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Dietary Fat Intake and Endogenous Sex Steroid Hormone Levels in Postmenopausal Women

From the Channing Laboratory, Department of Medicine, and Division of Preventive Medicine, Harvard Medical School and Brigham and Women’s Hospital; Department of Obstetrics and Gynecology, Brigham and Women’s Hospital; Departments of Epidemiology, Nutrition, and Biostatistics, Harvard School of Public Health, Boston, MA.

Address reprint requests to Michelle D. Holmes, MD, DrPH, Channing Laboratory, Department of Medicine, Harvard Medical School and Brigham and Women’s Hospital, 181 Longwood Ave, Boston, MA 02115.

	ABSTRACT

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PURPOSE: To examine the relationship between plasma levels of reproductive sex steroid hormones in postmenopausal women and their reported fat intake.

METHODS: We measured plasma sex steroid hormones levels in plasma collected in 1989 and 1990 from 381 healthy postmenopausal women. For each woman, we measured fat intake in 1986 and 1990 by a food-frequency questionnaire. The cross-sectional associations between the percentage of energy from total and specific types of dietary fat intake and plasma hormone levels were assessed by linear regression, controlling for energy intake, obesity, and protein intake.

RESULTS: The plasma estradiol level was 4.3% lower (95% confidence limits, -8.3%, -0.2%) for a substitution of 5% of energy from fat intake for an equivalent amount of energy from carbohydrate when adjusted for obesity and other covariates. Estradiol was also inversely associated with all other fat types except trans fat; the inverse associations with vegetable fat and marine omega-3 fats were statistically significant.

CONCLUSION: We observed an inverse association between total fat intake averaged over 4 to 5 years and estradiol levels. This result is inconsistent with the hypothesis that fat intake predisposes to breast cancer risk by raising endogenous estrogen levels.

	INTRODUCTION

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ENDOGENOUS reproductive hormones have been strongly implicated in the etiology of breast cancer; most studies in postmenopausal women show increased risk with elevated estrogen levels.^1,2 On the basis of animal studies,^3,4 international comparisons,^4-6 and a meta-analysis of case-control studies,⁷ high intake of total dietary fat has been postulated to increase breast cancer risk. The fact that a large randomized trial, the Women’s Health Initiative, is attempting to lower breast cancer risk with a low-fat diet attests to the appeal of this hypothesis.

However, mechanisms by which high intake of dietary fat could increase breast cancer risk have not been established. If high dietary fat intake were to increase estrogen levels, this would provide a plausible mechanism. In a recent meta-analysis of metabolic studies, significantly lower estradiol levels were found in the low-fat intervention groups.⁸ However, most of the component studies in the meta-analysis had no concurrent control group and were confounded by weight loss in the intervention group.⁹

Prospective studies of dietary fat and breast cancer, which unlike case-control studies are not prone to recall bias, do not support the dietary fat hypothesis.^10-12 For a protective effect to be evident, some authors have suggested that fat intake must account for 20% of energy. These authors maintain that such a protective effect has not been found in cohort studies because they were conducted in Western populations, in whom the level of fat intake is rarely this low.⁴ In a pooled analysis of seven international cohorts with nearly 5,000 breast cancer cases, the multivariate relative risk for consuming less than 20% of energy from fat compared with consuming 30% to 35% of energy from fat was 1.06 (95% confidence interval, 0.83 to 1.37).¹¹ In addition, in 14 years of follow-up of the Nurses’ Health Study (NHS) using a diet assessed at four different times, we found a 15% greater risk of breast cancer in women who consumed 20% of energy from fat than in those who consumed 30% to 35% of energy from fat. Although this increased risk was not statistically significant, the overall linear trend for higher risk with lower fat intake was P (for trend) = .03.¹² In this study, we examined the relationship between plasma levels of reproductive sex steroid hormones in 381 postmenopausal women on the basis of data collected in 1989 and 1990 and their reported fat intake, averaged over two dietary assessments made in 1986 and 1990.

