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Original Studies |
University of Massachusetts Medical School (C.L.), Worcester, Massachusetts 01655; and New England Research Institutes (H.A.F., J.B.M., A.B.A.), Watertown, Massachusetts 02172
Address all correspondence and requests for reprints to: Dr. C. Longcope, University of Massachusetts Medical School, Worcester, Massachusetts 01655.
| Abstract |
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Analyzed by multiple regression, controlling for testosterone and estradiol levels, age (P < 0.001) and fiber intake (P = 0.02) were positively correlated to SHBG concentration, whereas body mass index (P < 0.001) and protein intake (P < 0.03) were negatively correlated to SHBG concentration. The intakes of calories, fat (animal or vegetable), and carbohydrate were not related to SHBG concentration. We conclude that age and body mass index are major determinants of SHBG concentrations in older men, and fiber and protein intake are also significant contributors to SHBG levels, but total caloric intake and the intake of carbohydrate or fat are not significant. Thus, diets low in protein in elderly men may lead to elevated SHBG levels and decreased testosterone bioactivity. The decrease in bioavailable testosterone can then result in declines in sexual function and muscle and red cell mass, and contribute to the loss of bone density.
| Introduction |
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Several small scale studies of the relation between dietary composition (fiber, caloric, and protein intake) and SHBG levels show conflicting results. In women, a high fiber diet was shown to decrease SHBG levels (13, 14), whereas vegetarians (women and men) were reported to have increased SHBG levels compared to nonvegetarians (14, 15, 16). In another study, women with anorexia who were given increased calories had a decrease in SHBG levels (17), whereas other research indicates that a very low calorie diet results in a doubling of SHBG levels over a short term in women with polycystic ovary syndrome (18). Reed et al. (12) noted that normal men fed a high fat diet had a decrease in SHBG levels, whereas a diet low in fat resulted in an increase in SHBG levels. Vermuelen et al. (19) noted that a high protein diet increased SHBG levels. However, in rabbits fed a diet low in protein, there was a marked increase in SHBG levels (20).
Given these conflicting findings and the potential importance of dietary composition in regulating the circulating concentrations of SHBG (which will, in turn, affect the levels of bioavailable testosterone and estradiol), the purpose of this report was to investigate the relation between dietary components and SHBG with data from the Massachusetts Male Aging Study.
| Subjects and Methods |
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The MMAS participants were typically Caucasian (95%), employed (78%), and married (75%). Nearly half were Catholic (48%). Most had completed high school (71%), and many had earned at least a bachelors degree (42%). The low representation of racial minorities (4%) was consistent with the composition of the Massachusetts population. The distributions of body mass index (BMI), blood pressure, and serum cholesterol in the MMAS sample closely matched those in the second National Health and Nutrition Examination Survey. The 1563 men who completed the dietary assessment (91.5%) had a slightly higher mean age than those who did not complete it (mean, 55.4 vs. 53.2 yr) and a lower prevalence of current cigarette smoking (23.4% vs. 35.2%), but did not differ with respect to body weight, BMI, waist/hip ratio (WHR), alcohol intake, or serum concentration of SHBG, testosterone, or estradiol.
Data collection
A trained technician visited each subject in his home between 08001000 h and obtained written informed consent. Height, weight, and waist and hip circumferences were measured by standardized methods developed for large scale field work (22). Dietary intake was measured by the Willett semiquantitative 1-yr food frequency questionnaire (23). Current cigarette smoking was determined by self report. The subjects customary alcohol intake was estimated by self report of beer, wine, and liquor consumption, accounting for frequency, quantity, and binge drinking, using the Khavari formula (24).
Blood samples were drawn from the antecubital space within 2 h of the subjects awakening to control for diurnal variation. Two tubes were taken 30 min apart for hormone assays and were pooled in equal aliquots at the time of assay to smooth out episodic secretion (25). Blood was kept in an ice-cooled container for transport and was centrifuged within 6 h. Serum was stored in 5-ml scintillation vials at -20 C, shipped to the laboratory on dry ice within 1 week by same-day courier, and stored at -70 C until the time of assay. SHBG was measured by filtration assay (26), with a within-assay coefficient of variation of 8.0% and a between-assay coefficient of variation of 10.9%. Testosterone was measured by RIA (Diagnostic Products, Los Angeles, CA). Estradiol was measured by RIA after solvent extraction and Celite chromatography (27). For both testosterone and estradiol, the inter- and intraassay coefficients of variation were less than 10%.
Data analysis
Serum concentrations of SHBG and estradiol and daily alcohol intake were log transformed for analysis to reduce the influence of extreme values. The resulting distributions were virtually normal as judged by the Shapiro-Wilk statistic (P > 0.25).
Pearson correlation coefficients were used to assess the simple association of log SHBG with the following independent variables: age, weight, BMI, WHR, total energy intake (kilocalories per day), serum testosterone and estradiol concentrations, current cigarette smoking, and daily intakes of protein, carbohydrate, fiber, and fat (animal, vegetable, and total).
