Welcome to Part III of the Protein Series - Protein Quality. If you haven't already done so, it maybe worth visiting a few basics from my previous posts
Protein - The Sexiest Macronutrient. Why we need it, what are its building blocks, how to combine complementary proteins to achieve complete protein.
How much protein do we need. What is the minimum protein requirement vs optimal health, requirement for athletes, weight management and healthy ageing.
What is Protein Quality? If you are going to forget everything else I say here, just remember that protein quality refers to how much of your dietary protein is effectively digested, absorbed and used by the body. Whatever protein food you put in your mouth, your stomach needs to digest it into smaller, simpler protein molecules and amino acids, then your intestine needs to absorb and your cells need to use these molecules to build, grow and repair. Ultimately this is how protein helps us maintain health and bodily functions.
The easier it is to digest, absorb and use these protein molecules, the higher the protein quality.
Animal and Plant-Based Sources of Protein Let's compare animal and plants sources in the context of
Protein Content Animal protein sources tend to have higher protein content than plant-based sources.
Source: USDA Food Database
Beef, pork, and chicken typically provide 30g of protein per portion (110g-140g)
Fish, shrimp and seafood can provide about 20g per portion (113g)
Legumes such as tofu, lentils and chickpeas are about 10% protein
Nuts and seeds such as almonds, walnuts and chia are richer in protein but each portion (3 Tbsp to 1/4 cup) provides about 5g protein.
Hemp seeds are pretty exceptional, providing 32g protein for every 100g of food
Quinoa is about 5% protein.
Chlorella and Spirulina
It is worth mentioning two impressive plant-based proteins, chlorella and spirulina. Both are complete protein and unusually protein-rich (50% and 67%). However, we can't really take these algae in substantial amounts, mainly because they taste funny and normal dosages are about 1-5 grams. They are usually taken as supplements or added to smoothies or yoghurt.
Calorie Density It is quite easy to get enough protein from meat without eating too many calories. For example, we can get roughly 100g protein from 300-400g of meat for less than 1,000 kcal. However, if we want to get the same amount of protein from plants, we need about >1kg of tofu, lentils and chickpeas for close to 1,500 kcal. Likewise, we need 2 kilograms of quinoa for over 2,600 kcal. Nuts and seeds are calorie dense because they are high in fat. We can get 100g protein from a mere 500g to 700g of walnuts, almonds, cashew or chia seeds, but for as much as 3,000 -4000 kcal.
I know this looks ridiculous and no one eats that much quinoa or nuts and seeds in a day. The above is only to illustrate "how much protein for calorie". Even if you are not concerned about the high calorie, there is a physical and practical time limit to how much quinoa, almonds and chia we can consume. Amino Acid Profiles - Complete and Incomplete Animal sources provide complete proteins - i.e. the full set of essential amino acids that cannot be synthesized in our body and have to be obtained from our diet. Plant proteins tend to be incomplete, meaning they lack one or more essential amino acids, e.g. methionine, lysine, tryptophan. The one exception is soybean, tofu and other soy derived foods, are considered complete protein.
Quinoa, a delicious and popular pseudo cereal, is actually NOT a complete protein. Although quinoa does contain all the essential amino acids, the amount of lysine in quinoa is quite low and noticeably less than soy. Lysine is effectively the limiting amino acid in quinoa that disqualifies it from being a complete protein. If you are a vegetarian You can achieve complete protein through a plant-based diet by complementing proteins such as grains and legumes, click here to learn more.
Include as much variety of food as possible to cover all requirement for protein and micronutrients (minerals, vitamins, fatty acids)
Use complementary protein to obtain all essential amino acids/ complete protein. Examples include. hummus (legumes) and pitta bread (grain), rice (grain) and beans (legumes), bean burgers.
Use traditional methods such as rinsing, soaking, fermenting and high heating grains and legumes to improve digestion and absorption of nutrients such as calcium and iron. Click to read about grains and antinutrients.
If you are an ovo-lacto-vegetarian who is able to consume eggs, yoghurt and cheese, include them in your diet. Although these can be high in calories, their protein is easier to absorb and more bioavailable, also accompanied by a full nutrient package.
Nutrient Package Unless you are munching on a bag of potato chips, deep fried chicken, or a highly processed candy bar void of nutrition, our foods are always more than the caloric content of protein, carbohydrates and fats. They also come with micronutrients such as vitamins, minerals, fatty acids and phytonutrients.
Animal protein sources are generally higher in micronutrients which are also more bioavailable for us. The same nutrients may be present in plant foods but in smaller amounts. Moreover, the presence of antinutrients in grains can limit the bioavailability of these nutrients (e.g. iron).
