Whey vs Casein vs Plant Protein Powders

The first choice in the protein aisle is the source: whey, casein, a plant protein or a blend. Labels talk about "fast" and "slow" proteins and "complete" proteins. This guide sets out what the research reviews and public sources we read say about how the three families differ, for healthy adults. For the full set of buying checks, see our protein powder buying guide.

What "protein quality" means

The International Society of Sports Nutrition (ISSN) position stand on protein and exercise explains that the body needs nine essential amino acids (EAAs) from food because it cannot make them. It says protein quality is typically defined by how effectively a protein stimulates muscle protein synthesis, and that animal and dairy proteins contain the highest percentage of EAAs. It singles out leucine, one of the EAAs: dairy protein is about 9% to 11% leucine, and the stand lists plant proteins among the sources low in it. Several ISSN authors consult for protein-supplement companies, so read it beside the independent reviews below.

U.S. labels treat quality in a different way. FDA's rule works out the percent Daily Value for protein from a corrected amount: the grams of protein multiplied by an amino acid score corrected for digestibility. A low-scoring protein can therefore show a big gram figure and a smaller percentage. Our guide to protein claims explains the rule.

Whey

Whey is the milk protein that stays in the liquid when casein is separated during cheesemaking. Powders come as concentrate, isolate or hydrolysate. The U.S. Dairy Export Council defines whey protein isolate as at least 90% protein, with typical lactose of 0.5% to 1.0%; concentrates are sold at lower protein percentages, so more of the powder is something else. The ISSN lists whey and egg whites, soy and very lean meat as faster-digesting sources, calls whey hydrolysates and isolates the most extensively researched for use around resistance exercise, and describes one study in which muscle protein synthesis after whey was about 93% higher than after casein and 18% higher than after soy at rest. That measures a few hours, not gains over months.

Whey comes from milk. Milk is one of the nine major food allergens, and the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says the word "whey" in an ingredient list means the product contains lactose. People with a milk allergy should avoid it, and people with lactose intolerance should ask a clinician or dietitian before trying one.

Casein

The ISSN calls casein a slowly digested protein. Its stand reports that 30 to 40 grams of casein before sleep increased overnight muscle protein synthesis and metabolic rate in the studies it reviewed. Many tubs blend whey and casein; the stand notes that evidence is building for combining sources and cites one 10-week training study in which a whey and casein combination produced the largest rise in fat-free mass of the groups compared. Casein is a milk protein too, so the allergy and lactose points above apply. Micellar casein on Amazon (opens in a new tab) is a search for it.

Plant proteins

Soy, pea, rice and blends of them are the common choices. The ISSN stand calls soy a complete protein but notes that it holds lower amounts of EAAs and leucine than milk protein, and that in one study muscle protein synthesis after soy was higher than after casein and lower than after whey. It says soy has by far the most research of the plant sources. For rice, it describes an eight-week study in experienced lifters taking 48 grams a day of rice protein or whey protein, with similar gains in strength, muscle thickness and body composition.

A 2021 systematic review and meta-analysis in Nutrients pooled 16 trials, whose plant proteins were soy, pea and rice. Protein source did not change gains in absolute lean mass or muscle strength. Animal protein favored percent lean mass, and adults under 50 gained more absolute lean mass with animal protein, by 0.41 kg on average. Total protein intakes in the trials were generally above the Recommended Dietary Allowance. The authors concluded that animal protein tends to be more beneficial for lean mass, especially in younger adults. Read together, the sources suggest the difference is modest and that total daily protein matters more than the source.

Two plant-specific points. Soy is a major allergen, so a soy-based powder must declare it. And in Consumer Reports' (CR) 2025 tests of 23 products, plant-based powders, all built on pea protein, averaged about nine times the lead of dairy-based ones, with exceptions and no established cause; CR also found several dairy-based products above its level of concern. The industry group Council for Responsible Nutrition disputed CR's threshold. If you choose a plant powder, look for a maker that publishes heavy-metal results. Our vegan products page is a place to start for plant-based foods.

Egg, beef and collagen

Egg and beef proteins are sold as powders. The ISSN describes an eight-week study in which 46 grams a day of hydrolyzed chicken protein, beef protein isolate or whey protein concentrate gave similar strength gains. Collagen is a protein too, but it is incomplete; our guide to protein claims explains how that affects the label.

Reading the ingredient list

The source is named in the ingredients, so look for terms such as whey protein concentrate, whey protein isolate, micellar casein, calcium caseinate, pea protein isolate or soy protein isolate. NIDDK lists milk, lactose, whey and curds among the words that mean a product contains lactose. A blend names several of them, and a long run of sweeteners and gums after the protein is a prompt to check the panel.

Picking one

• If you tolerate dairy: whey is the most researched. Casein digests more slowly, and blends combine both.
• If you avoid dairy: a plant protein or blend, with the lead point and the soy allergen in mind.
• If you have a milk or soy allergy: read the "Contains" statement on every flavor and size, because formulas change.
• If you want the least added: an unflavored powder, for example this search for unflavored whey isolate on Amazon (opens in a new tab) or unflavored pea protein on Amazon (opens in a new tab).

Whatever the source, the grams per serving, the sweeteners and any testing mark matter as much, and our label guide covers them. To turn a daily target into grams, see how much protein you need.

Cautions

The research above concerns healthy adults. NIDDK says some people with chronic kidney disease may need moderate protein and should work with a dietitian or health care professional. NIH's National Center for Complementary and Integrative Health notes that many supplements have not been tested in pregnant women, nursing mothers or children. FDA advises talking to your doctor, pharmacist or other health care professional before using a supplement, and lists milk and soybeans among the nine major allergens. The label on the pack is the authority on ingredients, serving size and warnings, and a doctor, pharmacist or registered dietitian is the right person to ask about your own health.