Protein: Research

by Jack Norris, RD

Contents

• Summary
• Introduction: Assessing Protein Needs of a Population
• Dietary Reference Intakes
  • Setting the DRIs for Protein
  • Women Have the Same RDA as Men
  • Criticism of the Protein RDA
    • Protein Needs for People Over 60
    • Protein Needs of Athletes
• Protein and Amino Acid Content of Selected Plant Foods
  • Good Sources of Lysine among Plant Foods
• Plant Protein Balance Research
  • Nitrogen Balance Study on Vegans
  • Nitrogen Balance Studies using Plant Protein
  • Serum Albumin Studies of People on Plant-Based Diets
  • Creatinine Clearance Study on Vegans
  • Summary of Plant Protein Balance Research
• Amino Acid Intakes and Blood Levels of Vegans
  • Amino Acid Intakes of Vegan Men
    • Amino Acid Intakes of Vegan Women
  • Amino Acid Blood Levels of Vegan Men
  • Arginine and the Vegan Diet
• Appendix A: Protein Quality Scoring
  • FAO Recommends the PDCAAS in 1989
  • FAO Recommends the DIAAS in 2011
  • Criticism of the DIAAS
• Appendix B: The DIAAS Reference Protein
• Bibliography

Summary

The recommended dietary allowance (RDA) for protein is 0.8 g per kg of healthy body weight per day (g/kg/day). Research on protein balance suggests that vegans should aim for at least 1.0 g/kg/day.

Vegans should choose plenty of lysine-rich foods which include legumes, soyfoods, seitan, quinoa, pistachios, and pumpkin seeds (see our Protein Recommendations for more info).

Introduction: Assessing Protein Needs of a Population

Discussions about protein needs tend to focus on three different lines of reasoning:

1. Protein quality scoring
2. Muscle synthesis studies
3. Protein balance studies

Protein quality scoring is mostly relevant to populations at risk for malnutrition while muscle synthesis studies are mostly relevant to athletes. Protein balance studies are the most relevant studies for assessing the protein needs of a well-nourished population.

We include an in-depth discussion of protein quality scoring in Appendix A: Protein Quality Scoring, including explanations of the protein efficiency ratio (PER), the protein digestibility–corrected amino acid score (PDCAAS), and the digestible indispensable amino acid score (DIAAS).

We cover muscle synthesis and plant-based diets in our article, Weightlifting for Vegans.

Here, we’ll focus on protein balance studies in discussing the protein needs of the general vegan population.

Dietary Reference Intakes

The United States Institute of Medicine (IOM) sets the dietary reference intake (DRI). The DRI for protein is given in grams per kilogram per day (g/kg/day) of “body weight” (IOM, 2005). Fat mass doesn’t require much protein for maintenance, so “body weight” is generally interpreted to mean “ideal” or “healthy” body weight even though this isn’t specified in the DRI.

The table below lists the DRI for protein. The table also includes the DRI for the amino acid, lysine, because lysine tends to be the essential amino acid least plentiful in the average vegan diet; if a vegan is meeting their needs for lysine, they’re most likely meeting their needs for all the essential amino acids.

U.S. Dietary Reference Intakes for Protein and Lysine
Age	Protein (g/kg)A	Lysine (mg/kg)A
7–12 mos	1.20	89
1–3	1.05	58
4–8	0.95	46
9–13 male	0.95	46
9–13 female	0.95	43
14-18 male	0.85	43
14-18 female	0.85	40
≥ 19	0.80	38
Pregnancy	1.1	51
Breastfeeding	1.3	52
Aper kg of body weight

The table below lists the adult RDA for protein and the essential amino acids.

Adult RDA for Protein and Amino Acids
	PRO (g)	HIS (mg)	ISO (mg)	LEU (mg)	LYS (mg)	MET+CYS (mg)	PHE+TYR (mg)	THR (mg)	TRP (mg)	VAL (mg)
RDA per kg of body weight	0.8	14	19	42	38	19	33	20	5	24
RDA for a 140 lb. person	51	891	1,209	2,673	2,418	1,209	2,100	1,273	318	1,527
PRO-protein, HIS-histidine, ISO-isoleucine, LEU-leucine, LYS-lysine, MET+CYS-methionine plus cysteine, PHE+TYR-phenylalanine plus tyrosine, THR-threonine, TRP-tryptophan, VAL-valine

Setting the DRIs for Protein

In setting the DRSs for protein, the IOM relied on nitrogen balance studies, especially a 2003 meta-analysis by Rand et al. (2003).

