There is regular increment in the height and weight of growing adolescents. The weight comprises of fat mass and fat free mass. The blood pressure varies with the age of the child and it is closely related to height and weight. Significant increases in blood pressure occur during adolescence, and many temporary variations take place before the more stable levels of adult life are attained (16).

There are several studies associating blood pressure in children and adolescents with body fatness i.e. stating that high body fat is associated with high blood pressure in children and adolescents (5, 6,16). Most of them are done in children and adolescents of foreign populations (12, 2, 15). Some investigators (1) studied only adults in this regard.

As per our review of literature, none of the above references where adolescent populations were studied, analyzed correlation of FMI and FFMI with blood pressure. None of the such studies has been done in Gujarati Indian ethnic adolescents.

In many studies (18, 20), BMI has been considered as good indicator of fatness while certain other studies consider body fat percentage (7) and Fat Mass Index (FMI) as a better measure of fatness than BMI.

According to the new recommendations for defining childhood obesity, FMI approach is more sensitive than the BMI approach so, we decided to compare correlation of FMI and FFMI with blood pressure. The expression of a change in FM in absolute value fails to allow an appropriate comparison among subjects of different body size so, to define childhood obesity, FMI is better than FM (7).

The objective of this study was to determine which portion of the BMI, FMI or FFMI has more strong influence on blood pressure in Gujarati Indian school going adolescents.

A randomized cross sectional non-interventional study was conducted after the approval of the ethical committee of the institute and obtaining the informed consent from the parents/guardians and the principals of the schools.733 schoolchildren of 10-18 years of both genders of all socioeconomic classes were selected by multistage sampling from five schools of Anand district of Gujarat. The subjects below 10 years were not included in the study because according to W.H.O., adolescent age is 10-19 years. (19) In addition, the body fat monitor (OMRON HBF-306) does not measure body fat in subjectsbelow 10 years. None of the selected subject was above 18 years. The subjects with presence or a history of any acute or chronic disease state that would affect the study variables were excluded.

Measuring blood pressure

The Systolic blood pressure (SBP) and Diastolic blood pressure (DBP) were measured by Omron T8 automatic Blood Pressure instrument (Accuracy: BP: ± 4mmHg, PR: ± 5) validated by Association for the Advancement of Medical Instrumentation (AAMI) and British Hypertension Society (BHS) (13). A small- sized cuff for thin subjects (arm circumferences of 17 to 22 cm (at the center of brachium) with a small arm circumference and a medium-sized cuff with a bigger arm circumference for bigger adolescents (for arm circumferences of 22 to 32 cm (at the center of brachium) were used (17).

Our purpose was to get correlation of blood pressure with body composition so, to avoid circadian variation in the blood pressure recordings, we measured the blood pressure during before lunch periods in all the subjects uniformly.

The children and their class teachers were well explained the experiment. In each session of BP recording, the procedure was demonstrated. The students were given sufficient rest before starting the blood pressure recording. In addition, we behaved as friends to the children to avoid white coat rise in blood pressure.

The subjects were given a 10 minutes' rest and no intake of tea, coffee, food, water in last half an hour was allowed. The blood pressure was measured in sitting posture with a back rest, with cubital fossae at the level of heart (17).

Pulse rate and Blood pressure readings of each subject were recorded at the intervals of 1 minute. The average of the three consecutive readings was used for statistical analysis.

Body composition

The body mass index (BMI) and fat percentage were measured by OMRON HBF-306, a body fat monitor based on bioelectrical impedance principle. This fat monitor is based on the principle of bioelectrical impedance. The body weight was recorded bare footed in kg to the nearest 0.5 kg. The height was measured using meter scale without footwear to the nearest 1 cm. The BMI was computed by the machine as the weight (kg) divided by the square of height (m). Body Fat Percentage (BF %) and Total Body Fat Mass (FM) were assessed by bioelectrical impedance technique using Omron Body Fat Monitor 2 HBF -306 with standardized clothing.

Statistical analysis was done by computing Pearson correlation coefficients and linear regressions. On the basis of body mass, fat mass and lean body mass various indices were calculated. The associations of indices of body fat and lean body mass with blood pressure were computed using correlations.

