Feb. 14, 2020 -- Cow’s milk is creamy, filling, and delicious ice-cold, and decades of advertising have sold it to Americans as a food that “does a body good.” Dairy products are rich in calcium and protein, and they have long been promoted as important for helping kids grow and helping kids and adults build and maintain strong bones. But does dairy deserve its health halo?
The current U.S. dietary guidelines recommend that just about everyone eat three servings of dairy a day.
Now, in a new review, Walter Willett, MD, DrPH, a professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health, and his co-author, David Ludwig, MD, PhD, a professor of pediatrics and nutrition at Harvard, say the science behind those dietary recommendations is thin. And they say eating too much dairy may cause harm to both our bodies and the planet.
“If we’re going to recommend something, it obviously should be based on strong evidence,” says Willett. He reviewed the risks and benefits of drinking milk for The New England Journal of Medicine.
“The basis of calcium recommendations is, I think, fundamentally flawed in the United States,” he says.
He’s not the only one who feels that way.
Elizabeth Jacobs, PhD, is a professor of epidemiology, biostatistics, and nutritional sciences at the University of Arizona Mel & Enid Zuckerman College of Public Health in Tucson. She and her colleagues recently reviewed the science behind the dairy recommendations and concluded that the U.S. should follow Canada’s lead and ditch dairy as a separate food group. Instead, they recommended placing dairy foods in the protein category, making them one choice among many that would help people meet their protein requirements. Their paper is published in Nutrition Reviews.
The two papers come at time when the U.S. dietary guidelines are under review. A new version of the guidelines will be issued by a panel of experts later this year, and for the first time will include advice for pregnant women and for children under age 2.
“We’re not saying milk is dangerous or harmful,” Jacobs says. “No matter how you slice it, Americans are moving away from milk. So let’s adapt to this change and give people more opportunity to meet their nutritional needs.”
Willett also points out that dairy farming is hard on the environment. While that might not have been a big consideration 20 years ago, climate change makes it critical to consider now. “If it’s going to have a major adverse environmental impact, we better take a serious look at our recommendations as well and see what we’re going to do to mitigate that,” he says.
Slim Evidence Behind Dairy’s Health Claims
While we’re drinking less dairy as a beverage, we’re still consuming more of it overall. According to the U.S. Department of Agriculture, the average American ate and drank about 9% more dairy in 2018 than we consumed per person in 1975. Data shows that we’re eating more cheese and yogurt but drinking a lot less milk. Milk consumption has fallen about 40% since 1975. But because it takes more milk to make products like cheese and yogurt, dairy consumption is up overall.
The current dietary guidelines for dairy are based on the idea that we need milk to help meet daily calcium requirements.
Yet Willett says those recommendations come from studies that were relatively small -- including just 155 men and women. And those studies were short -- following people for 2 to 3 weeks. Researchers measured how much calcium they ate and drank, and compared it to how much they were excreting in stool and urine. The idea was to find out how much calcium the body needs to keep it in balance.
In adults, who are done growing, calcium balance should be net zero. That is, people should excrete about the same amount as they eat or drink. In Americans, who tend to eat a lot of calcium compared to people in other countries, the studies concluded 741 milligrams of calcium a day was enough for balance. In other countries, like Peru, where diets typically aren’t as rich in calcium and dairy products, the amount needed for balance was much less -- around 200 milligrams. Willett says this is consistent with the idea that the body can change how much calcium it absorbs from food. When people eat less calcium, the body may simply absorb more to meet its needs.
He also points to large population-based studies that offer snapshots of how people eat and what happens to their health. These kinds of studies have consistently shown that in countries where people eat the most dairy, they also have higher rates of fractures.
“That raises sort of a red flag that there’s something wrong here,” Willett says.
Those studies can’t prove that eating more dairy causes hip fractures, but Willett believes it makes sense because eating dairy products in childhood is known to accelerate growth and lengthen bones. The risk appears to be highest for men who drank a lot of milk in childhood.
“That’s probably because of basic mechanics. If you have long bones, they’re easier to break than short bones,” he says.
Not everyone agrees with the study’s conclusions. In a written statement, the National Dairy Council, which represents dairy farmers, said the study didn’t include the “total body of evidence” on dairy foods.
“Dairy remains an important part of a balanced diet and provides lasting and meaningful nourishment for people, the planet and communities,” Gregory Miller, PhD, chief global science officer at the National Dairy Council, said in a written statement.
In additional to bone health, milk has been touted as being helpful for weight loss. The review found no evidence to support that.
Research shows that dairy products can help control blood pressure, but only when they’re part of an overall healthy diet. That makes it tough to tease out whether milk or dairy products were responsible for the benefit.