Nitrogen balance studies are used because nitrogen is a component of protein and isn’t found in the other macronutrients (i.e., fat, carbohydrate, and alcohol). Nitrogen balance studies studies measure how much nitrogen someone eats and then subtracts how much nitrogen they lose through urine, feces, hair, sweat, etc. If they lose more nitrogen than they eat, they’re in negative nitrogen balance and require more protein. If they lose as much as they eat, they’re in nitrogen balance and are obtaining enough protein to meet their needs.

The IOM determined that the estimated average requirement (EAR) for protein for the average adult aged 19-50 years is 0.66 g/kg/day (IOM, 2005). They increased the EAR to set the recommended dietary allowance (RDA) at 0.8 g/kg/day which is intended to cover the needs of 97–98% of the population.

Women Have the Same RDA as Men

Given that women have, on average, a lower percentage of lean body mass than men, it might be surprising that they have the same RDA for protein (and amino acids). In setting the RDAs, the IOM says (IOM, 2005, p. 644):

Although the data indicate that women have a lower nitrogen requirement than men per kilogram of body weight, this was only statistically significant when all studies were included, but not when the analysis was restricted to the primary data sets. This difference may be due to differences in body composition between men and women, with women and men having on average 28 and 15 percent fat mass, respectively. When controlled for lean body mass, no gender differences in the protein requirements were found. However, in view of the uncertain significance of the difference between the genders, the same protein EAR on a body weight basis for both men and women is chosen.

Criticism of the Protein RDA

A group of researchers from The Hospital for Sick Children in Toronto (Elango, 2010) argue that the methods for determining the RDA for protein underestimates needs. One of the authors, Dr. Paul B. Pencharz, was a member of the Panel on DRIs for macronutrients and a member of the Joint WHO/FAO/United Nations University Expert Consultation on Protein and Amino Acid Requirements in Human Nutrition. Elango et al. write:

The current recommendations for protein intakes in adults are primarily based on the reanalysis of existing nitrogen balance studies. The nitrogen balance technique has inherent methodological limitations, which lead to an underestimation of the requirement estimate. Furthermore, the application of a single linear regression analysis to identify zero nitrogen balance is not appropriate because the nitrogen intake response relationship is not linear. On the basis of these concerns, we reanalyzed published nitrogen balance studies using two-phase linear regression analysis. We also applied the [isotope-based indicator amino acid oxidation (IAAO)] method to determine total protein requirements in adults. The mean and population-safe intakes based on the reanalysis were determined to be 0.91 and 1.0 g protein/kg/day and 0.93 and 1.2 g/kg/day, respectively, based on the IAAO method. These new values are approximately 40% higher than the current recommendations, and therefore, there is an urgent need to reassess recommendations for protein intake in adult humans.

This group of researchers considers the average protein requirement to be 0.91–0.93 g/kg/day and the amount to cover 97%–98% of the population to be 1.0–1.2 g/kg/day.

Protein Needs for People Over 60

Some researchers argue that people over 60 years old are less efficient at maintaining muscle and bone and therefore are healthier on 1.0–1.3 g/kg/day of protein (Gaffney-Stomberg, 2009, Paddon-Jones, 2008, Morais, 2006), with some of this research being supported by or connected to animal agriculture trade organizations (Gaffney-Stomberg, 2009, Paddon-Jones, 2008).

There is some conflicting research. A randomized trial of 92 men (average age 73 years) found no difference in lean body mass, muscle performance, physical function, well-being, or anabolic response to testosterone between those who consumed 0.8 g/kg/day and those who consumed 1.3 g/kg/day of protein for 6 months (Bhasin, 2018).

Protein Needs of Athletes

The RDA doesn’t include a higher protein recommendation for athletes. However, in a joint position paper on nutrition and athletic performance, the American College of Sports Medicine, the Academy of Nutrition and Dietetics, and Dietitians of Canada recommend 1.2 to 2.0 g/kg/day for athletes (Thomas, 2016). They don’t differentiate between strength and endurance athletes.

A meta-analysis of protein intake among strength athletes concluded that a daily protein intake of 1.6 g/kg/day, separated into 0.25 g/kg doses, is sufficient for muscle protein synthesis (Morton, 2018).

