They say a person can’t last three days without water, but any parent knows that if you run out of milk, you’ve got about a day to replace it before things start to fall apart.

But what if you’re considering replacing the milk itself?

For generations, cow’s milk has been sold as the gold standard of U.S. nutrition, the preferred choice for kids to eat with their lunches at school, and with all that protein, it’s practically a meal in a glass.

However, with lactose intolerance and milk allergies on the rise, as well as a growing concern about the environmental impact of the dairy industry and the possible connection between cow’s milk and a number of health issues — including ear, nose and throat problems, obesity and even cancer — millions of families are switching to a number of milk alternatives made with various nuts, grains, seeds and legumes.

In the past year, sales of milk alternatives have increased 17.5 percent, while the total number of gallons of milk sold have dropped about two percent, according to market research company SymphonyIRI.

Brenda Bourgeois of Marietta switched from cow’s milk to soy nearly 10 years ago.

“I’m always trying to find healthy alternatives and thought I’d try it to see how it was,” said Bourgeois. “I didn’t know if I’d like it or not, but it was fine.”

Bourgeois, 72, said that as she has aged, she has became more concerned about weight gain and discovered that she could avoid putting on extra pounds with soy and it contained less cholesterol than cow’s milk.

No matter the reason you’re bringing home something other than cow’s milk, it’s important to know that just because it looks like milk and is labeled “milk,” the liquid made from soy beans, rice, hemp, coconut, almond or flaxseeds has different nutritional profiles than real milk.

“People mistakenly think that just because it’s soy or almond that it’s going to be the best for you all around ... or that just because it says milk that it has the same benefits of cow’s milk,” says Sally Bowman, a registered and licensed dietitian who works as a consultant at Central Texas Nutrition Consultants. The dairy industry has tried to push back against manufacturers calling these products “milk,” but so far haven’t won over the Food and Drug Administration.

Know the nutrition

Regardless of the name, it’s important to read the nutritional labels before making a decision about what to drink. Almost all have added calcium and vitamin D, the two big selling points for cow’s milk in the first place, but few have as much protein. Many of the chocolate- and vanilla-flavored milks contain added sugar, as much as 22 grams per serving, and if they don’t have much protein, their nutritional profile can be closer to juice than it is to milk, Bowman says.

You can get protein from a number of sources, but calcium can be harder to come by. Bowman says that companies that are making these milk alternatives are aiming for a similar nutritional profile by adding about the same amount of calcium and vitamin D per serving that cow’s milk provides.

Rachel Brandeis, an Atlanta-based registered dietitian, said calcium and vitamin D fortification are very important for seniors because those in that group are at higher risk for bone fractures and vitamin D deficiencies.

School-age kids need 800 to 1,300 milligrams of calcium per day, as well as vitamin D to help them absorb it, which you can usually get in three glasses of cow’s milk or an alternative, but it’s best to include a variety of calcium sources, such as green vegetables and legumes, instead of relying on one source.

People making the switch to almond milk can miss out on some essential amino acids, the building blocks for protein.

“Almond milk is not a complete protein and, like soy milk, you need to make sure it is fortified with calcium and vitamin. D,” Brandeis said. “Also, we lose muscle mass as we age, so it’s important that your diet does contain complete protein sources to help prevent muscle wasting. If you are lactose intolerant or don’t like the taste of regular skim or 1 percent cow’s milk, switching to soy is a fine alternative.”

How they compare

Most grocery stores sell soy and almond milks in the refrigerated section and a wider variety in hermetically sealed, shelf-stable boxes at a slightly higher cost per ounce.

When it comes to baking, you can use any of the alternative milks in place of regular milk with similar results.

Now, about the taste. For most of us, cow’s milk has a flavor and texture that is ingrained in our memory that alternative milks just can’t replicate, but some of them are closer than others.

Almond, coconut and hemp milks are the creamiest, but hemp milk has a strong, slightly funky flavor. Vanilla-flavored rice and soy milks are popular because they are sweeter than the plain varieties, that can taste a little chalky and watery. The best rice milks taste like horchata, the Mexican rice milk drink flavored with vanilla and sugar.

AJC staff writer Gracie Bonds Staples contributed to this article.