	METHODS

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NHS Subjects and Blood Sample Collection
In 1976, the NHS cohort was established when 121,700 female registered nurses from across the United States, aged 30 to 55 years, answered a mailed questionnaire on risk factors for cancer and cardiovascular disease. Every 2 years since, we have sent follow-up questionnaires to NHS participants. For this analysis, information on height and age at menarche was assessed in 1976 and on weight at the age of 18 in 1980. Information on parity and participant’s age at the birth of the participant’s first child was last assessed in 1984, when participants were 38 years of age. Information on age at menopause, smoking status, weight change since the age of 18, family history of breast cancer, and personal history of benign breast disease was assessed in 1990. Physical activity was also measured in 1990 by a validated and reproducible questionnaire¹³ that requested self-report of weekly leisure time activity estimated for the past year. Scores were calculated in Metabolic Equivalent Task (MET) hours per week, with one MET-hour defined as the energy expended in sitting quietly for one hour. Higher MET values for other activities reflect multiples of energy expenditure needed for sitting quietly; for example, 10 MET-hours would represent 21/2 hours of brisk walking.

Blood samples were collected in 1989 and 1990 from 32,826 NHS participants who were from 43 to 69 years of age at the time, as previously detailed.¹⁴ Each woman was sent a kit that contained all supplies needed for blood collection, plus a supplemental questionnaire about menopausal status, recent postmenopausal hormone use, time since last meal, and time of day of blood sampling. Participants arranged to have their blood drawn and then mailed the whole-blood sample cooled with an enclosed ice pack via overnight mail. We have previously documented the stability of the hormones during the period of transport.¹⁵ After receipt in our laboratory, samples were centrifuged, divided, and frozen in the vapor phase of liquid nitrogen freezers (-130°C or colder). Hormone levels for the plasma samples used in this analysis were assayed in three batches between February and September 1993. This study was approved by the Institutional Review Board of the Brigham and Women’s Hospital, Boston, MA.

Women included in this analysis were control subjects in a nested case-control study of plasma hormone levels and breast cancer risk.¹⁶ In addition, 49 women who had reported low fat intake (< 25% and < 19% of energy on the 1986 and 1990 food-frequency questionnaires [FFQs], respectively) were added to increase the range of fat intake. These cut points were chosen to maximize the number of women with low fat intakes on both questionnaires, and all women who met these criteria were included in this analysis. These samples were interspersed with the control samples and were assayed at the same time. Women included were all postmenopausal (no menses for at least 12 months before blood sampling) and had not used hormones for at least 3 months before the blood collection. Participants had no previously diagnosed cancer (except nonmelanoma skin cancer) and could not have implausible scores for total energy intake (< 500 kcal or > 3,500 kcal/d).

Semiquantitative FFQs
In 1980, a 61-item FFQ designed to assess dietary intake was added to the biennial NHS questionnaire. In 1984, 1986, and 1990, an expanded FFQ was used. The FFQs have been described in detail, and their validity and reproducibility have been documented elsewhere.^17,18 The percentage of total energy intake accounted for by fat intake was calculated, including alcohol intake in total energy. In this analysis, nutrient intake from the 1986 and the 1990 FFQs were averaged for each woman; for the 0.8% of women who did not fill out the 1986 FFQ, only the 1990 values were used.

Laboratory Analysis
Except for estrone sulfate, all hormones were analyzed at the Nichols Institute (San Juan Capistrano, CA) by radioimmunoassay. The first two batches of estrone sulfate were assayed by C. Longcope, University of Massachusetts Medical School, and the third batch at Nichols Institute. Details of the laboratory methods have been previously reported.¹⁶

In each batch of samples, we assessed laboratory precision by the inclusion of replicate samples unidentifiable to laboratory personnel. Within-batch laboratory coefficients of variation ranged from 6% (percentage bioavailable estradiol) to 13.6% dehydroepiandrosterone (DHEA).¹⁶

Statistical Analysis
Statistical analyses were performed with SAS software (SAS Institute, Cary, NC). First, the distribution of levels for each hormone was determined. We excluded zero to nine women per hormone whose values were greater than the absolute value of the 75^th percentile plus three times the interquartile range. Adjusted geometric least squares mean hormone levels across categories of fat were calculated by regressing the natural logarithm of hormone levels on potential confounders, adding the mean log hormone level to the average of the residuals, and exponentiating this value. These mean hormone levels were the predicted levels at the reference values of the confounders.

