Testosterone deficiency in men, aka hypogonadism, is associated with increased total and abdominal fat mass, and reduced muscle mass, which negatively impacts body composition.[1, 2] This contributes to development of risk factors like insulin resistance, chronic inflammation, and atherogenic dyslipidemia (a triad of increased blood levels of small, dense LDL particles and triglycerides, and decreased levels of HDL particles), which increase the risk for cardiovascular disease, metabolic syndrome and diabetes.[1, 3-16]
Previous studies have shown that testosterone replacement therapy ameliorates these risk factors in testosterone deficient (hypogonadal) men; it increases insulin sensitivity [17-20] and HDL (the "good" cholesterol) [9, 10, 20, 21], and reduces waist circumference [9, 20, 22], fasting blood glucose [9, 20] triglycerides (blood fats), LDL (the "bad" cholesterol) [19, 22-24], and several inflammatory markers.[17, 25]
A 2011 meta-analysis concluded that testosterone replacement therapy improves metabolic control, as well as reduces abdominal obesity. Many studies have shown that testosterone replacement therapy in hypogonadal men increases muscle mass and reduces fat mass.[19, 26-32] Further, adding testosterone (50 mg/day for 1 year, administered as a transdermal gel) to a diet and exercise program results in greater therapeutic improvements of glycemic control and reverses the metabolic syndrome.
Testosterone also has direct (non-obesity mediated) beneficial effects on many metabolic and cardiovascular risk factors [12, 33-37], and reduces death risk independently of body fat status. In line with all these effects, low testosterone levels are associated with increased risk of cardiovascular complications , and all-cause and cardiovascular disease death [40-42]. Low testosterone may thus be a predictive marker for men at high risk of cardiovascular disease. In a group of men aged 50-91 who were followed for 20 years, it was found that men whose total testosterone levels were in the lowest quartile (241 ng/dl or lower) were 40% more likely to die than those with higher levels, independent of age, adiposity, lifestyle or presence of cardiovascular risk factors.
Thus, treatment of testosterone deficient men with testosterone has demonstrated considerable health benefits. Despite this, critics state that most of the studies on testosterone replacement therapy were too small. They also argue that the studies were of too short duration (most of them lasting 6-12 months), and that the long-term effects of testosterone on body composition are not known.
Two 5 year long studies were just published that addressed the duration and small study size shortcomings in previous research...
A few days ago, Jan 29th 2014, a controversial study  was published showing that men aged 65 years and older, had a two-fold increase in the risk of heart attack in the 90 days after filling an initial TT
prescription, regardless of cardiovascular disease history. Among younger men below 65 years of age with a history of heart disease, the study reported two to three-fold increased risk of MI in the 90 days following an initial TT prescription.
This study has stirred up heated discussions and media headlines. Let's dissect it and look under the hood...