Testosterone and Body Fat
Testosterone and body fat are intimately related for us guys. Either one
can strongly affect the other and even lead to a viscious cycle of increased
weight gain and lowered testosterone.
Let's start with what happens when you put on extra body fat: you slowly
begin to increase cortisol, lower testosterone and decrease human growth
hormone. If you reach medical obesity, all three of these hormones are negatively impacted
very significantly. 
Why does increased body weight lead to a reduction in testosterone levels?
The reasons are many, but one of the most important is increased aromatization.
Fat cells contain aromatase and as you increase weight your levels of this
enzyme increase. Unfortunately, aromatase converts testosterone to
estrogen (estradiol) and so the all important testosterone/estradiol ratio is
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This creates a viscious cycle because the less testosterone you have, the easier it is
to lose muscle and, since "muscle burns fat", weight gain comes much more easily,
leading to further losses in testosterone, etc. The loss in growth
hormone and the increase in cortisol contribute to the same phenomenon.
In fact, cortisol will also create a similar viscious cycle, especially for those with an "apple
shape", which most guys are. Studies show that abdominal fat, or visceral fat as
it is called in the medical world, has much higher levels of cortisol. 
Therefore, as you inflate that spare tire around your middle, more cortisol is
created and cortisol will definitely reduce testosterone. Again, you can
see the viscious cycle here: lowered testosterone will lead to an increase
in overall body fat including abdominal and you have a very negative downward
spiral going on.
Put this all together and it explains the reason that visceral fat and low testosterone are
intimately and negatively related in the studies.  Either one can contribute to the
other. And both are associated with increased risk of diabetes,
cardiovascular disease and insulin resistance.  (See my link on Low
Testosterone Symptoms as well.)
NOTE: Visceral fat is also linked to increased risk of cardiovascular
disease and certain cancers as well.
So it is critical to get your testosterone tested regularly and keep your weight
down. Read my links on How to Improve Your Testosterone Naturally for
ideas as well.
1) Metabolism, Sep 1995, 44(Supplement 3):Pages 21-23 "Endocrine
abnormalities of obesity"
2) Science, Dec 7 2001, 294(5549):2166-2170, "A Transgenic Model of Visceral
Obesity and the Metabolic Syndrome"
3) Metabolism, Sep 1990, 39(9):897-901, "Visceral fat accumulation in men is
positively associated with insulin, glucose, and C-peptide levels, but
negatively with testosterone levels"
4) Obesity, 2006, 14:16S–19S, "Why Visceral Fat is Bad: Mechanisms of the