PROTEIN QUALITY EXPLAINED : From Nitrogen Balance to DIAAS & Eggs Constant
- INTRODUCTION
 |
| Nitrogen balance to useable protein |
Part 1: Nitrogen Balance and Nitrogen Need
Part 2: Rough Protein Need (in grams)
Part 3: DIAAS (Digestible Indispensable Amino Acid Score)
Part 4: Bioavailability Based on “Eggs Constant” Concept
Part 5: How to Calculate Rough Bioavailability (Using DIAAS × Eggs Constant)
🧬 Nitrogen Balance and Nitrogen Need
The most accurate scientific basis for understanding protein requirements is nitrogen balance ,
as protein is the body's primary source of nitrogen.
Nitrogen balance indicates whether the body is in an anabolic (muscle-building) or catabolic (breakdown) state.
1. Nitrogen Balance Concept
| Balance Type | Meaning | Physiological Effect |
|---|---|---|
| Positive | Intake > Loss | Muscle growth, repair, recovery |
| Neutral | Intake = Loss | Maintenance state |
| Negative | Intake < Loss | Muscle breakdown, fatigue |
2. Nitrogen–Protein Conversion
Protein is around 16% nitrogen
Therefore, we can easily convert protein and nitrogen:
3. Nitrogen Need per kg Body Weight
According to scientific references (FAO/WHO 2013), the human body needs a daily supply of nitrogen to maintain a neutral nitrogen balance.
Average physiological requirement ≈ 0.128 g nitrogen per kg body weight per day
(This value is derived from direct nitrogen balance studies.)
4. Nitrogen Supply Table (per kg BW)
| Activity Level | Nitrogen Need (g/kg BW) | Nitrogen Supply Purpose |
|---|---|---|
| Sedentary / Basal | 0.128 | Maintenance of basic metabolism |
| Lightly active | 0.16 | Mild tissue turnover |
| Moderate activity | 0.224 | muscle adaptation |
| heavy training | 0.288 | Strong anabolic demand |
| Growth / Athletic Phase | 0.32 | Positive nitrogen balance |
5. Example: Daily Nitrogen Supply
Example person:
Weight = 70 kg
Target = Positive nitrogen balance (training phase)
Nitrogen need ≈ 0.288 g/kg BW
That means a 70 kg person needs to supply around 20 grams of nitrogen per day so that he can maintain an anabolic state.
Key Takeaway
-
Nitrogen balance tells us whether we are growing or in a breakdown state.
-
This value forms the foundation of our rough protein intake (in grams), which will be converted in the next section (Part 2).
🧬Rough Protein Need (in Grams) — Based on Nitrogen Requirement
In Part 1, we saw that an average adult needs 0.128g of nitrogen per kilogram of body weight per day
Now, we'll convert this nitrogen requirement into protein gram equivalents to get a rough daily protein requirement.
1. Nitrogen to Protein Conversion
Protein is ≈ 16% nitrogen.
So the conversion factor is:
That means if the body needs 1 g nitrogen, then for that ≈ 6.25 g protein will have to be taken.
2. Rough Protein Need per kg Body Weight
Now we convert the nitrogen values from Part 1 into protein.
| Activity Level | Nitrogen Need (g/kg BW) | Protein Equivalent (g/kg BW) | Physiological State |
|---|---|---|---|
| Sedentary / Basal | 0.128 | 0.8g/kg | Maintenance |
| Light activity | 0.16 | 1.0g/kg | Mild turnover |
| Moderate exercise | 0.224 | 1.4g/kg | Adaptation Phase |
| Strength Training / Athletes | 0.288 | 1.8g/kg | Positive nitrogen balance |
| Growth / Heavy Training | 0.32 | 2.0g/kg | High anabolic steroid demand |
3. Example: Daily Protein Requirement
Example person:
Weight = 70 kg
Activity = Moderate training
Protein need = 1.4 g/kg BW
This means that a 70 kg individual will need to consume approximately 98 grams of protein per day to maintain sufficient nitrogen for muscle adaptation and recovery.
