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American Journal of Epidemiology Advance Access originally published online on February 3, 2008
American Journal of Epidemiology 2008 167(8):935-943; doi:10.1093/aje/kwm397
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American Journal of Epidemiology © The Author 2008. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

ORIGINAL CONTRIBUTIONS

Dietary Intake Related to Prevalent Functional Limitations in Midlife Women

Kristin M. Tomey1, MaryFran R. Sowers1, Carolyn Crandall2, Janet Johnston3, Mary Jannausch1 and Matheos Yosef1

1 Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor, MI
2 David Geffen School of Medicine, University of California at Los Angeles, Los Angeles, CA
3 Department of Epidemiology, University of Pittsburgh, Pittsburgh, PA

Correspondence to Dr. MaryFran Sowers, University of Michigan, 339 E. Liberty, Suite 310, Ann Arbor, MI 48104 (e-mail: mfsowers{at}umich.edu).

Received for publication August 3, 2007. Accepted for publication December 14, 2007.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Physical functioning measures are considered integrated markers of the aging process. This prospective investigation examined relations between dietary intake of women at midlife in 1996–1997 and prevalence of physical functioning limitations 4 years later, defined by the Medical Outcomes Study Short-Form 36. The sample included 2,160 multiethnic women, aged 42–52 years, from six geographic areas participating in the Study of Women's Health Across the Nation (SWAN). Associations between measures of diet quality and number of fruit and vegetable servings and prevalent physical functional limitations (no, moderate, or substantial limitations) were tested by logistic regression. The prevalence of moderate and substantial functional limitations was 31% and 10%, respectively. Women in the highest quartile of cholesterol intake had 40% greater odds (odds ratio = 1.4, 95% confidence interval: 1.1, 1.8) of being more limited versus those in the lowest quartile. Women in the highest quartile of fat and saturated fat intakes were 50% and 60% more likely to be more limited, with respective odds ratios of 1.5 and 1.6 (95% confidence intervals: 1.2, 2.0 and 1.2, 2.1) versus those in the lowest quartiles. Lower fruit, vegetable, and fiber intakes were related to reporting greater functional limitations. Modifying dietary practices could be important in minimizing physical limitations.

body mass index; diet; disabled persons

Abbreviations: CI, confidence interval; OR, odds ratio; Y04, fourth examination


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
About one third of older Americans have functional limitations, defined as difficulty performing basic body functions such as walking, climbing, lifting, carrying, and seeing (1, 2). Physical functional limitations in older women are common (2); among women aged 70 years or older, 42 percent have one limitation while 20 percent had four or more physical limitations (2). Functional limitations are not strictly a later-life phenomenon. A recent study showed that these limitations were present in 12–25 percent of middle-aged women, depending on how the functional limitation was characterized (3).

It is widely recognized that chronic conditions and diseases, including heart disease and arthritis, are associated with the development of functional limitations (4–12). Many of these same conditions are also linked to obesity and, in fact, high body mass index is a risk factor for functional limitations (7, 13–15). There is limited information about the direct association of dietary risk factors and functional limitations (16). However, dietary factors may contribute to functional limitations indirectly through the initiation and exacerbation of obesity, chronic diseases, and other conditions associated with limited physical functioning.

The authors hypothesized that, in a multiethnic national cohort of women at midlife, dietary intake evaluated at the baseline examination would be less favorable in those who subsequently characterized themselves as functionally limited compared with those free of functional limitations. A better understanding of this relation in a time period in which women are becoming functionally limited may identify a key opportunity for intervention (17–19).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
The Study of Women's Health Across the Nation (SWAN) is a multiethnic longitudinal study with a baseline enrollment of 3,302 women. Recruitment procedures and study design are described fully elsewhere (20). Briefly, enrollees were recruited from Boston Massachusetts, Chicago Illinois, the Detroit, Michigan, area, Los Angeles, California, Newark, New Jersey, Pittsburgh, Pennsylvania, and Oakland, California. Eligibility criteria included being female, being aged 42–52 years, having a uterus and at least one intact ovary, reporting a menstrual period within the past 3 months, not having taken hormone medications (including birth control pills, estrogen, or progesterone preparations) in the last 3 months, residing in an above-mentioned geographic area, and self-identifying as African American, Caucasian, Chinese, Hispanic, or Japanese. The analytical sample used for this report includes women with nutrition and lifestyle data at the 1996–1997 baseline examination and physical functioning information reported during their fourth examination (Y04) (n = 2,093). Data from the New Jersey site were excluded because of incompleteness and inconsistencies in the diet and physical functioning data that could not be resolved. Institutional review board approval was granted for the study protocol at each study site, and informed consent was obtained from each study participant.

