There is a strong link between men's low levels of testosterone and having two or more chronic diseases such as arthritis, high blood pressure, and type 2 diabetes — even in the under-40s.
This was the conclusion of a study that, for the first time, examined the relationship between total testosterone, age, and specific chronic conditions in a representative sample of the adult male population of the United States.
"If we look at data for men from a population level," states lead author Mark Peterson, an assistant professor of physical medicine and rehabilitation at the University of Michigan in Ann Arbor, "it has become evident over time that chronic disease is on the rise in older males."
He goes on to explain, however, that they are also finding that obesity and physical inactivity may be responsible for testosterone decline "even at younger ages."
The researchers report their findings in a paper now published in the journal Scientific Reports.
The traditional view of testosterone is that it is mainly involved in sex-specific development of the male human body. However, it is becoming increasingly evident that the hormone plays many different roles, including some that are not sex-related, in both males and females.
We know, for example, that testosterone is important for bone health and cardiovascular function, and that it can have a beneficial impact on "lean muscle mass and body fat."
Testosterone decline and disease risk
The new study concerns the importance of testosterone in male human health, and what effects might be linked to its decline.
Previous studies have demonstrated that testosterone levels fall in men as they get older, and that deficiency of the hormone is also linked to the sorts of chronic diseases that often accompany obesity.
"But it hasn't been previously understood," explains Prof. Peterson, "what the optimal levels of total testosterone should be in men at varying ages, and to what effect those varying levels of the hormone have on disease risk across the life span."
In their new paper, he and his colleagues note that while "normal ranges" for total testosterone in "young healthy men" have been defined, these are based on studies of groups that are not representative of the "growing, ethnically diverse U.S. population."
In addition, those studies excluded people with "chronic multimorbidity" — that is, those with more than one chronic illness.
Therefore, Prof. Peterson and colleagues designed their study to examine the relationship between total testosterone deficiency and chronic multimorbidity in "a large, population-representative sample of U.S. men." They also designed it to look at the effect in different age groups.
Data drawn from national health survey
The team used data from the National Health and Nutrition Examination Survey, which assesses the health and dietary status of the U.S. population using a combination of interviews and physical exams.
Their analysis included 2,161 men aged 20 and older for whom there was complete information on: age and other demographics, diagnosed chronic illnesses, testosterone levels as measured from blood samples, grip strength, and laboratory-confirmed risk factors for cardiometabolic disease.
First, they calculated the prevalence of nine chronic conditions in the representative sample. These were: arthritis, cardiovascular disease, clinical depression, high blood pressure, high cholesterol, high triglycerides, pulmonary disease, stroke, and type 2 diabetes.
They then examined how the prevalence of having two or more of these conditions varied across "young, middle-aged, and older men with and without testosterone deficiency."
Young men were defined as aged 20–39.9 years, middle-aged men as 40–59.9 years, and older men as aged 60 years and above.
Significant 'dose-response relationship'
The results from across all age groups revealed that having two or more chronic diseases was more common in men with low total testosterone than in those with normal levels, and it was particularly striking among the young men and older men.
Further analysis showed that there was a significant "dose-response relationship" at play, which Prof. Peterson says "means that men should be concerned about declining total testosterone, even if it has not reached a level to warrant a clinical diagnosis [...]."
The level of clinical diagnosis he refers to is if total testosterone falls below 300 nanograms per deciliter, or 10.4 nanomoles per liter.
The scientists suggest that while the study does not prove that the decline of testosterone causes chronic diseases, it should prompt further research and expand clinicians' understanding of the hormone.
They also hope that their findings will raise men's awareness about testosterone deficiency. "A lot of men may not be aware of the risk factors for testosterone deficiency because of their current lifestyle," says Prof. Peterson.
"And more importantly, that declining levels could be contributing to a silent decline in overall health and increased risk for chronic disease."
Prof. Mark Peterson