GW BGZ 2024 Case 8 - A glass of (soy) milk before sleep

What is sarcopenia?

Sarcopenia is the progressive and generalized loss of skeletal muscle mass, strength, and function that occurs with aging. It affects both muscle quantity and quality, meaning that remaining muscle fibers often generate less force. It is considered a geriatric syndrome with multifactorial origins including metabolic, hormonal, nutritional, and neuromuscular factors.

Key consequences of sarcopenia include:

• Frailty

• Reduced mobility and walking speed

• Increased risk of falls and fractures

• Loss of independence and reduced quality of life

When does age-related muscle loss begin, and at what rate?

• Muscle loss starts gradually after age 30, with noticeable decline accelerating during the fourth or fifth decade.

• After age 50, muscle mass declines at an estimated rate of 0.5–1% per year, while strength declines faster at 1–2% per year.

• After age 60, both mass and strength losses accelerate.

Mechanism:

• Loss results from a chronic imbalance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB), with synthesis insufficient to counteract breakdown.

Contributing factors:

• Hormonal changes (decline in testosterone, growth hormone, IGF-1)

• Reduced physical activity

• Chronic inflammation

• Inadequate protein intake

• Neuromuscular changes

How does sarcopenia affect muscle strength and physical performance?

• Strength declines faster than mass, reflecting both reduced muscle fiber quality and neuromuscular impairments.

• Functional consequences include:

  • Reduced walking speed and balance

  • Difficulty performing daily activities (e.g., climbing stairs, rising from chairs, carrying objects)

  • Increased susceptibility to injury

• Severe sarcopenia contributes to frailty, hospitalization, and higher mortality risk.

What is the physiological basis of sarcopenia?

Sarcopenia develops due to multiple interacting mechanisms:

1. Chronic imbalance in muscle protein turnover:

   • Muscle is in constant turnover (~1–2% per day).

   • Aging shifts balance toward net protein breakdown, causing gradual muscle loss.

2. Anabolic resistance:

   • Aging muscles respond less efficiently to anabolic stimuli such as protein intake or resistance exercise.

   • Requires higher protein doses to achieve comparable muscle protein synthesis (MPS) to younger adults.

3. Reduced physical activity:

   • Less mechanical loading → decreased MPS, smaller fiber size, fat infiltration.

   • Immobilization accelerates muscle loss.

4. Hormonal changes:

   • Declines in testosterone, growth hormone, and IGF-1 reduce muscle maintenance and growth capacity.

5. Neuromuscular degeneration:

   • Loss of motor neurons → fiber denervation → atrophy or altered fiber composition.

6. Chronic low-grade inflammation (“inflammaging”):

   • Impairs anabolic signaling and protein synthesis, reduces muscle regenerative capacity.

7. Reduced dietary protein intake:

   • Due to lower appetite, dental issues, digestive problems, or socioeconomic factors, limiting amino acids for MPS.

What are the main mechanisms of muscle loss with aging?

1. Inadequate protein intake: Older adults often consume less protein than required.

2. Anabolic resistance: Reduced efficiency of muscles to use ingested protein for MPS.

3. Increased protein needs: Illness, chronic disease, and inflammation raise protein requirements.

4. Negative energy balance: Calorie deficits exacerbate protein breakdown.

5. Secondary contributors: Sedentary behavior, hormonal changes, chronic inflammation.

Bottom line: Aging and lifestyle factors tip the balance toward net muscle loss.

How does nutrition regulate muscle protein metabolism?

• Muscle mass is regulated by balance between MPS and MPB.

• Protein and amino acids:

  • Stimulate MPS (especially essential amino acids, notably leucine)

  • Leucine activates the mTOR pathway, a master regulator of protein synthesis

  • Insulin from meals suppresses MPB

• Protein dose and distribution:

  • Older adults require ~30–40 g protein per meal (vs. ~20 g for young adults)

  • 4–6 evenly distributed protein-rich meals per day maximize daily MPS

  • Pre-sleep protein intake supports overnight MPS

• Protein type:

  • Fast-digesting, leucine-rich proteins (whey) → highest MPS

  • Slow-digesting proteins (casein, soy) less effective unless fortified with leucine

How does exercise influence sarcopenia?