The association between the percentage of energy from fat intake and hormone levels was also assessed by linear regression. The robust variance was used to ensure valid inference even if the regression residuals were not normally distributed.¹⁹ Four-knot restricted cubic spline models for the regression of hormone levels on percentage of energy from type of fat, adjusted for other covariates, were used to examine linearity.²⁰ These models were compared with those that assumed a linear association to obtain a likelihood ratio test for nonlinearity. In linear regression, the difference in plasma hormone levels was modeled on the natural logarithm scale. Solving (1-e^ß) x 100, where ß is the estimated regression slope and is the specified incremental difference in fat intake, represents the percentage difference in that hormone level. The effect of total fat and types of fat were expressed as an incremental increase in percentage of energy from fat. That increment was 5% of energy for total fat, (animal, vegetable, saturated, monounsaturated, and polyunsaturated fats), 1% of energy for trans-unsaturated fat, and 0.01% of energy for omega-3 fats from fish.

A secondary analysis was performed to determine whether results could be attributed to a few points having undue influence. In these linear regressions, dfbetas, which approximate the change in the beta coefficient that would result from deletion of the observation from the data set, were calculated for each observation. Participants were omitted whose dfbeta for the multivariate adjusted slope of hormone levels on percentage of energy from fat type was greater than the absolute value of the median dfbeta plus or minus three times the interquartile range of the dfbetas.²¹

	RESULTS

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Characteristics of the 381 women included in the study are listed in Table 1. The mean age was 62.6 years, mean height 64.4 in, and mean body mass index in 1990 25.7 kg/m². They consumed an average of 1,750 kcal per day, including 6.8 g of alcohol, 74 g of protein, and 20 g of fiber. The average percentage of energy intake from fat was 29% (range, 10% to 46%), which reflects the oversampling of women with low fat intake. Of total fat intake, 55% was from animal fat and 45% from vegetable fat. The mean percentage of energy intake from carbohydrates was 51%. Fourteen percent had a first-degree family history of breast cancer, 36% had a personal history of benign breast disease, and 13% were current smokers.

This reversal of association was caused mainly by control for measures of obesity: body mass index at the age of 18 and weight gain since the age of 18. In a multivariate model not adjusted for obesity, a 5% increase in energy from total fat had a nonsignificant positive association with estradiol level, 0.3% (95% CL, -4.2, 4.9). However, in a model that contained as the only covariates age, energy, and obesity, a 5% increase in energy from fat was negatively associated with estradiol level, -2.3% (95% CL, -6.1, 1.6). Results for bioavailable estradiol level, estradiol free percentage, estrone level, SHBG level, and testosterone level were similar.

Fiber intake was not directly associated with hormone levels in this study. However, dietary fiber may modify the association between fat intake and hormone levels, with the greatest inverse association seen in women having the lowest fiber intake. The change in estradiol levels associated with a 5% of energy increase in fat intake across tertiles (lowest to highest) of energy-adjusted fiber intake were -14.5% (95% CL, -19.1, -9.7), 1.5% (-6.6, 10.4), and 2.9% (-6.4, 1.3) (P [for the interaction term] = .25). Results were similar for bioavailable estradiol and estrone sulfate levels.

Tables 5 and 6 are similar to Table 4 except that the relationship between intake of specific types of fat and plasma hormone levels is examined. In each table, all the major types of fat that make up total fat intake are included in the same model; therefore, the estimated effect of each type of fat can be interpreted as a substitution for an equivalent amount of energy from carbohydrates, holding all other aspects of the data constant.

Table 5 shows the relationship of plasma hormone levels with animal and vegetable fat intake. Estradiol level remained inversely associated with both animal and vegetable fat intake, and the linear association with vegetable fat intake was statistically significant (P = .03). SHBG level remained positively associated with animal fat intake (P = .02).

