Whey vs soy protein is a choice between a dairy protein and a plant one. ICMR-NIN's Dietary Guidelines for Indians, 2024, name both as sources of protein powder, whey from cheese and paneer making and soy from soybeans.1 Which one suits you depends far more on how you eat than on any single number.
Whey and soy are both complete proteins
Harvard's Nutrition Source notes that, unlike some plant proteins, soy protein is considered a complete protein, with all nine essential amino acids that the body cannot make.2 Whey is also a complete protein, according to the review of protein sources by Hoffman and Falvo.3 So on the question of whether each protein carries the full set, the two stand level.
How much protein whey and soy give
In their most refined forms, the protein shares are close. USDA lists soy protein isolate at 88.32 g of protein in 100 g.4 Whey isolate comes to 90% protein or more in the same review that describes it as complete.3 A 25 g scoop of soy isolate therefore gives about 22 g of protein, and the same scoop of whey isolate gives at least 22.5 g.
Most people meet soy as a food, though. Firm tofu has 17.27 g of protein in 100 g in USDA's table, so a 150 g slab gives about 26 g.5 Soy chunks and soya milk are other everyday soy foods, while whey usually arrives as a powder or in the milk you drink.
Soy protein benefits beyond the protein
Soy foods bring other nutrients with them. Harvard lists B vitamins, fibre, potassium and magnesium, along with high-quality protein, among the nutrients in soy foods.2 Firm tofu, for example, carries 2.3 g of fibre in 100 g.5 Harvard's page also sorts soy foods into unfermented ones, such as tofu and soy milk, and fermented ones, such as miso and tempeh.2
Those soy protein benefits come from eating soy as a food. A soy protein isolate powder, which USDA lists with no fibre at all, keeps the protein and leaves most of the rest behind.4
Choosing between soy vs whey protein
The choice usually starts with your diet. If you avoid dairy, or follow a vegan diet, whey is ruled out, because the NHS describes vegans as people who do not eat foods from animals, including dairy products.6 Soy then becomes the complete plant protein that fits. For dairy-free drinks, the same NHS page suggests fortified soya drinks and yoghurts, choosing lower-fat and lower-sugar options.6
Lactose is the other dairy question. The NHS describes lactose as a sugar found in dairy products, which means whey carries some.7 The review puts lactose at 10 to 55% of a whey concentrate and about 0.5% of an isolate, while soy protein carries none from milk.3
If you eat dairy and lactose is not a concern, both are open to you, and the deciding points are practical ones. Look at the protein given for one serving, the sugar and sweeteners in the tub, and whether you will actually enjoy the taste every day.
A whey soy protein blend
Some powders combine the two. ICMR-NIN's guidelines mention powders that draw on several protein sources at once.1 A whey soy protein blend names both on the ingredients list, and the protein next to the serving covers the two together. That kind of blend can suit someone who wants dairy and plant protein in one scoop, and you read it like any other tub.
The protein in Horlicks Protein
Horlicks Protein uses a different protein source from both. Unflavoured lists fermented yeast protein together with whey protein concentrate, while Chocolate and Coffee carry fermented yeast protein. A 36 g sachet carries 24 g of protein for 200 ml of cold water or milk. For a dairy option, the bottled milkshake contains 20 g of ultra-filtered milk protein.
ICMR-NIN's guidelines work out daily protein at 0.83 g per kg, so someone who weighs 65 kg needs about 54 g. A sachet supplies 24 g of that, whichever protein the grams came from.1
Summary
Whey is a dairy protein and soy is a plant protein, and both are complete. In their most refined forms they carry similar shares of protein, with soy isolate at 88.32 g in 100 g and whey isolate at 90% or more. Soy foods such as tofu also bring fibre, B vitamins, potassium and magnesium. If you avoid dairy or lactose, or eat vegan, soy is the one that fits, and if you eat dairy, the label, the sugar and the taste decide it. Blends of the two exist, and Horlicks Protein lists fermented yeast protein, adding whey protein concentrate in Unflavoured, for 24 g of protein a sachet.
Sources
1. ICMR-National Institute of Nutrition. Dietary Guidelines for Indians, 2024. https://nin.res.in/dietaryguidelines/pdfjs/locale/DGI_2024.pdf
2. Harvard T.H. Chan School of Public Health. The Nutrition Source, Soy. https://nutritionsource.hsph.harvard.edu/soy/
3. Hoffman JR, Falvo MJ. Protein, which is best? Journal of Sports Science and Medicine, 2004. PubMed 24482589. https://pmc.ncbi.nlm.nih.gov/articles/PMC3905294/
4. USDA FoodData Central. Soy protein isolate. FDC 174276. https://fdc.nal.usda.gov/food-search/?query=174276&type=SR%20Legacy
5. USDA FoodData Central. Tofu, raw, firm, prepared with calcium sulfate. FDC 172475. https://fdc.nal.usda.gov/food-search/?query=172475&type=SR%20Legacy
6. NHS. The vegan diet. https://www.nhs.uk/live-well/eat-well/how-to-eat-a-balanced-diet/the-vegan-diet/
7. NHS. Lactose intolerance. https://www.nhs.uk/conditions/lactose-intolerance/