Its effects on other health outcomes are mixed. Willett says observational studies have found strong links between eating dairy and some kinds of cancer, such as prostate cancers. Again, these studies can’t show that milk causes cancer. There were no links found between milk and getting diabetes. And there was no link between lifespan and eating dairy.
Taken together, the science shows that “milk is not essential for health,” says Marion Nestle, PhD, a retired professor of nutrition, food studies, and public health at New York University who was not involved in the study.
“This tells me that milk is a food like any other, meaning that its effects depend on everything else people are eating or doing. People who like milk can continue drinking it. Those who don’t like it don’t have to,” she says. “It’s just a food.”
Willett agrees. He says if you’re a dairy underachiever, you shouldn’t worry about it. If you’re not getting any dairy in your diet at all, it’s not a bad idea to take a calcium supplement, but don’t take gobs -- 500-600 milligrams a day should be enough.
What About Kids?
“It’s complicated for adults, but it’s even more complicated for kids, and we have even less data,” Willett says.
The calcium needs of kids are trickier to figure out. They’re growing, so they’ll naturally need more. But the role dairy should play in meeting their calcium needs isn’t clear.
There is good evidence that kids who drink cow’s milk grow taller than those who don’t.
It’s not known exactly how milk accelerates growth. But the study authors say cows are often pregnant when they’re milked, which increases hormones like estrogen and progesterone. Cows have also been bred to produce more of another hormone, called insulin-like growth factor, which increases milk production, but those hormones may also promote growth in people.
There’s also some worry that hormones in milk may lead to the cancer later in life, but the evidence for that is mostly circumstantial.
Kids need calcium for building strong bones, Willett says, but studies don’t show that adding a lot more dairy makes a difference.
One study, for example, randomly assigned 240 kids, ages 8 to 15, who weren't getting enough calcium in their diets, to a meal plan with three added daily servings of dairy, or to continue on their normal diets. After 18 months, the study found no difference in bone density between the kids who had more dairy and the ones who didn’t.
Willett also notes that while the U.S. recommends that kids ages 4 to 8 get 1,000 milligrams of calcium in their diets, the U.K. recommends about half that much, just 450 to 550 milligrams a day.
That doesn’t have to come from milk, he says. Other foods like kale, broccoli, tofu, nuts, and beans all count toward the goal. One important point, he says, is if dairy is off the table at your house, make sure your kids are getting vitamin D, though a dietary supplement.
Jean Welsh, PhD, who researches nutrition as an associate professor of pediatrics at Emory University, praised the reviewers for raising important questions about dairy. But she urged caution when it comes to taking dairy off the table for kids.
“What always makes me nervous when we talk about these key features of our diets is if we promote a change, what’s going to replace it?” says Welsh, who was not involved in the review.
“The study authors say that if you have a good-quality diet, you don’t need milk. Well, yeah,” Welsh says. “It’s not like we’re eating well.”
On average, many kids probably don’t get enough broccoli, kale, or other sources of calcium in their diets to meet all their needs, she says.
Milk is better than sugar-sweetened beverages, she says, especially for kids.
Welsh recently tested several brands of conventional and organic milk for pesticides, antibiotics, and hormones. While pesticides and antibiotics were sometimes found in the conventionally farmed milk samples, none were found in the organic milk samples. Hormone levels were also higher in the conventionally farmed samples, compared to the organic samples.
She says that if organic milk is too pricey, parents shouldn’t worry. Milk is still good for kids. Especially if they’re picky eaters.
“While there are advantages to drinking organic milk in that it’s free of chemicals often used in milk production, we do not have evidence that this makes a difference in children’s health,” Welsh says. “What we do know is that milk, organic or not, is a readily available source of nutrients important in the diets of children.”
Environmental Impacts of Dairy
Even if you’ve loved dairy for a long time, there are reasons to reconsider, not least of which is climate change. Willett notes that considering different sources of protein, the costs of dairy to the environment are probably five to 10 times greater than plant-based protein sources. Dairy farms consume more water. They can contribute to water pollution. Large-scale dairies may depend on antibiotics to keep their animals healthy, which contributes to antibiotic resistance in people. He says limiting dairy production would make a “major contribution” to reaching greenhouse gas targets.
Some dairy alternatives have their own environmental issues. Almonds, for example, are a water-intensive crop.
Miller, of the National Dairy Council, says dairy farmers are working to green their operations.
“U.S. dairy only accounts for approximately 2% of total U.S. greenhouse gas emissions. Farmers continue to make even more environmental progress. For example, producing a gallon of milk in 2017 involved 30% less water, 21% less land, 19% smaller carbon footprint and 20% less manure than in 2007,” he says.