In Table 6, which examines saturated, monounsaturated, polyunsaturated, trans-unsaturated, and omega-3 (from fish) fat intake, estradiol level remained inversely associated with intake of all fat types except trans fats; the linear association with omega-3 fats from fish was statistically significant. Estrone level also had a statistically significant inverse association with omega-3 fats from fish. Strong relationships were seen between some types of fat and some of the androgens. Polyunsaturated fat intake was inversely associated with androstenedione, testosterone, DHEA, and dehydroepiandrosterone sulfate (DHEAS): a 5% higher energy intake from polyunsaturated fat was associated with 19% to 46% lower plasma levels of these hormones. In addition, a 5% higher energy intake from monounsaturated fat was associated with a 35% higher plasma level of DHEA and DHEAS.

The expression of a linear relationship can be distorted by a few influential points. The linear regressions in Tables 4 and 6 were repeated using the procedure for deletion of influential points reported in Statistical Methods. In each model, an average of 8% of the points (range, 3% to 13%) were removed for undue influence. All results remained substantially the same.

	DISCUSSION

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We observed inverse associations between reported dietary intake of total fat averaged over 4 years and five plasma estrogens and testosterone when adjusted for obesity and other covariates; this association was statistically significant for estradiol. Estradiol level, as well as estrone sulfate, estrone, and DHEAS levels, has been positively associated with breast cancer risk in this population.¹⁶ Therefore, the present results are contrary to the hypothesis that high fat intake predisposes to breast cancer risk by raising endogenous estrogens. They are, however, consistent with our previously reported finding of a modest inverse relationship between total fat intake and breast cancer risk.¹²

The inverse association between fat intake and hormone levels that we observed depended primarily on control for obesity. It is important to examine this issue critically. If obesity were an intermediate step between high fat intake and elevated hormone levels (ie, high fat intake leads to obesity, and increased adipose tissue leads to higher hormone levels), then adjustment for obesity would not be appropriate. However, if obesity is independently associated with both fat intake and hormone levels (possibly because health-conscious women restrict both fat intake and total energy) and not an intermediary, then it is a confounder and should be adjusted for.

We examined the cross-sectional association between dietary fat intake (assessed in 1980 and updated in 1984, 1986, and 1990) and body mass index from 1980 through 1994 in the NHS. We used regression methods for clustered data, because this analysis was based on 415,077 observations in 85,804 women.²² We adjusted for the following factors found to be associated with obesity in a previous study²³: smoking status, race, marital status, level of physical activity, parity, body mass index at age 18, alcohol use, and total energy intake. We additionally adjusted for protein intake to determine the association of body mass index with substituting carbohydrate for fat intake.

Cross-sectionally, women who consumed a higher percentage of total energy from fat in this cohort were slightly (0.08 kg/m²) heavier (95% confidence interval, 0.07 to 0.09; P [for a linear association] .0001) per 5% increment in energy consumed from fat.

We also conducted a longitudinal analysis with 450,013 observations using similar methods in the same women. We examined how the percentage of energy from fat intake, cumulatively averaged and updated over time, predicted the change in body mass index over the next 2-year period. Longitudinally, fat intake did not predict any increase in body mass index. An increment of 5% of energy in dietary fat was associated with -0.003 kg/m² in body mass index (95% CL, -0.005, -0.001; P [for an inverse linear association] = .02).

Thus, this cross-sectional association seen between fat intake and body mass index is probably a result of confounding by health consciousness: women who strive to be lean because they believe it to be healthy also consume a lower fat diet because they have been told that it is healthy. This lack of association between dietary fat and body fat is also consistent with randomized trials that have lasted 1 year or longer.²⁴

Because fat intake does not predict weight gain over time in this cohort, obesity does not seem to be an intermediary between fat intake and hormone levels, and there is no reason not to adjust for obesity. Because obesity is associated cross-sectionally with fat intake in this cohort and is also associated with hormone levels,¹⁴ it is a confounder and should be adjusted for. For these reasons, we believe that the multivariate associations of fat intake with hormones, including adjustment for obesity, are the appropriate ones to examine. We note, however, that even when we did not adjust for obesity, the association between dietary fat and estrogen levels was essentially null rather than significantly positive.