• Resistance exercise is the most effective intervention to stimulate MPS.

• Effects of resistance training:

  • Increases muscle fiber size and strength

  • Improves neuromuscular coordination

  • Enhances insulin sensitivity

  • Primes muscle to better respond to dietary protein (overcoming anabolic resistance)

• Post-exercise MPS remains elevated for 16–48 hours depending on training status.

• Synergistic effect when combined with protein ingestion results in greater net muscle protein accretion.

How can sarcopenia be reduced or prevented?

Primary strategies:

1. Adequate dietary protein intake:

   • 1.2–1.5 g protein/kg body weight/day for older adults

   • Spread evenly across 4–6 meals

   • Include leucine-rich proteins and consider pre-sleep protein

2. Resistance exercise:

   • Stimulates MPS and improves muscle quality and strength

   • Counteracts anabolic resistance

3. Maintain daily physical activity:

   • Avoid prolonged inactivity

   • Improves metabolic health and preserves muscle mass

Secondary strategies:

• Address chronic inflammation, hormonal imbalances, and underlying diseases

• Optimize nutrition (calories, micronutrients, hydration)

Key takeaway: Sarcopenia is multifactorial but can be slowed or partially reversed with combined nutritional and exercise interventions.

How does protein type and timing influence MPS in older adults?

• Protein type:

  • Fast-digesting, leucine-rich proteins (whey) maximize MPS

  • Slow-digesting proteins less effective unless fortified with leucine

• Protein timing:

  • Post-exercise protein intake enhances recovery and MPS

  • Pre-sleep protein provides amino acids during overnight fasting

  • Total daily protein distribution is more important than a single “anabolic window”

Optimal strategy: 4–6 evenly spaced protein-rich meals with 30–40 g protein per meal for older adults.

What is the role of leucine and mTOR in muscle protein synthesis?

• Leucine: Essential amino acid that acts as a signal and substrate for MPS.

• Mechanism:

  • Leucine activates the mTOR (mechanistic target of rapamycin) pathway, which regulates cell growth and protein synthesis.

  • mTOR activation leads to ribosomal activation and increased translation of muscle proteins.

• Other signals: Insulin and resistance exercise also activate mTOR.

• Practical implication: Leucine-rich proteins are essential to counteract anabolic resistance in older adults.

What is anabolic resistance?

Anabolic resistance is the reduced ability of skeletal muscle, particularly in older adults, to increase muscle protein synthesis (MPS) in response to anabolic stimuli, such as dietary protein intake, essential amino acids, and resistance exercise.

In younger individuals:

• Protein ingestion leads to a robust increase in MPS.

In older adults:

• The same protein intake produces a blunted MPS response.

• Higher protein doses are required to achieve maximal stimulation

What is the primary physiological manifestation of anabolic resistance?

The primary manifestation is a blunted stimulation of muscle protein synthesis (MPS) after protein or amino acid ingestion.

Key characteristics:

• Reduced postprandial (after-meal) increase in MPS

• Possibly reduced suppression of muscle protein breakdown (MPB)

• Higher amino acid threshold required to activate anabolic pathways

Importantly:

• The maximum capacity for MPS is often preserved if sufficient protein is consumed.

• The issue lies in reduced sensitivity, not total inability.

Thus, anabolic resistance is best described as a rightward shift in the dose-response curve of protein intake versus MPS.

How does anabolic resistance contribute to sarcopenia?

Anabolic resistance contributes to sarcopenia by promoting a chronic negative muscle protein balance over time.

Mechanism:

• Each meal stimulates less MPS than in younger individuals

• Daily protein intake becomes insufficient to maintain muscle mass

• Small deficits accumulate over years

Consequences:

• Progressive loss of muscle mass

• Decline in strength and physical function

• Increased risk of frailty and disability

Thus, anabolic resistance is a key physiological barrier to maintaining muscle mass with aging.

What are the main mechanisms underlying anabolic resistance?