Protein and Amino Acid Content of Selected Plant Foods

See our online spreadsheet, Protein and Amino Acid Content of Selected Plant Foods.

Good Sources of Lysine among Plant Foods

The United States Food and Drug Administration (FDA) defines a “good source” of a nutrient as 10% to 19% of the DRI or Daily Value (FDA, 2023). Because vegans might have slightly higher protein needs than non-vegans, we’ll use the midpoint (14.5%). Using a reference body weight of 70 kg for men and 57 kg for women (IOM, 2005, p. 680), a “good source” of lysine contains 350 mg per serving. Here’s a list of good sources of lysine (weights are for one serving of ready-to-eat food):

• Legumes – 1/2 cup cooked
  • Beans – garbanzo (chickpea), kidney, pinto, navy (125-150 grams)
  • Lentils (100 grams)
  • Peas – split (100 grams) or green (80 grams)
  • Soyfoods – edamame (80 grams), tofu (125 grams), tempeh (165 grams), soy milk (1 cup or 250 mL), soy meats (3 oz or 85 grams)
  • Peanuts – 1/4 cup (35-40 grams)
• Seitan – 3 oz (85 grams)
• Quinoa – 1 cup cooked (185 grams)
• Pistachios – 1/4 cup (30 grams)
• Pumpkin seeds – 1/4 cup roasted (35 grams)

Plant Protein Balance Research

How much protein do vegans need?

The IOM says, “In conclusion, available evidence does not support recommending a separate protein requirement for vegetarians who consume complementary mixtures of plant proteins (p. 662, IOM, 2005).”

But what is the evidence?

Nitrogen Balance Study on Vegans

There has been only one nitrogen balance study conducted on vegans.

Bartholamae and Johnston (USA, 2023) performed a nitrogen balance study on 18 minimally active, healthy male vegans aged 31.6 ± 6.2 years. They fed them a diet with a protein content of 0.8 g/kg/d. Nitrogen balance was determined after 5 days using the equation:

The mean absolute nitrogen balance was −1.38 ± 1.22 g/d, and was statistically lower than the nitrogen equilibrium score of zero (95% CI: −2.00 to −0.75), indicating that the average vegan wasn’t in nitrogen balance. Only 3 vegans appeared to be in positive nitrogen balance.

There were some limitations to this study. Instead of measuring non-UUN nitrogen losses directly, they used a factor of 4 g/d. They also allowed only 5 days for an adaptation feeding period which is on the short end of standard protocol.

Another limitation is that the equation for nitrogen balance used the typical protein nitrogen conversion factor of 6.25. The protein nitrogen conversion factor is the amount of protein typically represented from an amount of protein. Using a protein nitrogen conversion factor of 6.25 for plant proteins has been criticized (Mariotti, 2008, Craddock, 2021); Mariotti et al. recommends using a protein nitrogen conversion factor of 5.6 rather than 6.25 for mixed diets using animal and plant protein.

In our spreadsheet, Bartholomae & Johnston’s nitrogen balance study on vegans, we recalculated the average nitrogen balance results using a conversion factor of 5.6 which results in a nitrogen balance of -0.25 g/d (95% CI: -0.87 to 0.38). Because Bartholamae and Johnston provide a sample menu used in the study, we were also able to create a conversion factor of 5.435 based on data provided by Mariotti, 2008. Using a conversion factor of 5.435 results in a nitrogen balance of 0.08 g/day (95% CI: -0.54 to 0.71). Although these are just estimates, they indicate that the vegans, on average, were likely close to nitrogen balance.

The RDA for protein is supposed to cover the needs of 97–98% of a population and so while the average vegan in this study was probably close to nitrogen balance on 0.8 g/kg/d of protein, this amount of protein probably isn’t enough to cover the needs of 97-98% of non-athletic vegans.

Nitrogen Balance Studies using Plant Protein

Doyle et al. (1965) conducted a study in two stages. In the first stage, eight young men were fed a vegan diet with 0.50 g/kg/day of protein and an amino acid profile matching milk. Unsurprisingly, on such a low protein intake, the men weren’t in nitrogen balance. In the second stage, protein was increased to 0.75 g/kg/day using 0.25 g/kg/day of soy protein and the men were, for the most part, in nitrogen balance.