Previously, the association of dietary fat and endogenous estrogens has been examined in short-term interventions in which women were fed a low-fat diet and their plasma hormones were measured. In a recent meta-analysis of the results from 13 interventions, including four among postmenopausal women, the authors reported a 23% decline (95% CL, -27.7%, -18.1%) in serum estradiol levels among postmenopausal women who consumed 10% to 24% of energy from fat over a period of 3 weeks to 5 months.⁸

However, the validity of this meta-analysis was compromised by the validity of the component studies. None of the four component studies included a simultaneous control group.^25-28 In addition, in three of the four studies, women experienced statistically significant weight loss,^25,27,28 and a strong positive association between adiposity and postmenopausal estrogen levels has been well documented.^16,29,30

Although small losses in weight are often seen in short-term studies of dietary fat reduction, they are not sustained in longer-term studies.³¹ Thus, studies that last for only a few weeks or months may prompt misleading conclusions about the long-term effects on blood hormone levels. In our present analysis, we found that control for measures of obesity caused a reversal in the associations seen.

Although not included in the meta-analysis because it was conducted among women with breast cancer, one randomized trial among postmenopausal women has been reported. Rose et al³² reported a significant reduction in serum estradiol levels among women with initially higher levels, after an 18-month low-fat diet intervention. However, as described elsewhere,¹⁸ the comparisons in this analysis were within the intervention group, not between the intervention and control groups. In addition, women with the lowest initial estradiol levels were excluded, leaving the reduction to be seen only among women with initially high levels. Because a similar reduction in estradiol levels was seen in the control group, the results are consistent with regression toward the mean and no effect of dietary fat.

In addition to intervention studies, several cross-sectional studies have examined the relationship between low-fat diet and plasma estrogens. In one of the largest of these, with 325 perimenopausal women, London et al³³ reported no association between fat intake and serum estrogen levels. Likewise, a cross-sectional study of 253 postmenopausal women in Wisconsin showed no association between fat intake and serum estrone level.³⁴ In another cross-sectional study of 88 Greek women, no association was noted between fat intake and urinary estrogen levels.³⁵ However, in two cross-sectional studies that included 24 and 93 women, respectively, vegetarian women who consumed a low-fat diet were compared with nonvegetarian women who consumed a higher-fat diet. Both found statistically significantly lower serum or urinary estrogen levels among the vegetarians.^36,37 However, in both of these studies, vegetarian women were less obese than nonvegetarian women, and the groups had many dietary differences other than fat intake. In the larger of these two studies, the study by Armstrong et al,³⁶ adjustment for body mass index diminished the difference in urinary estrogens between vegetarian and nonvegetarian women, although it was still statistically significant; the smaller of these two studies, by Barbosa et al,³⁷ did not adjust for obesity at all. In addition, Barbosa et al report a statistically significantly higher intake of fiber among the vegetarian women, but they did not adjust for it in the analysis,³⁷ even though fiber intake has been hypothesized to lower estrogen levels.³⁸

Little has been written about the effect of low-fat diet on hormones other than estrogens. Breast cancer risk has been positively associated with androgens.^16,39 Of the interventions previously mentioned, only one, that by Ingram et al,²⁸ examined the effect of a low-fat diet on serum androgens among postmenopausal women. A nonsignificant decrease in testosterone level and essentially no change in DHEAS level were reported.²⁸ Both this study and the intervention by Prentice et al²⁷ report a nonsignificant decrease in SHBG level with low-fat dietary intervention. The cross-sectional study from Wisconsin showed no association of dietary fat intake with serum androgen or SHBG levels.³⁴ In the two cross-sectional studies that compared vegetarian women with nonvegetarian women, one reported no significant differences in plasma testosterone levels,³⁶ the other no significant difference in DHEAS levels,³⁷ and both no statistically significant difference in SHBG levels.