BMI and FFMI in boys and girls were not much different from each other but, the FMI in girls (7.0±9.7) was much more than those in boys (2.8±1.8). This was so because in the age group of 10-18, in girls, the fat deposition is more than that in the boys due to pubertal changes. Blood pressure values overall also were not significantly different in boys and girls.

The major finding of this study is that all the blood pressure variables are more positively correlated with FFMI than with FMI in both boys and girls. Moreover, comparing association of FMI and FFMI with blood pressure: in all subjects (n=733); FFMI was more strongly associated with blood pressure than FMI. These findings may signify that in growing adolescents there is growth in both fat mass and lean body mass and size of vascular tree also grows with growth in the body size. And, as we know, according to the Poiseuille’s law, resistance to the blood flow is directly proportional to the length of the blood vessels; with increase in the size (length) of the vascular tree, resistance to blood flow will also increase and hence blood pressure will also increase (9).

On the contrary, in adults, Ahmed M-el B et al (1) found a lower correlation of FFMI than that of FMI with blood pressure. In adults, this may be due to the almost fixed size of the vascular tree. The significant negative correlation of SBP in girls with their FMI may be owing to the protective action of estrogens against high BP (3). Estrogens cause vasodilation by release of nitric oxide (EDRF = endothelium derived relaxing factor) and EDHF (endothelium derived hyperpolarizing factor) (14).

According to a review by Kienitz T et al (10), longterm net effect of androgens appears to be vasoconstriction via upregulation of thromboxane A 2 expression, norepinephrine synthesis, angiotensin II expression, and endothelin-1 action. Furthermore, androgens cause cardiac hypertrophy, promote atherosclerosis, vascular remodeling and stimulate renal prohypertensive processes involving the reninangiotensin-aldosterone system.

According to WHO, the puberty in boys also begins at about 10 years’ age (19). So in the age group of 10-18, boys are generally having higher levels of testosterone than those below 10 years. Similarly the girls of study sample will also be having higher estrogen levels. Thus these hormones may be contributing to the differences between the BP: FMI correlations of boys and that of girls.

The mean blood pressure (MBP) was strongly correlated with BMI is all subjects and boys showed stronger correlation with MBP than the girls. The SBP was strongly correlated with BMI in boys while girls showed no significant correlation of SBP with BMI. As far as DBP is concerned, both boys and girls showed strong correlation with BMI. These differing influences of BMI on blood pressure in adolescents may also be owing to the effects of estrogen and testosterone in girls and boys respectively.

On regressing FMI and FFMI on mean blood pressure (Tables IV, V and VI), in all three groups (all subjects, boys and girls), The FFMI was having more significance on blood pressure than the FMI. In case of girls, MBP was not having any statisticalsignificance with FMI in comparison to that with FFMI.

Still, in morbidly obese children and adolescents, to avoid cardiovascular risks, body fat reduction may be a better means of getting rid of body mass as fat mass can be reduced but fat free mass reduction is difficult. Further, obesity in long term can cause hypertension and other cardiovascular morbidities by several other mechanisms. For instance, according to Braunwald’s Heart Disease: A textbook of cardiovascular medicine, with weight gain, reflex sympathetic activation is thought to be an important compensation to burn fat but at the expense of sympathetic overactivity in target tissues (i.e., vascular smooth muscle and kidney) that produce hypertension. Near-maximal rates of sympathetic firing are seen in hypertensive patients with the metabolic syndrome with or without new-onset type 2 diabetes. Although the sympathetic activation associates with insulin resistance, the precise stimulus to sympathetic outflow is unknown, withleptin, other adipokines, and angiotensin II being potential candidates (4).

Conclusion

The blood pressure was found to be more strongly associated with lean body mass index than to the fat mass index in adolescents. As far as prevention of hypertension is concerned, reducing body fat may remain as important measure to prevent hypertension; as body fat mass is reducible while lean body mass may not be reducible and, in long term, obesity can lead to hypertension by various mechanisms.