Adipose tissue in muscle: a novel depot similar in size to visceral adipose tissue

Abstract

Background: The manner in which fat depot volumes and distributions, particularly the adipose tissue (AT) between the muscles, vary by race is unknown.

Objective: The objective was to quantify a previously unstudied and novel intermuscular AT (IMAT) depot and subcutaneous AT, visceral AT (VAT), and total-body skeletal muscle mass in healthy sedentary African American (AA), Asian, and white adults by whole-body magnetic resonance imaging. IMAT is the AT between muscles and within the boundary of the muscle fascia.

Design: Analyses were conducted on 227 women [AA (n = 79): body mass index (BMI; in kg/m(2)), 29.0 +/- 5.5; age, 45.7 +/- 16.9 y; Asian (n = 38): BMI, 21.7 +/- 2.9; age, 47.2 +/- 19.9 y; whites (n = 110): BMI, 24.9 +/- 5.4; age, 43.7 +/- 16.2 y]) and 111 men [AA (n = 39): BMI, 25.6 +/- 3.2; age, 45.5 +/- 18.8 y; Asian (n = 13): BMI, 24.9 +/- 2.5; age, 45.6 +/- 25.0 y; white (n = 59): BMI, 25.8 +/- 3.8; age 44.5 +/- 16.3 y].

Results: IMAT depots were not significantly different in size between race groups at low levels of adiposity; however, with increasing adiposity, AAs had a significantly greater increment in the proportion of total AT (TAT) than did the whites and Asians (58, 46, and 44 g IMAT/kg TAT, respectively; P = 0.001). VAT depots were not significantly different in size at low levels of adiposity but, with increasing adiposity, VAT accumulation was greater than IMAT accumulation in the Asians and whites; no significant differences were observed in AAs.

Conclusion: Race differences in AT distribution extend to IMAT, a depot that may influence race-ethnicity differences in dysglycemia.

FIGURE 1

Intermuscular adipose tissue (IMAT) predicted from sex, age, weight², total adipose tissue (TAT), and TAT-by-race interaction, shown in relation to TAT (kg) in African American ([unk]; n = 118), Asian (*; n = 51), and white (△; n = 169) women and men. Linear regression lines are shown for African Americans (- - -), Asians ([unk]), and whites (—). All slopes were significantly different from zero (P < 0.05). All intercepts were significantly different from zero (P < 0.001). The overall SEE = 0.366 kg, as described in the text. African Americans: IMAT =−0.414 + 0.187 (male) + 0.008 (age) + 0.059 (TAT) − 0.00006 (weight²). Asians: IMAT =−0.148 + 0.187 (male) + 0.008 (age) + 0.044 (TAT) − 0.00006 (weight²). Whites: IMAT =−0.306 + 0.187 (male) + 0.008 (age) + 0.047 (TAT) − 0.00006 (weight²).

FIGURE 2

Intermuscular adipose tissue (IMAT; [unk]) and visceral adipose tissue (VAT; △) as a function of total adipose tissue (TAT) in women. Linear regression lines are shown for IMAT ([unk]) and VAT (- - -). All slopes were significantly different from zero (P < 0.001). None of the intercepts were significantly different from zero, except for the regression of VAT in white women (P < 0.001). African American women (n = 79): IMAT = −0.123 + 0.049 × TAT (SEE = 0.495 kg); VAT = −0.423 + 0.067 × TAT (SEE = 0.712 kg). Asian women (n = 38): IMAT = 0.070 + 0.041 × TAT (SEE = 0.322 kg); VAT =−0.729 + 0.129 × TAT (SEE = 0.782 kg). White women (n = 110): IMAT = 0.034 + 0.037 × TAT (SEE = 0.421 kg); VAT =−0.662 + 0.086 × TAT (SEE = 0.782 kg).

FIGURE 3

Intermuscular adipose tissue (IMAT; [unk]) and visceral adipose tissue (VAT; △) as a function of total adipose tissue (TAT) in men. Linear regression lines are shown for IMAT ([unk]) and VAT (- - -). All slopes were significantly different from zero (P < 0.001). None of the intercepts were significantly different from zero, except for the regression of VAT in white men (P < 0.001). African American men (n = 39): IMAT =−0.217 + 0.058 × TAT (SEE = 0.432 kg); VAT =−0.859 + 0.145 × TAT (SEE = 1.184 kg). Asian men (n = 13): IMAT =−0.179 + 0.064 × TAT (SEE = 0.295 kg); VAT =−1.876 + 0.238 × TAT (SEE = 0.965 kg). White men (n = 59): IMAT = −0.182 + 0.044 × TAT (SEE = 0.276 kg); VAT = −1.425 + 0.195 × TAT (SEE = 0.957 kg).

FIGURE 4

Mean (±SE) predicted intermuscular adipose tissue (IMAT; ■) and visceral AT (VAT; □) according to the regression equations assuming 35 kg total AT in African Americans (n = 79 women and 39 men), Asians (n = 38 women and 13 men), and whites (n = 110 women and 59 men). Means with different lowercase letters are significantly different, P < 0.05 (adjusted for Tukey-Kramer multiple comparison procedure).

FIGURE 5

Mean (±SE) amounts of visceral adipose tissue (VAT) plus intermuscular AT (IMAT) as a percentage of total AT (TAT) in African American (n = 118), Asian (n = 51), and white (n = 169) women (□) and men (■). Means with different superscript letters are significantly different, P < 0.001 for race. P values adjusted for Tukey-Kramer multiple comparisons are based on log-transformed (VAT + IMAT) as a percentage of TAT.