Muscle fiber-type distribution predicts weight gain and unfavorable left ventricular geometry: a 19 year follow-up study.
BACKGROUND
Skeletal muscle consists of type-I (slow-twitch) and type-II (fast-twitch) fibers, with proportions highly variable between individuals and mostly determined by genetic factors. Cross-sectional studies have associated low percentage of type-I fibers (type-I%) with many cardiovascular risk factors.


METHODS
We investigated whether baseline type-I% predicts left ventricular (LV) structure and function at 19-year follow-up, and if so, which are the strongest mediating factors. At baseline in 1984 muscle fiber-type distribution (by actomyosin ATPase staining) was studied in 63 healthy men (aged 32-58 years). The follow-up in 2003 included echocardiography, measurement of obesity related variables, physical activity and blood pressure.


RESULTS
In the 40 men not using cardiovascular drugs at follow-up, low type-I% predicted higher heart rate, blood pressure, and LV fractional shortening suggesting increased sympathetic tone. Low type-I% predicted smaller LV chamber diameters (P < or = 0.009) and greater relative wall thickness (P = 0.034) without increase in LV mass (concentric remodeling). This was explained by the association of type-I% with obesity related variables. Type-I% was an independent predictor of follow-up body fat percentage, waist/hip ratio, weight gain in adulthood, and physical activity (in all P < or = 0.001). After including these risk factors in the regression models, weight gain was the strongest predictor of LV geometry explaining 64% of the variation in LV end-diastolic diameter, 72% in end-systolic diameter, and 53% in relative wall thickness.


CONCLUSIONS
Low type-I% predicts obesity and weight gain especially in the mid-abdomen, and consequently unfavourable LV geometry indicating increased cardiovascular risk.
Karjalainen
Jouko
J
Unit for Sports and Exercise Medicine, University of Helsinki, Finland. jouko.karjalainen@pp.inet.fi
Tikkanen
Heikki
H
Hernelahti
Miika
M
Kujala
Urho M
UM
D000328:Adult; D017677:Age Distribution; D000368:Aged; D001707:Biopsy, Needle; D002318:Cardiovascular Diseases / Q000000981:diagnostic imaging / Q000453:epidemiology*; D015331:Cohort Studies; D015150:Echocardiography, Doppler; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D018656:Muscle Fibers, Fast-Twitch / Q000473:pathology* / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000473:pathology* / Q000502:physiology; D018482:Muscle, Skeletal / Q000473:pathology; D009200:Myocardial Contraction / Q000502:physiology; D009765:Obesity / Q000175:diagnosis* / Q000453:epidemiology; D011237:Predictive Value of Tests; D011336:Probability; D012016:Reference Values; D012044:Regression Analysis; D012189:Retrospective Studies; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D013997:Time Factors; D018487:Ventricular Dysfunction, Left / Q000453:epidemiology / Q000473:pathology*; D015430:Weight Gain