Physical functioning status was assessed at Y04 with responses to a 10-question subscale of the Medical Outcomes Study Short-Form 36 (SF-36) (21, 22). Participants indicated whether they were limited a lot, a little, or not limited in activities including bathing, dressing, carrying groceries, bending, moderate and vigorous athletic activities, walking, and climbing stairs. Scores ranged from 0 to 100, with higher scores representing better functioning. Women with a score below the population norm (<50 points) were classified as having substantial limitations. Such individuals could have reported no limitations on, at most, five of the 10 activities. Those with a score of 51–85 were classified as having moderate limitations; for example, a woman with 85 points could have reported no limitations on, at most, eight of 10 activities, thus allowing for some limitations in vigorous and moderate activities. Women with 86–100 points were considered not limited.

Dietary data were obtained at baseline from a modified version of the 1995 Block interviewer-assisted food frequency questionnaire (23). The questionnaire was administered in English, Chinese, or Japanese and reflected the "usual" dietary pattern for the previous year. In addition to the 103-item core food list, the Chinese and Japanese versions included additional culture-specific items. Micronutrient (i.e., minerals, vitamins, and vitamin derivatives) values included both dietary and supplement intakes.

Absolute and relative nutrient intake values are reported. DietSys software (http://appliedresearch.cancer.gov/DietSys/software.html) was used to assign nutrient values. These values were obtained from the US Department of Agriculture nutrient database for standard reference, Bowes and Church's Food Values of Portions Commonly Used (24), commercial food manufacturers' websites (e.g., Kellogg), and food labels (especially for ethnic foods). Relative nutrient intakes were expressed per a standard amount of energy consumed (1,000 kcal = 4,184 kJ) to provide information on diet quality (25) and were categorized as quartiles to provide comparisons for likelihood of greater functional limitations.

Dietary data were also expressed in reference to national dietary guidelines and then related to level of physical functioning. The national dietary guidelines criteria were as follows: fat (20–35 percent of total energy), saturated fat (<10 percent of total energy), cholesterol (<300 mg), and sodium (<2,300 mg) (26), along with trans fats (<1 percent energy) (27) and fruits + vegetables (at least five servings per day) (28). Because dairy products were less likely to be primary calcium sources for African-American and Asian women (29), participants were classified as meeting the calcium recommendation if they consumed at least 67 percent of the adequate intake of calcium based on the Dietary Reference Intakes (30), that is, at least 670 mg/day for females aged 31–50 years and at least 804 mg/day for women aged 51–70 years (31, 32).

Fruits and vegetables were each quantified as number of 1/2-cup daily servings. For most fruits, a serving was quantified as one medium piece, except watermelon (one slice), cantaloupe (1/4 medium), and mangoes or papayas (1/2 medium). Although most vegetable servings consisted of 1/2 cup, the exceptions were green salad (one medium bowl), French fries (3/4 cup), and white potatoes (1/2 cup); 1 cup = 236.6 ml.

