Are Protein Requirements Different for Aging Athletes Compared to Younger Athletes?

We are all well aware of the importance of protein intake as a means to help repair, regenerate and build muscle tissue, as protein provides the essential building blocks for the remodeling of muscle and body proteins.

Evidence has clearly established that daily protein requirements for any type of athlete are elevated above the current minimum RDA of 0.8g/kg/day across the spectrum of sport disciplines.¹ 

However, most of the research that has contributed to these increased recommendations was performed in active young athletes (18–35 years).

The aging of these younger athletes and a greater awareness of an active lifestyle to support successful aging has contributed to an expansion of athletes above the age of 35 years ², which is the typical delineation for 'Master athlete'.

Therefore, there is a need to clearly define the nutrient requirements of these older athletes to support optimal training, adaptation, and health.

Muscle mass, strength, and aerobic capacity generally peak in early adulthood (<30 years) after which they begin a slow decline (~0.5 – 1%/year) that can accelerate in the 8^th decade. The high levels of physical activity that are typical of Master athletes tend to spare them from the normal loss of muscle and fitness that occurs in their sedentary peers.³

A recent review of the literature showed that Master athletes’ aerobic capacity, muscle strength, and body composition are generally on par with young, untrained adults.⁴ 

While there is an inevitable decline in exercise capacity and performance with age, Master athletes are generally regarded as models of successful aging.³

Therefore, it’s not totally clear whether other consequences of aging that may impact protein requirements are observed in Master athletes.

In this article, I’ll summarize the available evidence on the primary factors associated with typical age-related anabolic resistance and discuss to what extent they may be present in Master athletes. 

Anabolic Resistance of Aging 

Decrements in muscle size and function due to aging are multifactorial and could potentially include decreases in neuromuscular function, motor unit/muscle fiber loss (especially type II fibers), muscle stem cell dysregulation, and/or atrophy of skeletal muscle, the latter of which is largely supported by an “anabolic resistance” to dietary protein. 

Anabolic resistance is characterized by a blunted muscle protein synthetic response to a bolus (i.e. meal) dietary amino acid ingestion at rest ⁵ and after resistance exercise in older untrained adults.⁶ 

As an example, an estimated average meal protein requirement of 0.4 g/kg of high-quality, leucine-enriched protein (i.e. whey) is required to maximize protein synthesis at rest in untrained older adult. This is approximately 65% greater than for younger adult.⁵ 

Similarly, 20 g of high-quality protein (i.e., whey) is sufficient to maximally increase protein synthesis after exercise in young, trained men, whereas older, untrained adults require up to 40g.⁷ 

Thus, this anabolic resistance to suboptimal meal protein intakes has contributed to the recommendations that older untrained adults require a greater meal protein intake to maximize muscle protein synthesis at rest ⁸ and after resistance exercise.⁹ 

Anabolic resistance to dietary amino acids is multifactorial and could include:

• - Chronic inflammation
• - Reduced capillarity and/or vasodilatory capacity (and hence amino acid delivery)
• - Attenuated muscle uptake (e.g., amino acid transporter expression/function)
• - Reduced ribosomal protein content or activity

Amino Acid Delivery

There is a blunted muscle protein synthetic response to exogenous amino acids due to aging.  The age-related decline in muscle capillarity seems to be influenced by habitual activity as training can induce angiogenesis (i.e., development of new blood vessels) in older adults ¹⁰ whereas inactivity may cause decrements in muscle capillarity.

In addition, as little as 45 mins of moderate paced walking can improve insulin-induced nutritive blood flow (i.e. microvascular perfusion), which is concomitant with an increase in intramuscular anabolic signaling and muscle protein synthesis to exogenous insulin and/or amino acids.¹¹ 

Collectively, these data highlight the important role for habitual activity in capillarity and microvascular responsiveness to dietary nutrients.

Given Master athletes would have training volumes that surpass their untrained peers and possess capillary networks that are on par with their younger contemporaries ¹², it is unlikely that nutrient (insulin and amino acids) delivery to skeletal muscle is diminished in these athletes.¹³

Amino Acid Uptake

Amino acid uptake into skeletal muscle has been suggested to represent a locus of control for skeletal muscle anabolism that may be compromised in older muscle.¹⁴ 

Uptake of amino acids is facilitated by transmembrane proteins that may also act as “transceptors” as a feedforward mechanism to enhance protein synthesis.¹⁵

There is a lack of understanding for the potential role for these transporters in any age-related anabolic resistance.

Although research in Master athletes are lacking, the observation that inactivity can reduce amino acid transporter expression ¹⁶, whereas activity can increase its expression in young and older adults could collectively suggest the presence of sufficient amino acid transporters in Master athletes. 

This in fact, would not represent a rate-controlling step for post-exercise anabolism in concurrently training Master athletes.¹³

Inflammation 

Aging may be linked with chronic subclinical systemic and intramuscular inflammation, which is thought to contribute to the anabolic resistance associated of aging.¹⁷

However, both inactivity and/or obesity may worsen this inflammatory situation, highlighting potential modifiable risk factors for its etiology. For example, a very interesting finding demonstrated that combined aerobic and resistance exercise, but not weight loss alone, decreases the intramuscular expression of inflammatory markers in frail obese older adults.¹⁸

Therefore, it’s not surprising that Master athletes have similar resting levels of inflammatory markers as young adults and experience attenuated exercise-induced increases in some of these markers compared to untrained older adults.¹⁹

Essentially, chronic intramuscular inflammation (potentially linked to anabolic resistance) is not likely to be present in Master athletes.  

Do active older adults have reduced post-exercise Anabolic Resistance? 

