More than 90% of the time, people allergic to cow's milk are allergic to goat's milk, Sicherer adds: "If someone's allergic to cow's milk, I tell them to stay away from mammalian milks."
Because soy milk is made from a plant, it contains no cholesterol and negligible amounts of saturated fat: just half a gram per cup.
Compared with whole cow's or goat's milk, it is lower in calories too, but a glass still provides the same levels of key nutrients present in those milks, including calcium, protein, vitamin A, vitamin D and potassium. That's partly because soybeans contain calcium, protein and potassium. But soy milk is also fortified to be nutritionally comparable to cow's milk.
Soy milk's lack of cholesterol and low levels of saturated and total fat have made it a popular choice for people looking to improve their heart health, says Stacey Krawczyk, a research dietitian with the National Soybean Research Laboratory at the University of Illinois at Urbana-Champaign. For the last 10 years, soy foods have been allowed to bear the FDA-approved claim that a low-fat, low-cholesterol diet containing 25 grams of soy protein per day may reduce the risk of heart disease.
Soy milk may have another benefit: In recent decades, several large population studies have suggested consuming soy may be linked to a lower risk of cancer, including prostate, colorectal and breast cancer.
But the relationship between soy milk consumption and cancer remains unclear -- largely because most studies have focused on populations, such as those in Asia, that consume whole soy products, such as tofu, tempeh and edamame, as a large part of their diet. Studies on the general U.S. and European populations have not been able to replicate the findings, in part because soy consumption levels here are much lower, Kushi says.
This protective effect against cancer, if there is one, is thought to be at least partly due to estrogen-like compounds in soy that may compete with human estrogen in the body, hindering it from prompting the cell proliferation that can trigger cancer. But the link between soy consumption and cancer may invert in women after menopause, when natural estrogen levels plummet. "The evidence is still unclear," Kushi says.
Soy can be a good dairy alternative for most people with allergies to cow's milk. Soy allergies affect 0.4% of children -- more common than most food allergies but far less common than ones to milk. Soy milk allergy in children is often outgrown. And though people allergic to cow's milk are often likely to have another food allergy, the differences in the two milks' proteins means an allergy to one doesn't automatically translate into an allergy to the other, McNairn says.
Soy milk also lacks lactose, so it's easier for people with lactose intolerance to digest it.
A downside? Because soybeans have an inherently bitter taste, soy milk is often heavily processed -- and sweetened -- to mask that flavor, says Kantha Shelke, a food chemist with the Chicago-based food-science think tank Corvus Blue. Sweeteners are often high on the list of ingredients in soy milks, adding sugar and calories that consumers might not be aware of. Still, with about 5 grams of sugar per cup, even the more sugary soy milks contain fewer sugars than the 12 grams per cup in cow's milk. (Soy milks labeled "unsweetened" contain about 1 gram.)
Soy milk presents its own digestibility challenges, Kazaks says. The milk contains high levels of oligosaccharides, carbohydrates that are hard for the body to break down. "It can really cause a lot of gas in some people," she says.
"With almond milk, it's more about what you don't get" than what you do, says Sam Cunningham, an independent food scientist and consultant specializing in nuts, who helped develop almond milk for Sacramento-based Blue Diamond Growers as an employee of the almond processor in the 1990s.
Like soy milk, almond milk contains zero cholesterol. It's free of saturated fats, so it's a healthful option for people with, or at risk for, heart disease. It doesn't contain lactose, so it's an option for people with lactose intolerance. And it's even lower in calories and total fat than soy milk: a glass contains just 60 calories and 2.5 grams of fat to soy milk's 100 calories and 4 fat grams.
But although almonds, among nuts, are a good source of calcium and protein, almond milk's calcium and protein levels don't compare to the levels in cow's, goat's or soy milks. A glass of almond milk provides just 1 gram of protein. Some brands provide up to 20% of the daily recommended calcium intake (about 10% less than the other milks), but other brands provide none.
Almonds are also a good source of iron, riboflavin, vitamin E and some essential fatty acids. A cup of the ground-up nuts contains more than 11 grams of omega-6 fats (but very few omega-3s).
In recent years, several studies have hinted at a link between nut consumption and lower blood cholesterol and a reduced risk of heart disease. Since 2003, the Food and Drug Administration has allowed almonds (and other nuts) to bear the claim that eating 1.5 ounces of nuts daily, as part of a diet low in saturated fat and cholesterol, may reduce risk of heart disease.
Still, nuts are one thing -- almond milk is another. The fraction of almond milk that's actually comprised of finely blended almonds varies between products and can be minimal, Kazaks says. In many commercially available almond milks, almonds are the second or third ingredient, after water and sweeteners. (The same is true for many soy milks as well.) So despite the high vitamin E and omega-6 content of almonds, a glass of almond milk may contain none of the vitamin and just 300 to 600 milligrams of the omega-6s.