What is an amino acid score? Can you reach 100 with plant-based sources?

What does the Amino Acid Score measure?

Have you ever heard that protein from meat and fish is high-quality, while protein from soy and grains is of lower quality? This article explains what the Amino Acid Score actually measures and whether plant-based proteins can also achieve a high score.

What is the difference between amino acids and protein?

Proteins are made up of amino acids linked together like a chain. The protein found in meat, fish, soy, and eggs is not a single, uniform "protein" substance; rather, the combination of types and amounts of amino acids differs for every food. This variation in combination is why the amino acid score changes depending on the food.

The Ministry of Education, Culture, Sports, Science and Technology (MEXT) food composition tables list the amount of protein in foods calculated by summing individual amino acids, separate from the protein calculated based on nitrogen content. Even if two products are both labeled "10g of protein," the amino acid composition inside is different. When checking protein content on nutritional labels, looking at the ingredients used alongside the total amount will help you get a better picture of the amino acid composition.

Why are essential amino acids called "essential"?

Some amino acids can be produced by the body, while others cannot. The MEXT food composition tables list nine "indispensable amino acids" that the human body cannot produce or cannot produce in sufficient quantities relative to needs. The term "essential" is used because obtaining these from food is mandatory.

The amino acid score is a figure representing how well these nine essential amino acids are balanced within a food. If even one amino acid is lacking, the entire score is pulled down by that limited amount. In the next chapter, we will look specifically at the reference amounts for each of these nine. Knowing these standards will help clarify where plant-based foods might fall short and where they are sufficient.

Additionally, when choosing general foods including protein bars, it is important not just to look at protein content, but also to check other nutritional information like calories, fats, and carbohydrates, and to consider them as part of your total daily intake.

How is the Amino Acid Score determined?

The amino acid score is determined by comparing each of the nine essential amino acids against a standard amount; the proportion of the one that is most deficient becomes the score for the entire food. Understanding these standard amounts reveals which foods are more likely to achieve a high score.

The concept of the "first limiting amino acid"

The amino acid score is determined by the single amino acid with the lowest quantity among the nine. The MEXT food composition tables define the "first limiting amino acid" as the limiting amino acid with the lowest ratio relative to the reference pattern (MEXT Standard Tables of Food Composition in Japan, 2020 Edition (8th Revision), Amino Acid Composition Table Section). The calculation method is also shown in the same document: it is calculated by dividing the amount of the first limiting amino acid in 1g of food protein by the reference amount for that same amino acid in the scoring pattern, then multiplying by 100.

For example, if 8 out of 9 essential amino acids in a food meet the reference amount, but the remaining one is only at 70% of the standard, the amino acid score for that food is 70. No matter how much of the other amino acids are present, they cannot raise the score. Just like a wooden barrel with one short stave will leak water regardless of the height of the others, if even one of the nine amino acids is extremely low, the overall score is dragged down to that level. Which amino acid becomes the first limiting amino acid varies from food to food.

The nine essential amino acids and their reference amounts

To know which is the first limiting amino acid, you need to know the reference amount for each of the nine. The Kyoiku Tosha Amino Acid Composition Table includes the 2007 version of the amino acid scoring pattern, which shows the requirements for those aged 18 and over per 1g of protein. The values are: isoleucine 30mg, leucine 59mg, lysine 45mg, sulfur-containing amino acids 22mg, aromatic amino acids 38mg, threonine 23mg, tryptophan 6.0mg, valine 39mg, and histidine 15mg, for a total of 277mg for all nine.

Looking at the numbers, some require large amounts like leucine and valine, while others like tryptophan satisfy the criteria with only a tiny amount. By comparing the amount of each amino acid in a food against these standards and identifying the one with the lowest percentage, the first limiting amino acid and the amino acid score are established. If all nine exceed the reference amounts, the score will be 100, regardless of whether the source is plant or animal.

What does an Amino Acid Score of 100 guarantee?

When we see an amino acid score of 100, it is easy to assume the food is perfectly absorbed by the body. However, this figure only indicates the balance of essential amino acids and is a separate matter from how easily it is digested and absorbed.

A different issue from ease of digestion and absorption

The Food and Agriculture Organization (FAO) of the United Nations explains that an amino acid score of 100 signifies that the most deficient essential amino acid in the food protein has reached 100, not that the digestion and absorption rate is 100% (FAO/WHO Joint Expert Consultation). In other words, the score of 100 means "the balance of contained amounts meets the standard," but it does not account for how much of what is eaten is actually absorbed by the body.