Anabolic resistance is multifactorial, involving impairments at several levels:

1. Digestive and absorptive limitations

2. Reduced amino acid availability

3. Impaired muscle perfusion and delivery

4. Reduced insulin sensitivity

5. Defective intracellular signaling (mTOR pathway)

6. Physical inactivity

7. Chronic low-grade inflammation

8. Neuromuscular changes

These mechanisms interact across the entire pathway:
Digestion → absorption → circulation → delivery → uptake → intracellular signaling → protein synthesis

How do digestion and absorption contribute to anabolic resistance?

With aging, there may be impairments in protein digestion and amino acid absorption, leading to reduced systemic availability of amino acids.

Key points:

• Slower or less efficient breakdown of dietary protein

• Reduced absorption efficiency in the gut

• Increased splanchnic extraction, meaning more amino acids are retained in the gut and liver

Consequences:

• Fewer amino acids enter systemic circulation

• Reduced availability of amino acids for skeletal muscle

• Blunted stimulation of MPS

This is often described as amino acids “remaining in the gut” rather than reaching muscle tissue.

How does impaired muscle perfusion contribute to anabolic resistance?

Muscle perfusion refers to blood flow to skeletal muscle, which is essential for delivering amino acids and insulin.

In aging:

• Insulin-mediated vasodilation is reduced

• Capillary recruitment is impaired

• Blood flow to muscle after meals is diminished

Possible causes:

• Endothelial dysfunction

• Increased vasoconstrictors (e.g., endothelin-1)

Consequences:

• Reduced amino acid delivery to muscle

• Impaired nutrient uptake

• Blunted anabolic signaling

Thus, even if amino acids are present in circulation, they may not effectively reach muscle tissue.

What is the role of insulin resistance in anabolic resistance?

Insulin plays a permissive role in muscle protein metabolism by:

• Suppressing muscle protein breakdown (MPB)

• Facilitating nutrient uptake and blood flow

In aging:

• Insulin resistance develops

• Reduced ability to suppress MPB

• Impaired nutrient delivery and utilization

Consequences:

• Less favorable net protein balance

• Reduced efficiency of post-meal anabolic response

Although insulin does not strongly stimulate MPS directly, it is essential for maintaining overall protein balance.

How does impaired intracellular signaling contribute to anabolic resistance?

The most critical intracellular pathway is the mTOR (mechanistic target of rapamycin) signaling pathway, which regulates protein synthesis.

In young muscle:

• Amino acids (especially leucine) activate mTOR

• mTOR stimulates translation and MPS

In aging muscle:

• Reduced sensitivity of mTOR to amino acids

• Impaired activation of downstream targets (e.g., S6K)

• Possible reduction in ribosomal content

Consequences:

• Higher amino acid threshold needed to activate MPS

• Reduced efficiency of protein synthesis

This represents a central cellular defect in anabolic resistance.

What is the role of physical inactivity in anabolic resistance?

Physical inactivity is one of the most important and modifiable contributors to anabolic resistance.

Effects of inactivity:

• Reduced basal and postprandial MPS

• Impaired mTOR signaling

• Decreased amino acid transport into muscle

• Reduced insulin sensitivity

• Diminished muscle blood flow

Even short-term inactivity (e.g., reduced daily steps, bed rest) can:

• Rapidly induce anabolic resistance

• Accelerate muscle loss

Key insight:

• Anabolic resistance is not solely due to aging, but strongly influenced by inactivity

• Inactivity can mimic or accelerate the aging process in muscle

How does chronic inflammation contribute to anabolic resistance?

Aging is associated with chronic low-grade inflammation (“inflammaging”), characterized by elevated cytokines.

Effects on muscle:

• Impairs mTOR signaling

• Reduces muscle protein synthesis

• Promotes protein breakdown

• Inhibits muscle regeneration and satellite cell activity

Consequences:

• Reduced anabolic responsiveness

• Impaired recovery and adaptation

Inflammation creates a catabolic environment that opposes muscle maintenance.

Can anabolic resistance be reversed?

Anabolic resistance cannot be completely reversed, as it is partly driven by intrinsic aging processes.

However, it can be significantly attenuated or partially overcome through targeted interventions.

Key concept:

• Aging reduces sensitivity, but capacity for muscle growth remains under optimal conditions

Thus, appropriate lifestyle and nutritional strategies can restore much of the anabolic response.

What nutritional strategies help reduce anabolic resistance?

