“Plant protein is inferior”

Amino-acid completeness

Evidence quality: Moderate (Mariotti and Gardner, 2019; Lim et al., 2021). The label "incomplete protein" can imply that a plant food entirely lacks one or more indispensable amino acids, which is not an accurate description of plant-food amino-acid content (Mariotti and Gardner, 2019). A review of protein and amino-acid adequacy in vegetarian diets states that all plant foods contain all 20 amino acids, including the nine that humans must obtain from food (Mariotti and Gardner, 2019). The relevant differences are the proportions of amino acids and the extent of digestion and absorption, not the categorical presence or absence of an amino acid (Mariotti and Gardner, 2019).

Grains tend to provide lysine in a lower proportion relative to human requirements, while legumes tend to provide lower proportions of sulfur-containing amino acids (Mariotti and Gardner, 2019). These profiles are complementary, but complementation does not have to occur within the same meal because amino-acid adequacy is evaluated across total intake over time (Mariotti and Gardner, 2019). The review concluded that legumes, nuts, and seeds can support full protein adequacy in adults consuming vegetarian or vegan diets when overall food and energy intake are sufficient (Mariotti and Gardner, 2019).

Protein quality and soy

Protein quality measures combine indispensable-amino-acid composition with digestibility (Gorissen et al., 2018). Laboratory analysis of commercial protein isolates found that plant sources varied substantially; on average they contained less indispensable amino acid and leucine than the animal isolates examined, and some—but not all—were relatively low in lysine or methionine (Gorissen et al., 2018). This supports a precise distinction: some individual plant proteins score lower on specific quality metrics, but that does not establish diet-level inadequacy (Gorissen et al., 2018; Mariotti and Gardner, 2019).

Soy is an important exception to broad generalizations about lower plant-protein quality (van den Berg et al., 2022). A quantitative review of in-vivo measurements reported a mean protein digestibility-corrected amino-acid score of 92.4 and concluded that most soy products had high protein-quality scores, with processing affecting the result (van den Berg et al., 2022). Soy and many legumes are therefore substantial protein sources, while a varied combination of legumes, grains, nuts, seeds, and soy can offset differences in amino-acid profile across the diet (Mariotti and Gardner, 2019; van den Berg et al., 2022).

Adequacy and longer-term outcomes

The Academy of Nutrition and Dietetics' 2025 position paper states that appropriately planned vegetarian and vegan patterns can be nutritionally adequate for nonpregnant, nonlactating adults (Raj et al., 2025). This is a diet-level position and does not mean that every plant-based diet is automatically adequate; total energy, total protein, food selection, and individual requirements still apply (Raj et al., 2025; Mariotti and Gardner, 2019).

Longer-term muscle outcomes provide a more practical test than amino-acid scores alone because acute synthesis and chronic changes are different outcomes (Lim et al., 2021). A systematic review and meta-analysis included 18 randomized-trial articles, with 16 in meta-analysis, and found no source-related difference in absolute lean-mass change or muscle strength when total protein intake was generally above the recommended allowance (Lim et al., 2021). Animal protein favored change in percentage lean mass, and subgroup analyses favored animal protein for absolute and percentage lean mass in adults younger than 50 years; the authors noted limited studies and variation in background diet (Lim et al., 2021). The evidence therefore does not support a universal claim that source never matters, but it also does not support a categorical claim that plant protein cannot maintain strength or lean mass (Lim et al., 2021).

Older adults and low intake

Protein needs can be harder to meet when appetite or energy intake is low (Bauer et al., 2013). The PROT-AGE group recommended 1.0–1.2 grams of protein per kilogram of body weight per day for adults over 65, with higher amounts for many active or ill older adults, while noting that severe kidney disease can require restriction (Bauer et al., 2013). It also identified meal distribution, protein quality, and leucine content as potentially relevant, although evidence was insufficient for detailed source-specific prescriptions (Bauer et al., 2013).