Changes in muscle mass are expected throughout the lifespan. However, after about 50 years of age, muscle mass decreases by 1 to 2% per year. Sarcopenia is a progressive skeletal muscle disorder characterized by significant loss of muscle mass, strength, and function. It is diagnosed in 5-13% of adults between 60 and 70 years and the risk rate increases to 11-50% for those 80 years and over. (1, 2) Basic activities like getting out of a chair can be challenging for people with sarcopenia.

Pathogenesis of Sarcopenia

There are two classes of sarcopenia: primary and secondary. Primary sarcopenia is age-related loss of muscle mass. Secondary sarcopenia is disease-mediated loss of muscle mass and function. (3) For this article, we’re going to focus on primary or age-related sarcopenia.

The underpinning pathophysiology of primary sarcopenia is still unclear, but many influences, in addition to aging, are worth considering. (4) These can include: inflammation and increased cytokines, decreased neuronal motor units, decreased vascular supply, decreased growth hormone and IGF-1, decreased testosterone, and decreased DHEA. (3) Notably, calorie and protein intake, oxidative stress, vitamin D status, and physical activity are also involved in sarcopenia development. (2)

Nutrition Assessment

It is important to recognize clinical symptoms associated with sarcopenia as a part of a comprehensive nutrition assessment. The SARC-F questionnaire is a five-question screening tool related to strength, assistance in walking, rising from a chair, climbing stairs, and falls. Questions from the SARC-F include:

  • How much difficulty do you have lifting and carrying 10 pounds?
  • How much difficulty do you have walking across a room?
  • How much difficulty do you have transferring from a chair or bed?
  • How much difficulty do you have climbing a flight of 10 stairs?
  • How many times have you fallen in the past year?

Calf circumference and grip strength are also core measures of sarcopenia. (3) Consider laboratory testing, including nutrient levels (amino acids, Vitamin D, etc.), organic acids for oxidative stress markers, cytokine, and hormone panels.

 

Nutritional Strategies

Let’s now look at specific nutrients and functional nutrition interventions for individuals with sarcopenia.

Total calorie intake

A large body of evidence links overall poor nutrient intake to muscle loss in older adults. Insufficient food intake is connected to a drop in sensory perception, poor oral health, adverse effects of medications, social isolation, and an overall reduction in appetite, also known as “anorexia of aging.” All of the above combined can lead to weight loss, decreased body fat, and loss of muscle mass from muscle catabolism. (5,6,7)

A lack of diet consistency and nutrient diversity is also a potential risk factor for muscle loss and sarcopenia. (8) More diet education around the impact of whole-food nutrition and healthy living patterns could impact many facets of health and the aging process, muscle health included.

Protein Intake

Research suggests that protein requirements are higher in older adults. A 2015 study evaluated the differences in protein intake in women over 65 with sarcopenia (n = 35) and women without sarcopenia (n = 165). The results showed that muscle mass was more significant in women who consumed at least 1.2g/kg protein per day. (9) However, meeting total protein and calorie targets can be challenging for the aging population as appetite and senses, like taste and smell, decline. According to Yanai H, “protein intake and physical activity are the main anabolic stimuli for muscle protein synthesis.” (10).

Leucine, in particular, is a vital branched-chain amino acid required for muscle protein synthesis (MPS). (11) In fact, leucine is considered the master dietary regulator of muscle protein turnover. Consuming leucine-rich protein sources has the most significant influence on muscle mass preservation and function in older age. (12) Examples of leucine-rich foods include:

  • Whey protein
  • Chicken
  • Beef
  • Pork
  • Salmon
  • Lentils

Beta-hydroxy-Beta-methylbutyrate (HMB), a metabolite of the amino acid leucine, positively affects muscle mass and strength. Studies show that HMB stimulates mTOR anabolic pathways and down-regulates muscle catabolic pathways, thus promoting muscle protein synthesis. HMB supplementation has been studied in different clinical conditions where muscle wasting is expected, like bed-ridden illnesses, cancer cachexia, and HIV. Some studies show small yet promising effects with HMB supplementation on muscle mass in aging people, even in the absence of exercise. A dose of 3g of HMB per day is recommended to maintain or improve muscle function with minimal to no known adverse effects. (13,14,15,16)

