Predictors of South African male testosterone levels: the THUSA study

The Paternal Provisioning Hypothesis: effects of workload and testosterone production on men's musculature

OBJECTIVES

Testosterone supports male reproduction through a broad range of behavioral and physiological effects, including the maintenance of sexually dimorphic muscle used in male-male competition. Although it is often assumed that a persistent relationship exists between men's testosterone production and musculature, most studies either fail to find evidence for such a relationship, or document very weak associations. In nonhuman primates, by contrast, correlations between testosterone and muscle mass are higher. Here, we propose the "Paternal Provisioning Hypothesis," which predicts that men's skeletal muscle is less dependent on the effects of androgens than that of other primates, and more sensitive to the physical demands of men's work. This permits human fathers to downregulate testosterone, which has negative impacts on pair-bonding and parenting effort, but without sacrificing the strength and musculature necessary to provision mates and offspring.

METHODS

We tested predictions of the Paternal Provisioning Hypothesis by assessing parental status, salivary testosterone levels, anthropometry, and strength among 122 men (ages 18-78) at the Mogielica Human Ecology Study Site in rural Poland. We chose this population because men practice subsistence agriculture, regularly engaging in physically demanding labor. Grip and chest strength were assessed using a dynamometer, and upper-body musculature was estimated from arm muscle circumference.

RESULTS

In this population, testosterone showed no association with measures of strength or musculature, and was lower in older men and pair-bonded fathers. Marital and parental status and workload, by contrast, were positive predictors of muscle mass and strength measures.

DISCUSSION

These findings offer support for the Paternal Provisioning Hypothesis.

Association of testosterone levels with socio-demographic characteristics in a sample of Ugandan men

BACKGROUND

Testosterone, a male reproductive hormone, affects several physiological processes, such as sperm production, energy, strength, sexual behavior, sleep and the general well being of men. Normal levels of testosterone are necessary to effect these physiological processes. The objective of this study was to study the association between testosterone levels in a sample of Ugandan men with socio-demographic characteristics, and compare the testosterone levels of Ugandan men with that of men in other countries.

METHODS

Eighty men were enrolled from the medical campus at Makerere University. Blood samples were drawn from 7.00 - 8.00 a.m. and total testosterone was measured using radioimmunoassay. The free and bioavailable testosterone was calculated after measuring sex hormone binding globulin (SHBG) and albumin in the blood samples. Self-administered questionnaires were used to obtain socio-demographic characteristics of the subjects. Biometric measurements including weight, height and waist circumference were also recorded.

RESULTS

Serum testosterone levels of Ugandan men were within the normal physiological ranges. Married participants and those with dependents had lower testosterone than unmarried participants and those without dependents respectively. Sexually active participants had lower testosterone levels than those who were not sexually active.

CONCLUSION

Testosterone levels were lower in association with several socio-demographic characteristics including being married, having dependents, and daily coital frequency. Further research is warranted into the relationship between testosterone levels and contributory sexual behavior that may be important in understanding the spread of HIV/AIDS, given its high prevalence in sub-Saharan Africa.

Racial variation in sex steroid hormone concentration in black and white men: a meta-analysis

Sex steroid hormones are associated with chronic diseases and mortality with risk associations that differ between racial and ethnic groups. However, it is currently unclear whether sex steroid hormone levels differ between black and white men. The aim of this study was to assess racial variation in circulating testosterone, free testosterone, sex hormone-binding globulin (SHBG) and estradiol levels in men. We searched PubMed for articles comparing circulating hormones in black and white men. A meta-analysis was performed using weighted mean differences (WMD) to compare hormones levels between black and white men. Fifteen eligible studies were identified; three did not report adjusted means. After age adjustment, free testosterone levels were significantly higher in black than in white men (WMD = 4.07 pg/mL, 95% CI 1.26, 6.88). Depending on the free testosterone concentration in white men, this WMD translates into a racial difference ranging from 2.5 to 4.9%. Total testosterone (WMD = 0.10 ng/mL, 95% CI -0.02, 0.22), estradiol (WMD = 0.67 pg/mL, 95% CI -0.04, 1.38) and SHBG (WMD = -0.45 nmol/L, 95% CI -1.75, 0.85) concentrations did not differ comparing blacks with whites. After adjustment for age, black men have a modestly but significantly 2.5 to 4.9% higher free testosterone level than white men. Based on previous studies on effects of sex steroid hormones on risk of chronic diseases or mortality, this modest difference is unlikely to explain racial differences in disease risk.

Testosterone is associated with self-employment among Australian men

Testosterone has pronounced effects on men's physiological development and smaller, more nuanced, impacts on their economic behavior. In this study of 1199 Australian adult males, we investigate the relationship between the self-employed and their serum testosterone levels. Because prior studies have identified that testosterone is a hormone that is responsive to external factors (e.g. competition, risk-taking), we explicitly control for omitted variable bias and reverse causality by using an instrumental variable approach. We use insulin as our primary instrument to account for endogeneity between testosterone and self-employment. This is because prior research has identified a relationship between insulin and testosterone but not between insulin and self-employment. Our results show that there is a positive association between total testosterone and self-employment. Robustness checks using bioavailable testosterone and another similar instrument (daily alcohol consumption) confirm this positive finding.