4. Protein Need vs Protein Utilization
These are “rough” values – they assume that all protein is being absorbed 100%,
but in reality, the usable protein is slightly less in terms of absorption and amino acid digestibility (DIAAS).
Example:
If the effective utilization of a food is 80%, then out of 100g of labeled protein, the body will use only 80g.
So next in Part 3 we will look at how actual usable protein is determined based on DIAAS (Digestible Indispensable Amino Acid Score) – and how this influences nitrogen efficiency.
🧬Understanding DIAAS — Protein Quality and True Digestibility
Protein isn't just quantity, it's quality that matters too.
Even if two foods provide the same 20g of protein, their usable (absorbable) protein content is not necessarily the same—
this difference is measured by DIAAS , the Digestible Indispensable Amino Acid Score .
⚖️ 1. What is DIAAS?
DIAAS is a scientific scale that tells
how much percentage of essential amino acids (which the body cannot produce on its own) of a food are digested and utilized in the body.
It measures:
-
Digestibility of each essential amino acid
-
Absorption efficiency after digestion
-
Real nitrogen retention (protein usable fraction)
📊 2. DIAAS Scale Interpretation
| DIAAS Range | Protein Quality | Description |
|---|---|---|
| ≥ 1.00 | Excellent | Complete protein, fully digestible |
| 0.75 – 0.99 | Good | High quality, minor limitations |
| 0.50 – 0.74 | Moderate | Partial digestibility |
| < 0.50 | Low | Incomplete amino acid profile |
3. Why DIAAS is More Accurate Than PDCAAS
The older method PDCAAS (Protein Digestibility-Corrected Amino Acid Score) uses fecal digestibility,
while DIAAS measures ileal digestibility — meaning, those amino acids that can be absorbed in the small intestine.
👉 That's why DIAAS gives the true picture of protein quality.
4. DIAAS Values for Common Foods
| Food Source | Type | DIAAS (Approx.) | Quality Note |
|---|---|---|---|
| Eggs | Animal | 1.13 | Reference complete protein |
| Milk | Animal | 1.08 | Excellent digestibility |
| Chicken Breast | Animal | 1.10 | High quality, lean |
| Fish (Tuna/Salmon) | Animal | 1.05 | High omega-3 + digestible |
| Whey Protein | Animal | 1.09 | Fast absorbing, complete |
| Soy Protein Isolate | Plant | 0.866 | Best plant-based complete protein |
| Tofu | Plant | 0.97 | Good plant option |
| Lentils (Dal) | Plant | 0.72 | Incomplete amino acid set |
| Chickpeas (Chole) | Plant | 0.84 | Moderate digestibility |
| Rajma (Kidney Beans) | Plant | 0.72 | Needs soaking/cooking |
| Peas | Plant | 0.68 | Low in methionine |
| Wheat Protein (Gluten) | Plant | 0.45 | Poor amino acid balance |
| Quinoa | Plant | 0.92 | Rare complete plant source |
| Almonds | Plant | 0.63 | Moderate; fat-rich matrix |
| Peanuts | Plant | 0.65 | Good but incomplete |
| Cheese (Hard) | Animal | 1.00 | Balanced amino acids |
| Prawns | Animal | 1.02 | Lean and digestible |
| Greek Yogurt | Animal | 0.95 | High digestibility dairy |
| Oats | Plant | 0.57 | Combine with milk/legumes |
| Soy Milk | Plant | 0.80 | Good vegan protein drink |
5. Key Insight
DIAAS tells you what the actual digestible amino acid content is behind the protein numbers .
Higher DIAAS = Better nitrogen utilization = Higher anabolic efficiency.
But this is still a raw score —we can't extract protein directly from it into usable protein for the body.
For that, we need a bioavailability correction factor —
which we'll introduce in the next section:
👉 Part 4: Bioavailability Based on “Eggs Constant” Concept
(This section will explain how we convert DIAAS into real usable protein through formula.)