It is also important to consider changes in protein digestion and absorption in the aging population. A 2020 meta-analysis examined how protein type, protein dose, and age specifically impact dietary protein digestion and plasma phenylalanine availability. The study found reduced protein digestion and phenylalanine absorption in older men compared to younger men. Gorissen et al. explained that “such a seemingly small impairment in dietary protein digestion and absorption with each meal at an older age could have a substantial impact on muscle mass maintenance in the long term.” As stated before, eating higher amounts of protein per meal is recommended with age. Additionally, choosing milk proteins, like whey and casein, can enhance postprandial plasma phenylalanine availability. (15)

Several other studies have evaluated the connection between protein intake and muscle mass in the aging population (15, 16). Based on the data so far, protein intake of at least 1.0-1.2 g/kg body weight per day is recommended with a minimum of 25-30 grams protein at mealtimes. (14, 17) Higher doses of protein (1.5-2.0g/kg) may be indicated in some individuals, especially when there is evidence of a catabolic state. (3)

Omega 3’s and Antioxidants

As functional nutrition practitioners, we know that oxidative stress and inflammation can cause a cascade of cellular damage. In the case of sarcopenia, higher reactive oxygen species (ROS) and lower dietary antioxidant intake can interrupt signaling pathways (e.g., MPS pathway) and lead to damaged muscle tissue. An epidemiological study called the Invecchiare in Chianti, or InCHIANTI study found that lower blood concentrations of selenium, carotenoids, and alpha-tocopherols were associated with lower muscle strength. However, there’s inconclusive repeatable evidence to show how various antioxidants play a role in muscle health and sarcopenia. (4)

Visceral fat and low-grade inflammation play a role in many age-related conditions and may also impact sarcopenia. While the data is still being studied and extrapolated, it’s hypothesized that long-chain polyunsaturated fats, omega-3s in particular, help maintain muscle strength. (4) Regardless of the evidence to date, encouraging intake of exogenous antioxidants and omega-3 fatty acids via whole foods is a benign recommendation with more pros than cons. Furthermore, a comprehensive nutritional profile &/or Omega 3- Index, both discussed in IFN Academy training and available through Rupa Health, could help guide essential fatty acid and antioxidant recommendations.

Vitamin D

Vitamin D deficiency is common across the lifespan, despite gender and race. Beyond poor bone health, low vitamin D 25(OH) is connected to insufficient muscle strength in the aging population. According to Bauer et al., “vitamin D supplementation specifically for sarcopenia was found to have insufficient evidence, though there is evidence that persons with low vitamin D levels may improve their strength with vitamin D supplementation.” (3) Monitoring vitamin D status in the aging population is critical, especially in those at high risk of sarcopenia. (4,18)

Nutrition and Lifestyle Recommendations

To date, a specific diet hasn’t been extensively studied for sarcopenia, but our clinical knowledge as functional nutrition practitioners can undoubtedly point us in the right direction. Below are five basic nutrition recommendations for muscle support in the aging population. Consider using the acronym “STAIN” taught extensively throughout IFNA to dive deeper into root causes and possible contributors.

  1. Focus on high-quality protein at mealtimes, ideally achieving 25-30 grams of protein per meal. Smaller meals may need to be divided throughout the day, depending on the individual.
  2. Fill in the gaps with higher protein options like hard-boiled eggs, Greek yogurt with berries and seeds, or a protein fruit smoothie.
  3. Increase whole fruits and non-starchy vegetable intake for antioxidant and gut microbiota support.
  4. Supplement strategically to achieve nutrient optimization. Consider a whey protein supplement, if tolerated. Whey protein is an excellent source of leucine required for MPS. HMB is also a supplement of consideration.
  5. Combine whole food, consistent nutrition with regular exercise, specifically resistance and strength training. If willing and able, consider working alongside a certified personal trainer to create a strength training program to parallel with a nourishing diet.