Register et al. (1967) fed a vegan diet of 0.91 g/kg/day of protein, of which 0.55 g/kg/day was legume protein, to participants for 3 weeks. On the vegan diet, 9 out of 12 of the participants were in nitrogen balance. They also found that the protein on a vegan diet was 2.6% less digested than the protein on a non-vegetarian diet.

A third study placed young men on a near-vegan diet (except for 41 g of dried, skim milk) for 90 days, using 1 g/kg/day of protein(Yáñez, 1986). Some of the protein was from legumes (amount not reported). Seven out of the 8 subjects were in nitrogen balance.

Serum Albumin Studies of People on Plant-Based Diets

Albumin is a protein in the blood that responds to different amounts of dietary protein and can be used to measure protein needs.

Caso et al. (2000) found a 12% reduced rate of albumin synthesis among men eating a diet for 10 days of 63% plant protein compared to 26% plant protein; total protein was 78 g/day. Albumin synthesis returned to normal after 18 g/day of soy protein was added, increasing plant protein to 78% and total protein to 96 g/day. Participants were eating about 1.09 g/kg/day without the soy and 1.34 g/kg/day with the soy. The synthesis of two other proteins, prealbumin and transferrin, were also lower on the diet with 63% plant protein. The lower synthesis of transferrin, an iron transport protein, is interesting because lysine supplements have been found to increase iron absorption.

In a cross-sectional study, Haddad et al. (1999) found vegans (10 men and 15 women) to have significantly higher serum albumin levels than non-vegetarians. We estimated the protein intake of vegans to be 0.93 g/kg/day (calculations); 10 of the 15 vegan women didn’t meet the protein RDA of 0.8 g/kg/day. The authors stated, “Although serum albumin may not be a sensitive indicator of protein nutriture, the higher concentrations suggest that the diets of the vegan participants were adequate in protein.”

Creatinine Clearance Study on Vegans

Using data collected during the 1980s, Andrich et al. (2011) found similar muscle mass among vegan and non-vegan middle-aged women despite differences in protein intake of 1.0 g/kg/day for vegans and 1.3 g/kg/day for omnivores. Muscle mass was estimated using formulas based on creatinine clearance (a byproduct of muscle metabolism).

The vegan women’s lysine intake of 30 mg/kg/day didn’t meet the RDA of 38 mg/kg/day. However, the study estimated vegan women to be consuming an average of only 1,511 kcal/day vs. 1,866 kcal/day for the omnivores despite similar body mass indexes (20.0 and 20.7 respectively), suggesting that food intake for the vegans was underestimated possibly due to a lack of data on vegan foods.

Summary of Plant Protein Balance Research

The results of protein balance studies of people on plant-based diets is compiled in the table below. It’s not obvious what they indicate for the protein needs of vegans, but an estimate is that vegans might benefit from at least 1.0 g/kg/day of protein.

Protein Balance Studies Using Plant-Based Diets
Study	Protein (g/kg/day)A	Legume Protein	Result
Doyle, 1965	0.5	Amino acids matched milk	Subjects not in nitrogen balance
Doyle, 1965	0.75	Amino acids matched milk	100% in nitrogen balance
Bartholamae, 2023	0.8	Protein was mostly legumes	Average vegan was possibly in nitrogen balance but some probably were not
Register, 1967	0.91	0.55 g/kg/day	75% in nitrogen balance
Haddad, 1999	0.93	1.2 servings per day	Normal albumin levels
Yáñez, 1986	1.0	Small amount	7 out of 8 in nitrogen balance
Andrich, 2011	1.0	Lysine intake 79% RDA	Muscle mass similar to omnivores
Caso, 2000	1.09	Not reported	12% lower albumin synthesis than controls
Caso, 2000	1.34	At least 0.25 g/kg/day	Normal albumin synthesis
Agrams per kg of body weight per day

Amino Acid Intakes and Blood Levels of Vegans

Researchers from EPIC-Oxford analyzed the dietary intake and blood levels of amino acids in various diet groups in adult men (Schmidt, 2015). The study included 98 men for each diet group (vegan, lacto-ovo-vegetarian, pescatarian, and meat-eater). On average, participants had followed their diet for several years.

Protein and Amino Acid Intakes of Vegan Men

The vegan men in Schmidt et al.’s analysis of EPIC-Oxford were eating 0.85 g/kg/day of protein (calculations). Although the men were apparently healthy, protein balance wasn’t measured. It should also be noted that the protein and amino acid intakes were measured by a food frequency questionnaire which the authors say might have underestimated the intakes of vegans.