🧬Bioavailability Based on “Eggs Constant” Concept
We know that DIAAS indicates the amino acid digestibility of a food –
but actual protein bioavailability (ie, the efficiency with which a protein is absorbed and used in the body)
depends on factors other than DIAAS – such as metabolic utilization and post-absorptive retention .
To quantify this, we introduce an experimental constant –
🧠 “Eggs Constant” , which is the linking factor between bioavailability and DIAAS .
1. Why “Eggs Constant”?
We consider eggs a standard protein source –
because their amino acid ratio is almost identical to human metabolic requirements.
Based on research:
-
Egg bioavailability ≈ 0.92
-
Egg diameter ≈ 1.13
If we find the ratio of both the values:
This value represents a “conversion efficiency” —
that is, a scale for estimating actual usable bioavailability from DIAAS.
2. Core Formula: From DIAAS to Bioavailability
We can calculate approximate bioavailability (BA) for each protein source:
That is, if the DIAAS of a food is 1.00,
then its usable bioavailability will be ≈ 1.00 × 0.814 = 0.814 (81.4%) .
3. Example Calculations
| Food Source | DIAAS | Bioavailability = DIAAS × 0.814 | Efficiency Note |
|---|---|---|---|
| Eggs | 1.13 | 0.92 | Reference constant |
| Milk | 1.08 | 0.88 | Highly efficient |
| Chicken | 1.10 | 0.90 | Excellent |
| Soy Protein Isolate | 0.90 | 0.73 | Best plant-based option |
| Lentils | 0.72 | 0.59 | Moderate digestibility |
| Peas | 0.64 | 0.52 | Low bioavailability |
| Quinoa | 0.83 | 0.68 | High for plant |
| Wheat Gluten | 0.45 | 0.36 | Poor amino acid ratio |
4. Interpretation
Eggs Constant is a normalization factor –
it tells us
how efficiently other foods retain nitrogen if we consider eggs as a “perfect utilization reference.”
Yani higher DIAAS + higher bioavailability =
👉 better nitrogen retention
👉 better anabolic potential
5. Formula Summary
| Expected Bioavailability = DIAAS × 0.814 |
|---|
We'll use this in the next part –
where we'll learn how rough usable protein (in grams) can be calculated using this formula,
and create a comparison chart of the top 20 foods on "actual usable protein efficiency."
🧬 How to Calculate Rough Bioavailability Using DIAAS (with Top 20 Foods)
Now we understand:
1️⃣ Nitrogen requirement defines true protein need.
2️⃣ DIAAS tells amino acid digestibility.
3️⃣ “Eggs Constant” connects DIAAS to real bioavailability.
Now let's combine all these concepts to find "usable protein efficiency"
—
the amount of protein the body actually uses , not just consumes.
1. Formula Recap
| Expected Bioavailability = DIAAS × 0.814 |
|---|
That is, if a food contains 25g of protein and its DIAAS is 0.90,
then its usable protein = 25 × (0.90 × 0.814) = 18.3g
2. Example Calculation (Step-by-Step)
Food: Soy Protein Isolate
-
Total Protein = 25g
-
DIAAS = 0.90
-
Eggs Constant = 0.814
That means 25g protein of soy isolate in the body approx. Provides 18g of usable amino acids .