Vitamin B12 is present in meat, especially organ meats, fish, seafood, eggs and dairy products. Most plant foods lack vitamin B12, and one exception is nutritional yeast.
Vitamin D is found in oily fish, eggs and dairy, which is important for the absorption of calcium.
DHA, an omega 3 fatty acid, is commonly found in fatty fish and seafood. Flax and chia seeds are high in omega 3 but only in the form of alpha linoleic acid (ALA). ALA cannot be directly used by humans and needs to be converted into EPA and DHA. The conversion rate is 6% for EPA and about 4% for DHA.
Heme-iron is only found in meat, fish and poultry, which is the most bioavailable form of iron. Non-heme iron is found in plants such as beans, dark leafy greens, dark chocolates, nuts and seeds. Unfortunately non-heme iron is not well absorbed, vegans and vegetarians are recommended to increase their intake of iron-rich foods to 1.8 times RDA (8mg/ 18mg for male/female, 14mg/ 33 mg for vegetarians). Cooking with iron utensils and adding vitamin C can also enhance iron absorption.
Now, do you get a sense that animal proteins are higher quality than plant proteins? Care to know how scientists assess protein quality?
How to Measure Protein Quality (DIAAS) The digestible indispensable amino acids score (DIAAS) is a system that measures protein quality. DIAAS most recently replaced the protein digestibility-corrected amino acid score (PDCAAS). DIAAS is based on measures at the end of small intestines (ileal digestibility) whereas PDCASS uses full total fecal tract digestibility. DIAAS measures digestibility of each essential amino acid in a protein food, i.e. how much of each essential amino acid in a food is available for the human body to use.
a DIAAS score of <75% has no protein quality claim
a score from 75%-99% represents good protein quality
a score of 100% or more is excellent or high protein quality
Animal Proteins Have Higher DIAAS Scores In general, meat, poultry, fish, eggs and dairy are scored as excellent (100% or higher). Most plant proteins like grains and legumes have a score of less than 75%, thus they cannot claim to have protein quality. One exception is soy, which has a score (75%) as a good quality protein. Click here to see a table of DIAAS for meat, animal derived foods, plants and grains.
Limitations of DIAAS DIAAS is a better measure of protein quality than PDCAAS, however I wouldn't rely on DIAAS alone to determine my protein choices, for the following reasons:
The system is based on tests done on pigs and not humans.
The score on the same foods can vary hugely depending on the source, cooking and processing methods. In my research, I found a DIAAS score of 77 for dehulled oats on one study and a score of 43 on oats on another.
The DIAAS system focuses solely on essential amino acids and doesn’t take into account of the nutrient package which is also important for our health.
Conclusion
Animal proteins are higher quality than plant proteins because they (1) are easier to digest and absorb, i.e. more bioavailable, (2) have lower calorie density (3) have higher bioavailability of essential amino acids, and (4) a more bioavailable nutrient package.
Vegetarians should focus on diversifying their diet to ensure they cover all nutritional requirements.
DIAAS score measures amino acid profiles. The system has a few limitations and does not address the nutrient package that comes alongside the protein.
At the end of the day, please don't get bogged down by your food and any scoring system. We should all try to take in a wide range of fresh whole foods to ensure we get the full spectrum of nutrients.
How to Put This Into Practice? Whilst I really enjoy researching and learning about protein quality, I don't mean for anyone to measure and weigh every bit of their food in the kitchen or at the table. Here is a quick presentation to help you figure out how much protein you may need and what foods you to choose. Either click on the link or go into the slide below, swipe sideway to get to the next slide.
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References USDA Food Database
Gerster, H., (1998), Can adults adequately convert alpha-linolenic acid (18:3n-3) to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)?, International Journal for vitamin and nutritional research, 68 (3): 159-73. https://pubmed.ncbi.nlm.nih.gov/9637947/
Iron, The Nutrition Source, Harvard T. H. Chan Public School of Health
Millward, D.J., Layman, D.K., Tome, D., Schaafsma, G., (2008). Protein quality assessment: impact of expanding understanding of protein and amino acid needs for optimal health. The American Journal of Clinical Nutrition, 87 (5): 1576S-1581S. DOI: 10.1093/ajcn/87.5.1576S
Baily, H.M., Hans, H.S., (2019), Can the digestible indispensable amino acid score methodology decrease protein malnutrition, Animal Frontiers, 9, (4), 18-23. DOI: 10.1093/af/vfz038
Mathai, J.K, Lui, Y., Stein, H.H., (2017), 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, 117 (4): 490-499. DOI: 10.1017/S0007114517000125.