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Lower insulin sensitivity found In Mexican-Americans, regardless of behavioral and metabolic factors

Dietary fat intake may offer a clue to controlling spread of type-2 diabetes in this highly susceptible population group

Perhaps the most significant -- and least known -- consequence of the growth of this demographic group is to the nation's health care system. Not because of the current issues surrounding the delivery of primary care services – but for the future services that will be necessary to diagnose and treat a population highly susceptible to type 2 diabetes.

Background Mexican-Americans are diagnosed with type 2 diabetes mellitus (T2DM) two to three times more frequently than are non-Hispanic white (NHW) Americans. Scientists have found that insulin resistance is key in the cause for the disease; studies have revealed that Mexican-Americans of both genders and all ages demonstrate greater levels of insulin resistance when compared to the NHW population.

Although the reasons are unclear, some suggest that genetic factors could explain the higher prevalence of insulin resistance in this Hispanic population group, considering that 35 percent of their genetic Mexican-American make-up is attributable to Native American ancestry.

It is also possible that lifestyle factors, including diet and exercise, contribute to the ethnic differences in insulin resistance. Visceral adiposity (obesity), exercise, and dietary fat have all been shown to impact peripheral insulin resistance.

The Study A new research study that compares behavioral, metabolic, and molecular behavior between Mexican-Americans and non-Hispanic whites offers new clues on why public health officials need to act on this potential problem now. At the same time, the current study findings can provide an important first step to this problem that could easily be undertaken by school systems, restaurants, and all institutions that meet the dietary needs of our citizens and residents from the south.

The authors of "Behavioral, Metabolic, and Molecular Correlates of Lower Insulin Sensitivity In Mexican-Americans," are Richard C. Ho, Kevin P. Davy, Matthew S. Hickey, Scott A. Summers, and Christopher L. Melby, all from Colorado State University, Fort Collins, Colorado. Their findings appear in the October 2002 edition of the American Journal of Physiology--Endocrinology and Metabolism, a journal of the American Physiological Society (APS).

[In previous studies, Mexican-Americans have been shown to exhibit lower insulin sensitivity independently of body fat and body fat patterning. However, this issue is not entirely resolved given that studies that have documented diminished insulin sensitivity in non-obese, non-diabetic Mexican-Americans compared with NHW utilized less sensitive methods to estimated Mexican-American body fat and central adiposity (obesity). Furthermore, the possible contribution of lower physical activity and physical fitness in this demographic group as well as differences in dietary intake were not accounted for in these studies. Because Mexican-Americans compared with NHW tend to exhibit greater central adiposity, are less physically active, and consume a more atherogenic diet, it is important to examine these factors as possible contributors to the lower insulin sensitivity in Mexican-Americans.]

This study had two specific aims: First, to determine whether or not differences in insulin sensitivity persist between these two groups after controlling for the effects of acute and chronic exercise, abdominal fat distribution, and dietary intake. Second, to ascertain whether Mexican-Americans exhibit lower skeletal muscle protein concentrations of IR , PI3K p85, Akt1, Akt2, and GLUT4 compared with NHW after controlling for these same potential confounders.

Methodology: Thirteen (13) non-obese Mexican-Americans (seven females, six males) were matched to 13 non-obese NHW (seven females, six males) age 18-40, on the basis of gender, age, and aerobic fitness.

Subjects were eligible for participation on the basis of the following characteristics: being nonsmoking, apparently healthy individuals with no overt signs or symptoms of disease as determined by a medical history, and having normal fasting blood glucose, no past or present history of endocrine disorders, and resting blood pressure of 110/90 mHg. To be appropriately identified as Mexican-Americans, each participant traced his/her ethnicity to all four grandparents. The Colorado State University Human Research Committee approved the study protocol.

The researchers looked at a number of variables, including body mass, height, and composition.

• Body weight was measured on a balance scale to the nearest 100 g. The percentage of body fat, absolute fat mass, and fat-free mass were measured in all subjects.
• Abdominal visceral fat: A measure of the total, visceral, and subcutaneous fat in the abdominal region was conducted.
• Dietary intake: Subjects were instructed to accurately record food intake (e.g., portion sizes, food preparation methods, brand names of products) over a four-day period by using two-dimensional food models.
• Insulin sensitivity: For the determination of insulin sensitivity, subjects were instructed to fast for 12 hours before blood collection. Subjects also refrained from participation in any form of exercise for 48 hours before the study. Four estimates of insulin sensitivity were used.
• Muscle biopsies: Muscle biopsies were obtained from all subjects to examine insulin-signaling pathway intermediates and glucose transporters in skeletal muscle.

Results

The researchers found that:

• Mexican-Americans were found to be significantly less insulin sensitive compared with their NHW counterparts.
• There were no significant differences between the two groups with regard to skeletal muscle protein abundance of IR , PI3K p85, Akt1, Akt2, or GLUT4. Skeletal muscle protein abundance of IR was significantly associated with fasting plasma insulin.
• Percent total energy intake from palmitoleic acid was significantly higher among Mexican-Americans, with a trend toward higher percent total energy intake from palmitic acid and oleic acid and lower fiber intake among Mexican-Americans.

Conclusions

There are three major significant findings in this study:

• First, non-obese, non-diabetic Mexican-Americans adults were less insulin sensitive compared with NHW adults, even when the potential roles of cardiorespiratory fitness, acute exercise, and total and regional adiposity were accounted for.
• Second, skeletal muscle protein abundance of IR , PI3K p85, Akt1, Akt2, and GLUT4 was not significantly different between the two groups and therefore does not account for the group differences in insulin sensitivity.
• Finally, group differences in insulin sensitivity were attenuated to losing statistical significance after dietary intakes of palmitic acid, palmitoleic acid or skeletal muscle IR protein content were accounted for.

This study demonstrates that lower insulin sensitivity persists in non-obese, non-diabetic Mexican-Americans compared with their non-Hispanic white counterparts, even after acute and chronic effects of exercise and abdominal fat distribution are accounted for. Furthermore, protein abundance of skeletal muscle IR , PI3K p85, Akt1, Akt2, or GLUT4 does not explain these differences. Differences in insulin sensitivity are lost when dietary intakes of palmitate and palmitoleate are accounted for, suggesting the possibility that these factors may contribute to the lower insulin sensitivity seen in Mexican-Americans.

Source: October 2002 edition of the American Journal of Physiology--Endocrinology and Metabolism.

The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.

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