WORCESTER, MASSACHUSETTS. Both total and bioavailable testosterone levels decline as men age and the decrease is often accompanied by a decline in sexual function, loss of bone density and muscle mass, and an increase in cholesterol level. The decline in bioavailable testosterone is partially due to an age-related increase in sex hormone-binding globulin (SHBG) which removes the free testosterone from circulation. Researchers at the University of Massachusetts Medical School have just completed a major study aimed at determining the relationship between diet and the level of SHBG in middle-aged men. The study was carried out in 1987-1989 as part of the Massachusetts Male Aging Study and involved 1522 men aged 40 to 70 years. All study participants were interviewed in person, had blood samples drawn and were evaluated for height, weight, and waist and hip circumferences. They also completed the Willett semiquantitative 1-year food frequency questionnaire.
Multiple regression analysis of the collected data showed that a high intake of protein
corresponded to a lower level of SHBG and thus by inference, a higher level of free
testosterone. A high fiber intake, on the other hand, was clearly correlated with a high
SHBG level. Other research has shown that vegetarians tend to have higher SHBG levels
than do non-vegetarians. The data analysis also found that advanced age and a high
serum testosterone level were associated with higher SHBG levels whereas a greater
body mass index and waist/hip ratio both correlated with a lower SHBG level. No
correlation was found with total calorie intake or intakes of fats and carbohydrates.
The researchers conclude that elderly men can increase their bioavailable testosterone
levels by increasing their protein intake.