An overall diet quality index was based on the Diet Quality Index-Revised algorithm adapted for use with the food frequency questionnaire (33). The Diet Quality Index-Revised algorithm incorporates 10 elements worth up to 10 points each, for a possible total of 100 points; higher scores represent better overall diet quality. The first eight elements addressed goals for fat, saturated fat, and dietary cholesterol; number of fruit, vegetable, and grain servings; calcium; and iron. The diversity element was composed of points given for consuming a greater variety of foods, with consideration given to grains, vegetables, fruits (including juice), and meat/dairy. Up to 2.5 points were given for each food category, for a total diversity score of 10 points. Moderation, the final element, consisted of points given for lower consumption of alcohol (0–2.5 points), sodium (0–2.5 points), and sugar and fat (0–5 points).

Covariates assessed at baseline included body size, as well as health and demographic variables. Weight and height were measured by use of calibrated scales and stadiometers and were used to calculate body mass index (weight (kg)/height (m)2). Body mass index was classified into four groups: <18.5 kg/m2 (underweight), 18.5–24.99 kg/m2 (normal weight), 25.0–29.99 kg/m2 (overweight), and ≥30.0 kg/m2 (obese) (34).

At baseline, women self-reported the presence of high blood pressure, diabetes, heart disease, and arthritis. Trouble sleeping was defined when women reported difficulty sleeping at least 6–8 times in the previous 14 days. The presence of depressive symptomatology was defined as a score of 16 or greater on the 20-item Center for Epidemiologic Studies-Depression Scale (35).

Active and environmental tobacco smoke exposure was assessed at baseline (36, 37). Total smoke exposure was classified as 1) nonsmoker with no environmental tobacco smoke exposure, 2) nonsmoker with at least 1 hour of environmental tobacco smoke exposure per week, 3) smoker with no environmental tobacco smoke exposure, or 4) smoker with at least 1 hour of environmental tobacco smoke exposure per week (37).

Women reported whether it was very hard, somewhat hard, or not hard at all to pay for basics such as food, housing, and health care as a measure of economic stress. Baseline menopausal status categories included premenopause (no decrease in the regularity of menstrual bleeding during the previous year) and early perimenopause (decreased menstrual regularity in the 3 months before the interview). Other covariates, assessed via questionnaire, included race/ethnicity, marital status, and perception of overall health.

Statistical analyses were conducted with SAS, version 9.1, software (SAS Institute, Inc., Cary, North Carolina). Analysis of covariance (SAS PROC GLM) was used to generate least-squared means and 95 percent confidence intervals for each dietary variable according to the three-level functional limitation classification, adjusted for race/ethnicity, economic stress, self-reported presence or absence of four baseline health conditions (high blood pressure, diabetes, heart disease, and arthritis; assessed as separate variables), baseline total tobacco smoke exposure, depressive symptom questionnaire score, trouble sleeping, menopausal status, and study site. Overall differences in means among the three functional limitations categories were compared by use of an F test.

Ordinal logistic regression was used to model the three-level dependent functional limitation variable (none, moderate, or substantial limitation at Y04) with dietary variables (at baseline) as the independent variables. The odds of being in a worse functional limitations category of those in the highest versus lowest quartile of intake for fat, saturated fat, trans fat, and cholesterol or in the lowest versus highest quartile of fruit, vegetable, fiber, micronutrient (magnesium, potassium, iron, vitamin C, vitamin E, vitamin A), or carotenoid (beta-carotene, lutein, lycopene) intake were tested. The chi-square proportional odds assumption was met for all models, and associations included adjustment for covariates previously described. Covariate selection was based on published associations (4, 5). The reference group for race/ethnicity was Caucasian women, and the reference group for study site was the Michigan site. Interaction terms for race/ethnicity and economic stress were tested in each model (38, 39). In the case of a statistically significant interaction, stratum-specific results are presented. The alpha was set to 0.05, and all p values are two tailed.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Nineteen percent of African-American participants were classified as having substantial limitations in comparison with 8 percent, 6.3 percent, and 3.8 percent of Caucasian, Chinese, and Japanese women, respectively (table 1). A larger proportion of women reporting economic stress had substantial limitation (32.4 percent) compared with women with less economic stress (13.5 percent and 6.9 percent among those with some and no economic stress, respectively).