 

Conclusion

As we know, it’s far more effective to develop healthy and consistent nutrition and lifestyle habits early in life to reduce the risk of disease. This is why our role as nutrition practitioners and educators is essential throughout the lifespan. Muscle mass maintenance and support, in particular, change with age and is influenced mainly by overall diet quality and quantity. In the case of sarcopenia, nutrition intervention plays a pivotal role in slowing the condition’s progression. Considering total calorie, antioxidant, and protein intake, with an emphasis on leucine and HMB, is essential for people with sarcopenia. Additional labs and biometrics, like grip strength, calf circumference, organic acids, and inflammatory markers, are also helpful in this population. Most importantly, collaborating with other healthcare professionals to create a patient-centered care plan can help support quality of life and reduce other risk factors associated with sarcopenia.

To learn more about a functional nutrition approach, register for your IFNA training today!

by Tori Eaton, MS, RD, IFNCP


References

  1. Haehling S, Morley J, Anker S. An overview of sarcopenia: facts and numbers on prevalence and clinical impact. J Cachexia Sarcopenia Muscle. 2010;1(2):129-133. doi: 10.1007/s13539-010-0014-2
  2. Cruz-Jentoft AJ, Bahat G, Bauer J, et al. Sarcopenia: revised European consensus on definition and diagnosis. Age Ageing. 2019; 48(1): 16–31. doi: 10.1093/ageing/afy169
  3. Bauer J, Morley JE, Schols AMWJ, et al. Sarcopenia: a time for action. An SCWD position paper. J Cachexia Sarcopenia Muscle. 2019;10(5):956-961. doi: 10.1002/jcsm.12483.
  4. Robinson S, Granic A, Sayer A. Nutrition and muscle strength, as the key component of sarcopenia: an overview of current evidence. Nutrients. 2019;11(12), 2942. https://doi.org/10.3390/nu1112294
  5. Mathieu ME, Reid R, King N. Sensory profile of adults with reduced food intake and the potential roles of nutrition and physical activity interventions. Adv Ntr. 2019;10(6):1120-1125. doi: https://doi.org/10.1093/advances/nmz044
  6. Kiesswetter E, Poggiogalle E, Migliaccio S, et al. Functional determinants of dietary intake in community-dwelling older adults: A DEDIPAC (DEterminants of DIet and Physical ACtivity) systematic literature review. Health Nutr. 2018;21(10):1886-1903.doi:10.1017/S1368980017004244
  7. Cox N, Ibrahim K, Sayer A, et al. Assessment and treatment of the anorexia of aging: a systematic review. Nutrients. 2019;11(1):144. doi:https://doi.org/10.3390/nu11010144
  8. Bloom I, Shand C, Cooper C. Diet quality and sarcopenia in older adults: a systematic review. Nutrients. 2018;10(3):308. doi:10.3390/nu10030308
  9. Yanai H. Nutrition for sarcopenia. J Clin Med Res. 2015; 7(12): 926–931. doi: 10.14740/jocmr2361w
    Celis-Morales CA, Petermann F, Steell L, et at. Associations of dietary protein intake with fat-free mass and grip strength: a cross-sectional study in 146,816 UK biobank participants. Am J Epidemiol. 2018;187(11):2405-2414. doi: 10.1093/aje/kwy134
  10. Dorner B, Posthauer MB. Nutrition’s role in sarcopenia prevention. Today’s Dietitian. 2012;14(9):62
  11. Coelho-Júnior HJ, Milano-Teixeira L, Rodrigues B, et al. Relative protein intake and physical function in older adults: a systematic review and meta-analysis of observational studies. Nutrients. 2018;10(9):1330. doi: 10.3390/nu10091330.
  12. Gorissen S, Trommelen S, Kouw I , et al. Protein type, protein dose, and age modulate dietary protein digestion and phenylalanine absorption kinetics and plasma phenylalanine availability in humans. J Nutr. 2020;150(8):2041-2050. doi: 10.1093/jn/nxaa024. 
  13. Fanelli Kuczmarski, M, Pohlig, RT, Stave Shupe E, et al. Dietary protein intake and overall diet quality are associated with handgrip strength in African American and white adults. J. Nutr. Health Aging. 2018;22:700–709.doi: https://doi.org/10.1007/s12603-018-1006-8
  14. Holecek M. Beta-hydroxy-beta-methylbutyrate supplementation and skeletal muscle in healthy and muscle-wasting conditions. J Cachexia Sarcopenia Muscle. 2017;8(4). https://doi.org/10.1002/jcsm.12208
  15. Bear D, Langan A, Dimidi E, et al. β-Hydroxy-β-methylbutyrate and its impact on skeletal muscle mass and physical function in clinical practice: a systematic review and meta-analysis. AJCN. 2019;109(4). https://doi.org/10.1093/ajcn/nqy373
  16. Din USU, Brook MS, Selby A, Quinlan J, et al. A double-blind placebo controlled trial into the impacts of HMB supplementation and exercise on free-living muscle protein synthesis, muscle mass and function, in older adults. Clin Nutr. 2019;38(5):2071-2078. https://doi.org/10.1016/j.clnu.2018.09.025
  17. Paddon-Jones D, Rasmussen BB. Dietary protein recommendations and the prevention of sarcopenia. Curr Opin Clin Nutr Metab Care. 2009;12(1):86–90. doi: 10.1097/MCO.0b013e32831cef8b.
  18. Remelli F, Vitali A, Zurlo A, Volpato S. Vitamin D deficiency and sarcopenia in older persons. Nutrients. 2019;11(12):2861. doi:10.3390/nu11122861

