The tide has turned on cow's milk.

Americans are simply drinking less of the stuff, with the changing tastes of consumers, concerned about the environmental toll of dairy and their own health, switching to plant-based dairy alternatives.

The U.S. Department of Agriculture reports that from 2009 to 2018, traditional milk consumption declined by nearly 19%.

“Because of this global shift, the largest dairy companies in the U.S. are investing in plant based alternatives and the ones that aren’t face a very uncertain future," said Alesia Soltanpanah, the executive director of World Animal Protection US, to USA TODAY.

It's taken a toll on milk producers. Dean Foods, America's largest milk manufacturer, filed for bankruptcy. In 2018 the Dairy Farmers of America, which represents about 30% of total U.S. milk production, reported a 7.5% decrease in sales — largely due to reduced milk prices.

But, even as American tastes are changing, some questions still remain: Do adults need to drink cow's milk? Long story short, not really, experts say.

And are plant-based beverages really healthier than the cow-based counterpart? Well, that depends.

Milk is 'not necessary' for adults, but it's good for kids

The idea that having a glass of milk with dinner is a bit outdated, but it still persists -at least in the federal government.

MyPlate, the USDA's current nutritional guide enacted in 2011, suggests 2 to 3 cups of dairy for adults daily, but its definition is broadened to include yogurt and cheese, as well as calcium-fortified soy milk. (Notably, it does not include other plant- and nut-based beverages as viable alternatives.) Still, the MyPlate visualizer shows a glass of milk alongside a glass of water for an average meal.

The USDA is in the process of developing its updated edition of Dietary Guidelines for Americans, but a spokeswoman wouldn't comment whether guidelines on milk would change.

Milk isn't necessary in an adult's diet, says Vasanti Malik, an assistant professor of nutrition at Harvard's School of Public Health,"but it can be good for you."

She told USA TODAY that cow's milk does have a lot of nutrients -vitamin D, protein and calcium. And it's an especially convenient way for kids -who are less likely to have a broader diet and experience rapid growth -to get these nutrients in their diet.

For adults, they tend to get those nutrients elsewhere.

Plant milk's benefits may be overrated

The largest beneficiary of the decline in cow's milk are plant-based alternatives. Nut- and plant-based milk alternative sales nearly tripled, up to 9%, from 2017 to 2018, according to the Plant Based Foods Association, and are expected to continue rising.

Some experts caution that their health benefits might be overhyped.

According to the American Society for Nutrition, most plant- and nut-based beverages have less fat and calories than low-fat milk. But that's not the whole story.

"If you do choose to drink alternative or plant-based milks, it’s important to check for the amount of protein and sugar on the nutritional label of the product," Glatter said.

Many brands are fortified with vitamin D and calcium, but not every option does, he said.

Further, both Glatter and Malik warned, they may have added sugars compared to regular cow's milk, which has lactose -a natural sugar -and generally does not have added sugars.

But if health benefits are secondary to environmental concerns, all plant- and nut-based milk is better for the environment than cow's milk, according to a study by the University of Oxford.

Soy, almond, oat and rice milk all produce less carbon and greenhouse gas emissions, while using less land and water than cow's milk.

"If you’re conscientious about the environmental impact, then opting for another source of those nutrients and protein would be a better option for you," said Malik.