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  		TABLE 1. Characteristics of SWAN* participants in 1996–1997 according to functional limitation status assessed in the year 2000

 
Baseline fat and cholesterol intake and functional limitations 4 years later
Failure to comply with the dietary intake guidelines for cholesterol, total fat, or saturated fat was associated with being more limited 4 years later. Women with no limitations (85 percent) were more likely to meet the recommendation of consuming less than 300 mg of cholesterol, the recommended cutpoint, compared with those with moderate (79 percent) or substantial (62 percent) limitations (table 2). Mean cholesterol nutrient densities/1,000 kcal were significantly different according to functional limitations categories; adjusted least-squared dietary cholesterol means = 128.9 (95 percent confidence interval (CI): 118.7, 139.1), 131.4 (95 percent CI: 121.2, 141.6), and 143.6 (95 percent CI: 132.6, 154.6) mg/1,000 kcal among those not, moderately, and substantially limited, respectively (table 3). Women in the highest quartile of cholesterol intake (≥143.8 g/1,000 kcal) had 40 percent greater odds of being more limited compared with those in the lowest quartile of consumption (≤89.9 g/1,000 kcal: odds ratio (OR) = 1.4, 95 percent CI: 1.1, 1.8) (table 4).


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  		TABLE 2. Compliance of SWAN* participants with dietary and body mass index recommendations in 1996–1997 by functional limitations status assessed in the year 2000

 

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  		TABLE 3. Least-squared means of dietary variables assessed in 1996–1997 by functional limitations status in the year 2000 in SWAN{dagger} participants

 

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  		TABLE 4. The odds of being in a poorer functional limitations category in the year 2000 based on quartile of nutrient density or daily food serving in 1996–1997 in SWAN* participants

 
A significantly higher proportion of those with no limitations (63 percent) consumed the recommended total fat intake (20–35 percent of total energy) compared with 54 percent or 48 percent in those with moderate or substantial limitations, respectively (table 2). Baseline nutrient density of total fat was significantly different according to functional limitations status; adjusted least-squared means = 35.8 (95 percent CI: 34.0, 37.6), 36.9 (95 percent CI: 35.1, 38.7), and 38.0 (95 percent CI: 36.2, 39.8) fat g/1,000 kcal in those who reported none, moderate, and substantial limitations, respectively (table 3). Women in the highest quartile of total fat intake (≥41.9 g/1,000 kcal) were 50 percent more likely to be in a greater limitations category compared with those in the lowest quartile (≤30.7 g/1,000 kcal: OR = 1.5, 95 percent CI: 1.2, 2.0) (table 4).

A significantly greater proportion of women with no limitations (55 percent) consumed recommended amounts of saturated fat (<10 percent of total energy) compared with those with moderate or substantial limitations (44 percent or 37 percent, respectively) (table 2). Baseline saturated fat nutrient densities were significantly different among women according to functional limitations classification; adjusted least-squared means = 10.8 (95 percent CI: 10.2, 11.4), 11.3 (95 percent CI: 10.7, 11.9), and 11.4 (95 percent CI: 10.8, 12.0) g/1,000 kcal among those reporting none, moderate, and substantial limitations, respectively (table 3). Women in the highest intake quartile for saturated fat (≥13.5 g/1,000 kcal) were 60 percent more likely to be more limited compared with those in the lowest quartile (≤9.0 g/1,000 kcal: OR = 1.6, 95 percent CI: 1.2, 2.1) (table 4). Trans fat intake was not associated with reported functional limitations status.

Baseline fruit, vegetable, and fiber intake and functional limitations 4 years later
The proportion of those consuming "five-a-day" fruit + vegetable servings varied quite modestly according to physical limitation status (table 2), although those who were not limited consumed somewhat more fruit than those with functional limitations; adjusted least-squared means = 0.9 (95 percent CI: 0.7, 1.1), 0.8 (95 percent CI: 0.6, 1.0), and 0.7 (95 percent CI: 0.5, 0.9) daily fruit servings for those with no, moderate, and substantial limitations, respectively (table 3). Women in the lowest quartile of fruit intake (<0.7 daily servings) were 60 percent more likely to be more limited than were those in the highest fruit intake quartile (≥1.8 daily servings: adjusted OR = 1.6, 95 percent CI: 1.3, 2.1) (table 4).

