SUMMARY
Lean adolescents with increased risk for metabolic syndrome
The aim of the present study was to determine in adolescents the relationship between insulin levels and body mass index (BMI), body fat distribution, diet, life style and lipid profile. We studied 167 adolescents (68 boys and 99 girls) whose ages ranged from 14 to 17 years. A detailed medical (including pubertal stage) and nutritional record was obtained from each subject. Biochemical measurements included fasting serum insulin, glucose, total cholesterol (TC), triglycerides (Tg), HDL-C, LDL-C and VLDL-C. HOMA insulin resistance (IR) and HOMA b -cell function (b -cell) were calculated. Insulin levels were over 84 pmol/L (cut off normal value in our lab) in 56% of the boys and 43% of the girls. Thirty-seven percent of lean adolescents whose BMI was 21.5 ± 1.9 kg/m2 presented higher fasting insulin levels, HOMA IR, Tg, systolic (SBP) and diastolic blood pressure (DBP) values when compared to a lean normoinsulinemic group. Insulin levels were correlated (p< 0.01) with body mass index. Both boys and girls in the highest BMI quartile (BMI > 24 kg/m2) had significantly higher serum insulin, HOMA b -cell, and Tg levels, and the lowest HDL-C levels. A high-energy intake rich in saturated fat and low physical activity were found in this lean but metabolically altered adolescents. We conclude that even with a BMI as low as 21 kg/m2 an inappropriate diet and low physical activity might be responsible for the high insulin levels and dislipidemias in adolescents.

Key words: Insulin, body mass index, diet, physical activity, insulin resistance, adolescents.

RESUMEN
Adolescentes delgados con alto riesgo de presentar Sindrome Metabólico
El objetivo del presente estudio fue determinar la relación entre los niveles de insulina y el Indice de Masa Corporal (IMC), distribución de grasa corporal, dieta, estilo de vida y perfil lipídico en adolescentes. A cada adolescente se la realizó una historia clínica detallada (incluyendo estadío puberal), además de una encuesta nutricional. Dentro de los parámetros bioquímicos se determinaron en suero los niveles de glicemia e insulina basal, colesterol total (CT), triglicéridos (TG), HDL-C, LDL-C, y VLDL-C. Se calcularon el HOMA-IR, y el HOMA-ß-cell. Se encontró que el 56% de los varones y el 43% de las hembras tenía valores de insulina > 84pmol/L (valor de referencia para nuestro laboratorio). Un 37% de los adolescentes delgados, con un IMC promedio de 21,5 ± 1,9 kg/m2, presentó niveles elevados de insulina, HOMA-IR, TG, presión arterial sistólica y diastólica comparados con el grupo de adolescentes delgados normoinsulinémicos. Se encontró una correlación positiva y significativa (p< 0,01) entre los niveles de insulina y el IMC. Tanto los varones como las hembras ubicados en los cuartiles más altos de IMC (IMC>24 kg/m2 ), tenian niveles significativamente más altos de insulina, HOMA- ßcell y TG, acompañados de niveles más bajos de HDL-C. Una alta ingesta de energía rica en grasa saturadas y una baja actividad física, se encontraron en estos adolescentes delgados pero metabólicamente alterados. En conclusión, niveles tan bajos de IMC como 21 kg/m2, una dieta inapropiada y una baja actividad física pueden ser responsables de los altos niveles de insulina y dislipidemias en los adolescentes.

Palabras clave: Insulina, índice de masa corporal, dieta, actividad física, insulino resistencia, adolescentes.

