Protective Effect of Total Carotenoid and Lycopene Intake on the Risk of Hip Fracture: A 17-Year Follow-Up From the Framingham Osteoporosis Study

Authors

  • Shivani Sahni,

    1. Dietary Assessment and Epidemiology Research Program, Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) and Friedman School of Nutrition Science and Policy (FSNSP), Tufts University, Boston, Massachusetts, USA
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  • Marian T Hannan,

    1. Institute for Aging Research, Hebrew SeniorLife and Harvard Medical School, Boston, Massachusetts, USA
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  • Jeffrey Blumberg,

    1. Antioxidants Research Laboratory, HNRCA and FSNSP, Tufts University, Boston, Massachusetts, USA
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  • L Adrienne Cupples,

    1. Department of Biostatistics, Boston University School of Public Health, Boston, Massachusetts, USA
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  • Douglas P Kiel,

    1. Institute for Aging Research, Hebrew SeniorLife and Harvard Medical School, Boston, Massachusetts, USA
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  • Katherine L Tucker

    Corresponding author
    1. Dietary Assessment and Epidemiology Research Program, Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) and Friedman School of Nutrition Science and Policy (FSNSP), Tufts University, Boston, Massachusetts, USA
    • Address correspondence to: Katherine L. Tucker, PhD, Dietary Assessment and Epidemiology Research Program, Jean Mayer USDA HNRCA at Tufts University, 711 Washington Street, Boston, MA 02111-1524, USA
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  • The authors state that they have no conflicts of interest.

  • Published online on January 5, 2009

Abstract

In vitro and in vivo studies suggest that carotenoids may inhibit bone resorption, yet no previous study has examined individual carotenoid intake (other than β-carotene) and the risk of fracture. We evaluated associations of total and individual carotenoid intake (α-carotene, β-carotene, β-cryptoxanthin, lycopene, lutein + zeaxanthin) with incident hip fracture and nonvertebral osteoporotic fracture. Three hundred seventy men and 576 women (mean age, 75 ± 5 yr) from the Framingham Osteoporosis Study completed a food frequency questionnaire (FFQ) in 1988–1989 and were followed for hip fracture until 2005 and nonvertebral fracture until 2003. Tertiles of carotenoid intake were created from estimates obtained using the Willett FFQ adjusting for total energy (residual method). HRs were estimated using Cox-proportional hazards regression, adjusting for sex, age, body mass index, height, total energy, calcium and vitamin D intake, physical activity, alcohol, smoking, multivitamin use, and current estrogen use. A total of 100 hip fractures occurred over 17 yr of follow-up. Subjects in the highest tertile of total carotenoid intake had lower risk of hip fracture (p = 0.02). Subjects with higher lycopene intake had lower risk of hip fracture (p = 0.01) and nonvertebral fracture (p = 0.02). A weak protective trend was observed for total β-carotene for hip fracture alone, but associations did not reach statistical significance (p = 0.10). No significant associations were observed with α-carotene, β-cryptoxanthin, or lutein + zeaxanthin. These results suggest a protective role of several carotenoids for bone health in older adults.

INTRODUCTION

Osteoporosis is a major public health problem in the aging population. It has been estimated that almost 10 million Americans have osteoporosis,(1) and there are ∼1.5 million osteoporotic fractures in the United States each year.(2,3) Among 50-yr-old non-Hispanic white women, the lifetime risk of hip, spine, or forearm fracture is ∼40%.(4) Of these fractures, hip fractures are the most serious, because they almost always result in hospitalization and lead to death for ∼20% and to permanent disability for ∼50%.(5) Studies have consistently shown that higher fruit and vegetable intake has positive effects on bone mineral status.(6–14) There is evidence that reactive oxygen intermediates may be involved in the bone-resorptive process(15–18) and that fruit- and vegetable-specific antioxidants, such as carotenoids, are capable of decreasing this oxidative stress.(19–21) Therefore, carotenoids may help in preventing osteoporosis.(22) In particular, an inverse relation of carotenoid and lycopene with biochemical markers of bone turnover has recently been shown.(21)

Data from several in vitro(23–26) and in vivo(21,27,28) studies suggest that further investigation into the relationship between carotenoids and bone health is warranted. To our knowledge, no longitudinal observational study has examined the association between intake of carotenoids (other than β-carotene(29)) and the risk of hip fracture. A previous study by our group in the Framingham Osteoporosis Study reported a protective effect of carotenoids against 4-yr loss in trochanter BMD in men and in lumbar spine in women.(30) Therefore, we evaluated associations between intake of total carotenoids and individual carotenoids (α-carotene, total β-carotene, β-cryptoxanthin, lycopene, lutein + zeaxanthin) and the risk of hip fracture as well as nonvertebral osteoporotic fracture in the Framingham Osteoporosis Study.