There is no research investigating the post-exercise ingested bolus protein dose–response in Master athletes. However, some evidence for a reduced anabolic resistance after exercise may be drawn from populations of older adults who presumably have different levels of habitual physical activity or reside in communities that may be more conducive to greater levels of non-exercise adaptive thermogenesis (e.g. walking or cycling for transport).

In direct comparisons of the anabolic response of muscle protein synthesis to exogenous amino acids in trained older adults, mixed muscle protein synthesis is ~20% greater after 12 weeks of resistance training in older adults.²⁰

Therefore, available evidence from acute dose–response studies, comparisons of trained and untrained older adults, and the general ‘young’ muscle phenotype of Master athletes would suggest they are unlikely to suffer from the typical age-related anabolic resistance, which incidentally is thought to be exacerbated by physical inactivity.²¹

In light of this evidence, I will focus the rest of this discussion on the current understanding of protein requirements in young adults as a reference point for older athletes. 

Resistance Exercise Protein Requirements

Current recommendations for young, resistance trained males is ~ 20 g of high-quality (i.e. leucine and essential amino acid-enriched) protein to maximize post-exercise rates of mixed muscle protein synthesis and myofibrillar protein synthesis.⁷

Due to the fact that there is little evidence for the presence of ‘anabolic resistance’ in Master athletes, a relative protein intake of ~0.3 g/kg post-resistance exercise represents a suitable target for older athletes to enhance muscle protein remodeling.²²

Fig: Schematic representation of the activity-related differences in muscle morphology a between Master athletes and older untrained adults that would influence the rested or post-exercise increase in muscle protein synthesis after meal protein ingestion (b). ‘Anabolic resistance’ is a rightward shift in the protein dose–response curve that is characteristic of an inactive lifestyle, especially with aging. Shaded area represents the meal protein intake that would maximize muscle protein synthesis yet minimize dietary amino acid oxidation in Master athletes (current)

The main goal of most strength athletes is to maximize muscle mass and quality. Therefore, nutrition practices for these individuals would ideally support enhanced tissue remodeling (i.e. breakdown and synthesis of muscle protein) and a positive net protein balance.

Therefore, the repeated consumption of mixed macronutrient meals with an adequate quantity of protein to support myofibrillar protein synthesis and carbohydrate to support training demands would be the cornerstone of their nutrition plan.

This mirrors the practice of many younger elite athletes who consume 4-5 meals per day. ²³  

In addition, balancing your distribution of protein is ideal.  Research shows that a balanced distribution of protein intake as compared to a skewed intake supports moderately greater gains in lean body mass during resistance training in younger adults.²⁴ 

The recommendation of repeated ingestion of moderate-protein containing meals every ~4 hrs (~4-5 meals/day) to optimize muscle mass in trained populations would also extend to Master athletes. Although muscle mass does not necessarily translate to greater muscle strength, it’s noteworthy that even small differences in daily protein intake (i.e., ~1.21 vs. ~1.34 g/kg/day) may be associated with detectable differences in muscle quality in Master athletes.²⁵ 

Endurance Exercise Protein Requirements

Endurance training has consistently been shown to increase daily protein requirements.  This is  due in part to the requirement to replenish exercise-induced amino acid oxidative losses.²⁶

Important research using nitrogen balance that included both younger (~ 27 years) and middle aged (~ 52 years) athletes reported identical requirements of ~ 1.2 g/kg/day.²⁷ 

This tells us that protein requirements in younger Master athletes are not substantially different from athletes half their age. Recent evidence highlights that protein requirements are towards the upper range of the current sports nutrition guidelines.¹ 

An intake of ~ 1.7 to 1.8 g/kg/day, which represents a modest ~ 15% of total energy, is generally met in younger athletes provided they consume adequate energy. This amount may potentially not be reached in some Master athletes Middle aged (~ 52 years) ²⁸ and older (~ 65 years) ²⁹ trained adults have been reported to have a greater contribution of fat to total energy expenditure than younger athletes. 

Thus, in view of the fact that this greater relative fat oxidation would also be accompanied by a reduction in amino acid oxidative losses, this would further suggest Master athletes would be unlikely to have requirements that are greater than younger athletes. I think after examining the current evidence, a protein intake of ~1.8 g/kg/day is ideal for hard training endurance Master athletes. 

Bone health & Protein Intake

As we age, our bones are predispose to an increased risk of fracture due to the gradual loss of bone density and strength.

Physical activity associated with the training of many Master athletes (especially those performing strength and power training) helps maintain bone mineral density.³⁰ 

In addition, there is also evidence that a daily protein intake greater than the current RDA of 0.8 g/kg/day is associated with greater bone mineral density ³⁰, especially when consumed with adequate calcium and potentially vitamin D intake.

Master athletes adhering to the current guidelines (as mentioned above) for protein intake would meet their needs for maintaining bone health. Potential caveats could be to ensure Master athletes also consume adequate calcium (i.e. > 600 mg/day), vitamin D (> 800 IU/day), and alkaline salts, the latter of which would help regulate acid–base balance, a risk factor for bone loss with age.³¹ 

Alkaline salts can be found in a balanced diet enriched with fruits and vegetables.

Summary

Master athletes, just like their younger counterparts; should ensure their nutrition plans include adequate dietary protein intake to support their training and recovery.

Fortunately, current evidence does not support the need for higher protein intakes than what is currently recommended for younger athletes.

The recommendation of increasing protein requirements in older adults to offset any age-related anabolic resistance is of little relevance to highly active Master athletes.

The ideal situation is for Master athletes to apply a dietary approach that targets an optimal meal protein intake and frequency to efficiently stimulate muscle protein remodeling.

In addition, other important aspects of recovery such as optimal carbohydrate and energy intake should also be considered. Refer to the table below for a summation of protein intake recommendations for endurance and resistance training Master athletes.

Table: Recommended meal protein intakes for Master athletes (adapted from ¹³)

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