The organization further defines amino acid digestibility as the proportion of ingested amino acids that are actually absorbed, noting it as a measurement concept distinct from the amino acid score (FAO). The organization recommends the Digestible Indispensable Amino Acid Score (DIAAS) as a metric that incorporates this digestibility, calculated by dividing the amount of digestible essential amino acids by the amount of the same amino acid in a reference protein and multiplying by 100 (FAO). If you have been looking only at the amino acid score when choosing food, knowing that nutrient content and ease of absorption are separate issues will likely change how you view labels.

A score below 100 does not mean the quality is poor

As mentioned above, the amino acid score is determined solely by the percentage of the single most deficient essential amino acid. Because of this, there are points to keep in mind for foods that do not reach 100. For instance, if a food has a score of 90, it means that 8 of the 9 amino acids meet the standard and the remaining one has reached 90% of the standard; it does not mean that all essential amino acids are uniformly at 90%.

If you compare numbers without understanding this mechanism, the difference between 90 and 100 can feel much larger than it really is. Once you understand that the calculation method lowers the score if even one amino acid is slightly below the standard, you will realize it is not a reason to reject foods that do not reach 100. When choosing food, it is recommended to look beyond the score itself and consider what other foods you are combining in your daily diet.

Is it true that plant protein has a low score?

You may have heard that "protein from soy and grains is of lower quality." However, when you distinguish between the types of amino acids that tend to be lacking and the differences between plant-based ingredients, it becomes clear that it is not as simple as it seems.

Amino acids often lacking in grains

Everyone has days where they eat grain-centered meals, such as a breakfast of only white bread or a lunch of only an onigiri (rice ball). A peer-reviewed paper published in Frontiers in Nutrition reported that plant proteins investigated often lacked lysine, leucine, and methionine (Frontiers in Nutrition).

Among these, lysine is one of the essential amino acids for which the requirement for those aged 18 and over is set at 45mg/g of protein in the 2007 amino acid scoring pattern (Kyoiku Tosha Amino Acid Composition Table). Protein derived from grains tends to have lower amounts of lysine relative to this standard, making it a commonly cited first limiting amino acid.

A peer-reviewed paper in Nutrients reported that essential amino acids in the plant proteins studied accounted for an average of 26±2% of total protein, while for animal protein, the figure was 37±2% (Nutrients). These figures show average trends for plant versus animal sources, but do not mean that the amino acid scores for individual foods are guaranteed to be low.

High-scoring plant ingredients like soy and peas

You might have seen labels like "pea protein" or "soy protein" in the ingredients section of protein-based food products. The Academy of Nutrition and Dietetics states that if sufficient energy is consumed and a variety of plant foods are eaten, essential amino acids can be adequately obtained, and it is not necessary to combine "incomplete" protein foods at every single meal (Academy of Nutrition and Dietetics).

Even if lysine or other amino acids are often lacking when looking at grains alone, incorporating different plant protein sources into your daily diet, such as soy or peas, can help balance your overall essential amino acids. Thinking about adding bean-derived protein to a grain-heavy diet is a practical approach.

When choosing plant-based foods, check the ingredient label to see if protein derived from soy or peas is used. Knowing the types of ingredients makes it easier to see what you should add to a grain-centered diet.

How to choose foods with a high Amino Acid Score?

There are differences in how essential amino acids are contained in meat, fish, eggs, and dairy compared to plant foods like beans and grains. Here is how to approach animal versus plant-based foods when deciding what to pick up at the store.

Trends in meat, fish, eggs, and dairy

As mentioned, animal protein tends to have a higher proportion of essential amino acids than plant protein. Because of this, animal foods like chicken, fish, eggs, milk, and yogurt are characterized by their ability to provide a balanced array of essential amino acids even in a single serving.

For example, if you add sautéed chicken breast as a main dish at dinner, you do not need to worry too much about what to add as a side dish, as that one item makes it easy to balance your amino acids. The same applies to adding an egg dish to breakfast or choosing yogurt as a snack. The convenience of choosing without worrying about combinations is unique to animal-based foods.

However, there are still differences in protein and fat content among animal products. We recommend choosing foods by considering fat and calorie content in addition to the amino acid balance.