Changes in muscle mass are expected throughout the lifespan. However, after about 50 years of age, muscle mass decreases by 1 to 2% per year. Sarcopenia is a progressive skeletal muscle disorder characterized by significant loss of muscle mass, strength, and function. It is diagnosed in 5-13% of adults between 60 and 70 years and the risk rate increases to 11-50% for those 80 years and over. (1, 2) Basic activities like getting out of a chair can be challenging for people with sarcopenia.

Pathogenesis of Sarcopenia

There are two classes of sarcopenia: primary and secondary. Primary sarcopenia is age-related loss of muscle mass. Secondary sarcopenia is disease-mediated loss of muscle mass and function. (3) For this article, we’re going to focus on primary or age-related sarcopenia.

The underpinning pathophysiology of primary sarcopenia is still unclear, but many influences, in addition to aging, are worth considering. (4) These can include: inflammation and increased cytokines, decreased neuronal motor units, decreased vascular supply, decreased growth hormone and IGF-1, decreased testosterone, and decreased DHEA. (3) Notably, calorie and protein intake, oxidative stress, vitamin D status, and physical activity are also involved in sarcopenia development. (2)

Nutrition Assessment

It is important to recognize clinical symptoms associated with sarcopenia as a part of a comprehensive nutrition assessment. The SARC-F questionnaire is a five-question screening tool related to strength, assistance in walking, rising from a chair, climbing stairs, and falls. Questions from the SARC-F include:

  • How much difficulty do you have lifting and carrying 10 pounds?
  • How much difficulty do you have walking across a room?
  • How much difficulty do you have transferring from a chair or bed?
  • How much difficulty do you have climbing a flight of 10 stairs?
  • How many times have you fallen in the past year?

Calf circumference and grip strength are also core measures of sarcopenia. (3) Consider laboratory testing, including nutrient levels (amino acids, Vitamin D, etc.), organic acids for oxidative stress markers, cytokine, and hormone panels.

