Intramyocellular Lipid and Impaired Myofiber Contraction in Normal Weight and Obese Older Adults

Abstract

Background: Evidence implicates the amount and location of fat in aging-related loss of muscle function; however, whether intramyocellular lipids affect muscle contractile capacity is unknown.

Methods: We compared both in vivo knee extensor muscle strength, power, and quality and in vitro mechanical properties of vastus lateralis single-muscle fibers between normal weight (NW) and obese older adults and determined the relationship between muscle lipid content (both intramuscular adipose tissue and intramyocellular lipids) and in vivo and in vitro muscle function in NW and obese individuals.

Results: The obese group had a greater percentage of type-I fibers compared to the NW group. The cross-sectional area of type-I fibers was greater in obese compared to NW; however, maximal shortening velocity of type-I fibers in the obese was slower compared to NW. Type-I and type-IIa fibers from obese group produced lower specific force than that of type-I and type-IIa fibers from the NW group. Normalized power was also substantially lower (~50%) in type-I fibers from obese adults. The intramyocellular lipids data showed that total lipid droplet area, number of lipid droplets, and area fraction were about twofold greater in type-I fibers from the obese compared to the NW group. Interestingly, a significant inverse relationship between average number of lipid droplets and single-fiber unloaded shortening velocity, maximal velocity, and specific power was observed in obese participants. Additionally, muscle echointensity correlated with single-fiber specific force.

Conclusions: These data indicate that greater intramyocellular lipids are associated with slower myofiber contraction, force, and power development in obese older adults.

Keywords: Muscle; Obesity; Sarcopenia.

© The Author 2015. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Figures

Figure 1.

Functional characteristics of single-muscle fibers. Type-I and type-IIa fiber cross-sectional area (CSA) (A), maximal shortening velocity (B), absolute maximal Ca2+-activated force (C), and specific maximal Ca2+-activated force relative to CSA (D) in normal weight (NW; black) and obese (gray) groups. *Significant difference between groups within fiber type; #significant difference between fiber types within group.

Figure 2.

Power-generating capacity of single-muscle fibers. Type-I and type-IIa muscle fiber absolute power (A) and power normalized to fiber size (B) in normal weight (NW; black) and obese (gray) participants. *Significant difference between groups within fiber type; #significant difference between fiber types within group.

Figure 3.

Oil-red-O (A–C) and adenosine triphosphatase (B–D) staining of muscle cross-sections from a representative normal weight (A–B) and obese (C–D) study participant.

Figure 4.

Relationship between intramyocellular lipid and cross-sectional area (CSA) of type-I and type-IIa fibers from normal weight (NW) and obese older adults. Scatterplots of CSA of individual fibers with total lipid droplet area (A and D), number of lipid droplets (B and E), and average area of each lipid droplet (C and F) for individual fibers from NW (gray) and obese (red) participants.

Figure 5.

Relationship between ultrasound-derived muscle echointensity and single-fiber specific force and intramyocellular lipid. Scatterplots of muscle bulk standard deviation (SD) echointensity with percent area of myofiber fat (A), bulk muscle mean (B), and SD (C) values with myofiber specific force. R: Pearson correlation coefficient. (D) Representative ultrasound image showing subcutaneous fat, rectus femoris, vastus intermedius, and femoral bone.