Soy milk lacks sugar and carbs (lactose) and will give a lower spike in blood glucose as compared to cow's milk. It also has more protein which is great in meeting protein requirements for vegetarians and vegans.
the main reason for the increasing popularity of soy milk seems to be health concerns, such as inflammatory bowel disease and lactose intolerance.
The milk from a cow (or goat, or sheep) is complete food for the growth and development of a young animal. It contains all the essential amino acids (the protein building blocks that your body is unable to make for itself) as well as a complex mixture of fats, carbohydrates, vitamins and minerals including calcium, phosphate and vitamin B12.
A soybean is also complete food – for the growth and development of a soy plant seedling. The nutritional needs of plants are obviously quite different from those of animals, and accordingly, the nutritional profile of unadulterated soy milk is very different from that of animal milks.
Fresh soy milk, made from grinding and then straining soaked dried soy beans, has less fat and carbohydrate than animal milks, and only a small amount of calcium. And it’s missing some of the vitamins present in animal milks as well.
The protein content of soy milk is similar to cow’s milk, and all the essential amino acids are present but in smaller amounts than in cow’s milk. Because it’s plant food, soy milk contains small amounts of fibre, and twice as much folate as animal milks.
The fat content is similar in both cow and soy milk, and low fat or “light” varieties are available for both. The type of fat in full-cream cow’s milk is butterfat, high in saturated fat, while soybean oil is mostly polyunsaturated. The fats added to soy milk are usually canola or sunflower oil, again rich in polyunsaturated and monounsaturated fats. This means that soy milk is a source of “good” fats.