded by various national cancer societies, ministries of health and government research organizations.
The NIH-AARP diet and health study — a long-term study of nearly 500,000 older Americans published in the Archives of Internal Medicine in 2009 — found no link between dairy intake and the overall risk of cancer. However, women who consumed the most dairy were about 30% less likely than women who consumed the least to develop colorectal cancer. To a lesser extent, dairy seemed to protect men from colorectal cancer, too.
The PCRM website says that milk raises the risk of breast cancer, but even Barnard isn’t convinced. " Breast cancer is unclear,” he says, adding that he doesn’t often look at the organization’s website. A 2005 report from a researcher with the Australian equivalent of the Dairy Council combined results of 52 previous studies examining the issue. When put together, the studies didn’t show any connection between dairy and breast cancer.
Ultimately, the science on milk — like reliable nutrition advice — may be somewhere between two extremes.