• Higher protein intake per meal:

  • ~0.4 g/kg body weight per meal

  • Ensures sufficient amino acid stimulus

• Higher total daily protein intake:

  • ≥1.2 g/kg/day

• Leucine-rich proteins:

  • Leucine activates mTOR and stimulates MPS

  • Whey protein is particularly effective

• Protein distribution:

  • Evenly spaced meals maximize repeated MPS stimulation

• Pre-sleep protein ingestion:

  • ~40 g protein supports overnight MPS

• Use of rapidly digestible proteins:

  • Improves amino acid availability and peak MPS response

How does resistance exercise reduce anabolic resistance?

Resistance exercise is the most effective intervention to improve anabolic sensitivity.

Mechanisms:

• Activates mTOR signaling

• Increases muscle blood flow

• Enhances amino acid transport into muscle

• Improves insulin sensitivity

• Increases ribosomal capacity

Effects:

• Restores responsiveness to protein intake

• Enhances MPS for up to 24 hours post-exercise

Key concept:

• Exercise primes muscle to respond more effectively to amino acids

Why is the combination of exercise and protein intake important?

Exercise and protein intake have a synergistic effect:

• Exercise:

  • Increases sensitivity of muscle to amino acids

• Protein intake:

  • Provides the building blocks for protein synthesis

Together:

• Maximize MPS

• Improve net protein balance

• Promote muscle maintenance and growth

Exercise alone is insufficient without amino acids, and protein alone is less effective without the stimulus provided by exercise.

What are the main physiological effects of protein supplementation in older adults?

• Stimulates muscle protein synthesis (MPS)

• Improves net protein balance (↑ MPS vs MPB)

• Helps overcome anabolic resistance

• Maintains or increases lean body mass

• Improves muscle strength and function

• Reduces risk/progression of sarcopenia

• Supports recovery and muscle remodeling

• Useful when dietary protein intake is insufficient

Why is whey protein particularly effective for older adults?

• Rapid digestion → quick rise in blood amino acids

• High in essential amino acids (EAAs)

• Very rich in leucine → strong mTOR activation

• Produces greater MPS response vs other proteins

• Helps overcome anabolic resistance

• Convenient and easy to consume (important with low appetite)

How does casein protein differ from whey protein?

• Slow digestion → gradual amino acid release

• Lower peak MPS compared to whey

• Better for sustained amino acid availability

• Useful for pre-sleep protein intake

• Less effective alone for overcoming anabolic resistance

What is the role of leucine in protein supplementation?

• Key essential amino acid for MPS

• Activates mTOR pathway

• Acts as a signal + substrate

• Higher leucine needed in older adults

• Helps overcome anabolic resistance

• Found in high amounts in whey protein

What is the recommended daily protein intake for older adults?

• General recommendation:

  • ≥1.2 g/kg body weight/day

• Higher needs in:

  • Illness, inflammation, or injury → up to 1.5 g/kg/day

• Higher than young adults (~0.8 g/kg/day)

• Needed to compensate for anabolic resistance

Why is protein distribution across the day important?

• ~0.4 g/kg per meal

• Typically 30–40 g protein per meal

• Higher than young adults (~20 g)

• Required to maximize MPS

• Helps overcome anabolic resistance thresholdMPS is stimulated per meal, not continuously

• Even distribution → multiple MPS peaks

• Prevents long periods of low amino acid availability

• Example: 3–5 protein-rich meals/day

• More effective than skewed intake (e.g., protein only at dinner)

What is the role of pre-sleep protein intake?

• Provides amino acids during overnight fasting

• Supports overnight MPS

• Reduces muscle breakdown at night

• Typical recommendation: ~40 g protein before sleep

• Often uses casein (slow digestion)

What types of proteins are there (amino acid classification)?

Essential amino acids (EAAs):

• Cannot be synthesized → must come from diet

• 9 EAAs: leucine, isoleucine, valine, lysine, methionine, phenylalanine, threonine, tryptophan, histidine

• Leucine = key trigger of MPS (via mTOR)

• Required for muscle protein synthesis (MPS)

Non-essential amino acids (NEAAs):

• Can be synthesized by body

• Examples: alanine, glycine, glutamine, serine

• Important for metabolism but do not stimulate MPS alone

What is protein quality?

