Effect of Age, Body Weight and Smoking on Testosterone in Men

 This study showed that 

 1- Men with higher Body Mass Index (a measure of weight that takes into account height, calculate yours here: http://1.usa.gov/XBIO5K ) had lower testosterone as they aged compared to thinner men. In fact, "a change in BMI from nonobese to obese may be equivalent to a 15 yr fall in T" Note: BMI is a very rough measure that does not take into account muscle or fat mass.  A muscular man with little body fat may have high BMI (in my case, my BMI is 30). A better study would measure fat mass, but that tends to be cumbersome and expensive.

 2- Smoking did not affect testosterone, but it made the pituitary gland send a higher LH signal to testicles, which could be caused by progressive testicular dysfunction. 

 3- Not surprisingly, sex hormone binding globulin increased with age. SHBG binds to testosterone and renders it ineffective. Higher insulin levels and lower IGF-1 levels caused by aging may account for this increase in SHBG.

4- Aging causes primary testicular dysfunction with maintained total T and progressively blunted free T associated with higher LH. This interpretation is supported by the age-related attrition of the testicular Leydig cell population  and other factors involved with testicular function. 

Reference: http://jcem.endojournals.org/content/93/7/2737.long

Ethnicity, Smoking and Body Composition Influence Testosterone and Estradiol Levels in Healthy Young Adult Men in Malaysia: A Pilot Study

Background: Variations in sex hormone levels can be caused by differences in ethnicity, smoking habits, and body composition and may be related to racial differences in the prevalence of certain diseases. 

Objectives: This study examined the effects of ethnicity, smoking, and body composition on testosterone and estradiol levels in a group of young Malaysian men. 

Materials and Methods: In this cross-sectional study, 189 Malay and Chinese men aged 20-39 years answered a detailed questionnaire, underwent body anthropometric measurements, and had their blood drawn for hormone assays. 

Results: The results indicated no differences in testosterone levels between races (P>0.05), but estradiol levels were significantly higher among Malay compared with Chinese men (P<0.05). No difference was detected in sex hormone levels between smokers and non-smokers (P>0.05). However, smokers with more than 10 years of cigarette smoke exposure had a significantly higher estradiol level than smokers with 1-5 years of exposure (P<0.05). Testosterone (total, free, and non-SHBG-bound) levels correlated inversely and significantly (P<0.05) with body mass index (BMI), waist-to-hip circumference ratio (WHR), and percentage of body fat. By multiple stepwise regression, body fat percentage was the most influential predictor of testosterone (β=-0.456 for total, β=-0.279 for free, β= -0.297 for non-SHBG-bound fraction) and SHBG levels (β=-0.172). 

Conclusions: Estradiol levels are influenced by ethnicity and duration of smoking, whereas testosterone levels are governed by body fat percentage in Malaysian young adult males.

Ref: http://endometabol.com/?page=article&article_id=3937