Very few cross-sectional studies have examined the correlation between specific types of dietary fat and endogenous sex steroid hormones in postmenopausal women. London et al³³ reported no association between serum estrogens and either saturated fat or linoleic acid. Likewise, Katsouyanni et al³⁵ report no association between urinary estrogens and saturated, polyunsaturated, and monounsaturated fats.

Use of diet records to validate the FFQ measure of fat intake in this study has been criticized because of hypothesized correlated error between the two self-reported measures of dietary intake.⁴⁰ However, we used the same analysis as in Table 3 to predict fasting serum triglyceride levels, which are known to be inversely related to fat intake,⁴¹ in 185 women. These women included 84 from the sex steroid hormone analysis for whom we had triglyceride levels, 41 who were diagnosed with breast cancer more than 2 years after dietary assessment, and 60 who were controls in a study of myocardial infarction. All were postmenopausal, nondiabetic, not taking cholesterol-lowering drugs, and had fasted overnight. Controlling for fiber intake and the covariates listed in Table 3, we found the geometric mean serum triglyceride levels (mg/dL) across categories of percentage of energy from fat intake to be as follows: 156, 139, 129, 103, 85, and 70 mg/dL, corresponding to 20%, 20.1% to 25%, 25.1% to 30%, 30.1% to 35%, 35.1% to 40%, and greater than 40% of kilocalories. These results are comparable to those found in an intervention study among hypercholesterolemic men by Knopp et al.⁴² That study used diet records to measure reported fat intake among four intervention groups with progressively lower fat intakes. After 1 year of the interventions in which mean percentage of energy from fat was 27%, 26%, 25%, and 22%, the corresponding serum triglyceride levels increased by 10%, 3%, 22%, and 39%. Thus, our findings for plasma fasting triglyceride levels provide clear objective evidence that the dietary questions used in this study are sensitive to dietary fat.

In summary, although a low-fat diet has been hypothesized to reduce levels of endogenous sex hormones, previous intervention studies have been inconclusive because of limitations in design or analysis. Our study was larger than those of exclusively postmenopausal women and had the advantage of averaging dietary intake over 4 to 5 years, which reduced measurement error. Also, we were able to extend the range of fat intake by oversampling women with low fat intake and to cover multiple types of fat as well as multiple hormones. We measured hormones with excellent precision, having previously documented that a single measure reasonably reflects average long-term hormone levels.⁴³ We were also able to control for potential confounding factors, such as obesity and alcohol intake.

Limitations of this study include its cross-sectional nature. We were also limited in our ability to examine women with extremely low fat intake, as only 15% of the total population reported 20% or less of total energy from fat.

We found no evidence that higher fat intake is associated with higher levels of any reproductive hormones in this group of postmenopausal women. In fact, we found that the levels of serum estradiol, a hormone associated with increased breast cancer risk, were inversely associated with fat intake. Although not consistently statistically significant, this inverse relationship was true for most types of fat, including animal, vegetable, saturated, monounsaturated, polyunsaturated, and omega-3 (from fish) fats; the exception was a nonsignificant positive association with trans-unsaturated fat intake. These results are consistent with our previous finding of a lack of association between low intake of total fat over 14 years of follow-up and a decreased risk of breast cancer.¹² In addition, we found that DHEA and DHEAS levels had a strong positive association with monounsaturated fat intake and a strong inverse association with polyunsaturated fat intake; these novel findings need to be duplicated in other studies. Our findings suggest that adoption of a low-fat diet during midlife does not lower long-term endogenous estrogen levels, which has been the hypothesized mechanism by which a low-fat diet might prevent breast cancer.

	ACKNOWLEDGMENTS

 
Supported by the National Institutes of Health grants no. CA40356 and CA49449. D.J.H., G.A.C., and W.C.W. were supported by the Harvard Center for Cancer Prevention, Boston, MA. S.E.H. was partially supported by a career development award DAMD17-96-1-6021 from the United States Army Medical Research and Material Command, Fort Detrick, MD.

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Submitted December 19, 1999; accepted June 14, 2000.

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