The table below compares vegan amino acid intakes in EPIC-Oxford to the United States RDA for amino acids.

Essential Amino Acid Intakes of Vegan Men in EPIC-Oxford
Amino Acid	Intake (g/day)	RDA (g/day)	Percentage of RDA
Isoleucine	2.47	1.29	191
Leucine	4.33	2.85	152
Valine	2.95	1.63	181
Histidine	1.52	0.95	160
Lysine	2.82	2.58	109
Methionine+Cysteine	1.72	1.29	133
Phenylalanine+Tyrosine	4.79	2.24	214
Theronine	2.19	1.36	161
Tryptophan	0.77	0.34	226
Weight of the participants wasn’t reported, but vegans had an average body mass index of 22.1 kg/m2. If we assume an average height of 175.3 cm for a British male (BBC, 2010), the average weight of the vegans was 67.9 kg.

Vegan men met the RDA for all essential amino acids.

Lysine was the limiting amino acid in the diets of the vegan men; they surpassed the RDA by only 9%. The 95% confidence interval for lysine intake was 2.69-2.95 g/day, with a lower margin of 104% of the RDA.

The vegan men surpassed the RDA for methionine by the next lowest amount of 33%. Methionine is traditionally the amino acid of second most concern in vegan diets.

Vegans in the United States might eat higher amounts of protein than those in the United Kingdom. The Adventist Health Study-2 from the United States found an average protein intake of 71 g/day for men and women combined (Average Nutrient Intakes), considerably more than the 58 g/day of this subset of men in EPIC-Oxford (Schmidt, 2015).

Amino Acid Intakes of Vegan Women

As a percentage of body weight, the RDA for protein and amino acids is the same for women as for men.

Vegan women in EPIC-Oxford ate 56 g of protein per day (Average Nutrient Intakes), only marginally less than vegan men Schmidt et al.’s subset of EPIC-Oxford, and so their amino acid intakes are likely similar.

As with men, Seventh-day Adventist vegan women in the United States most likely eat more protein than vegan women in the United Kingdom (Average Nutrient Intakes), suggesting that Seventh-day Adventist vegan women are surpassing the RDA for lysine and other amino acids.

Amino Acid Blood Levels of Vegan Men

Schmidt et al. (2015) compared the blood levels of amino acids between diet groups. Vegans had lower levels of lysine, methionine, tryptophan, and tyrosine, and higher levels of alanine and glycine. The researchers didn’t seem concerned by these differences.

The table below compares the plasma levels of amino acids of vegan men in Epic-Oxford to the reference ranges given by the U.S. National Library of Medicine (NLM). The NLM notes that these numbers are dependent on the specific laboratory methods used (NLM, 2016).

Plasma Amino Acid Levels in Vegan Men
Amino Acid	Plasma µmol/l (95% CI)	Reference Range µmol/l
Alanine	621 (595, 648)	230-510
Arginine	44 (39, 48)	13-64
Asparagine	98 (95, 102)	45-130
Aspartate	69 (66, 72)	0-6
Citrulline	40 (38, 42)	16-55
Glutamate	262 (248, 277)	18-98
Glutamine	547 (529, 566)	390-650
Glycine	452 (434, 470)	170-330
Histidine	117 (113, 120)	26-120
Isoleucine	96 (92, 100)	42-100
Leucine	191 (184, 199)	66-170
Lysine	210 (201, 219)	150-220
Methionine	27 (26, 28)	16-30
Ornithine	205 (197, 215)	27-80
Phenylalanine	97 (93, 101)	41-68
Proline	244 (233, 256)	110-360
Serine	197 (190, 205)	56-140
Theronine	165 (159, 171)	92-240
Tryptophan	65 (63, 68)	n/a
Tyrosine	73 (70, 76)	45-74
Valine	217 (209, 225)	150-310
The NLM provides a reference range for cystine (two cysteine molecules combined), but Schmidt et al. didn’t list plasma levels for cystine or cysteine. The NLM provides no reference range for tryptophan, with no explanation.

Vegans had levels higher than the reference range for alanine, glutamate, glycine, leucine, ornithine (a non-protein amino acid), phenylalanine, and serine; aspartate was also higher, but the reference range is surprisingly low. It’s unclear if these elevated levels have clinical significance.