Multiple regression analysis was conducted to identify a maximal set of independent variables that maintained a statistically significant association with SHBG when controlled for all other variables in the model. An adjusted effect size for each independent variable was constructed from the corresponding log regression coefficient by calculating the percent difference in SHBG resulting from a 1 SD change in the independent variable (in the case of age, a 10-yr change). Statistical Analysis System software was used for all computations (28).
| Results |
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Multiple regression analysis produced a set of 6 variables (Table 3
) that were all significantly associated
with SHBG when controlled for one another. The model was first
identified by a backward elimination procedure, beginning with the 15
variables listed in Table 2
. An exhaustive model-testing algorithm
confirmed that this model accounted for more variance in SHBG than any
other 6-variable model constructed from those 15 predictors. Deletion
of 8 outliers (extremely low testosterone, high body size, or high
fiber intake) did not affect the selection of variables or parameter
estimates. Although no additional variables significantly improved the
model, log estradiol was added as a seventh variable because it
improved the Cp goodness of fit statistic (29)
and because the effect estimates for other variables were thereby
adjusted for both major sex steroids. The fraction of variance
explained by the 7-variable model was r2 =
25%.
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Protein and fiber, both of which showed weak associations in simple correlation analysis, entered the multiple regression model with slight gains in statistical significance (P = 0.03 and P = 0.02, respectively). None of the three fat variables (animal, vegetable, and total) was associated with SHBG in multiple regression, whether entered singly or in pairs. Cigarette smoking became insignificant, and total energy, carbohydrate, and alcohol remained insignificant when controlled for other variables. Subjects were questioned as to recent loss of appetite, and men who gave a positive answer to that question did have a slightly higher mean SHBG concentration (36.6 vs. 32.0 nmol/L; P = 0.05). However, when the appetite variable was added to the multiple regression model the significance greatly diminished (P = 0.12), indicating that its effect was explained by the other variables in the model.
To compare effect sizes among the predictors, we used the fitted
regression coefficients to calculate the percent change in SHBG
corresponding to a 1 SD change in each significant
independent variable (Table 3
; for age, we used a 10-yr change).
Testosterone and age were strongest in effect size as well as
statistical significance, producing changes on the order of 15% in
SHBG for a 1 SD change in the predictor. The anthropometric
variables (BMI and WHR) showed about half that effect (67%). The
dietary effects were, in turn, half as large (3%).
| Discussion |
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The concentration of SHBG was significantly correlated with age and anthropometrics. These results confirm the findings of others (30, 31). However, we found that weight, which is often used as a predictor of SHBG concentration (10), was not an independent predictor of SHBG when controlled for BMI and WHR. Future investigations should consider measuring BMI and WHR rather than (or in addition to) weight.
The dietary components that correlated best with SHBG levels were protein and fiber. Protein intake, which is marginally significant when tested by simple correlation, is more strongly significant when tested using multiple regression. Thus, the lower the protein intake, the higher the concentration of SHBG. This mirrors our findings in rabbits (20) and indicates that protein intake can be an important control of SHBG level.
The mechanism by which protein intake can be a controlling factor on SHBG concentration is uncertain. One of the major controlling factors on SHBG synthesis is insulin. This intake of protein has been shown to increase insulin levels (32), and insulin has been shown to reduce SHBG levels (33, 34). The effect of protein on SHBG could be mediated in part by its effect on insulin, with a low protein intake leading to low insulin levels and release of the inhibition of SHBG synthesis. If this were to be the mechanism by which protein effects SHBG levels, one would expect that carbohydrate (CHO) intake, a stimulus for insulin release, would also effect SHBG levels. However, we could find no significant relationship between CHO intake and SHBG levels when tested by simple correlation or controlling for other factors. Therefore, it is likely that the relationship of protein intake to SHBG levels involves more than a possible effect on insulin, but it is unclear from our data what that may be. It should be noted that low protein intake was directly correlated with CHO, fat, and caloric intake, so that the lower intake of protein was not being replaced by increased CHO or fat.
It has been suggested that fat intake may be related to SHBG levels (35, 36). In this sample the simple correlation between animal fat and SHBG is significant. However, when controlled for potential confounders such as age, hormones, and anthropometrics, the association no longer remains.
There is conflicting evidence on the importance of fiber intake to SHBG levels. Our finding that fiber intake is correlated positively to SHBG levels, even after controlling for age, testosterone and estradiol, BMI, WHR, and protein intake is at variance with an earlier report indicating a negative correlation between fiber and SHBG. However, other research indicates that increasing fiber intake is associated with higher SHBG. Why our present results are at variance with those of Dorgan et al. (37) is not clear, but in that study the caloric intake was almost twice that in the present study, and the study design and analysis were different from those of the present study and those of the study by Adlercreutz et al. (38).
The results of this study have implications for research and clinical practice. In future research of diet and SHBG, the examination of relations between diet and SHBG levels should control for the potential confounding effects of numerous factors, such as age, hormone profiles, and anthropometrics. With regard to practice, in our previous work (39) an increase in SHBG and a related decrease in testosterone have been noted to occur in men as they age. With regard to practice, the inverse relationship between protein and SHBG suggests that in elderly men a high protein diet could increase bioavailable testosterone and mitigate the effects of the age-related decrease in that hormone. Intervention studies will be necessary to verify this.
Received June 11, 1999.
Revised September 15, 1999.
Accepted October 4, 1999.
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