Muscle Quality（筋肉の質）と筋肉量の関連

Muscle Quality（筋肉の質）と筋肉量の関連をみた論文を紹介します。

Sébastien Barbat-Artigas, Marie-Eve Filion, Stéphanie Plouffe, and Mylène Aubertin-Leheudre: Muscle Quality As a Potential Explanation of the Metabolically Healthy but Obese and Sarcopenic Obese Paradoxes. Metabolic Syndrome and Related Disorders. -Not available-, ahead of print. doi:10.1089/met.2011.0092.

抄録しか読んでいないので詳細不明ですが、この論文ではサルコペニアをType I とTypeII に分けています。分類は以下の通りで、TypeIIがサルコペニア、Type Iがサルコペニアのリスク群といえます。

Type I sarcopenia is defined as an SMI value of 1 to 2 SDs below the values for young adults from a reference population, whereas type II sarcopenia represents an SMI of 2 SDs or more below the same value.

研究の仮説をPECOにすると、以下の通りです。

P：閉経後の女性では、
E：筋肉量が少ない（サルコペニア）と、
C：筋肉量が多い場合と比較して、
O：Muscle Quality（筋肉の質）が高い＝筋肉量が少ない割に筋力が強い

結果ですが、骨格筋の筋肉量と筋力は正常体重群と過体重群（BMI25-30）では有意な相関を認めましたが、肥満群（BMI30以上）では相関を認めませんでした。筋肉量（筋肉量指数も）と筋肉の質は3群とも有意な逆相関を認めました（筋肉量が多いほど筋肉の質が低い）。TypeIIサルコペニアではType Iサルコペニア（リスク群）より筋肉の質が有意に高かったです。

以上より、仮説は検証されたという結論です。肥満やサルコペニア肥満の中には、筋肉の質がよいために代謝異常を認めない方がいる可能性があります。

Muscle Qualityについて触れている論文は多少ありますが、日本語訳が定まっていないようです。「筋肉質」では筋肉量が多い体格の意味になってしまうので、「筋肉の質」と訳しました。筋肉量減少よりも筋力低下のほうがADLへの影響は大きいので、サルコペニアでも筋肉の質がよければADLへの支障は少なめになります。

一方、サルコペニアでなくても筋肉の質が悪ければADLに支障が出る可能性もあります。筋肉の質と筋肉量の関連が生理的なものなのか、レジスタンストレーニングで十分改善できるものなのか興味深い領域です。確かに筋肉量が増えなくても筋力が改善することはしばしばありますし。

Abstract
Background: Obesity has been associated with metabolic complications. However, two subgroups of obese individuals (namely “sarcopenic obese” and “metabolically healthy but obese” individuals) with low muscle mass appear to have a better metabolic profile. Despite having a lower muscle mass, these individuals appear to have higher muscle strength, suggesting higher muscle quality. Studies have reported insulin sensitivity to be related to muscle quality. Therefore, the aim of the present study was to test the hypothesis that low muscle mass was associated with better muscle quality in obese individuals.

Methods: A total of 112 postmenopausal women aged between 50 and 77 years (means, 61±6 years) were recruited. Body composition (bioelectrical impedancemetry), muscle strength (hand dynamometer), and functional capacity (chair stand test, alternate step test and balance test) were measured.

Results: Skeletal muscle mass (SM; kg) was significantly positively correlated with muscle strength (kg) in normal weight (r=0.43; P=0.003) and overweight (r=0.42; P=0.020) women, but not in obese (r=0.28; P=0.13) women. Muscle mass (kg) was significantly negatively correlated with muscle quality (kg/SMkg) in normal weight (r=−0.41, P=0.005), overweight (r=−0.59; P<0.001) and obese (r=−0.59; P<0.001) individuals. Skeletal muscle mass index (kg/m2) was significantly negatively correlated with muscle quality in normal weight (r=−0.33, P=0.025), overweight (r=−0.47, P=0.008), and obese (r=−0.43, P=0.015) women. Finally, type II sarcopenic individuals had higher muscle quality than type I sarcopenic (P=0.002) and nonsarcopenic (P=0.001) individuals.

Conclusions: Our results support our hypothesis and showed that muscle quality increased as muscle mass and muscle mass index decreased. Thus, muscle quality may potentially explain the favorable metabolic profile observed in metabolically healthy but obese and sarcopenic obese postmenopausal women. Further studies including metabolic data are needed to confirm our results.