Arginine and the Vegan Diet

People with herpes are often advised to limit their intake of the amino acid arginine in order to reduce symptoms. EPIC-Oxford found vegans to have a non-significant trend toward lower levels of arginine in the blood and also a lower arginine intake than meat-eaters (3.92 g/day for vegans, 4.13 g/day for meat-eaters); lacto-ovo vegetarians had the lowest intake at 3.36 g/day (Schmidt, 2015).

Appendix A: Protein Quality Scoring

One of the earliest ways to assess protein quality was the protein efficiency ratio (PER) which determined a score based on the growth of rats fed different proteins. The PER was criticized both for using rats and for not taking into account protein maintenance needs which led to the Food and Agriculture Organization (FAO) of the United Nations recommending other ways to determine protein quality.

FAO Recommends the PDCAAS in 1989

In 1989, the FAO recommended replacing the PER with the protein digestibility–corrected amino acid score (PDCAAS) as the “most suitable regulatory method for evaluating protein quality of foods and infant formulas (p. 42, FAO, 1991).” The FAO considered the PDCAAS to be an improvement over the PER because the PDCAAS based protein scores on the needs of humans, specifically preschool-age children (p. 8, FAO, 1991).

The PDCAAS ranks a food’s protein quality based on its limiting amino acid which is the essential amino acid that the protein contains the least amount of in comparison to the reference protein, typically the milk protein casein (p.5, FAO, 1991). The PDCAAS scores each essential amino acid in a protein and then uses the lowest PDCAAS score for the overall protein, truncated at 1.00 (p. 35, FAO, 1991):

(mg of limiting amino acid in 1 g of test protein / mg of same amino acid in 1 g of reference protein ) x fecal true protein digestibility percentage

Table 11 (p. 39, FAO, 1991) provides the PDCAAS scores known by the FAO at the time. Only three animal foods are listed: casein (1.00), egg white (1.00), and beef (0.92). There are just over a dozen familiar plant proteins listed and their PDCAAS scores range from 0.26 to 0.69.

Although the PDCAAS ranks proteins according to the needs of humans, it’s been criticized for calculating amino acid absorption, aka digestibility, based on a protein fecal analysis of rats (Schaafsma, 2000). Rat digestion is different from humans and a fecal analysis doesn’t take into account that bacteria in the colon can change the amino acid composition of the feces. The FAO recommended changing from fecal digestibility to ileal digestibility when agreed upon procedures and sufficient data are available (p. 32, FAO, 1991).

The FAO implied that some of the PDCAAS might be based on human digestibility studies; they say “when human balance studies cannot be used, the standardized rat faecal-balance method of Eggum or McDonough et al. is recommended (p. 31, FAO, 1991).” But the abstracts for the citations of the PDCAAS scores given in Table 11 (p. 39, FAO, 1991) don’t indicate that any human data was used.

The PDCAAS has also been criticized for truncating protein scores at 1.00 which prevents the score from reflecting a food’s ability to complement other foods in the diet (Schaafsma, 2000). Only animal foods have PDCAAS scores high enough to be truncated.

The 1991 FAO report relied mostly on data by Sarwar and colleagues in listing the PDCAASs of foods (p. 39, FAO, 1991). Sarwar later published a paper suggesting that the PDCAAS method overestimates the quality of proteins for foods containing anti-nutritional factors (Sarwar, 1997) which are primarily plant foods.

FAO Recommends the DIAAS in 2011

In 2011, the FAO Expert Consultation on Protein Quality Evaluation in Human Nutrition was held to review the adequacy of the PDCAAS (FAO, 2013). The panel recommended changing how proteins are scored from the PDCAAS to the digestible indispensable amino acid score (DIAAS). The DIAAS uses the digestibility of specific amino acids rather than of entire proteins.

The DIAAS for a food is the lowest score for any of the essential amino acids (p. 3, FAO, 2013):

100 x (mg of digestible dietary indispensable amino acid in 1 g of the dietary protein / mg of the same dietary indispensable amino acid in 1 g of the reference protein)

DIAAS scores are not truncated at 100%. The ileal digestibility of each amino acid is preferably determined in humans, followed by growing pigs, and then growing rats (p. 4, FAO, 2013).