• Based on:

  • EAA content (especially leucine)

  • Digestibility

• High-quality protein:

  • High EAAs

  • Easily digestible

  • Strong MPS stimulation

• Animal proteins:

  • Higher quality

  • 85–95% digestible

• Plant proteins:

  • Lower quality (on average)

  • 50–75% digestible

  • Due to fiber & anti-nutritional factors

What is the difference between animal and plant proteins?

Animal proteins:

• High EAAs (complete proteins)

• High leucine

• High digestibility (85–95%)

• Strong MPS response

Plant proteins:

• Lower EAA content (often incomplete)

• Limiting amino acids:

  • Lysine (grains)

  • Methionine (legumes)

• Lower digestibility (50–75%)

• Weaker MPS per gram

Why do plant proteins stimulate less MPS?

• Lower leucine content

• Lower total EAAs

• Presence of anti-nutritional factors:

  • Fiber

  • Phytates

  • Tannins

• Slower digestion & absorption

• Higher splanchnic extraction

How can plant protein quality be improved?

• Increase total protein intake

  • ~10–20% more needed

• Combine proteins (complementary):

  • Grains (low lysine) + legumes (high lysine)

• Choose high-quality plant proteins (soy, pea)

• Use protein isolates/concentrates:

  • ↑ digestibility (~90%)

  • ↓ anti-nutritional factors

• Fortify with amino acids:

  • Leucine

  • Lysine

  • Methionine

• Processing methods (to increase digestibility):

  • Fermentation

  • Hydrolysis

  • Heating

What is the difference between vegan, vegetarian, and omnivorous diets?

Vegan diet:

• No animal products

• Protein: legumes, soy, grains, nuts

• Requires careful planning (EAAs)

Vegetarian diet:

• No meat/fish

• May include:

  • Dairy (lacto)

  • Eggs (ovo)

• Better protein quality than vegan

Omnivorous diet:

• Includes plant + animal foods

• High protein quality

• Usually no planning needed

What is muscle reconditioning and how does muscle exhibit plasticity?

• Muscle reconditioning refers to the ability of skeletal muscle to adapt to environmental, lifestyle, or physiological conditions, either positively (hypertrophy, increased function) or negatively (atrophy, decreased function).

• Muscle is highly plastic due to constant protein turnover, which involves breakdown and synthesis of proteins at a daily rate of 1–2% of total muscle protein.

• This turnover allows adaptation to stimuli such as:

  • Resistance or endurance exercise

  • Nutritional intake (amino acids, protein)

  • Hormonal changes

• Plasticity is critical for maintaining functional capacity, metabolic health, and recovery after illness or immobilization.

What is muscle deconditioning and which conditions contribute to it?

• Muscle deconditioning occurs when muscle adapts negatively due to inactivity, disease, or aging.

• Causes include:

  • Immobilization: casting, bed rest

  • Sarcopenia: age-related loss of muscle mass and strength

  • Cancer cachexia: rapid muscle loss due to cancer treatment or tumor metabolism

  • Chronic diseases: COPD, cardiovascular disease, type 2 diabetes (affects muscle quality, mass, and glucose uptake)

• Muscle deconditioning results in decreased functional capacity, insulin resistance, and reduced quality of life.

Do carbohydrates or fats affect muscle protein synthesis?

• Carbohydrates: increase insulin but do not stimulate muscle protein synthesis directly.

• Fat: may delay gastric emptying and amino acid absorption, but no direct effect on synthesis itself.

• Protein is the primary driver of anabolic response; timing relative to exercise is crucial.

What is the catabolic crisis model?

• Episodes of reduced physical activity (illness, hospitalization) cause rapid muscle loss.

• Post-crisis, the muscle may not fully recover to pre-event levels → progressive decline in functional capacity over time.

• Highlights the importance of prevention, early mobilization, and rehabilitation.

Take home messages

• You are what you ate and if you are physically active you are more of what you just ate (athletes)

• Opposite in care

• Protein ingestion and muscle contraction stimulate muscle tissue protein synthesis

• Physical (in)activity (de)sentisizes skeletal muscle tissue to the anabolic properties of dietary protein ingestion

• Protein is required to support muscle conditioning in both health and disease