Again, I am not saying that there hasnt been a decline in T. What I am saying is that the role of environmental factors play hasnt been exactly elucidated. Obesity, diabetes and illicit substance abuse have all skyrocket in the last 2 to 3 decades. The role these factors play is to my opinion downplayed and the role of environmental factors exaggerated.
The devil is in the details. Lets compare for example the two studies linked below.
Travis, 2008 found a population decline in T. They used blood samples from 3 waves of the MMAS; T1 around 1988, T2 around 1996 and T3 around 2003.
Travis et al followed the subjects as they aged, so its the same cohort.
The limitations of this study:
- its not a comparison between young, healthy men in 1988 and 2003 using the same T assays.
- Travis measured the T levels of the ageing men and had to ‘calculate back’ T levels on the basis of the known (or believed to be known) decline in T levels during aging to do an age matched comparison
- As men aged, they got sicker and needed significantly more medication. We dont know how much this contributed to the decline in T and if the assumption made to allow the age matched comparison was actually correct
- T levels were measured at different time points, potentially leading to a bias in results (T1 assays were performed in 1994, whereas T2 and T3 samples were assayed soon after in-home visits)
The study by Platz et al 2019 in contrast used samples of the NHANES cohorts from around 1989 and compared them to the cohort measured around 2001. They found that in never-smoking lean men without comorbidities the T concentration was the same (624 vs 626 ng/dl). So no decline of T levels in this period. The strenghts of this study:
- Direct comparison of age matched samples; they were never-smoking, lean men without aging-associated comorbidities
- Platz measured all samples in the same lab with the same method and the same instrumentation. Method precision was around 2 times higher in this study as compared to the Travis study (CV 4.5% vs 8 to 9%)
So to summarize, while Travis found a decline of about T levels of around 70 to 80 ng/dl within less than a decade, Platz et al found no such difference in the same period of time.
https://academic.oup.com/jcem/article/92/1/196/2598434
Two interesting studies were published on that topic in the last year. The first one talks about contributing factors such as obesity, T2D, illicit drug abuse, environmental factors on low T levels in yound adults.
The most recent one discusses a decline in T levels in US young mean. Unfortunately I dont have access to the article, but it will be interested to take a closer look at eg the exact T assay used in this study.
‘Limitations include the influence of confounding variables such as environmental factors and the use of differing assays for TT measurement.’