Intake of vegetable servings varied significantly according to functional limitations status; adjusted least-squared means = 1.7 (95 percent CI: 1.5, 1.9), 1.5 (95 percent CI: 1.3, 1.7), and 1.5 (95 percent CI: 1.3, 1.7) daily vegetable servings, respectively, for those with no, moderate, and substantial limitations. Women with less than or equal to one daily serving (the lowest quartile of vegetable intake) were 50 percent more likely to be more limited than were those consuming at least 2.4 daily servings (the highest quartile of intake: adjusted OR = 1.5, 95 percent CI: 1.2, 2.0) (table 4).

Like fruit and vegetable intakes, fiber consumption varied significantly but only modestly by functional limitations classification; adjusted least-squared mean = 6.3 (95 percent CI: 5.7, 6.9) versus 5.9 (95 percent CI: 5.5, 6.3) and 5.7 (95 percent CI: 5.3, 6.1) g/1,000 kcal, respectively (table 3). Women in the lowest quartile of baseline fiber intake (≤5.0 mg/1,000 kcal) were 80 percent more likely to be more limited compared with those in the highest quartile of fiber intake (≥8.4 mg/1,000 kcal) in adjusted prospective analyses (OR = 1.8, 95 percent CI: 1.4, 2.3) (table 4).

Baseline micronutrient intake and functional limitations 4 years later
Those in the lowest quartile of magnesium (≤117 mg/1,000 kcal) nutrient density were 50 percent more likely to be more limited than were those in the highest quartile (≥166 mg/1,000 kcal: OR = 1.5, 95 percent CI: 1.1, 1.9) (table 4). Those in the lowest quartile of lycopene (<345 µg/1,000 kcal) nutrient density were 40 percent more likely to be more limited than were those in the highest quartile (≥1,025 µg/1,000 kcal: OR = 1.4, 95 percent CI: 1.1, 1.9). Odds ratios from adjusted models were not statistically significant for iron, vitamin C, vitamin E, vitamin A, beta-carotene, or lutein.

Baseline diet quality and functional limitations 4 years later
The Diet Quality Index was significantly different according to functional limitations category; least-squared means = 65.1 (95 percent CI: 62.4, 67.7) versus 63.0 (95 percent CI: 60.4, 65.6) and 62.0 (95 percent CI: 59.3, 64.8) points, respectively, in those reporting none, moderate, and substantial limitations, respectively, after adjustment for covariates (F test: p = 0.002). There was a borderline association between lower baseline Diet Quality Index scores and higher likelihood of being in a greater substantial limitations category 4 years later (OR = 1.02, 95 percent CI: 1.01, 1.02) after adjustment for covariates.

Unique race/ethnic differences
Japanese women who exceeded the recommended sodium intake (<2,300 mg) were 2.2 times more likely to be more limited than were those who were within the recommended range (OR = 2.2, 95 percent CI: 1.2, 3.9). In addition, African-American women in the lowest quartile of potassium intake (<1,213 mg/1,000 kcal) were 2.5 times more likely to have substantial limitations (OR = 2.5, 95 percent CI: 1.5, 4.2) than were women in the highest quartile (≥1,698 mg/1,000 kcal).

Other nutrients, including calcium, were not identified as having statistically significant associations with functional limitations status after adjustment for covariates.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
Higher baseline intake of dietary cholesterol, lower fiber intake, and failure to consume recommended amounts of total and saturated fat were each consistently associated with greater functional limitations assessed 4 years later in middle-aged women, even following adjustment for demographic and health variables. Associations between cholesterol and fat were observed whether the dietary data were expressed in relation to recommended intakes or in relation to nutrient intake quality with respect to total energy consumed. Greater subsequent functional limitations were also associated with lower baseline magnesium and lycopene intakes, as well as fewer baseline fruit and vegetable servings.