MATERIALS AND METHODS

Participants

The Framingham Study began in 1948 to examine risk factors for heart disease. The original subjects (5209 men and women; age, 28–62 yr) were selected as a population-based sample of two thirds of the households in Framingham, MA, USA, and have been examined biennially for >50 yr.(31) In 1988–1989 (20th exam), there were 1402 surviving subjects from the original cohort who participated in the Framingham Osteoporosis Study. We excluded 426 subjects with missing or incomplete food frequency questionnaires (FFQs; based on the criteria of >12 food items left blank in the FFQ) or with energy intakes <2.51 or >16.74 MJ (<600 or >4000 kcal/d) at the 20th exam. Of 976 subjects with complete FFQ and fracture information, we excluded 30 subjects with prior hip fracture (Fig. 1). We further excluded 17 participants with missing covariate information on body mass index (BMI), multivitamin use, or current estrogen use. The final analytic sample (n = 929) was followed for incident hip fracture from the date when they completed the FFQ to the end of 2005. For analyses of nonvertebral osteoporotic fractures, 11 additional subjects with prior nonvertebral fracture and 17 with missing covariate information were excluded for a final analytic sample of 918. The subjects were followed for an incident nonvertebral osteoporotic fracture from the date when they completed the FFQ to the end of 2003. No participants met the further exclusion criteria based on energy intakes <2.51 or >16.74 MJ (600 or 4000 kcal)/d. All participants provided informed consent for their participation. The Institutional Review Board for Human Research at Boston University, Hebrew Rehabilitation Center, and Tufts University approved this study.

Figure Figure 1.

Flow chart showing total number of subjects enrolled in the Framingham Heart Study and the final number of subjects included in the analyses. 1Framingham Heart Study. 2Food Frequency Questionnaire.

Assessment of carotenoid intake

Usual dietary intake was assessed in 1988–1989 (20th exam) with a semiquantitative, 126-item Willett FFQ.(32,33) Questionnaires were mailed to the study participants. They were asked to complete them, based on their intake over the previous year, and to bring them to the examination where they were reviewed with participants by clinic staff. This FFQ has been previously validated against biochemical measures for individual carotenoid intakes in this cohort.(34) Pearson correlation coefficients for women and men were as follows: α-carotene, 0.30 and 0.28; β-carotene, 0.34 and 0.31; β-cryptoxanthin, 0.45 and 0.36; lycopene, 0.36 and 0.31; and lutein + zeaxanthin, 0.24 and 0.14 (adjusting for age, energy intake, BMI, plasma cholesterol concentration, and smoking) and are similar to those published in other validation studies. The FFQ performed better among women than men. However, in men, the correlations improved after adjustment for confounders. Because the plasma measures, like dietary measures, may be subject to day-to-day fluctuations, the use of a single day may introduce random error that will attenuate the observed correlation. Furthermore, the error associated with the plasma measures is unlikely to be correlated with the error in the FFQ estimations. Therefore, it can be assumed that the true associations between the dietary and plasma measures are greater than those observed. The investigators of this validation study reported that this FFQ provided reasonably valid information about major individual carotenoids except for lutein + zeaxanthin. The FFQ produced estimated intakes for each carotenoid in our study. However, the U.S. Department of Agriculture (USDA) national nutrient database lists the combined content of lutein + zeaxanthin.(35) Therefore, these carotenoids were used as one observational unit in this study. In this study, we calculated total carotenoid intake as the sum of the intake of five individual carotenoids. Because carotenoids other than β-carotene are not generally used in supplemental form, only β-carotene intake included intake from supplements as well as from diet.

