Whey vs plant protein for weight loss: protein quality (DIAAS), muscle trials, the one weight head-to-head, protein per calorie and lead testing, side by side.
Fact-checked against FDA labels and official sources; not yet reviewed by a licensed clinician. This page is education, not medical advice. Talk to your doctor before starting, stopping or changing any treatment.
Key takeaways
- Whey protein isolate scored 125 on DIAAS, soy protein isolate 98 and pea protein concentrate 73 (100+ is "excellent", 75-99 "good").[1]
- Across 9 trials with strength training, soy and animal proteins such as whey led to similar gains in strength and lean mass.[2]
- Protein supplements added about 0.30 kg of fat-free mass during training, with no further gains above about 1.6 g/kg a day of total protein.[3]
- In a 23-week trial, whey (but not soy) led to 1.8 kg lower weight than a carbohydrate drink; whey and soy did not differ from each other.[5]
- Consumer Reports found plant-based protein products averaged about nine times the lead of dairy-based ones.[8]
In short: Whey and plant protein powders can both help you reach a higher protein intake while you lose weight. Whey scores higher on protein-quality tests and usually packs more protein per calorie, but in trials of people who strength train, soy and other plant proteins built muscle and strength about as well when total protein was high enough. In independent testing, plant powders carried more lead on average, so certification and how often you use them matter.
This page sets the two side by side using protein-quality research, randomized trials and maker-stated labels. It does not name a winner. For specific products, see our researched list of the best protein powders for weight loss, and for the bigger picture, our guide to the high-protein diet explains why protein helps during weight loss. This is general education; if you have kidney disease, a milk allergy or another health condition, talk with your clinician or a registered dietitian before adding a protein supplement.
The two in one minute
| Whey protein | Plant protein | |
|---|---|---|
| Comes from | Milk | Pea, soy, rice, pumpkin seed and other plants, often blended |
| Protein quality (DIAAS, adult pattern) | 125 (isolate) to 133 (concentrate): “excellent” | 98 (soy isolate): “good”; 73 (pea concentrate): below the cut-off for a quality claim |
| Protein per 100 calories (labels in our list) | 20.0 to 22.2 g | 14.3 to 15.7 g |
| Muscle and strength with training | Gains in trials | Similar gains in trials when total protein was matched |
| Lead in Consumer Reports’ tests | Lower on average | About 9 times dairy on average |
| Allergens on labels | Contains milk | Varies by blend (one in our list lists coconut, a tree nut) |
| Price per serving (our list, checked Oct 6, 2026) | About $1.78 to $2.80 | About $1.85 for the one with a clear per-serving price |
Why protein matters when you’re losing weight
Whichever powder you use, the reason to use one is the same: getting enough protein while eating less. A 2015 review in the American Journal of Clinical Nutrition found that diets with about 1.2 to 1.6 grams of protein per kilogram of body weight a day, including about 25 to 30 grams at meals, improved appetite, body weight or heart-risk factors in studies. Higher protein also helps limit the muscle lost along with fat. Our protein calculator turns those ranges into grams for your weight, and the macro calculator fits protein into your daily calories.
A powder is a convenience, not a requirement. Eggs, Greek yogurt, fish, chicken, tofu, beans and lentils all count. A shake earns its place when it helps you reach your protein on busy days without many extra calories, and it works best inside a modest calorie deficit.
Protein quality: what DIAAS measures
Proteins differ in how many essential amino acids they contain and how well they are digested. The Food and Agriculture Organization of the United Nations (FAO) recommends a score called DIAAS (digestible indispensable amino acid score) for this. It measures digestibility of each amino acid at the end of the small intestine. A score of 100 or more counts as an “excellent” protein source, 75 to 99 as “good,” and under 75 is too low for a protein-quality claim. The older PDCAAS score caps every protein at 100 (written as 1.0), which hides real differences at the top.
In this study, all four dairy proteins scored 100 or more, soy protein isolate scored 98 and pea protein concentrate 73. The limiting amino acids were the sulfur-containing ones (methionine and cysteine) in soy and pea. That is why many plant powders blend pea with rice or seeds: a mix can fill one source’s gaps with another’s. Single-ingredient plant powders and blends will score differently, and most labels don’t list a DIAAS.
Leucine is one amino acid where the two differ. Working from the same study’s ingredient analysis, whey protein isolate had about 11.6 grams of leucine per 100 grams of protein, soy protein isolate about 8.0 and pea protein concentrate about 7.4. A 2018 review notes that whey, likely because of its higher leucine, stimulates muscle protein building more than soy in short-term lab studies. The question is whether that shows up over months.