There are few data linking dietary factors with functional limitations, especially among middle-aged persons. Results from two large investigations, however, are consistent with our findings. In the Whitehall study (16), self-reported prevalence of poor physical functioning was evaluated in 3,413 middle-aged women characterized according to their dietary intake 5 years earlier. Women with healthier diets were half as likely to have functional limitations compared with women reporting less healthy diets. In the Atherosclerosis Risk in Communities (ARIC) Study, higher fruit and vegetable intakes along with higher dairy consumption by middle-aged biracial participants were protective for subsequently reporting impaired lower extremity function (39).

In our investigation, Japanese women who consumed at least 2,300 mg of sodium at baseline were more likely to be more functionally limited 4 years later than were Japanese women who met the sodium recommendation. Notably, about half of Japanese and Caucasian women met the recommendation for appropriate sodium intake, while the proportion meeting the recommendation for appropriate sodium intake was almost two thirds in Chinese and African-American women. Further, African-American women in the lowest quartile of potassium (mg/1,000 kcal) intake at baseline were over twice as likely to be more functionally limited at Y04 than were African-American women in the highest quartile of intake. In fact, African-American participants in the Study of Women's Health Across the Nation reported consuming relatively less potassium than did women of other races/ethnicities. This is important considering the frequency of hypertension treatment among African Americans and that certain treatments are potassium wasting, posing an additional requirement for adequate potassium nutriture.

The designation "functionally limited" characterizes a disruption in physical functioning based on the ability to perform selected physical activities. This disruption likely reflects the dysfunction of one or more underlying biologic, neurologic, or psychological systems; however, very limited information exists on what processes may be at work in this context. Cardiovascular disease and related processes have been associated with functional limitations (4). Dietary risk factors for cardiovascular disease include high fat, saturated fat, and cholesterol intakes, as well as low antioxidant intake (16), and diet modification is the fundamental tenet of the Adult Treatment Program III in coronary heart disease prevention (40). Our findings consistently showed that dietary factors associated with increased heart disease risk were also related to increased prevalence and severity of subsequent prevalent functional limitations. It is currently unknown whether these interview-based measures of physical functioning at midlife really represent a surrogate measure of subclinical cardiovascular disease. By extension, it is currently unknown if these dietary associations with functional limitations represent dietary associations with cardiovascular disease.

An advantage of the study was that diet was assessed 4 years prior to the assessment of physical functioning. However, it is not possible to conclude whether baseline diets high in cholesterol and fat and low in fiber are in part responsible for development of subsequent limitations, because the physical functioning data are prevalent rather than incident data. The authors cannot rule out that associations with dietary intake may be due to lifestyle changes that occurred after the decline in physical functioning, during which time it could have become more difficult to access healthier foods. Further, there may have been disease onset and/or changes in dietary intake that led to changes in functioning in the interval between dietary assessment and functional status evaluation. Although body mass index would most likely impact the association between dietary intake and physical functioning, we did not adjust for it in any analyses because it was hypothesized to lie in the causal pathway (13, 16) and, thus, was not considered a confounder. Finally, nutrient values obtained from food frequency questionnaires likely involved some misclassification error because of the difficulty in recalling and quantifying usual food consumption during the previous year.

This community-based investigation provides consistent evidence that dietary intakes of cholesterol, total and saturated fat, and fiber were related to prevalent physical functional limitations, assessed 4 years later. Further, these relations are occurring at midlife when there may be potential to recover physical function or delay further deterioration. Future longitudinal research will help to clarify the role of these lifestyle factors in the development of functional limitations.


   ACKNOWLEDGMENTS
 
The Study of Women's Health Across the Nation has grant support from the National Institutes of Health, Department of Health and Human Services, through the National Institute on Aging, the National Institute of Nursing Research, and the National Institutes of Health Office of Research on Women's Health (grants NR004061, AG012505, AG012535, AG012531, AG012539, AG012546, AG012553, AG012554, and AG012495).

Conflict of interest: none declared.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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