A seminar write-up on the effect of testosterone on BMI by James, Simeon and Anthonia

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 axillaryHYPERLINK "http://en.wikipedia.org/wiki/Axillary_hair" 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 AdolpheHYPERLINK "http://en.wikipedia.org/wiki/Adolphe_Quetelet" HYPERLINK "http://en.wikipedia.org/wiki/Adolphe_Quetelet"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).

CHAPTER THREE

3.1   EFFECT OF TESTOSTERONE ON BODY MASS INDEX

In the light of the effect of testosterone on the body as a whole, primarily, its anabolic effects (Bhasin S et.al 1996),   as stated above, testosterone could easily be said to have a ‘’dual’’ effect on the body mass index, i.e. it could either increase or decrease the body mass index.

Taking for instance, the effect of testosterone on the body’s muscles; testosterone tends to increase muscle mass and strength (Bhasin S et.al 1996), this then leads to an increase in overall body mass as the muscle mass contributes about 15% to the total body mass. This increase in body mass tends to increase the body mass index as mass, in KG, is a parameter used in calculating the body mass index and it is directly proportional to it.

Mathematically, for instance, a man weighs, say, 70kg and has a height of , say, 1.82 meters, this will give a BMI value of 21.13 kg/m²   , however, when there is an increase in weight to, say, 75kg, with the height remaining constant, there will be an increase in the body mass index to 22.64 kg/m²

To further buttress the fact that testosterone could increase BMI with respect to body weight, it is known that one of the anabolic effect of testosterone is causing an increase in bone mass and density which undoubtedly can result in an increase in body mass and thus the body mass index.

Furthermore, Testosterones can also drastically increase appetite due to which people eat more calories than the amount that is burnt. This in turn tends to make them gain weight thus causing an increase in the body mass index.

On the other hand, however, due to some of the effect of testosterone on the body, like stimulation of linear growth, probably due to the inhibition of the closure of bones epiphysis, testosterone could cause a decrease in BMI, as height, unlike weight, has an inverse effect on body mass index i.e. an increase in height would lead to a decrease in the body mass index. This fact is also substantiated with the fact that testosterone levels can affect the metabolism of the body. Men who have a balanced circulation of this hormone in their body have a healthy basal metabolic rate due to which additional fat is burnt down to increase energy and fat storage in the body decreases which to some extent can result in a reduction in body weight and thus a reduction in body mass index.

More so, studies have shown that as men age, testosterone level in the body decreases, this decrease is however ascribed to the conversion of testosterone to an estrogen derivative known as estradiol in a process known as aromatization catalysed by the enzyme aromatase produced by fat cells, (Meinhardt U, et.al 2002), which stimulates fat deposition thus increasing the body mass index.

3.2 CONCLUSION

From the illustrations above, it can be inference that Testosterone can either increase or decrease the body mass index due to its anabolic effects (Bhasin S et.al 1996) and its effect on appetite which in turn affects either body mass or height. However, by affecting the mass of the body, it can lead to an increase in body mass index, as mass, as illustrated above, is directly proportional to body mass index, i.e. an increase in mass will cause an increase in body mass index.

Also, due to the effect of testosterone on body height, it can lead to a decrease in body mass index as height has an inverse relationship with the body mass index i.e. an increase in height will lead to a decrease in body mass index.

REFERENCES

1.     Southren AL, Gordon GG, Tochimoto S, Pinzon G, Lane DR, Stypulkowski W (May 1967). "Mean plasma concentration, metabolic clearance and basal plasma production rates of testosterone in normal young men and women using a constant infusion procedure: effect of time of day and plasma concentration on the metabolic clearance rate of testosterone". J. Clin. Endocrinol. Metab. 27 (5): 686–94.

2.     Mechoulam R, Brueggemeier RW, Denlinger DL (September 1984). "Estrogens in insects". Journal Cellular and Molecular Life Sciences 40 (9): 942–944.

3.     Bhasin S, Storer TW, Berman N, Callegari C, Clevenger B, Phillips J, Bunnell TJ, Tricker R, Shirazi A, Casaburi R (July 1996). "The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men". N. Engl. J. Med. 335 (1): 1–7.

4.     De Loof A (October 2006). "Ecdysteroids: the overlooked sex steroids of insects? Males: the black box". Insect Science 13 (5): 325–338.

5.     "BMI classification". Global Database on Body Mass Index. WHO. 2006.

6.     MacKay, N.J. (2010). "Scaling of human body mass with height: The body mass index revisited". Journal of Biomechanics 43 (4): 764–6.

7.     Gadzik, James (2006). "'How much should I weigh?' Quetelet's equation, upper weight limits, and BMI prime". Connecticut Medicine 70 (2): 81–8.

8.     Korevaar, Nick (July 2003). "Notes on Body Mass Index and actual national data".

9.     Meinhardt U, Mullis PE (August 2002). "The essential role of the aromatase/p450arom". Semin. Reprod. Med. 20 (3): 277–84.