INTRODUCTION
The emerging worldwide epidemic of type 2 diabetes and cardiovascular disease establishes as research priorities to analyze risk factors in adolescents. There is an increasing evidence that acquired cellular resistance to insulin leads to impaired glucose tolerance, hyperglycemia, hyperinsulinemia and dyslipidemia. Genetic factors in combination with sedentary life-style, and a high-energy diet lead to obesity. Nearly 20 years ago, it was suggested that individuals exist who are not obese on the basis of height and weight, but who, like people with overt obesity, are hyperinsulinemic, insulin-resistant, and predisposed to type 2 diabetes, hypertriglyceridemia, and premature coronary disease (1). Since then, it has become increasingly clear that such metabolically obese, normal weight individuals are very common in the general population and they probably represent one end of the spectrum of people with insulin resistance syndrome. Metabolic obesity could account for the higher prevalence of type 2 diabetes and other disorders in people with body mass index (BMI) in the 20-27 kg/m² range who have gained modest amounts of weight (2–10 kg of adipose mass) in adult life. Such individuals might be characterized by hyperinsulinism and possibly by an increase in fat cell size compared to patients of similar age, height and weight (1). Bergström et al (2) demonstrated that features typical of the insulin resistance syndrome were already present in Swedish adolescents. In addition, Young and Rosenbloom (3) reported type 2 diabetes in a pediatric population. The measurements of serum insulin concentrations is an accepted way of estimating insulin resistance (4) but there are not reference values (normative data) available in the adolescent age group. Hyperinsulinemia is used as a biochemical marker for insulin resistance as it has itself been implicated in the development and maintenance of excess adiposity (5,6). However, little attention has been paid to the metabolic consequences of hyperinsulinemia in adolescents, even though such studies might provide important insights into the natural history of obesity and cardiovascular disease in adult life. The objective of the present study was to determine the relationship between insulin level and body mass index, body fat distribution, diet, life style and lipid profile in adolescents.

METHODS
Subjects
The study was performed on healthy adolescents with similar socioeconomic status, from an urban school in Maracaibo-Venezuela, and were selected in accordance with sex and age. The study involved 167 subjects aged between 14 and 17 y: 68 boys and 99 girls. All volunteers received verbal and written information on the purpose and content of the study, and the participation was voluntary. All subjects were documented to be in good health based on their health history that included height (m), weight (kg), pubertal age (estimated according to Tanner’s criteria), collection of data on chronic disease history (diabetes, hypertension, cardiovascular disease and obesity), medication and physical activity (min/week). None of the adolescents received medication.

Anthropometric assessment
Waist circumference was measured at the umbilical level and hip circumference as the largest measure around the hips. Biceps, triceps, subscapular and suprailiac skinfolds were measured with a Lange Skinfold Caliper. Height and weight were measured in the standard fashion, and BMI (kg/m²) was calculated from these values. We considered lean adolescents when BMI was = 25 kg/m² and obese adolescents when BMI was > 25 kg/m². These cutoffs’ BMI were established according to body mass index curves reported by FUNDACREDESA Venezuela Project 1994 (7).

Blood pressure
Systolic (SBP) and diastolic ([ DBP] Korotkoff 4^th phase) blood pressures were measured in the right arm in the sitting position using a standard mercury sphygmomanometer. The mean of two blood-pressure measurements obtained five minutes apart was used in the analyses.

Biochemical measurements
After a 12-h fasting blood samples (10 ml) were collected from each subject and transported and processed immediately after collection. Blood glucose was measured by the glucose oxidase method (Glucose liquicolor Human). Total cholesterol (TC), triglycerides (Tg), HDL-C and LDL-C were measured by enzymatic methods (Cholesterol liquicolor, Triglycerides liquicolor and HDL-cholesterol liquicolor test kits from Human, Germany and cholesterol LDL, PVS method from Boehringer); VLDL-C was determined by the formula: VLDL-C = TC – HDL-C – LDL-C. Serum insulin level was measured using a solid-phase ¹²⁵I radio immunoassay (Coat-A-Count Insulin, Diagnostic Products, USA). Insulin resistance and b -cell function were determined by the Homeostasis Model Assessment [HOMA] (8).