3. Usable Protein Table (Top 20 Protein Foods)
| Rank | Food | Protein (per 100g) | DIAAS | Bioavailability (DIAAS × 0.814) | Usable Protein (g/100g) |
|---|---|---|---|---|---|
| 1️⃣ | Eggs (whole) | 13 | 1.13 | 0.92 | 12.0 |
| 2️⃣ | Whey Protein | 80 | 1.09 | 0.89 | 71.2 |
| 3️⃣ | Chicken Breast | 32 | 1.10 | 0.90 | 28.8 |
| 4️⃣ | Fish (Tuna) | 26 | 1.05 | 0.85 | 22.1 |
| 5️⃣ | Milk | 3.4 | 1.08 | 0.88 | 3.0 |
| 6️⃣ | Greek Yogurt | 10 | 0.95 | 0.77 | 7.7 |
| 7️⃣ | Cheese (Hard) | 25 | 1.00 | 0.81 | 20.3 |
| 8️⃣ | Prawns | 24 | 1.02 | 0.83 | 19.9 |
| 9️⃣ | Soy Protein Isolate | 90 | 0.90 | 0.73 | 65.9 |
| 🔟 | Quinoa | 14 | 0.92 | 0.75 | 9.5 |
| 11 | Tofu | 8 | 0.97 | 0.79 | 5.8 |
| 12 | Lentils (Dal) | 9 | 0.72 | 0.59 | 5.3 |
| 13 | Chickpeas | 8.9 | 0.84 | 0.68 | 5.4 |
| 14 | Kidney Beans | 8.7 | 0.72 | 0.58 | 5.0 |
| 15 | Peas | 6.5 | 0.64 | 0.52 | 3.4 |
| 16 | Almonds | 21 | 0.63 | 0.51 | 10.7 |
| 17 | Peanuts | 25 | 0.65 | 0.53 | 13.3 |
| 18 | Wheat Gluten | 24 | 0.45 | 0.36 | 8.6 |
| 19 | Oats | 12 | 0.57 | 0.46 | 5.5 |
| 20 | Soy Milk | 3.3 | 0.80 | 0.65 | 2.1 |
4. Interpretation
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The usable fraction of animal proteins (eggs, whey, meat, milk) is 80–95% .
-
Plant proteins (legumes, grains, nuts) have a usable fraction between 50–80% .
-
Soy isolate and quinoa are the best plant-based choices — with high DIAAS and good retention.
5. Final Takeaway
The " Eggs Constant (0.814) " is a practical bridge—
converting DIAAS to actual bioavailability.
This formula allows us to roughly estimate how much protein from any food the body actually utilizes .
👉 This is the advanced observation through which we can understand nutrition not just in grams
but from the angle of usable efficiency .
🧬Summary, Key Insights & Final Takeaways
In nutritional science, simply looking at protein grams isn't enough—
the body only uses as much protein as digestible amino acids and nitrogen retention allow.
This is the basis of the "Usable Protein Efficiency" model.
1. Recap of Each Step
| Step | Core Concept | Key Value / Formula |
|---|---|---|
| Part 1 | Nitrogen balance defines protein need | 0.128 g N / kg BW per day |
| Part 2 | Convert nitrogen → protein grams | Protein (g) = N (g) × 6.25 |
| Part 3 | DIAAS measures amino-acid digestibility | ≥ 1.00 = Excellent quality |
| Part 4 | “Eggs Constant” links DIAAS → bioavailability | 0.814 = 0.92 / 1.13 |
| Part 5 | Usable Protein (g) = Total Protein × (DIAAS × 0.814) | practical use |
2. Key Insights
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Eggs, milk, whey, chicken → highest usable efficiency (≈ 85–90 %).
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Soy isolate, quinoa, tofu → best plant options (≈ 65–75 %).
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Beans + grains combos improve plant protein quality.
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High DIAAS ≠ full utilization — always apply Eggs Constant for realistic value.
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True protein planning = quantity × quality × bioavailability.
3. Quick Reference Chart
| Category | Best Source | DIAAS | Approx. Usable % at solo |
|---|---|---|---|
| Animal Protein | Egg / Whey | 1.1± | 88–92% |
| Dairy Protein | Milk / Yogurt | 1.0 ± | 80–88% |
| Plant Protein (Top) | Soy / Quinoa | 0.8–0.9 | 65–85% |
| Plant Protein (Low) | Chickpea / Pea / kidny beens | 0.6–0.7 | 35–65% |
💬Final Takeaway
“Protein nutrition isn't just about how much you eat —
it's about how much your body can actually use .”
By combining Nitrogen Balance + DIAAS + Eggs Constant ,
we get a practical, science-based way to estimate real usable protein for any food source.
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