What the trials found over months
Whey vs plant protein in longer trials
- 9 trials, 266 peoplesoy vs whey or other animal protein with strength training: no difference in strength or lean massMessina et al., Int J Sport Nutr Exerc Metab 2018
- +0.30 kgmore fat-free mass with protein supplements during strength training (49 trials, 1,863 people)Morton et al., Br J Sports Med 2018
- 1.6 g/kgdaily protein above which supplements added no further fat-free mass gainsMorton et al., Br J Sports Med 2018
- 1.2 kgleg lean mass gained in 12 weeks by both vegans and omnivores eating 1.6 g/kgHevia-Larraín et al., Sports Medicine 2021
Checked on October 7, 2026
Soy vs animal protein. A 2018 meta-analysis pooled 9 trials (266 people) lasting at least 6 weeks in which people did resistance training and took soy or an animal protein (whey, beef, milk or dairy). Both increased strength, and there was no difference between them for bench press, squat or lean body mass. The authors concluded that soy supplements produce similar gains to whey with training.
Protein supplements in general. A larger 2018 meta-analysis of 49 trials (1,863 people) found that protein supplements during at least 6 weeks of strength training added about 2.49 kg to one-rep strength and 0.30 kg to fat-free mass. Once total protein reached about 1.6 grams per kilogram a day, more protein added nothing further. The effect was smaller with older age and larger in people who already trained.
An all-plant diet. In a 12-week study of 19 young men who were habitual vegans and 19 omnivores, both groups trained twice a week and had protein raised to 1.6 g/kg a day with a supplement (whey for the omnivores). Leg lean mass rose by 1.2 kg in both groups, with no differences in muscle size or leg-press strength. The study was small and in young, healthy men, so it says less about older adults or people dieting.
The one weight-focused head-to-head trial
Most whey-vs-plant studies look at muscle, not weight. One USDA trial looked at weight directly. It randomly assigned 90 adults with overweight or obesity to drink whey protein, soy protein (about 56 grams of protein a day for each) or a carbohydrate drink with the same calories, twice a day for 23 weeks, with no other diet advice. Weight and body composition did not differ between the whey and soy groups, or between soy and carbohydrate. Compared with the carbohydrate group, the whey group weighed 1.8 kg less and had 2.3 kg less fat mass, and its waists were smaller. Lean mass didn’t differ among the groups.
| Item | Value |
|---|---|
| Whey protein | 1.8 kg |
Soy protein did not differ from either whey or the carbohydrate drink, so it has no bar. One trial with no diet advice; not a weight-loss program.
Source: Baer DJ et al., Journal of Nutrition 2011 (90 adults with overweight or obesity) (checked on October 7, 2026)
This is one trial with people eating their usual diets, so it is a hint rather than settled evidence. It also doesn’t show soy failing: soy sat between the two other groups without a clear difference from either.
Protein per calorie: reading the label
When you are counting calories, the useful number is grams of protein per 100 calories (protein grams divided by calories, times 100). In the labels in our list, every whey isolate came out ahead of the plant powders.
In practice the gap per shake is modest. Thorne Plant Protein lists 22 grams of protein for 140 calories, while the whey isolates in our list give 20 to 28 grams for 90 to 130 calories. For 25 grams of protein, a powder with 21 grams per 100 calories costs about 119 calories and one with 15 grams per 100 calories about 167, a difference of about 48 calories a shake. That matters more on a tight calorie budget than a relaxed one. Labels also differ in sugar, sweeteners and fiber (BPN Vegan Protein lists 3 grams of fiber), so read the whole panel.
Safety: lead, certification and allergens
Dietary supplements, including protein powders, are not approved by the FDA before they go on sale; the FDA says it “does not have the authority” to do that, and makers are responsible for their products. That leaves quality checks to you and to independent programs.
- Lead. In tests of 23 protein powders and shakes, published in October 2025 and updated in January 2026, Consumer Reports found that plant-based products averaged about nine times the lead of dairy-based ones. CR uses a level of concern of 0.5 micrograms of lead a day. Results varied widely between brands, so the brand matters, not only the protein type; our protein powder list shows the products it measured.
- NSF Certified for Sport. This mark means the product is screened for 290 substances banned in sport, the contents match the label, there are no unsafe levels of contaminants, and the factory is audited. It is the check our list of picks requires.
- Informed Choice. A separate program that runs monthly retail testing of finished supplements for banned substances.
- Allergens. Whey labels say “contains milk.” Plant blends vary; BPN Vegan Protein, for example, lists coconut, a tree nut. Read the allergen line on every label.
- Kidneys. For healthy adults, higher-protein diets have not harmed kidney function in trials, but people with chronic kidney disease need a personal protein target; see the kidney section of our high-protein diet guide.
What people weigh up
- Most protein for the fewest calories: whey isolates led on the labels we checked.
- Dairy-free, vegan or lactose-sensitive: plant proteins (soy in most of the trials) supported muscle gains when total protein was high enough.
- Lowest lead: dairy averaged less in Consumer Reports’ tests; for plant powders, certification and published test results matter more.
- Taste: flavors and textures differ by brand more than by type. The one you will actually drink is the one that helps.
- Cost: in our list, per-serving prices overlapped, from about $1.24 for plain casein to $2.80 for one whey isolate.