Macronutrient intake
Macronutrient intake was assessed from the subject’s daily record of food eaten during 7 days (including breakfast, lunch, dinner and snack). The questionnaires were processed using the Expert System Ceres, a program that converts declared quantities of food into quantities of nutrients on the basis of the food composition table by National Nutrition Institute, Foods Composition Table (Venezuela) (9).

Data analysis
Statistical analysis was performed using the Stat Most Framework for Windows version 3.0 software. For normally distributed data the differences between means were evaluated with the Student’s t-test. For non-normally distributed data differences between distributions were evaluated with the Mann Whitney U test and correlation analysis with the Pearson’s correlation coefficient.

Ethical considerations
The study was approved by Ethics Committee of the Institute of Clinical Investigations, and informed written consent was requested and obtained from parents in the school where the study was performed; all adolescents were freely consenting.

RESULTS
Adolescents data are shown in Table 1. Anthropometric measurements, HOMA IR Index and blood pressures were significantly different between boys and girls. According to the criteria of Tanner both boys and girls were pubertal adolescents (boys 89.7% Tanner IV-V pubic hair and genitals; girls 97.9% Tanner IV-V breast and pubic hair, after the onset of menses). They were normotensive and had fasting serum insulin levels that were above our reference value (84 pmol/L). In both boys and girls, BMI was positively (p<0.05) correlated to central fat distribution, estimated as subscapular/triceps ratio, and with both circumference and skinfold measurements (p<0.001) (Table 2). In boys, anthropometric measurements were correlated positively with serum insulin, HOMA insulin resistance (IR), TC and LDL-C values and negatively with HDL-C (Table 3). In contrast, anthropometric measurements in girls were correlated positively only with systolic and diastolic blood pressures and negatively with HDL-C (Table 3). Both boys and girls in the highest BMI quartile (boys > 23.7 kg/m², girls > 24 kg/m²) had significantly higher serum insulin levels, HOMA b -cell function (b -cell), and Tg values and lower HDL-C values than adolescents in the lowest quartile (boys and girls < 19.6 kg/m²). Boys also had higher mean HOMA IR, LDL-C and VLDL-C values in the 4^th quartile and girls presented higher SBP and DBP values in the 4^th quartile (Table 4). In both boys and girls, there were no significant correlations between serum insulin and serum lipid values (data not shown).

TABLE 1
Adolescents characteristics

TABLE 2
Associations between Body Mass Index,
Circumferences and Skinfolds in adolescents

TABLE 3
Associations between anthropometric measurements
and serum insulin, serum lipids and blood pressure

TABLE 4
Insulin levels, lipid profile and blood pressure values by body mass index quartiles

As shown in Table 5, among the 167 adolescents studied, we found that 139 subjects had BMI = 25 kg/m² (lean) and 28 with BMI >25 kg/m² (obese). They were divided in 4 groups (A, B, C and D) according to the cutoff insulin value (84pml/L): Group A that was lean and had insulin values £ 84 pmol/l. Group B had a slightly higher BMI and skinfolds values than Group A. Hyperinsulinemia (>84pmol/L) and significantly (p=0.00000) higher HOMA IR and HOMA b -cell index were found in Group B compared with Group A. Also Group B had significantly higher serum triglyceride and a higher systolic and diastolic blood pressure than Group A. The physical activity (min/week) was lower in Group B than in Group A. In addition, waist circumference was higher than the Group A. In summary, insulin resistance and hyperinsulinemia occurred together in these non obese, normoglycemic adolescents. They were metabolically more affected than the Group C.

TABLE 5
Clinical and metabolic characteristics of adolescents
by insulin levels and body mass index

Group C obese adolescents (BMI>25kg/m²) with normal insulin values (<84 pmol/L), (Table 5) had similar insulin levels and HOMA IR values and higher (p=0.004) HOMA b -cell values compared to Group A. Despite this, higher LDL-C, systolic blood pressure and the lowest HDL-C value were observed. These high levels might be related to an increase in BMI as it was established in Table 2. This group had similar BMI and skinfolds to the obese hyperinsulinemic group (Group D) and the lowest level of HDL-C of all groups (Table 5).