 

Nutritional Strategies

Let’s now look at specific nutrients and functional nutrition interventions for individuals with sarcopenia.

Total calorie intake

A large body of evidence links overall poor nutrient intake to muscle loss in older adults. Insufficient food intake is connected to a drop in sensory perception, poor oral health, adverse effects of medications, social isolation, and an overall reduction in appetite, also known as “anorexia of aging.” All of the above combined can lead to weight loss, decreased body fat, and loss of muscle mass from muscle catabolism. (5,6,7)

A lack of diet consistency and nutrient diversity is also a potential risk factor for muscle loss and sarcopenia. (8) More diet education around the impact of whole-food nutrition and healthy living patterns could impact many facets of health and the aging process, muscle health included.

Protein Intake

Research suggests that protein requirements are higher in older adults. A 2015 study evaluated the differences in protein intake in women over 65 with sarcopenia (n = 35) and women without sarcopenia (n = 165). The results showed that muscle mass was more significant in women who consumed at least 1.2g/kg protein per day. (9) However, meeting total protein and calorie targets can be challenging for the aging population as appetite and senses, like taste and smell, decline. According to Yanai H, “protein intake and physical activity are the main anabolic stimuli for muscle protein synthesis.” (10).

Leucine, in particular, is a vital branched-chain amino acid required for muscle protein synthesis (MPS). (11) In fact, leucine is considered the master dietary regulator of muscle protein turnover. Consuming leucine-rich protein sources has the most significant influence on muscle mass preservation and function in older age. (12) Examples of leucine-rich foods include:

  • Whey protein
  • Chicken
  • Beef
  • Pork
  • Salmon
  • Lentils

Beta-hydroxy-Beta-methylbutyrate (HMB), a metabolite of the amino acid leucine, positively affects muscle mass and strength. Studies show that HMB stimulates mTOR anabolic pathways and down-regulates muscle catabolic pathways, thus promoting muscle protein synthesis. HMB supplementation has been studied in different clinical conditions where muscle wasting is expected, like bed-ridden illnesses, cancer cachexia, and HIV. Some studies show small yet promising effects with HMB supplementation on muscle mass in aging people, even in the absence of exercise. A dose of 3g of HMB per day is recommended to maintain or improve muscle function with minimal to no known adverse effects. (13,14,15,16)

It is also important to consider changes in protein digestion and absorption in the aging population. A 2020 meta-analysis examined how protein type, protein dose, and age specifically impact dietary protein digestion and plasma phenylalanine availability. The study found reduced protein digestion and phenylalanine absorption in older men compared to younger men. Gorissen et al. explained that “such a seemingly small impairment in dietary protein digestion and absorption with each meal at an older age could have a substantial impact on muscle mass maintenance in the long term.” As stated before, eating higher amounts of protein per meal is recommended with age. Additionally, choosing milk proteins, like whey and casein, can enhance postprandial plasma phenylalanine availability. (15)

Several other studies have evaluated the connection between protein intake and muscle mass in the aging population (15, 16). Based on the data so far, protein intake of at least 1.0-1.2 g/kg body weight per day is recommended with a minimum of 25-30 grams protein at mealtimes. (14, 17) Higher doses of protein (1.5-2.0g/kg) may be indicated in some individuals, especially when there is evidence of a catabolic state. (3)

Omega 3’s and Antioxidants

As functional nutrition practitioners, we know that oxidative stress and inflammation can cause a cascade of cellular damage. In the case of sarcopenia, higher reactive oxygen species (ROS) and lower dietary antioxidant intake can interrupt signaling pathways (e.g., MPS pathway) and lead to damaged muscle tissue. An epidemiological study called the Invecchiare in Chianti, or InCHIANTI study found that lower blood concentrations of selenium, carotenoids, and alpha-tocopherols were associated with lower muscle strength. However, there’s inconclusive repeatable evidence to show how various antioxidants play a role in muscle health and sarcopenia. (4)

Visceral fat and low-grade inflammation play a role in many age-related conditions and may also impact sarcopenia. While the data is still being studied and extrapolated, it’s hypothesized that long-chain polyunsaturated fats, omega-3s in particular, help maintain muscle strength. (4) Regardless of the evidence to date, encouraging intake of exogenous antioxidants and omega-3 fatty acids via whole foods is a benign recommendation with more pros than cons. Furthermore, a comprehensive nutritional profile &/or Omega 3- Index, both discussed in IFN Academy training and available through Rupa Health, could help guide essential fatty acid and antioxidant recommendations.