Marriage and motherhood are associated with lower testosterone concentrations in women

Testosterone has been hypothesized to modulate the trade-off between mating and parenting effort in males. Indeed, evidence from humans and other pair-bonded species suggests that fathers and men in committed relationships have lower testosterone levels than single men and men with no children. To date, only one published study has examined testosterone in relation to motherhood, finding that mothers of young children have lower testosterone than non-mothers. Here, we examine this question in 195 reproductive-age Norwegian women. Testosterone was measured in morning serum samples taken during the early follicular phase of the menstrual cycle, and marital and maternal status were assessed by questionnaire. Mothers of young children (age ≤3) had 14% lower testosterone than childless women and 19% lower testosterone than women who only had children over age 3. Among mothers, age of the youngest child strongly predicted testosterone levels. There was a trend towards lower testosterone among married women compared to unmarried women. All analyses controlled for body mass index (BMI), age, type of testosterone assay, and time of serum sample collection. This is the first study to look at testosterone concentrations in relation to marriage and motherhood in Western women, and it suggests that testosterone may differ with marital and maternal status in women, providing further corroboration of previous findings in both sexes.

Do evolutionary life-history trade-offs influence prostate cancer risk? a review of population variation in testosterone levels and prostate cancer disparities

An accumulation of evidence suggests that increased exposure to androgens is associated with prostate cancer risk. The unrestricted energy budget that is typical of Western diets represents a novel departure from the conditions in which men's steroid physiology evolved and is capable of supporting distinctly elevated testosterone levels. Although nutritional constraints likely underlie divergent patterns of testosterone secretion between Westernized and non-Western men, considerable variability exists in men's testosterone levels and prostate cancer rates within Westernized populations. Here, I use evolutionary life history theory as a framework to examine prostate cancer risk. Life history theory posits trade-offs between investment in early reproduction and long-term survival. One corollary of life history theory is the 'challenge hypothesis', which predicts that males augment testosterone levels in response to intrasexual competition occurring within reproductive contexts. Understanding men's evolved steroid physiology may contribute toward understanding susceptibility to prostate cancer. Among well-nourished populations of Westerners, men's testosterone levels already represent an outlier of cross-cultural variation. I hypothesize that Westernized men in aggressive social environments, characterized by intense male-male competition, will further augment testosterone production aggravating prostate cancer risk.

Sex ratio at birth and mortality rates are negatively related in humans

Evolutionary theory posits that resource availability and parental investment ability could signal offspring sex selection, in order to maximize reproductive returns. Non-human studies have provided evidence for this phenomenon, and maternal condition around the time of conception has been identified as most important factor that influence offspring sex selection. However, studies on humans have reported inconsistent results, mostly due to use of disparate measures as indicators of maternal condition. In the present study, the cross-cultural differences in human natal sex ratio were analyzed with respect to indirect measures of condition namely, life expectancy and mortality rate. Multiple regression modeling suggested that mortality rates have distinct predictive power independent of cross-cultural differences in fertility, wealth and latitude that were earlier shown to predict sex ratio at birth. These findings suggest that sex ratio variation in humans may relate to differences in parental and environmental conditions.

Birth weight and early socio-economic disadvantage as predictors of sex hormones and sex hormone binding globulin in men at age 49-51 years

OBJECTIVES

A number of associations have been shown between early growth and later sex hormone levels in women, but less is known about this relationship in men. This study investigated life-course predictors of sex hormones in men in the Newcastle Thousand Families birth cohort.

METHODS

The Newcastle Thousand Families Study is a prospective study initiated in 1947. At age 49-51 years, 574 study members returned detailed self-completion questionnaires and 412 attended for clinical examination, including 172 men in whom blood samples were taken. Estradiol, follicle stimulating hormone (FSH), luteinizing hormone (LH), testosterone, and sex hormone binding globulin (SHBG) were measured. Free testosterone concentrations were also calculated.

RESULTS

Social class at birth independently predicted FSH and LH, with higher levels with increasing socioeconomic disadvantage. SHBG was higher with increasing standardized birth weight and lower with increasing contemporary body mass index (BMI). BMI also predicted LH, SHBG, and testosterone. None of the variables included within this analysis were significant predictors of estradiol. No other associations were seen with any of the variables included from across the life-course.

CONCLUSIONS

Our findings suggest that birth weight may be positively associated with SHBG and early socioeconomic status may be related to FSH and LH in men. These novel findings are independent of contemporary BMI. Given the links between sex hormones, SHBG and disease outcomes such as type II diabetes and osteoporosis, it is possible that sex hormones may play a mediating role in the associations between circumstances in early life and later risk of chronic disease.

Age-related patterns of body composition and salivary testosterone among Ariaal men of Northern Kenya

BACKGROUND AND AIMS

Loss of muscle mass and gain of adipose tissue are hallmarks of aging among men in western populations. However, the extent to which these changes reflect inherent senescence is unclear. To determine age-related patterns of body composition and testosterone (T) among men in a subsistence population we sampled 104 nomadic and 102 settled men, ages 20+ years, among the Ariaal of northern Kenya.

METHODS

Anthropometric measures included height, weight, skinfolds, and waist circumference. Saliva samples, collected in the morning and afternoon, were assayed for T. Results were analyzed using general linear models, based on 10 year age groups.

RESULTS

Average overall BMI was 17.9+/-2.0 kg/m2. Settled and nomadic males did not differ in fat free mass (FFM). FFM peaked in the 30s with a significant decline in the oldest age group. Percent body fat and waist circumference increased significantly across age groups. Morning mean (+/-SD) salivary T was higher among settled males (107.6+/-57.6 vs 63.4+/-38.1 pmol/L; p<0.001), but did not differ significantly across age groups. Salivary T was positively related to suprailiac skinfolds, percent body fat and waist circumference among the settled males.

CONCLUSIONS

Age-related patterns of body composition, but not salivary T, among Ariaal men are consistent with those observed in western populations. Together these results suggest that lower FFM after 60 may reflect senescent processes among human males, while age patterns of adiposity reflect energy balance. T does not appear to play an essential role in age-related patterns of body composition.