Group D, obese adolescents with insulin levels > 84 pmol/L, had anthropometric measurements, fasting insulin level, HOMA IR, TC, TG LDL-C, systolic and diastolic blood pressure values significantly higher and HDL-C lower than Group A. Central fat distribution (subscapular to triceps ratio) was significantly higher (p=0.003) in these obese adolescents with higher insulin levels compared with in Group A (Table 5), having boys a higher ratio [p<0.0000] (Table 1).

In general, daily intake of energy and nutrients among the adolescents were higher than required (calculated on the basis of age, weight, sex and physical activity). High protein, fat and carbohydrate intake was observed according to the intake to required ratio values. Dietary fat came mainly from saturated fatty acids. Monounsaturated fatty acids intake did not reach 14% of total fat intake and the P/S ratio was less than 2.00 (recommended ratio). Low intake of vegetables, legumes and fruits was observed. Although no statistically significant, Group B had the highest intake of protein and saturated fat and the lowest intake of monounsaturated fat (Table 6).

TABLE 6
Daily intake of energy and nutrients of the adolescents
by insulin levels and body mass index

DISCUSSION
This study found that 37% of the adolescents with BMI 21.5 ± 19 kg/m² (Group B) presented higher fasting insulin levels, HOMA IR, Tg, systolic and diastolic blood pressure values when compared to Group A.

The mean insulin values were similar in both boys and girls. Hoffman et al (10) have demonstrated that insulin sensitivity is greater in pubertal and early pubertal boys than girls and is primarily determined by body mass. It has been suggested that individuals exist who are not obese on the basis of height and weight, but who, like people with overt obesity, are hyperinsulinemic, insulin resistant and predisposed to type 2 diabetes, hypertriglyceridemia and premature heart disease (1,11). In addition, central abdominal adiposity, even when estimated crudely by anthropometric measures, predicts the development of type 2 diabetes (12), cardiovascular morbidity and mortality (13) and is now recognized as part of the metabolic syndrome that includes insulin resistance, dyslipidemia and hypertension (14). Central adiposity is strongly related to insulin resistance, which is considered the precursor of type 2 diabetes (15). The abdominal fat depot may induce deterioration in insulin sensitivity through its characteristic high lypolysis (16) and its rapid turnover of fatty acids (17). Both insulin resistance and central adiposity were present in adolescents in Group B. Arslanian et al (18) demonstrated, that in nonobese normal children, percent adiposity is an important correlate of in vivo insulin sensitivity. However, independent of degree of adiposity, there remains significant relationship between diastolic blood pressure and insulin sensitivity as well as insulin levels in prepuberty, and insulin levels and VLDL in pubertal subject, possibly signaling the initiation of syndrome X in childhood.

Ferrari et al (19) observed that lean offspring (BMI<22.5 kg/m²) with one hypertensive parent, had significantly higher systolic blood pressure and they were insulin resistant. This situation has also been reported in nonobese, normoglycemic offspring and/or first degree relatives of patients with type 2 diabetes (20). Similarly, our Group B were first or second degree relatives of patients with type 2 diabetes [49 % (31/63)] and/or hypertension [(76% (48/63)]. The family history for diabetes was slightly higher in Group B compared with group A (49% vs 44%). Both had similar percentage for family history of hypertension. According to this result the alteration observed in Group B maybe related to other factors.

