Vegans Who Meet Protein Requirements May Lack Key Amino Acids
A study shows long-term vegans meet protein intake but struggle with essential amino acids like lysine and leucine.
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As veganism gains traction worldwide, questions arise about the nutritional adequacy of plant-based diets.
A recent study conducted in New Zealand, published in PLOS ONE, has evaluated both the quantity and quality of protein intake among long-term vegans. The findings show that while most participants met total protein requirements, a significant portion fell short in essential amino acids.
Understanding protein quality challenges in vegan diets
The adoption of vegan diets continues to rise globally, driven by concerns about health, environmental sustainability and animal welfare. While these diets can support good health, they also present distinct nutritional challenges, particularly around protein quality.
Proteins are composed of amino acids, nine of which are indispensable and must be obtained through diet. Unlike animal products, many plant-based proteins contain lower levels of certain essential amino acids (EAAs) and their digestibility may also be reduced by fiber and anti-nutritional compounds.
This is where protein quality – not just quantity – is important.
“Vegan diets are the most restrictive form of plant-based eating, relying entirely on plant sources for all nutrients. Achieving high protein quality on a vegan diet requires more than just consuming enough protein – it also depends on the right balance and variety of plant foods to supply all the amino acids in the quantities that our body needs,” said the authors.
“Prolonged deficiencies in these essential nutrients can negatively affect overall protein balance, muscle maintenance and other physiological functions, especially in more vulnerable populations,” they added.
Previous research on vegan diets has typically focused on total protein intake, often concluding that most vegans consume enough protein overall. However, this overlooks the importance of amino acid profiles and digestibility, both of which determine how effectively the body can utilize dietary protein.
Since ~3% of the global population is estimated to follow a vegan diet, understanding these concerns is vital for public health.
Amino acid deficiency in vegans
To understand the importance of protein quality, the team analyzed detailed 4-day food diaries from 193 long-term vegans in New Zealand. Participants had adhered to a vegan diet for at least two years, and their dietary intake was assessed for both total protein and amino acid content, adjusted for digestibility using true ileal digestibility (TID) values.
True ileal digestibility (TID) values
TID values refer to the proportion of amino acids from dietary protein that are absorbed in the small intestine, specifically at the ileum. This measurement accounts for amino acids that are not digested and absorbed, providing a more accurate assessment of a protein's nutritional value.
They found ~75% of participants met the estimated average requirement for total daily protein. However, once digestibility was factored in, a different picture emerged.
Only around half of the cohort met the daily requirements for lysine and leucine, two key EAAs for muscle protein synthesis and overall metabolic health.
“Many plant foods generally contain lower quantities of these amino acids that can be absorbed and utilized by the body,” the team explained.
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Legumes and pulses were the primary contributors to both total protein and lysine intake, highlighting their role in enhancing protein quality in vegan diets. Participants with insufficient lysine and leucine intake also tended to have lower total protein and energy intake and higher body fat percentages.
Enhancing protein quality in vegan nutrition
The study illustrates that meeting total protein needs is not enough. For vegans, ensuring adequate intake of EAAs is key to maintaining muscle mass, metabolic health and overall physiological function. Improving protein quality in vegan diets depends on strategic combinations of plant foods. Pairing legumes, which are rich in lysine, with grains that complement their amino acid profiles can enhance the overall EAA content of meals.
“The inclusion of legumes, nuts and seeds emerged as valuable plant sources – not only to support overall protein intake but also to specifically increase lysine and leucine quantities in a vegan diet,” the team stated.
The results call for more targeted dietary guidance for vegans, suggesting that current protein intake recommendations may not fully reflect the challenges of achieving amino acid adequacy on a plant-only diet.
As plant-based eating patterns continue to expand, ensuring their nutritional robustness will be necessary, especially for populations with higher protein demands or restricted dietary options.
Reference: Soh BXP, Vignes M, Smith NW, Von Hurst PR, McNabb WC. Evaluation of protein intake and protein quality in New Zealand vegans. PLoS ONE. 2025;20(4):e0314889. doi: 10.1371/journal.pone.0314889
This article is a rework of a press release issued by PLOS. Material has been edited for length and content.
