Just as with milk, there are varying fat and calorie contents for milk alternatives. Most are lower in protein than milk, but since they're all plant-based, no milk alternative contains cholesterol. Vitamins and minerals are added to many of the alternatives, making them nutritionally similar to milk.
A few have more calcium and vitamin D than milk! In particular, soy "milk" has the added advantage of isoflavones, known as phytoestrogens. Though the jury is still out on phytoestrogens, these substances may mimic estrogen and contribute to the slowing of bone loss due to decreasing estrogen levels associated with menopause.
Some "milks," however, lack several nutrients, such as the calcium and vitamin D, that are rich in dairy milk. These two nutrients are key players in the prevention of osteoporosis. What's more, the calcium in milk alternatives, namely soy "milk," is not as well absorbed as the calcium in dairy milk.
For instance, it takes about 500 milligrams of calcium (typically tri-calcium phosphate or TCP) in soy "milk" to equal a 1-cup (8 fluid ounce) serving of dairy milk containing 300 mg of calcium. You might need to consume 2 cups of fortified soy "milk" to obtain 500 mg calcium.