Skeletal muscle fat oxidation is increased in African-American and white women after 10 days of endurance exercise training

Nutrition Research Newsletter, Oct, 2006

A number of epidemiological studies have identified differences in the development and maintenance of obesity in African-American women (AAW) compared with white women (CW). More recently, however, mechanism-based studies have suggested metabolic differences in obese AAW compared with obese CW. Therefore, although differences in income, socioeconomic status, or education may contribute to the higher incidence of obesity in AAW, inherent biochemical mechanisms also seem to play a role. Rates of skeletal muscle oxidation of fatty acids have been shown to be lower in obese compared with lean individuals. To date, however, few studies have compared the capacity for skeletal muscle to oxidize fat between AAW and CW. It is well established that acute aerobic exercise is associated with increased lipid mobilization and oxidation by skeletal muscle if the exercise intensity is light to moderate. It is equally well understood that aerobically trained individuals are characterized by an expansion in mitochondrial oxidative capacity, and recent data have shown a greater in vitro oxidation of fatty acids by skeletal muscle in trained versus sedentary individuals. The shift in efficiency for metabolizing lipids by skeletal muscle in lean subjects occurs early with exercise training, as only 7 to 10 days of regular aerobic exercise increased postabsorptive fat oxidation without changing body composition. These data support the use of exercise training as a component in the treatment of obesity. However, there are few studies comparing the effects of aerobic exercise training on skeletal muscle fatty acid oxidation (FAO) between obese and lean individuals, and those that do exist are currently equivocal. Given the greater prevalence and severity of obesity in AAW, the purpose of these studies was to determine whether there are differences in skeletal muscle FAO rates between AAW and CW. The researchers hypothesized that the observed lower rates of lipid oxidation in skeletal muscle from obese individuals is also present in lean AAW. Furthermore, because of the relative absence of data comparing skeletal muscle FAO after aerobic training in obese individuals as well as between AAW and CW, they determined the in vitro rates of FAO in non-obese and obese AAW and CW before and after 10 days of aerobic exercise training.

In vitro rates of FAO were assessed in rectus abdominus muscle strips using [[1-.sup.14]C] palmitate (Pal) from lean AAW and CW undergoing voluntary abdominal surgery. Lean AAW and CW and obese AAW (BMI 36 [ or -] 1.2 kg/[m.sup.2]) and CW (BMI40 [ or -] 1.3 kg/[m.sup.2]) underwent 10 consecutive days of endurance exercise training (EET) on a cycle ergometer (60 min/d, 75% peak oxygen uptake). FAO was measured in vastus lateralis homogenates as captured [sup.14]C[O.sub.2] using [[1-.sup.14]C] Pal, palmitoyl-CoA (Pal-CoA), and palmityl-camitine (Pal-Car).

Muscle strip experiments showed suppressed rates of FAO in lean AAW versus CW. Endurance exercise training (EET) increased the rates of skeletal muscle Pal oxidation in both lean AAW and CW. In obese subjects, Pre-EET Pal (but not Pal-CoA or Pal-Car) oxidation was lower in AAW versus CW. EET increased Pal oxidation 100% in obese AAW and 59% in obese CW. Similar increases in post-EET FAO were observed for Pal-CoA and Pal-Car in both groups.

The major findings of this study are 1) that, based on data from intact muscle strips, the capacity to oxidize fatty acids by skeletal muscle seems to be reduced in sedentary, lean AAW versus CW, perhaps predisposing this racial group toward obesity; 2) that, compared with CW, obese AAW have a reduced capacity to oxidize exogenous long-chain fatty acids, which seems to be caused (at least in part) by reduced activation of fatty acyl units to their CoA derivatives; and 3) that both lean and obese AAW and CW are able to significantly increase their rates of long-chain FAO after 10 days of endurance exercise training. The latter finding suggests that the prescription for endurance exercise is extremely important to combat problems in muscle lipid metabolism that manifest as obesity, especially for AAW.

In summary, these findings support previous literature in that obese AAW possess reduced rates of long-chain FAO by skeletal muscle. Reductions in oxidation rates of palmitate, but not palmitoyl-CoA or palmityl-carnitine, suggest an additional biological difference in obese AAW at the site of activation of long-chain fatty acids. In addition, the researchers report the novel finding that lean AAW also manifest reduced rates of long-chain FAO, with comparisons between intact versus homogenate rates of oxidation suggesting potential differences in membrane transport of fatty acids into the cell cytosol. These findings support the hypothesis of inherent physiological or biochemical differences between AAW and CW that may place the former race at greater risk for developing obesity and related diseases. Despite the reduced rates of FAO in the basal state, both lean and obese AAW and obese CW were metabolically flexible in response to endurance exercise training and showed significant improvements in fatty acid oxidative capacity. These findings support an important role for exercise as a therapeutic intervention to reduce the progression toward obesity and type 2 diabetes for individuals at risk for developing these disease states.