Protein Deep Dive | Episode 29 | Dr. Layne Norton Podcast

In this deep dive episode, Dr. Layne Norton breaks down everything you need to know about protein — from the molecular level to practical real-world application.

We cover:

What protein actually is, how it’s made and broken down in the body
The complete science of muscle protein synthesis (MPS) and muscle protein breakdown (MPB), including the critical roles of mTORC1, leucine, eIF4E, 4E-BP1, and more
How dietary protein is digested, absorbed, and used by your muscles
Why leucine content matters and how different protein sources (whey vs. wheat vs. collagen) dramatically affect MPS
The “muscle full effect,” protein thresholds by age, and why protein distribution across meals may (or may not) matter
How high-protein diets impact fat loss, satiety, energy expenditure, and long-term body composition
The truth about protein and health outcomes: mortality, cardiovascular disease, cancer, kidney function, and type 2 diabetes — with the latest evidence separated from the hype and confounding factors

Whether you’re a casual gym-goer, competitive athlete, bodybuilder, powerlifter, or just someone who wants to optimize muscle and health, this episode gives you the complete, evidence-based picture on protein intake, quality, timing, and safety.

Plus, tips for vegans and how to actually use this information day-to-day.

If you want to understand protein at a level most people (and even many coaches) never reach, this is the episode for you!

I. What is Protein and How is it Made and Broken Down?

A. Chains of amino acids linked together

B. Amino acids that make up protein are unique in that they are the only macromolecules that contain nitrogen

Nitrogen has different bonding properties that allow proteins to form unique shapes
Nitrogen makes it possible to form structures that things like carbohydrates and fats aren’t capable of

C. Proteins fold into their 3D shapes called ‘conformational shapes’ based on their amino acid sequences

These amino acid sequences are coded for in DNA.

D. Protein synthesis

Signal >>>> DNA transcription to mRNA
mRNA >>>> Protein translation via ribosomes
In muscle protein synthesis (MPS) is controlled at the translational level

E. Translation

Rate limiting step of translation is ‘translation initiation’ 1
The rate limiting step of translation initiation is the assembly of a complex called eIF4F which acts as a scaffold for the ribosome to attach to the mRNA. 2
The assembly of eIF4F is rate limited by the binding of eIF4E with eIF4G
- eIF4E is typically bound by 4E-BP1
- The stimulation of mTORC1 (via resistance training or protein) causes mTORC1 to phosphorylate 4E-BP1, which causes it to dissociate from eIF4E making it available to bind with eIF4G. 3
- mTORC1 also phosphorylates eIF4G which increases it’s affinity to bind with eIF4E
- mTORC1 also phosphorylates ribosomal protein p70S6K. p70S6K increases the synthesis of ribosomal proteins, thus increasing the capacity for MPS. 4

F. MPS is also balanced by Muscle Protein Breakdown (MPB)

Proteins are always being constantly synthesized and broken down
For muscle mass to increase, MPS must exceed MPB over time

G. MPB systems

ubiqutiin – proteasome
- Old proteins are ‘tagged’ with ubiquitin for degradation. The Proteasome is a large, barrel like complex that degrades proteins tagged with ubiquitin. 5
Lysosomal Protein Degradation (this is how autophagy occurs). 6
- Old proteins are ‘engulfed’ by the lysosome and broken down
While the proteasome and lysosomes are the two main ways proteins are broken down, there are also calpains and caspases, but these are less regulatory.
Of these systems, the Ubiquitin – Proteasome system is the most influential as it is responsible for breakdown of myofibrillar proteins and is influenced by resistance training and nutrition

II. How is dietary protein digested and absorbed?

A. Step 1: Stomach

Strong hydrochloric acid begins to denature dietary proteins (denature = unfold). The unfolding of these proteins makes them accessible for digestive enzymes to begin cleaving various bonds. The stomach contains proteases like pepsin and pepsinogen which begin to ‘chop up’ these proteins into smaller peptides

B. Step 2: Small intestine

Pancreatic proteases like trypsin and chymotrisin begin to ‘chop up’ the peptides into very short amino acid sequences (typically individual amino acids, di-peptides, and tri-peptides)

C. Step 3: Absorption

This mostly occurs in the small intestine where these amino acids are absorbed into the portal vein of the liver for first pass metabolism

D. Step 4: Circulation

Amino acids that pass through the liver then can enter the bloodstream and be available for the peripheral tissues

III. How does dietary protein affect muscle?

A. When sufficient dietary protein is consumed at a meal, this stimulates mTORC1 and increases MPS. 7

The amount of protein required to stimulate this response appears to be closely tied to it’s leucine content.