For infants, the DIAAS reference protein is the amino acid pattern found in human breast milk. For non-infants the DIASS uses a reference protein based on the needs of humans. Appendix B: The DIAAS Reference Protein explains how they calculated the reference protein.

The FAO provides few examples of DIAAS scores. Table 6 (p. 44, FAO, 2013) provides a DIAAS score for whole milk powder (122), wheat (40), and peas (64).

Criticism of the DIAAS

Craddock et al. (2021) published a list of critiques of the DIAAS with respect to plant-based diets.

Much of Craddock et al.’s criticism focuses on the digestibility of plant foods which they argue is underestimated by the DIAAS. They cite Tomé D. (2013) whose review paper provides a table of ileal food digestibility in humans. Rather than being markedly lower in digestibility than animal protein, Craddock et al. point out that “soy protein isolate, pea protein isolate, pea protein flour, wheat flour, and lupin flour exhibited 89–92% digestibility, approximating values of 90–95% for eggs, meat, and milk proteins.”

Here’s a list of Craddock et al.’s criticisms with brief explanations:

• Nitrogen-to-protein conversion factors influence animal and plant foods differently.
  • The ratio of essential amino acids to total protein can be underestimated for plant foods when PDCAAS and DIAAS scores are calculated because the FAO doesn’t require food-specific nitrogen-to-protein conversion factors when determining the amount of protein in a food.
• There’s a limited number of fruits and vegetables within the DIAAS.
  • Many plant foods won’t be included in an assessment of an entire diet because most don’t have DIAAS scores.
• A DIAAS diet score is of questionable relevance to those following plant-based dietary patterns in developed nations.
  • Plant-based diets in food-secure regions will easily meet the needs for total essential amino acids. (In this criticism, Craddock et al. treat total essential amino acids as a whole and don’t analyze individual amino acids.)
• DIASS scores are based on animal digestion.
  • Digestibility of amino acids in rats and pigs cannot be extrapolated to humans. (The FAO admits that rats are not an ideal model, but says that pig digestion approximates humans while obtaining human data for a wide range of foods is impractical.)
• DIAAS scores for foods change when they’re part of a mixed diet.
  • In mixed diets, the digestibility of proteins and amino acids changes, while DIAAS scores are based only on single foods and give animal foods an inflated advantage.
• Digestibility of plant foods improves with heat treatment and processing.
  • Raw foods are used for most DIAAS scoring, whereas protein-rich plant foods are typically cooked or processed making them easier to digest.
• The DIAAS scores were never intended for athletes consuming a plant-based diet.
  • The FAO endorsed the DIAAS as a tool to help the global population avoid malnutrition by meeting protein requirements but hasn’t endorsed the DIASS for ranking proteins on the basis of their muscle-building potential. Craddock et al. cite research suggesting that proteins with higher DIASS scores don’t increase muscle hypertrophy.

Craddock et al. conclude that “emphasizing protein quality in developed nations is unproductive.” They say that if protein quality is to be assessed, the PDCAAS is the most suitable tool for use in Western adults who eat a plant-based diet.

It’s not clear why Craddock et al. believe that the PDCAAS is superior to the DIASS given that most of their critique of the DIASS would similarly apply to the PDCAAS. Perhaps they prefer the PDCAAS because it truncates animal protein scores at 1.00.

Appendix B: The DIAAS Reference Protein

Because it’s hard to follow how the FAO determined the DIAAS reference protein for non-infants, I’ve included more details here which require following along with the tables in their document.

For non-infants, the reference protein used in DIAAS scoring is a calculation of the amino acid needs, represented in Table 3 (p. 27, FAO, 2013) and narrowed down in Table 5 (p. 29, FAO, 2013).

The numbers in the bottom section of Table 5, Scoring pattern mg/g protein requirement, are calculated by multiplying the sum of the Maintenance and the Growth factor by the Amino acid requirements. The numbers derived in Table 3 (p. 27, FAO, 2013) are taken from a 2007 FAO report which explains how they’re calculated (FAO 2007).

In the 2007 FAO report, Table 23 (p. 150) lists the mean adult requirement for each amino acid as determined by the committee based on a variety of methods and sources, typically human nitrogen balance studies in which specific amino acids are limited. The FAO’s 2007 amino acid requirements for adults don’t match the IOM’s estimated average requirement (EAR), but are relatively close.

Last updated March 2023

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