Assessment of fracture

As reported previously,(36) all records of hospitalizations and deaths for the study participants were systematically reviewed for occurrences of hip fracture. Beginning in 1983 (18th biennial examination in the Framingham Study), hip fractures were reported by interview at each biennial examination or by telephone interview for participants unable to attend an examination. Reported hip fractures were confirmed by a review of medical records and radiographic and operative reports. For this study, incident hip fracture was defined as a first-time fracture of the proximal femur, which occurred over follow-up after the dietary assessment at the 20th exam (1988–1989). Self-reported nonvertebral fractures were ascertained at biennial examinations. Because the literature reports that the percent of false positives is low for self-reported fractures at the hip, shoulder, wrist, elbow, ankle, and pelvis,(37) we categorized the group of nonvertebral fractures as the first self-reported occurrence of shoulder, wrist, elbow, ankle, or pelvis fracture, as well as confirmed hip fracture.

Potential confounding factors

Previous studies on this cohort have reported several risk factors for osteoporosis and research from this work and other cohorts, including the Rancho Bernardo Study and clinical trials cohorts, have informed our inclusions for potential confounding variables. These include age, female sex,(38) BMI,(39) smoking,(40) caffeine,(41) alcohol(42) current estrogen use in women,(38,43) low physical activity,(44) and low intake of calcium and vitamin D.(45,46) All the covariates used in this study were measured at the baseline 20th exam (1988–1989) except for height used to calculate BMI. BMI, a known risk factor for osteoporosis, was calculated in the Framingham Study from measurements of height at exam 1 (1948–1949), taken without shoes, in inches, and measurements of weight taken at the 20th examination in pounds (converted to kilograms) with a standardized balance-beam scale. Because BMI is a measure of relative weight, designed to be independent of height, we included both BMI and height at exam 1 in our equations to adjust for ponderosity and body stature, which may be related to dietary intake and fracture.(8)

Smoking status was assessed using a questionnaire at the 20th exam (1988–1089) as never having smoked cigarettes or having any history of smoking. Physical activity was measured with the Framingham physical activity index, an estimated measure of energy expenditure calculated from questions about number of hours spent in heavy, moderate, light, or sedentary activity and number of hours spent sleeping during a typical day. Each component of these 24-h summaries was multiplied by an appropriate weighting factor on the basis of the estimated level of associated energy expenditure and summed to arrive at a physical activity score.(47) For example, a person who reported sleeping and resting for 16 h and engaged in 8 h of slight activity would have an index score of 29.8. The physical activity index at exam 19 (1985–1986) was used for the subjects who had a missing physical activity index at exam 20 (1988–1989).

A previous study by our group supported the hypothesis that alkaline-producing dietary components such as potassium, present in fruit and vegetables, play a beneficial role in bone health.(10) Therefore, final models were adjusted for potassium intake and baseline BMD at the femoral neck, to examine whether the association of carotenoid intake with fracture was independent of these variables. BMD (g/cm2) was measured in the original cohort at exam 20 (1988–1989) at the femoral neck using a Lunar DP2 dual photon absorptiometer. Intake of carotenoids, total calcium, vitamin D, and caffeine were assessed using the food list section of the FFQ.

Statistical analysis

We tested the associations for effect modification by sex. If the p value for interaction was statistically significant (p < 0.05), we performed the analyses on men and women separately. The Utah study of nutrition and bone health reported that smoking status (never/ever smoker) was an effect modifier of the association between antioxidant intake and hip fracture.(48) This could be because tobacco smoke contains large amounts of oxidants and free radicals that induce oxidative stress,(49) a condition associated with reduced BMD.(18) Therefore, we tested the association between carotenoid intake and hip fracture for effect modification by smoking status. Carotenoid intakes were adjusted for total energy intake using the residual method.(33) Square root transformations were applied to the exposure to achieve normality, before creating residuals. Descriptive characteristics were calculated for a combined sample of men and women.

HRs and 95% CIs were estimated for men and women combined and separately by tertile of carotenoid intake (continuous and categorical form of energy adjusted total carotenoid intake and intake of α-carotene, β-carotene, β-cryptoxanthin, lycopene, and lutein + zeaxanthin) using Cox proportional hazards regression, adjusting for potential confounders and covariates. HRs were used to estimate the relative increase in the risk of hip fracture for each of the two higher tertiles compared with the lowest tertile (referent); it was also used to test for a linear trend in the HRs across all tertiles.

