Oat milk and soy milk can occupy the same glass without doing the same nutritional job. In current U.S. unsweetened examples, Silk Unsweet Soymilk provides 8 g protein per cup while Oatly Unsweetened provides 0 g. Both are fortified, so calcium and vitamin D can look much more similar than the macronutrients.
One cup side by side
| Silk Unsweet Soy | Oatly Unsweetened | |
|---|---|---|
| Calories | 90 | 40 |
| Protein | 8 g | 0 g |
| Fat | 4.5 g | 1.5 g |
| Carbohydrate | 4 g | 6 g |
| Fiber | 2 g | <1 g |
| Calcium | 470 mg | 320 mg |
| Vitamin D | 3 mcg | 4 mcg |
The two drinks are similar in how people use them, not in protein. An 8 g gap in one cup is large enough to change breakfast meaningfully, especially if the beverage replaces dairy milk rather than simply lightening coffee.
Soy is much closer to milk as a protein source
Eight grams per cup puts this soy milk in the same general protein range as dairy milk. The Oatly example contributes none. Other oat milks may provide a few grams, so this is a comparison of two real products rather than a claim that every oat milk has zero protein.
Fortification can make micronutrients look surprisingly similar
Calcium and vitamin D in these beverages are largely added during manufacturing. Silk lists 470 mg calcium and 3 mcg vitamin D per cup; Oatly lists 320 mg calcium and 4 mcg vitamin D.
That is why the source ingredient alone does not tell you whether a plant beverage is a good calcium or vitamin-D source. Check the label, and shake the carton as directed because fortified minerals can settle.
Oat milk carries more carbohydrate in this comparison
The oat drink provides 6 g carbohydrate per cup versus 4 g in soy. Neither has added sugar in the products compared here, but “unsweetened” still does not mean carbohydrate-free.
Look beyond one nutrient if it replaces dairy regularly
Protein, calcium, vitamin D, vitamin B12, iodine, and potassium can all matter depending on the rest of the diet. Formulas vary substantially between brands and between original, barista, and unsweetened versions.
Protein is the clearest nutritional difference when these drinks replace dairy rather than simply flavor coffee. Unsweetened soy milk commonly provides several grams of protein per cup because soybeans are naturally protein-rich. Oat milk begins with much less protein, so many versions remain closer to a carbohydrate drink even when they are fortified with calcium and vitamins. For someone using several cups a day, that protein gap can become meaningful.
Fortification makes label reading essential. Calcium, vitamin D, vitamin B12, and sometimes vitamin A are added rather than naturally present at the listed level. Two cartons of the same type can therefore differ considerably. Shake fortified plant milk before pouring because minerals can settle, and compare products on a per-cup basis rather than assuming all oat or soy drinks are nutritionally identical.
Added sugar is another source of variation. ‘Original’ versions may contain added sugars, while unsweetened products can have little or none. Oat drinks can still contain more carbohydrate even without added sugar because processing breaks some oat starch into smaller carbohydrates. If glucose control or carbohydrate intake matters, the total-carbohydrate line is more informative than the front-of-pack word unsweetened alone.
Taste and use still count. Oat milk often has a naturally sweet, creamy profile that works in coffee and cereal; soy milk is usually the stronger choice when protein replacement is important. Choosing a fortified, unsweetened product that fits the role you need is more useful than declaring one plant milk universally superior.
For children, pregnancy, older age, or people with limited diets, the choice of a dairy replacement deserves more attention because protein and micronutrient needs may be harder to cover. Plant drinks are not automatically nutritionally interchangeable with cow’s milk, and some low-protein products are poor substitutes if they displace a major source of protein, calcium, iodine, or vitamin B12. Individual needs differ, so anyone relying heavily on plant drinks for medical, allergy, or vegan reasons may benefit from professional dietary advice rather than choosing only by taste.
Culinary performance differs as well. Soy milk’s higher protein can help with foaming, sauces, and baking structure, while oat milk’s starch and carbohydrate can create a creamy mouthfeel. Barista versions often add oil or stabilizers to improve texture, so they may contain more fat or calories than a basic unsweetened carton. If the drink is used mainly in coffee, that may be perfectly acceptable; if it supplies several glasses a day, nutrition and fortification deserve more weight than foam quality.
The practical choice
If protein is the priority, soy has a decisive advantage in these examples. Oat milk offers a different flavor and carbohydrate profile. For calcium and vitamin D, the comparison is about fortification, so the label on the exact carton matters more than the words “oat” or “soy.”
Sources
- Silk — Unsweet Soymilk — Current per-cup label.
- Oatly — Unsweetened Oatmilk — Current per-cup label.
- FDA — Milk and Plant-Based Beverages — Comparison guidance.