CHAPTER ONE
1.1 AN
OVERVIEW ON TESTOSTERONE
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles,
birds, and other vertebrates. Like all
steroid hormones, it has a steroid nucleus. In mammals, testosterone is primarily secreted in
the testicles of males, specifically in the leydig cells and the ovaries of females, although small amounts are also
secreted by the adrenal glands. It is the
principal male sex hormone and an anabolic steroid.
In
men, testosterone plays a key role in the development of male reproductive
tissues such as the testis and prostate as well as promoting secondary sexual
characteristics such as increased muscle, bone mass, and the
growth of body hair.
In addition, testosterone is essential for
health and well-being as well as the prevention of osteoporosis. (Tuck SP, et.al 2009)
On
average, in adult human males, the
plasma concentration of testosterone is about 7–8 times as great as the
concentration in adult human females' plasma,( Southren AL, et.al 1967), but as the
metabolic consumption of testosterone in males is greater, the daily production
is about 20 times greater in men.
Females also are more sensitive to the hormone. Testosterone is observed
in most vertebrates. Fish make a slightly
different form called 11-ketotestosterone. Its counterpart in insects is ecdysone. (De Loof A 2006). These ubiquitous steroids suggest that sex hormones have an ancient evolutionary history.
1.2 BIOLOGICAL EFFECTS OF TESTOSTERONE
In
general, androgens promote protein synthesis and growth of those tissues with
androgen receptors. Testosterone
effects can be classified as virilizing or androgenic and anabolic, though the distinction is somewhat
artificial, as many of the effects can be considered both.
· Anabolic effects include growth
of muscle mass and strength,
increased bone density and strength,
and stimulation of linear growth and bone maturation. (Bhasin S et.al 1996).
·
Androgenic effects include maturation of the sex organs, particularly the penis and the formation of the scrotum in the fetus, and after birth (usually
at puberty) a deepening of
the voice, growth of the beard and axillary hair. Many of these
fall into the category of male secondary sex characteristics. (Mooradian AD, et.al 1987).
1.3 Biosynthesis of testosterone
Like other steroid hormones,
testosterone is derived from cholesterol,
whose transport into the inner mitochondria membrane from the outer membrane
for the initiation of the synthetic pathway of steroid hormones is catalysed by
a regulatory protein known as steroidogenesis acute regulatory protein (StAR). The first step in the biosynthesis
involves the oxidative cleavage of the sidechain of cholesterol by CYP11A(cytochrome p450, family 11, subfamily A, POLYPEPTIDE
1), a mitochondrial
cytochrome P450 oxidase with the loss of six carbon atoms to give pregnenolone.
In the next step, two additional carbon atoms are removed by the CYP17A
enzyme in the endoplasmic
reticulum to yield a variety of C19
steroids. In addition, the 3-hydroxyl group is oxidized by 3-β-HSD (3β-hydroxysteroid
dehydrogenase) to produce androstenedione.
In the final step, the C-17 keto group androstenedione is reduced by 17-β
hydroxysteroid dehydrogenase to yield
testosterone.
The largest
amounts of testosterone (>95%) are produced by the testes in men. It is also
synthesized in far smaller quantities in women by the thecal cells
of the ovaries, by the
placenta, as
well as by the zona reticularis of the adrenal cortex
and even skin in both
sexes. In the testes, testosterone is produced by the Leydig cells.
The male generative glands also contain Sertoli cells
which require testosterone for spermatogenesis.
Like most hormones, testosterone is supplied to target tissues in the blood
where much of it is transported bound to a specific plasma protein,
sex hormone
binding globulin (SHBG).
CHAPTER TWO
2.1
THE BODY MASS INDEX
The body
mass index (BMI), or Quetelet index, is a measure for human
body shape based on an individual's weight and height, (Eknoyan, Garabed 2007). It was devised between 1830 and 1850 by
the Belgian polymath Adolphe Quetelet during the
course of developing "social physics". Body mass index is defined as
the individual's body mass divided by the square of their height. The formulae
universally used in medicine produce a unit of measure of kg/m2.
BODY
MASS INDEX
(Gadzik, James 2006).
BMI can also be determined using a BMI chart,
which displays BMI as a function of weight (horizontal axis) and height
(vertical axis) using contour lines for different values of BMI or colors for
different BMI categories.
2.2
BODY MASS INDEX CATEGORIES
A
frequent use of the BMI is to assess how much an individual's body weight
departs from what is normal or desirable for a person of his or her height. The
weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as
muscularity also affect BMI significantly (see discussion below and overweight). The WHO regards a BMI of less than 18.5
as underweight and may indicate malnutrition, an eating disorder, or other health problems, while
a BMI greater than 25 is considered overweight and above 30 is considered obese. These ranges of BMI values are valid only as
statistical categories, (WHO.
2006).
Category
|
BMI
range – kg/m2
|
Very severely
underweight
|
less than 15
|
Severely
underweight
|
from 15.0 to
16.0
|
Underweight
|
from 16.0 to
18.5
|
Normal
(healthy weight)
|
from 18.5 to
25
|
Overweight
|
from 25 to 30
|
Obese Class I
(Moderately obese)
|
from 30 to 35
|
Obese Class II
(Severely obese)
|
from 35 to 40
|
Obese Class
III (Very severely obese)
|
over 40
|
2.3 Some
exceptions to the categories stated above
BMI does not differentiate between body fat and muscle mass. This means there are some exceptions to the BMI guidelines.
- Muscles – body builders and people who have a lot of muscle bulk will have a high BMI, but are not overweight.
- Physical disabilities – people who have a physical disability and are unable to walk may have muscle wasting. Their BMI may be slightly lower, but this does not necessarily mean they are underweight. In these instances, it is important to consult a dietitian who will provide helpful advice.
BMI is not totally independent of
height and it tends to overestimate obesity among
the shortest people and underestimate it among the tallest. Therefore,
BMI should not be used as a guide for adults who are very short (less than 150
cm) or very tall (more than 190 cm) (MacKay,N.J.2010).
BMI may not correspond to the same degree of fatness in different populations. Asians and Indians, for example, have more body fat at any given BMI compared to people of European descent. Therefore, the cut-offs for overweight and obesity may need to be lower for these populations. This is because an increased risk of diabetes and cardiovascular disease begins at a BMI as low as 23 in Asian populations. (Korevaar, Nick 2003).
BMI may not correspond to the same degree of fatness in different populations. Asians and Indians, for example, have more body fat at any given BMI compared to people of European descent. Therefore, the cut-offs for overweight and obesity may need to be lower for these populations. This is because an increased risk of diabetes and cardiovascular disease begins at a BMI as low as 23 in Asian populations. (Korevaar, Nick 2003).