What to look for when choosing plant-based foods

When choosing plant-based foods like beans, soy products, or grains, check the details in the ingredient label. According to Consumer Affairs Agency materials, while ingredients are listed in descending order of use, the order alone does not reveal the actual amount of each ingredient or the specific protein content per ingredient (Consumer Affairs Agency "Things to Know About Food Labeling"). Checking if soy or beans are listed near the beginning can help identify the main ingredients.

The same material also notes that for composite ingredients, the constituent ingredients may be listed in parentheses following the name (Consumer Affairs Agency "Food Labeling Guide"). Checking inside these parentheses makes it easier to find if soy or bean-derived ingredients are used.

Furthermore, we recommend looking at the protein amount on the nutritional information label. The Consumer Affairs Agency states that processed foods for general use are required to display nutritional information including protein, and this amount should be confirmed along with the serving unit (Consumer Affairs Agency "Nutritional information labeling is mandatory for processed foods for general use"). By looking at both the ingredients and the nutritional label, you can understand what the food is made of and how much protein it contains.

Compensate for lacking amino acids through food pairing

As mentioned, grains are characterized by having relatively lower amounts of certain essential amino acids. However, there is no need to worry about the score of each individual dish; if you know how to combine them, you can easily manage with your regular diet.

Pairing rice with natto or tofu

The classic Japanese meal combination is actually rational in terms of complementing amino acids. Soy products compensate for the amino acids that tend to be lacking in rice alone.

For example, a menu of rice, natto, and miso soup with tofu is a combination that naturally occurs without needing anything special. The same logic applies to having cold tofu or simmered beans as a side dish on a day where you ate a single-bowl meal for lunch. If you have the awareness of adding just one bean product to your rice, it is sufficient as a pairing strategy.

Conversely, if you try to calculate amino acids for every meal out of concern for deficiencies, it will likely be unsustainable. We recommend incorporating soy products you already have in the fridge, such as tofu, natto, or soy milk, to a manageable extent.

Why it doesn't need to be finished in one meal

As noted, the Academy of Nutrition and Dietetics states that essential amino acids can be adequately obtained if sufficient energy is consumed and a variety of plant foods are eaten, and it is not necessary to combine incomplete proteins at every meal. This indicates that the focus should be on the total diet over a day or a few days, rather than on a per-meal basis.

Even if you only have pasta for lunch on a busy day, you do not need to "make up for it" in the same meal, as long as you incorporate meat, fish, or bean products into your dinner or the next morning's meal. Rather than searching for the perfect combination for every single meal, it is easier to continue by focusing on whether you are including several food groups throughout the day.

It makes things easier to know that if you have a week of eating out or a day where you want to minimize the number of dishes, you can just adjust with your meals the next day. When planning your menu, we recommend looking back at whether your protein sources are balanced over a day or a few days, rather than fixating on the contents of a single meal.

What to look for when choosing protein bars?

The protein bar aisle is lined with products ranging from those made with soy or pea protein to those using whey. Here is a summary of whether you should look at the ingredient label or the nutritional information label, and what can be learned from each.

Checking the types of ingredients from the ingredient label

Looking at the ingredient label of a protein bar reveals what protein sources are used. "Soy protein," "whey powder," and "pea protein" are examples, and since ingredients are listed in descending order of use, those near the beginning can generally be considered the main ingredients.

However, it is premature to judge the quality of the score just by looking at the types of ingredients. Even with "soy protein," the amino acid balance varies by product depending on how it is combined with other ingredients. For points to watch out for when choosing protein bars using plant-based ingredients, please see our related article: What are the disadvantages of plant protein? An explanation of how to view protein, carbohydrates, taste, and ingredients.

Protein amount and amino acid balance are separate issues

The amount of protein written on the nutritional label is simply weight. While you can tell if a bar contains 10g or 15g, you cannot discern from this number how well-balanced the essential amino acids within it are.

In other words, choosing a bar with a higher protein amount does not automatically mean the amino acid balance is better. Some products may use large amounts of ingredients that are low in specific essential amino acids just to boost the total amount. Quantity and quality must be looked at as separate axes.

When choosing a protein bar, it is a good habit to verify the protein amount on the nutritional label and then check the ingredient label to see what ingredients are being combined.

Related content is introduced in Is it true that you can't get amino acids from plant protein? and What is pea protein? Features and effects derived from peas; please refer to these as well.

