Jessica Simpson posted an ad for milk — and some fans got upset. Here's why there's so much drama over dairy lately.

Jessica Simpson is getting backlash after posting an Instagram ad in support of “Gonna Need Milk” — an updated version of the dairy industry's famous “Got Milk?” campaign. The ad, which the pop star posted online earlier this week, is simple: It features Simpson posing in a kitchen in a white tank top and black pants, holding a glass of milk and staring into the camera. "Milk ... does a body good," she wrote in the caption.

Many commenters disagreed. "Not really! Milk is the worst beverage humans can drink," one person wrote. "Milk is actually pretty bad for our bodies, very bad campaign," another said. Simpson hasn't publicly responded to the backlash, which follows the criticism Hailey Bieber received last May after modeling a “Got Milk?” T-shirt on Instagram.

What's it all about? Milk products have been getting a lot of criticism lately, whether it's traditional dairy products or plant-based versions. Dunkin Donuts is facing a $5 million class-action lawsuit over charging extra for non-dairy milks, and oat milk is now coming under fire over allegations it can cause blood sugar spikes.

The negativity surrounding the singer's campaign and milk products in general raises a lot of questions about milk and its impact on health. Here's what experts have to say.

Do you need milk as an adult?

Experts agree that you don't need to drink milk as an adult. "Humans do not need to drink cow’s milk at all," says Dr. Deborah Cohen, a professor of clinical nutrition at Rutgers University, to Yahoo News. "The reason many people have historically consumed cow milk and why most health care professionals have traditionally recommended cow milk is because cow milk is a good source — and highly bioavailable source — of both calcium and vitamin D." (Bioavailable means how well nutrients are absorbed in the body.)

Besides yogurt and cheese, other sources of these nutrients just aren't as robust, Cohen says.

Nutritionist Karen Ansel, author of The Calendar Diet, agrees. "While adults don’t need milk, there are loads of good reasons to drink it," she tells Yahoo Life. "Milk contains one of the highest quality forms of protein, meaning that your body can use more of its protein for jobs like building muscles, hormones and antibodies. It’s so powerful that one of its main proteins, whey, is used specifically to make protein powder."

But while cow's milk is packed with nutrients, "it is possible to get those nutrients from other sources," Jessica Cording, nutritionist and author of The Little Book of Game Changers: 50 Healthy Habits For Managing Stress & Anxiety, tells Yahoo Life. "Technically, someone does not need to consume milk to meet their nutritional needs," she adds.

"You can absolutely get these nutrients elsewhere," Katherine Balantekin, a dietitian and assistant professor at the University at Buffalo, tells Yahoo Life.

There is a wide variety of plant-based milks and other fortified foods on the market, Cohen says, that make it unnecessary for people to rely on cow's milk.

Is there any harm in having milk?

Dairy milk is becoming less popular in the U.S. In 2022, members of Generation Z bought 20% less milk than the national average, according to the New York Times.

But calling milk "the worst beverage humans can drink," is a bold claim, and experts say that's an overreach. "More and more people have become interested in a plant-based diet and vegetarian and vegan diets, which means less cow's milk consumption," Cohen says. "There is some concern and confusion about the role of dairy in cardiovascular disease, cancer and obesity, but there is no good evidence to support the relationship between low-fat dairy and any of these claims."

Cohen also points to concerns about cows contributing to greenhouse gas emissions and global warming, as well as ethical concerns about having cows produce milk for such long periods of time.

Cording says she finds the backlash against milk "fascinating," adding, "milk gets people really upset — it's very polarizing." There are some conditions where someone may not feel as comfortable having milk, such as lactose intolerance or a milk allergy, Cording says. But, for the average adult, she says there's no harm in having a glass when you want.

Whether to have plant-based milks or not is "very individual," Cording says. "Just because something is plant-based doesn't mean it's healthy," she says, noting that these products tend to be heavy in sugar. If you prefer to use plant-based milk, Cording recommends checking the label to see how much sugar it contains, as well whether it's fortified with nutrients like calcium, vitamin D, vitamin B12 and protein.

"Plant-based milks are a great alternative for folks who want to drink milk or use milk in cereal and baking, but are vegans," she says. "There is currently a wide range of plant-based milks including soy, almond, oats, hemp, cashew, coconut, rice, macadamia and pea."

But if you want to pour yourself a glass of standard cow's milk, experts say there's no reason not to.