There have been a lot of media reports and other nonexperts claiming testosterone is declining and the reason for this decline is manmade chemicals acting as endocrine disrupters. Let’s talk a fresh unbiased view of the evidence and see what’s what.
The Decline Of Testosterone: USA
The most important study in this area is an observational study conducted in Boston, USA (1). It measured the testosterone of men at 3 separate points in time 1987-89, 1995-97, and 2002-04; including a total of 1532 participants. You can see from the graph below that those measured in 1987-89 had the highest testosterone. For instance, a 60 year old man in 1987, compared to a 60 year old man in 2002, had roughly 70ng/dL higher testosterone.
Further analysis reveals that men born in earlier generations had higher testosterone than those born in later generations. Overall, the study found a 1% decrease in testosterone per year. That is a very significant finding, as it means over the 17 year study period, they saw a 17% decrease.
The Decline Of Testosterone: Denmark
Another important study in this area was conducted by a research team at Copenhagen’s University Hospital (2). The researchers analysed data from 4 large surveys, including a total of 5350 men. The first graph below shows a dramatic decrease in testosterone by birth year. However, the second graph shows the same results but adjusted for evaporation. The researchers found that over time the oldest samples had suffered from very slight evaporation, which increased the concentration of testosterone. Thereby, artificially elevating the testosterone level of the sample. However, we can still see men born from 1921-26 (the red line) had significantly higher testosterone than those born later.
The Rise in BMI and The Decline in Testosterone
The Danish researchers also adjusted the results according to the subjects’ BMI. A high BMI had a negative effect on testosterone, and the men born later had higher BMIs than those born earlier. They state:
The observed age-independent decline in male serum testosterone levels could be explained by a concurrent secular increase in BMI.
So, we can see that the men born later had lower testosterone because they had higher BMI. In contrast, the American study also adjusted for BMI, but still found a 1% decline in testosterone per year.
Furthermore, the researchers also noted:
the concurrent increase in BMI explained only part of the secular trend in male SHBG serum levels
SHBG (Sex Hormone Binding Globulin) is a hormone that regulates the level of free testosterone. It does this by binding to the majority of the testosterone, rendering it inactive for the time being. When SHBG levels decrease, more testosterone becomes unbound and therefore active. In short, the more SHGB, the less active testosterone.
The researchers speculated that the rise in insulin resistance may have contributed to the higher levels of SHGB (as they are associated with each other). Insulin resistance is your body’s ability to store carbs efficiently. This is damaged by high sugar diets, which have risen dramatically in the 20th century.
What About The Decline In Sperm?
In 2017, a large meta-analysis (3) showed that:
the mean sperm count declined, on average, 1.4% per year with an overall decline of 52.4% between 1973 and 2011.
The researchers noted that ‘there was no statistical support for the use of a nonlinear model’. This means that they are predicting the 1.4% per year decrease to continue, with no signs of leveling off. They also explained that the decline was found in North America, Europe, Australia, and New Zealand, but not in non-western countries. However, there have been declines demonstrated in the rest of the world, but they tend to be less serve. One study found a 0.4% per year decrease in sperm concentration from 1965-2015, in Asian men (4). Compare that to the 1.6% per year decrease found in western men.
The Testosterone/ Sperm Relationship
Testosterone is critical in the development of healthy sperm. But, how much is needed? There is some association between testosterone and sperm count (5). However, there are few studies directly measuring and correlating sperm count and testosterone.
The worldwide decline in sperm count is indicative of declining testosterone levels, but they cannot be said to be one and the same. However, both sperm count and testosterone levels are parameters for healthy testicles and the decline in both suggests a decline in male reproductive health.
Why Has Testosterone Declined?
- Obesity. The Danish study found higher BMIs were associated with lower testosterone. Obesity rates have tripled since 1975, therefore much of the blame lies here (rise in obesity explained). Fat tissue synthesizes the enzyme aromatase, which converts testosterone to estrogen, lowering men’s testosterone levels (6). This is the most significant factor in the decline of testosterone .
- Nutrient Deficiencies. Since we eat so much poor quality, processed food, nutrient deficiencies are on the rise. Our food is full of calories but devoid of nutrients. Moreover, the soils our food is grown in are increasing becoming depleted of minerals. The main micronutrients for testosterone production are: Folate, B6, Vitamin D, Vitamin K, Magnesium and Zinc. Although a deficiency in anything may impact production.
- Less Sleep. Sleep times are decreasing. People work more and play more, both cutting into our precious rest time. The advent of the computer and smartphone give us access to endless entertainment for late nights. And, the blue light they emit keeps us awake by stimulating daytime sunlight. The relationship between sleep and testosterone is well established: one study showed a 10-15% decrease in testosterone after a week of 5hrs sleep per night (7).
- More Toxins. Many man-made chemicals we are exposed act as endocrine disrupters. This means they affect the way our hormonal system works. Many mimic estrogen and have been shown to decrease testosterone (8). Sources of these include plastics, canned foods, and personal care products. The level of these and other environmental pollutants have been increasing in the environment and food supply, since their creation. (Read about toxins in red meat).
- PhytoEstrogens. These are naturally occurring compounds in plants. They act like estrogen and can decrease testosterone levels in the body (9). The most significant source of these is soy and soy products. Soy has enjoyed a boom in popularity and is commonly known as a health food. However, this reputation is undeserved and the rise in soy consumption is contributing to the male fertility decline.
- Pharmaceutical Drugs. Many drugs are known to decrease testosterone including statins, opioids, and antidepressants (10). Moreover, there are trends in the data that link those taking multiple medications, to lower testosterone levels. The use of medications has risen substantially and is likely contributing to the decline of testosterone levels.
- Studies show that there has been a decline in testosterone over the past 40 years (at least). This is roughly 1% per year.
- There has also been a dramatic decline in sperm count and quality. This is more pronounced in western countries, compared to non-western countries.
- Both of these trends show that male reproductive health is declining, and shows no signs of leveling off.
- Obesity is the dominant factor contributing to this decline.
- Other major factors are nutrient deficiencies, reduced sleep, toxins (endocrine disrupters), soy consumption and pharmaceutical drugs.