Metabolic inflexibility in substrate use is present in African-American but not Caucasian healthy, premenopausal, nondiabetic women.
BACKGROUND
There is an increased prevalence of obesity and insulin resistance in African-American compared with Caucasian females. Metabolic inflexibility (MI) is the inability to switch the use of lipids and carbohydrates in the peripheral tissue (i.e. muscle) based upon substrate availability.


OBJECTIVE
We examined whether MI exists in African-American females. MAIN OUTCOME MEASURES AND DESIGN: We measured substrate use differences during eucaloric, macronutrient-manipulated diets [high fat (50% fat, 35% carbohydrate, 15% protein) vs. low fat (30% fat, 55% carbohydrate, 15% protein)] between Caucasian and African-American women. We also compared differences in substrate use in response to insulin infusion during two-step pancreatic-euglycemic clamps and epinephrine infusion during lipolysis studies. In each study, similar groups of Caucasian and African-American women were compared.


RESULTS
Caucasians had significantly higher fat oxidation (FO) (P = 0.01) and lower carbohydrate oxidation (P < 0.01) during the high-fat vs. low-fat diet, whereas no significant differences were observed in African-Americans. The African-American women also failed to significantly suppress FO during the second step of the pancreatic-euglycemic clamp despite a doubling of their fasting plasma insulin and failed to increase their FO or decrease their carbohydrate oxidation in response to epinephrine infusion as much as Caucasian women did. The response of free fatty acid turnover rates to insulin and epinephrine stimulation was similar between races.


CONCLUSIONS
The impaired substrate use observed in African-American women during these three studies demonstrates the existence of MI and may contribute to their greater prevalence of obesity and insulin resistance.
Berk
Evan S
ES
New York Obesity Research Center, St. Luke's-Roosevelt Hospital Center, 1111 Amsterdam Avenue, Department of Medicine, Babcock Building Room 1031, New York, NY 10025, USA.
Kovera
Albert J
AJ
Boozer
Carol N
CN
Pi-Sunyer
F Xavier
FX
Albu
Jeanine B
JB
D000328:Adult; D001741:Black or African American; D050260:Carbohydrate Metabolism / Q000187:drug effects; D003924:Diabetes Mellitus, Type 2 / Q000209:etiology; D004837:Epinephrine / Q000494:pharmacology; D005230:Fatty Acids, Nonesterified / Q000378:metabolism; D005260:Female; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D007333:Insulin Resistance; D050356:Lipid Metabolism / Q000187:drug effects; D008066:Lipolysis / Q000187:drug effects; D018482:Muscle, Skeletal / Q000378:metabolism*; D009765:Obesity / Q000453:epidemiology; D010084:Oxidation-Reduction; D017697:Premenopause; D044465:White People
D005230:Fatty Acids, Nonesterified; D007328:Insulin; D004837:Epinephrine