Vitamin D

Vitamin D deficiency is common across the lifespan, despite gender and race. Beyond poor bone health, low vitamin D 25(OH) is connected to insufficient muscle strength in the aging population. According to Bauer et al., “vitamin D supplementation specifically for sarcopenia was found to have insufficient evidence, though there is evidence that persons with low vitamin D levels may improve their strength with vitamin D supplementation.” (3) Monitoring vitamin D status in the aging population is critical, especially in those at high risk of sarcopenia. (4,18)

Nutrition and Lifestyle Recommendations

To date, a specific diet hasn’t been extensively studied for sarcopenia, but our clinical knowledge as functional nutrition practitioners can undoubtedly point us in the right direction. Below are five basic nutrition recommendations for muscle support in the aging population. Consider using the acronym “STAIN” taught extensively throughout IFNA to dive deeper into root causes and possible contributors.

  1. Focus on high-quality protein at mealtimes, ideally achieving 25-30 grams of protein per meal. Smaller meals may need to be divided throughout the day, depending on the individual.
  2. Fill in the gaps with higher protein options like hard-boiled eggs, Greek yogurt with berries and seeds, or a protein fruit smoothie.
  3. Increase whole fruits and non-starchy vegetable intake for antioxidant and gut microbiota support.
  4. Supplement strategically to achieve nutrient optimization. Consider a whey protein supplement, if tolerated. Whey protein is an excellent source of leucine required for MPS. HMB is also a supplement of consideration.
  5. Combine whole food, consistent nutrition with regular exercise, specifically resistance and strength training. If willing and able, consider working alongside a certified personal trainer to create a strength training program to parallel with a nourishing diet.

 

Conclusion

As we know, it’s far more effective to develop healthy and consistent nutrition and lifestyle habits early in life to reduce the risk of disease. This is why our role as nutrition practitioners and educators is essential throughout the lifespan. Muscle mass maintenance and support, in particular, change with age and is influenced mainly by overall diet quality and quantity. In the case of sarcopenia, nutrition intervention plays a pivotal role in slowing the condition’s progression. Considering total calorie, antioxidant, and protein intake, with an emphasis on leucine and HMB, is essential for people with sarcopenia. Additional labs and biometrics, like grip strength, calf circumference, organic acids, and inflammatory markers, are also helpful in this population. Most importantly, collaborating with other healthcare professionals to create a patient-centered care plan can help support quality of life and reduce other risk factors associated with sarcopenia.

To learn more about a functional nutrition approach, register for your IFNA training today!

