Amino Acids, Dietary Protein, Physical Activity and Muscle Mass

protein shakeThis scientific review article reviews over 150 original research papers about dietary protein, absorption, protein recommendations, how it changes with age, meal protein composition, and more. It’s pretty jargon heavy and rather long. However, if you have an interest in health and fitness, and building muscle in particular, I recommend reading it.

TLDR of the Review:

  • Endurance and strength athletes need more protein, but more than .75 grams of protein per pound of body weight provides no additional benefit
  • 25 grams of protein at once seem to be the threshold for maximizing Muscle Protein Synthesis; smaller meals of no more than 25 grams protein equally spaced is better for promoting muscle growth than fewer larger protein meals
  • Diets higher in essential amino acids (EAA) are better at triggering muscle growth; they are more effective when taken after a workout
  • Eating more protein then you need can stress the kidneys over time, particularly for older individuals (50+)
  • As you age, your body gets less efficient at processing protein

Here’s a more in-depth bullet-point summary:

  • Excessive protein intake is hard on the kidneys, so over time eating way more than what is needed isn’t best
  • Endurance athletes and weight lifters (bodybuilders and power lifters) have higher protein requirements and the ability to absorb more
    • Most people, even those who lift regularly, are not bodybuilders, power lifters, or endurance athletes
  • There is a positive correlation between dietary protein intake and changes in lean body mass (LBM)
    • A daily protein intake of 1.0 g/kg body weight (.45g per pound) is sufficient for “active” people
    • In sedentary men and women (both young and elderly) a daily protein intake of 0.85 g/kg body weight (.39g per pound) was adequate
  • People participating in resistance training may need a higher protein intake than sedentary people, which has become “common” knowledge in the fitness world.
    • If you take time off, it is recommended to stay at 1g/kg body weight (.45g per pound)
    • A study showed no muscle was lost or no loss in protein absorption after 7 days of bed rest
    • Increased protein intake did NOT prevent muscle loss or decline in whole body protein absorption during 14 days of bed rest
    • At this time frame (or greater) it didn’t matter if you ate .8g/kg body weight (.36g per pound) or 1g/kg body weight (.45g per pound)
    • This gives evidence that the body adapts during periods of activity vs inactivity
  • MPS = muscle protein synthesis
    • MPS rate levels off with increasing doses of protein in a single serving
    • This levels off at around 25g.
    • Supporting evidence shows that diets containing 1.5 or 3.0g protein/kg lean body mass per day (.68 – 1.36 grams per pound) in both young and elderly had NO difference in MPS
    • Higher protein diets (beyond the dietary needs of your body) do NOT enhance MPS
  • The main way of looking at protein absorption is analyzing plasma amino acid (AA) concentration
    • When consuming a protein rich meal, the protein will eventually be digested and absorbed as amino acids
    • When muscles need repairing, this starts a complex cellular cascade to repair the torn and damaged muscle tissue resulting from resistance training
    • When the plasma is enriched with amino acids it will stimulate myofibril MPS (important component of muscle fiber), but this only lasts for up to one (1) hour even if there is a high concentration of plasma amino acids for 4-6 hours
    • This means that saturating you system with protein or amino acids won’t continuously stimulate myofibril MPS (new muscle fibers).
  • Some proteins are better at enriching plasma with amino acids than others – this is why whey protein is so popular
    • 25 grams of whey is all that is needed to increase myofibrillar MPS to roughly 180 minutes
      • Another study showed that the plateau was reached at 20g of protein
      • The study fed people 10g, 20g, 30, and 90g of beef protein in young men and women and looked at plasma amino acids and muscle biopsies to see MPS
    • Note that 25 grams is the threshold and consuming more has no benefits
    • Some research has shown that high doses of protein can have the opposite effect
      • This is a phenomenon and is hypothesized in the paper as a result of inhibitory signaling involving the endoplasmic reticulum (ER) stress
        • The ER is a place in the cell where amino acids are put together making proteins
        • When this protein synthesis system is flooded, it shuts down
  • There is a lot of research looking at the best way to prolong new muscle synthesis (myofibril MPS)
    • One paper cited states that continues feeding of amino acids to the muscle for 8 hours is the best way
    • Since previous data shows over-flooding is bad, the logical deduction here is that eating smaller quantities of protein over the day is best for muscle growth
  • Evidence also suggests that diets higher in Essential Amino Acids (amino acids that our bodies can’t) make are better at triggering the synthesis of new muscle fiber
    • Leucine is an important Essential Amino Acid for muscle and is found in meat and plants
    • Eating a meal high in Essential Amino Acids after weight training is the best
  • Whole whey vs chocolate milk
    • Whey initiates Muscle Protein Synthesis faster
    • While casein (a protein component in milk) has less of an effect, Muscle Protein Synthesis (MPS) lasts longer but is not as robust
    • Fat content was found to effect casein absorbency and effectiveness: less fat is better

This article goes into the effects of aging quite a bit, mainly looking people 50+ yrs. Summary here is that the elderly need less protein — their bodies can’t absorb it. High protein diets are worse for the elderly as they are more strenuous on the kidneys.


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