Looking at the ingredients and amino acid balance of BRAIN PRO

Based on the mechanisms discussed so far, let's look specifically at the ingredient and nutritional labels of BRAIN PRO. You will see how it satisfies essential amino acids using only plant-based ingredients and how it differs from other protein bars.

Plant-based ingredients: Pea protein

The ingredient list includes: almond paste, pea protein, almonds, soy puffs, cocoa powder, coconut oil, cacao butter, inulin syrup, agave syrup, lion's mane mushroom powder, rock salt, flavorings, and sweeteners (stevia), in that order. Pea protein is a plant-based protein ingredient extracted from peas. Since pea protein is listed second, we can see that it is used more than anything else besides the almond paste. As the ingredients also include soy puffs, BRAIN PRO is a product that combines two types of plant proteins: peas and soy. Almonds and almond paste are also nuts that contain protein, making the product designed to supplement protein using only plant sources. Because it is made without animal protein, it is easy to choose for those who wish to avoid dairy or eggs. Lion's mane mushroom powder is also included, but as a type of mushroom, it is not a protein source. For more details on lion's mane, please refer to What are the effects of Lion's Mane? Current status of research, how to eat, and precautions. Note that because it contains soy and almonds, those with allergies to these should be careful.

Comparing nutritional information with other protein bars

A variety of products are lined up in the protein bar aisle, including those primarily using whey or those using only soy. Since the protein and fat content per bar will change based on the ingredients, it is important to compare the nutritional labels. BRAIN PRO contains 190kcal, 11.1g of protein, 13.5g of fat, 7.1g of carbohydrates (4.9g net carbs, 2.2g dietary fiber), and 0.4g of salt equivalent per 35g bar. It uses no white sugar, preservatives, wheat, casein, trans fats, animal ingredients, or artificial coloring. Compared to bars that use whey protein derived from dairy, BRAIN PRO differs in that it secures its protein content using only plant-based ingredients. Even if products appear similar when comparing only the protein amount, how you choose them will change depending on whether the ingredients are animal or plant-based. You can tell if a product uses soy puffs and pea protein or whey powder by checking the ingredient label. BRAIN PRO is recommended for those who want to avoid animal ingredients or those who want to keep their ingredient list as simple as possible. Why not incorporate it as an option for when you want to balance your essential amino acids using only plant-based ingredients? It is suitable for those who want to choose based on ingredients, whether as a snack or for a post-workout bar.

Product	Content	Sugar	Artificial Sweeteners	White Sugar	Protein (Source)
Myprotein Layered Protein Bar Birthday Cake	60g	21g	Used (Sucralose)	Used	20g / Dairy/Soy
Cream Genmai Bran Cacao	36g	17g	None	Used	5g / Soy
in Bar Protein Baked Choco	42g	11g	Used (Sucralose)	Used	15.8g / Soy/Dairy
SOYJOY Plant-Based White Choco & Lemon	25g	5.7g	None	Used	6g / Soy
BRAIN PRO	35g	4.9g	None	None	11.1g / Plant (Pea)

The figures for each product are based on nutritional information listed on the manufacturers' official websites as of September 25, 2026. They are subject to change due to product revisions.

Frequently Asked Questions

What does an amino acid score of 100 mean?

It is a figure indicating that all nine essential amino acids, including the one in the lowest quantity, have met the reference amount. Reference amounts are set per 1g of food protein, and if even one type falls below the standard, that ratio becomes the score. Since it is not a number that indicates ease of digestion or absorption, a food with a score of 100 is not necessarily guaranteed to be easily absorbed by the body.

What kinds of meals are rich in amino acids?

Chicken, fish, eggs, and dairy products are foods that tend to contain a well-balanced array of essential amino acids. Among plant-based options, protein derived from soy and peas is considered to have a relatively high score. Amino acids that are often lacking in grains alone can be compensated for by combinations like adding natto or tofu to rice, so it is important to think about the entire diet rather than focusing on a single dish.

Summary

The amino acid score is determined by the proportion of the most deficient of the nine essential amino acids. Understanding this mechanism shows that it is not guaranteed that plant-based food will have a low score; if you choose ingredients that meet the reference amounts, even plant-based food can achieve a high score. For daily meals, rather than fixating on the number of a single dish, we recommend eating a combination of both animal and plant-based foods.