High vegan protein intake does not guarantee sufficient quality, research flags

A New Zealand study evaluating protein intake and quality reveals that three in four vegans ate sufficient protein. Still, half didn’t meet daily requirements for the essential amino acids lysine and leucine when accounting for protein digestibility.

The Massey University researchers note that the levels of essential amino acids in plant-based foods vary more than in animal-sourced foods, making these building blocks of particular concern in vegan diets. 

However, they caution that research on protein in these diets does not typically consider the specific amino acid content or the digestibility of different foods. 

“Vegan diets are the most restrictive form of plant-based eating, relying entirely on plant sources for all nutrients,” say the authors. “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 our body needs.” 

“Prolonged deficiencies in these essential nutrients can negatively affect overall protein balance, muscle maintenance, and other physiological functions, especially in more vulnerable populations.”

Lysine has been linked to collagen production and may support healthy immune systems. Leucine is important for muscle health and maintaining blood sugar levels. 

Essential amino acids

For the study published in PLOS One, the researchers analyzed four-day food diaries kept by 193 long-term vegans living in New Zealand. They used information from the US Department of Agriculture and the New Zealand FoodFiles database to calculate participants’ intake of amino acids from foods consumed. 

Based on the estimated average protein requirement, the mean protein intake was sufficient for 78.8% of male and 73% of female participants. When adjusting for body weight, the participants met essential amino acid requirements. 

However, when the researchers adjusted their analysis for digestibility, the share of vegans with adequate protein and essential amino acid consumption decreased — only around 50% of participants met daily requirements for lysine and leucine. 

Plant protein sources of leucine or lysine include soybeans, lentils, kidney beans, and chickpeas.“In our study, lysine and leucine were the most commonly under-consumed amino acids in our vegan cohort and fall below the daily requirements needed by our body,” explain the authors. “This is because many plant foods generally contain lower quantities of these amino acids that can be absorbed and utilized by the body.” 

“However, 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.”

Lysine and leucine

Animal-sourced foods, such as meat, dairy, and eggs, are important sources of lysine and leucine. Plant-based sources of leucine include soybeans, lentils, pumpkin seeds, and hemp seeds. Lysine can be found in lentils, chickpeas, kidney beans, and tempeh. 

While these amino acids are linked to several health benefits, researchers caution against excessive intake of some amino acids. 

Earlier research suggests that overweight and obese US citizens may be eating more isoleucine — an isomer of leucine — than needed. The authors revealed that reducing isoleucine intake in mice made them leaner and less frail, reducing cancer and prostate problems. 

The authors say future research should focus on identifying food combinations for a balanced intake of all digestible essential amino acids.Another study suggested that consuming over 22% of dietary calories from protein can increase heart disease risk. The authors note that leucine drives the formulation of arterial plaque, which is one of the leading causes of heart attacks and strokes. 

Follow-up research

The researchers call for future research to explore how to boost the intake of leucine and lysine for vegans in a nutritionally balanced manner. 

The research authors say there is no consensus on the appropriate protein and essential amino acid requirements per meal and whether this may differ among sexes, age groups, and body composition patterns. 

“To improve protein quality in vegan diets, future research should focus on identifying optimal combinations of traditional and novel plant-based foods to achieve balanced intakes of all digestible essential amino acids, particularly for lysine and leucine, and this should ideally be examined within the context of meals,” reads the paper. 

“Although the global vegan population remains relatively small, it is important to generate evidence-based knowledge to ensure such diets have optimal quality due to the restrictive nature of consuming only plant-sourced foods.”