Putting protein to work
Using a protein powder during weight loss
- Know your targetUse the protein calculator to see your daily range in grams.
- Fill gaps, don't replace mealsAdd a scoop where a meal runs low, often breakfast or a snack.
- Pair it with liftingTwo or more days of strength training help the protein protect muscle.
- Check the labelProtein per 100 calories, sugar, allergens and a certification mark.
- Mix in whole foodsRotate powders with eggs, yogurt, fish, tofu and beans.
A scoop blended into a smoothie is one of the easiest ways to use either type; our weight-loss smoothies and high-protein breakfasts show how. Protein matters most alongside strength training, and our guide to body recomposition covers losing fat while gaining muscle. If you are thinking about a full shake that replaces a meal, that is a different product: see our meal-replacement shakes. For more ways to eat well while losing weight, visit the nutrition hub, the supplements hub and our list of filling foods. More side-by-side pages are in our comparisons hub.
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- How much protein a day is right for me while I am losing weight?
- Is a protein supplement safe with my kidney function or other conditions?
- Would whey or a plant protein suit my digestion, allergies or eating pattern better?
- How can I get enough protein from whole foods instead of powders?
- Should I have my strength or muscle mass checked during weight loss?
Frequently asked questions
Is whey or plant protein better for weight loss?
Both can help you reach a higher protein intake while eating less. Whey usually has more protein per calorie and scores higher on protein quality, while plant proteins supported similar muscle gains in trials when total protein was high enough.
Can you build muscle with plant protein?
Yes. A meta-analysis of 9 trials found soy and animal proteins gave similar strength and lean-mass gains with resistance training, and a 12-week study found vegans and omnivores eating 1.6 g/kg a day gained the same leg lean mass.
Do plant protein powders have more heavy metals?
On average, yes, in Consumer Reports testing: plant-based products averaged about nine times the lead of dairy-based ones. Levels vary by brand, so third-party certification and published test results help.
What does DIAAS mean on a protein label?
It is the protein-quality score the FAO recommends, based on how well each essential amino acid is digested. A score of 100 or more is excellent, 75 to 99 good, and under 75 too low for a quality claim.
How much protein powder should I use?
There is no set amount. Studies of weight loss used about 1.2 to 1.6 g of protein per kg of body weight a day from all foods. Use powder to fill gaps, and ask a dietitian or clinician if you have kidney disease.
References
- Mathai JK, Liu Y, Stein HH. Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS). British Journal of Nutrition, 2017. doi:10.1017/S0007114517000125 · PMID 28382889 (accessed October 7, 2026) Other
- Messina M, Lynch H, Dickinson JM, Reed KE. No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. International Journal of Sport Nutrition and Exercise Metabolism, 2018. doi:10.1123/ijsnem.2018-0071 · PMID 29722584 (accessed October 7, 2026) Meta-analysis
- Morton RW, Murphy KT, McKellar SR, et al.. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine, 2018. doi:10.1136/bjsports-2017-097608 · PMID 28698222 (accessed October 7, 2026) Meta-analysis
- Hevia-Larraín V, Gualano B, Longobardi I, et al.. High-protein plant-based diet versus a protein-matched omnivorous diet to support resistance training adaptations: a comparison between habitual vegans and omnivores. Sports Medicine, 2021. doi:10.1007/s40279-021-01434-9 · PMID 33599941 · NCT03907059 (accessed October 7, 2026) Other
- Baer DJ, Stote KS, Paul DR, et al.. Whey protein but not soy protein supplementation alters body weight and composition in free-living overweight and obese adults. Journal of Nutrition, 2011. doi:10.3945/jn.111.139840 · PMID 21677076 (accessed October 7, 2026) Randomized trial
- Leidy HJ, Clifton PM, Astrup A, et al.. The role of protein in weight loss and maintenance. American Journal of Clinical Nutrition, 2015. doi:10.3945/ajcn.114.084038 · PMID 25926512 (accessed October 7, 2026) Review
- Dietary protein quality evaluation in human nutrition: report of an FAO Expert Consultation (FAO Food and Nutrition Paper 92). Food and Agriculture Organization of the United Nations, 2013. (accessed October 7, 2026) Guideline
- Protein Powders and Shakes Contain High Levels of Lead. Consumer Reports, 2025. (accessed October 7, 2026) News (reported facts only)
- Questions and Answers on Dietary Supplements. U.S. Food and Drug Administration. (accessed October 7, 2026) Government page
- What Our Mark Means (NSF Certified for Sport). NSF. (accessed October 7, 2026) Other
- Informed Choice certification. LGC Assure (Informed Choice). (accessed October 7, 2026) Other
- NSF Certified for Sport: certified products. NSF. (accessed October 7, 2026) Other
- Devries MC, Sithamparapillai A, Brimble KS, et al.. Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis. Journal of Nutrition, 2018. doi:10.1093/jn/nxy197 · PMID 30383278 (accessed October 7, 2026) Meta-analysis
Facts checked on October 7, 2026
Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.