Bergström et al (2) reported that during midpuberty serum insulin values in both boys and girls were higher in the younger age group. In this study, the mean age in Group B was not different to that of adolescents with similar or higher BMI. It has been suggested that the peak in serum insulin level in midpuberty results from an increased resistance to insulin (21). In fact, Table 5 shows that Group B, with high fasting insulin levels, had HOMA IR values significantly higher (2 fold) than those of the Group A. Insulin is known to have a growth hormone-like effect, and an increased level may be caused by an increased secretion of insulin related to the pubertal growth spurt. In effect, Group B had significantly higher HOMA b -cell index, indicative of non-impaired insulin secretion. They also had higher triglyceride levels, a good marker for risk of atherosclerosis at an early age (22). Amiel et al (23) suggest that insulin resistance during puberty is restricted to peripheral glucose metabolism. Selective insulin resistance leading to compensatory hyperinsulinemia may serve to amplify insulin’s effect on amino acid metabolism, thereby facilitating protein anabolism during this period of rapid growth.

In general, increases in insulin resistance tend to parallel the increases in BMI. This was observed in the adolescents studied (Table 3). However, a small group (Group C) with BMI>25 kg/m² had fasting insulin levels as low as Group A. Parameters linked to insulin resistance (high insulin level, high triglyceride levels, low HDL-C and high blood pressure) were found in the subjects of this study and emphasize the importance of increases in BMI in adolescents. Universal definition of obesity related to cutoff in adolescents has not been established. In Venezuela, the 90^th and 97^th of body mass index for age and sex have been recommended to define overweight and obesity based on FUNDACREDESA Venezuela Project (7). Cole et al (24) proposed cutoff points to define child obesity. These cutoff points are recommended for international comparisons of prevalence of overweight and obesity. According to Cole et al data (24) the Group B, having a BMI=21.5 kg/m² (15.5 ± 0.9 years) can be considered lean. Adolescents with BMI>25 kg/m² will be obese. The Group B had an increase in adipose tissue mass (measured by skinfolds and waist circumference) compared with normoinsulinemic adolescents in Group A. BMI was positively correlated (p<0.05) to central fat distribution estimated as the subscapular to triceps ratio. Also, BMI was significantly correlated with circumferences (waist, hip and waist/hip ratio) and skinfolds (Table 2). Central obesity, like hyperinsulinemia and insulin resistance, not only accompanies but also antedates metabolic disorders such as type 2 diabetes (11,25) and coronary heart disease (26). That it may be a very early event is suggested by the finding of the Bogalusa Heart Study (27,28) that increases in central fat, as reflected by increases in subscapular, suprailiac and subcostal skinfold thickness, correlate closely with increases in plasma insulin, blood pressure, and triglycerides in school-aged (6-18 y) children.

This study also found variations in serum insulin levels, partly caused by variations in daily dietary intake. Daily intake of energy and nutrients were higher than the requirements calculated for gender, ages and physical activity. A recent report suggests that high-protein diets may contribute to the more rapid progression of type 2 diabetes from IGT (29). Kitagawa et al (30) demonstrated that an increased incidence of noninsulin dependent diabetes mellitus among Japanese schoolchildren correlates with an increased intake of animal protein and fat. Intake of animal fat increased the consumption of saturated fatty acids that may increase insulin levels and decrease P/S ratio; these nutritional alterations increase the cardiovascular disease risk.

Another factor present in the Group B was the lowest physical activity in comparison with Group A. These adolescents require neither the high intake of calories nor the larger amount of carbohydrates. Physical activity has a beneficial effect in reducing plasma insulin levels among people with and without type 2 diabetes (31). Moderate amounts of physical activity have a significant role in reducing the burden of hyperinsulinemia and diabetes among ethnic populations at highest risk for these conditions (32). Daily physical activity is important to decrease risk for insulin resistance. Group B with low physical activity, hyperinsulinemia and insulin resistance have high risk to develop diabetes.

In conclusion, this study shows that adolescents with BMI as low as 21.5 kg/m², but exhibiting hyperinsulinemia and insulin resistance, that co-exist with low physical activity and high energy, animal protein and fat intake, are at increased risk for obesity, cardiovascular disease and diabetes.

ACKNOWLEDGEMENTS
To Ing. Angel Casanova, Professor of Statistics for his valuable advise with the analysis of the data.

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