by Tori Eaton, MS, RD, IFNCP


References

  1. Haehling S, Morley J, Anker S. An overview of sarcopenia: facts and numbers on prevalence and clinical impact. J Cachexia Sarcopenia Muscle. 2010;1(2):129-133. doi: 10.1007/s13539-010-0014-2
  2. Cruz-Jentoft AJ, Bahat G, Bauer J, et al. Sarcopenia: revised European consensus on definition and diagnosis. Age Ageing. 2019; 48(1): 16–31. doi: 10.1093/ageing/afy169
  3. Bauer J, Morley JE, Schols AMWJ, et al. Sarcopenia: a time for action. An SCWD position paper. J Cachexia Sarcopenia Muscle. 2019;10(5):956-961. doi: 10.1002/jcsm.12483.
  4. Robinson S, Granic A, Sayer A. Nutrition and muscle strength, as the key component of sarcopenia: an overview of current evidence. Nutrients. 2019;11(12), 2942. https://doi.org/10.3390/nu1112294
  5. Mathieu ME, Reid R, King N. Sensory profile of adults with reduced food intake and the potential roles of nutrition and physical activity interventions. Adv Ntr. 2019;10(6):1120-1125. doi: https://doi.org/10.1093/advances/nmz044
  6. Kiesswetter E, Poggiogalle E, Migliaccio S, et al. Functional determinants of dietary intake in community-dwelling older adults: A DEDIPAC (DEterminants of DIet and Physical ACtivity) systematic literature review. Health Nutr. 2018;21(10):1886-1903.doi:10.1017/S1368980017004244
  7. Cox N, Ibrahim K, Sayer A, et al. Assessment and treatment of the anorexia of aging: a systematic review. Nutrients. 2019;11(1):144. doi:https://doi.org/10.3390/nu11010144
  8. Bloom I, Shand C, Cooper C. Diet quality and sarcopenia in older adults: a systematic review. Nutrients. 2018;10(3):308. doi:10.3390/nu10030308
  9. Yanai H. Nutrition for sarcopenia. J Clin Med Res. 2015; 7(12): 926–931. doi: 10.14740/jocmr2361w
    Celis-Morales CA, Petermann F, Steell L, et at. Associations of dietary protein intake with fat-free mass and grip strength: a cross-sectional study in 146,816 UK biobank participants. Am J Epidemiol. 2018;187(11):2405-2414. doi: 10.1093/aje/kwy134
  10. Dorner B, Posthauer MB. Nutrition’s role in sarcopenia prevention. Today’s Dietitian. 2012;14(9):62
  11. Coelho-Júnior HJ, Milano-Teixeira L, Rodrigues B, et al. Relative protein intake and physical function in older adults: a systematic review and meta-analysis of observational studies. Nutrients. 2018;10(9):1330. doi: 10.3390/nu10091330.
  12. Gorissen S, Trommelen S, Kouw I , et al. Protein type, protein dose, and age modulate dietary protein digestion and phenylalanine absorption kinetics and plasma phenylalanine availability in humans. J Nutr. 2020;150(8):2041-2050. doi: 10.1093/jn/nxaa024. 
  13. Fanelli Kuczmarski, M, Pohlig, RT, Stave Shupe E, et al. Dietary protein intake and overall diet quality are associated with handgrip strength in African American and white adults. J. Nutr. Health Aging. 2018;22:700–709.doi: https://doi.org/10.1007/s12603-018-1006-8
  14. Holecek M. Beta-hydroxy-beta-methylbutyrate supplementation and skeletal muscle in healthy and muscle-wasting conditions. J Cachexia Sarcopenia Muscle. 2017;8(4). https://doi.org/10.1002/jcsm.12208
  15. Bear D, Langan A, Dimidi E, et al. β-Hydroxy-β-methylbutyrate and its impact on skeletal muscle mass and physical function in clinical practice: a systematic review and meta-analysis. AJCN. 2019;109(4). https://doi.org/10.1093/ajcn/nqy373
  16. Din USU, Brook MS, Selby A, Quinlan J, et al. A double-blind placebo controlled trial into the impacts of HMB supplementation and exercise on free-living muscle protein synthesis, muscle mass and function, in older adults. Clin Nutr. 2019;38(5):2071-2078. https://doi.org/10.1016/j.clnu.2018.09.025
  17. Paddon-Jones D, Rasmussen BB. Dietary protein recommendations and the prevention of sarcopenia. Curr Opin Clin Nutr Metab Care. 2009;12(1):86–90. doi: 10.1097/MCO.0b013e32831cef8b.
  18. Remelli F, Vitali A, Zurlo A, Volpato S. Vitamin D deficiency and sarcopenia in older persons. Nutrients. 2019;11(12):2861. doi:10.3390/nu11122861