B. Leucine is a branched chain amino acid (BCAA) and is largely responsible for being the component of protein that stimulates MPS via it’s effects on mTORC1

Leucine content typically reflects overall protein quality, it survives first pass metabolism largely untouched, and it has passive transport across the cell that is concentration driven
- This makes it an ideal amino acid to sense and trigger MPS.
- Meals containing insufficient leucine do not trigger MPS
Leucine content of various protein sources has been demonstrated to differentially impact MPS
- For sources like whey (10-12% leucine), very little protein is required to stimulate MPS (15-25g) whereas something like wheat (~6-7% leucine) may require 30-40g protein to stimulate MPS and a very poor source of leucine like collagen (1-3% leucine) has not ever been demonstrated to increase MPS regardless of dose. 9

C. The threshold

In young people, the effect of protein on MPS is relatively linear, but changes with age. 10
In adulthood, a threshold of minimum protein/leucine intake at a meal is required to stimulate MPS. 11
In elderly, this threshold is significantly higher than for the young
- Due to a decrease in the components of the anabolic signaling pathway as well as their sensitivity to anabolic stimuli. 12
- Elderly CAN achieve the same rates of MPS compared to young, but it requires more protein/leucine. 13
When this threshold is reached, MPS appears to run for a defined period of time. 14
- Above a certain point, increasing protein/leucine does not further appear to increase the amplitude or duration of MPS (muscle full effect). 15
- This is the rationale for properly distributing dietary protein over multiple meals

D. Higher protein diets increase muscle mass

Consuming ~1.6g/kg protein has been demonstrated to increase lean mass in people undergoing resistance training compared to lower protein intakes. 16 17
There is some evidence that even higher protein intakes (>3g/kg) may still provide further benefits on lean mass 18

E. Protein distribution

Since MPS is capped at a meal and there is no storage for amino acids, some (myself included) have suggested that distributing daily protein over multiple higher protein meals may be advantageous for lean mass and body composition. 19
The studies are conflicted however with some showing no effect while others show a benefit to protein distribution. 20 21 22 23 24 25 26 27

IV. How does dietary protein affect fat loss?

A. High protein diets increase energy expenditure

Protein has a TEF of 20-30% compared to carbs and fats at 5-10% and 0-3% respectively. 28
High protein diets also increase EE even in the context of ultra processed foods. 29

B. High protein diets also may have a beneficial impact on satiety, but this is not a consistent effect and may be more of an individual food effect. 30

C. High protein diets improve fat loss efficiency by increasing the percentage of weight lost as fat compared to lean. 31

D. High protein diets lead to better retention of lean mass and this may help prevent future weight regain. 32

V. Dietary Protein and health

A. Mortality

Inconsistent effect, some studies show an increased risk of mortality and some show a decreased risk of mortality with greater protein intake, especially in elderly. 33
A large meta-analysis of protein intake showed decreased risk of mortality with total protein intake, plant protein intake, and a neutral effect of animal protein intake. 34
The ‘negative’ effects on protein intake are likely confounded by the fact that people who consume more protein, in particular animal protein, is possibly due to the fact that many animal sources of protein are high in fat and calories, accordingly higher protein intake is associated with greater body weight, which may explain studies that show increased risk. 35

B. Cardiovascular disease

Like mortality, the results are mixed, but when major confounders are accounted for, there appears to be little impact of protein on cardiovascular disease. 36 37 38

C. Cancer

Like mortality and CVD, there are mixed results with cancer, but when major confounders are controlled for, it does not appear protein intake is causative for cancer. 39

D. Kidney Disease

It has long been hypothesized that higher protein intakes could contribute to kidney disease and kidney failure. It was advised that patients with compromised kidney function limit their protein intake
The weight of the evidence from human RCTs shows that high protein diets do not harm healthy kidneys. 40
There is limited data that low protein diets actually improve outcomes in patients with kidney disease. They may reduce overall load on the kidneys but low protein intake also makes it difficult for tissues to repair

E. Type 2 Diabetes

Some correlational studies have shown a negative impact of high protein diets on type 2 diabetes, but human randomized controlled trials have shown that high protein diets improve insulin sensitivity when confounders are controlled, indicating any association may be reverse causality. 42 43 44 45 46

VI. Recommendations

A. General population – Health mindset and want adequate muscle mass but aren’t worried about maximizing muscle mass: 1.4-1.8g/kg body weight

B. Athletes who want more optimal body composition and lean mass: 2-3g/kg

C. Bodybuilders, powerlifters, and other strength athletes: 2.5-3.5g/kg

D. Total protein intake is most important but to optimize further, spread protein relatively evenly over 3 meals

E. If vegan, consider supplementing with an isolated plant protein like potato, soy, or pea isolate to help achieve total protein goals

References

Intro
Protein Chemistry Overview
Outwork Nutrition
Protein Digestion and Absorption
How Protein Affects Muscle
David Protein
Protein Distribution
Protein and Fat Loss
Protein and Mortality
Protein and Cardiovascular Disease
Protein and Cancer
Protein and Kidney Disease
Protein and Type 2 Diabetes
Recommendations
Outro