Bioavailability 101: Why Your Body Prefers Animal Protein Over Plant-Based

If you’ve ever tracked your macros, you know the drill: hit your daily protein goal to build muscle, recover faster, and stay satiated. But what if the number on your nutrition app doesn't tell the whole story?

The truth is, 100 grams of plant protein does not equal 100 grams of animal protein in the eyes of your digestive system. While plant-based diets offer incredible health benefits—including fibre, antioxidants, and healthy fats—when it comes to protein efficiency, animal sources have a distinct biological edge.

The secret to this difference lies in a crucial nutritional concept: bioavailability.

Here is a breakdown of what protein bioavailability is, why your body prefers animal sources, and how to optimise your diet regardless of your eating style.

What is Protein Bioavailability?

In nutrition, bioavailability refers to the proportion of a nutrient that is successfully digested, absorbed into your bloodstream, and utilised by your body for its intended functions (like repairing tissue or building muscle).

You can eat 30 grams of protein from a bowl of lentils, but your body might only be able to extract and use a fraction of it. If a protein source is highly bioavailable, your body absorbs almost all of it with minimal waste.

Why Animal Protein Wins the Bioavailability Game

There are two primary reasons why your body absorbs animal protein more efficiently than plant-based protein: amino acid profiles and digestibility.

1. The "Complete" Amino Acid Profile

Proteins are made up of building blocks called amino acids. There are 20 in total, but 9 are considered "essential," meaning your body cannot produce them on its own—you must get them from food.

• Animal Proteins: Meat, poultry, fish, eggs, and dairy are considered complete proteins. They contain all 9 essential amino acids in the exact ratios your body needs to stimulate muscle protein synthesis (MPS).

• Plant Proteins: Most plant proteins are incomplete. They are usually missing or exceptionally low in one or more essential amino acids, most commonly leucine, lysine, or methionine.

Why Leucine Matters: Leucine is the specific amino acid responsible for flipping the "switch" that tells your body to build muscle. Animal proteins, particularly dairy (whey) and meat, are incredibly rich in leucine.

2. Digestibility and "Anti-Nutrients"

Even if a plant food contains decent protein, getting that protein out of the food and into your bloodstream is a harder job for your digestive tract.

Plants contain fibrous cell walls and natural defence compounds—often called anti-nutrients—such as:

• Phytic acid (phytates): Found in grains and legumes; binds to proteins and minerals, reducing absorption.

• Trypsin inhibitors: Found heavily in soybeans; they block the enzymes your body uses to digest protein.

• Tannins and Lectins: Can irritate the gut or interfere with nutrient breakdown.

Animal proteins lack these anti-nutrients and fibre, meaning your digestive enzymes can break them down rapidly and efficiently.

The Scoreboard: Measuring Protein Quality

Nutrition scientists measure protein quality using a system called the DIAAS (Digestible Indispensable Amino Acid Score). This score looks at how well amino acids are absorbed at the end of the small intestine.

Here is the protein quality comparison reorganized into a clear, readable text format:

• Milk / Whey: With a DIAAS score of 1.14 – 1.30+, this is ranked as Excellent and sits at the top of the scale.

• Eggs: Scoring 1.13, eggs provide an Excellent source of high-quality protein.

• Beef: Coming in at 1.10, beef is also rated as an Excellent protein source.

• Soy Protein Isolate: With a score of 0.90, this is considered a Good plant-based option.

• Pea Protein: This source has a score of 0.82, giving it a Fair quality rating.

• Black Beans: Scoring 0.53, these are considered a Low quality protein source on this specific scale.

• Wheat / Gluten: At the bottom of the list with a score of 0.40, this is rated as Very Low.

Note: A DIAAS score above 1.0 is the "gold standard," indicating that the protein source exceeds the body's requirements for all essential amino acids.

Can You Still Build Muscle on a Plant-Based Diet?

Absolutely. The fact that animal protein is more bioavailable does not mean you can't be strong, muscular, or healthy on a vegan or vegetarian diet. It just means you have to plan smarter.

If you prefer plant-based proteins, you need to compensate for the lower bioavailability by:

1. Eating higher total volumes of protein (aiming for 20-30% more total protein than a meat-eater to get the same net absorption).

2. Combining sources (e.g., eating rice and beans together to combine their incomplete amino acid profiles into a complete one).

3. Utilising protein isolates (like pea or soy protein powders), which have had the fibre and anti-nutrients stripped away, vastly improving their digestibility.