Multivariable models were adjusted for sex, age (yr), BMI (kg/m2), height at exam 1 (1948–1949) (in), total energy intake (MJ), physical activity index, alcohol intake (none/moderate: <13.2 g of alcohol/d for women and <26.4 g of alcohol/d for men/high: ≥13.2 g of alcohol/d for women and ≥ 26.4 g of alcohol/d for men),(6) smoking (never/ ever smoked), intake of total calcium (mg/d) and vitamin D (IU/d), caffeine (mg/d), multivitamin use (yes/no), and current estrogen use (in women alone) at exam 20 (1988–1989). For analyses on the combined sample of men and women, we created an indicator variable to adjust for sex and current estrogen use simultaneously (referent group: men, group 2: never or former estrogen using women, and group 3: current estrogen using women). Starting with a full model, we performed sequential regression models, removing variables one at a time, such that any covariate that changed the β-coefficient of the primary exposure by >10% was included in the final parsimonious model. Based on this method, caffeine intake was excluded from the parsimonious models presented in this paper. Final models were adjusted for potassium intake (mg) and for femoral neck BMD for hip fracture alone.

All analyses were performed using SAS statistical software (SAS user's guide, version 9.1; SAS Institute, Cary, NC, USA). A nominal two-sided p < 0.05 was considered statistically significant for all the analyses.

RESULTS

Subject characteristics

Women represented two thirds (61%) of the study sample. The mean age of men and women in this study was ∼75 yr, and mean BMI was 25.5 kg/m2 (Table 1). Approximately one third of the subjects reported education beyond high school. More than one half of the subjects reported alcohol use and ever having smoked cigarettes. The majority of the women (95%) reported no current estrogen use. Approximately one fourth of the subjects reported multivitamin supplement use. The mean intake of dietary calcium was 721 mg/d, whereas that of total calcium (diet + supplements) was 803 mg/d (Table 2). The mean intake of total carotenoids was 16,118 μg/d. In the 17 yr of follow-up, 100 hip fractures (20 in men and 80 in women) were reported among 946 study participants. In the combined sample of men and women, incidence rates for hip fracture were 13.9 per 1000 person-years (44/3166) for the lowest tertile of total carotenoid intake, 8.8 per 1000 person-years (31/3527) for the second tertile, and 7.0 per 1000 person-years (25/3556) for the highest tertile of total carotenoids intake. In the 15 yr of follow-up, 175 nonvertebral osteoporotic fractures (25 in men and 150 in women) were reported among the 935 subjects. In the combined sample, incidence rates for nonvertebral fracture were 20.8 per 1000 person-years (65/3129) for the lowest tertile of total carotenoid intake, 17.3 per 1000 person-years (58/3335) for the second tertile, and 15.1 per 1000 person-years (52/3426) for the highest tertile of total carotenoid intake.

Table Table 1.. Basic Description of the Framingham Original Cohort at the 20th Examination in Framingham, MA
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Table Table 2.. Nutrient Intake in the Framingham Original Cohort at the 20th Examination in Framingham, MA
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In the analyses on hip fracture and nonvertebral fracture, no significant interactions were seen with sex or smoking status (p values ranged from 0.09 to 0.97). Similar associations were observed for men and women (Fig. 2). Therefore, results are presented for the combined sample of adults. To ensure complete adjustment for body weight, sensitivity analyses were conducted for the hip fracture models. These were repeated with (1) replacement of height and BMI with height at exam 1 and weight at exam 20 and (2) with further inclusion of percent weight change from exam 19 to exam 20. These results did not differ from those adjusted for BMI and height and we, therefore, present only the original models.

Figure Figure 2.

Association of total carotenoid intake1 and risk of hip fracture in (A) elderly men and (B) women of the Framingham Osteoporosis Study.2p = 0.54 for men and 0.03 for women. 1Total carotenoid intake = sum of the intake of five individual carotenoids examined in this study, where total β-carotene intake = dietary + supplemental intake and only dietary intake of other carotenoids was included. 2Full models were adjusted for age at exam 20 (yr), body mass index (BMI; kg/m2), height at exam 1 (in), total energy intake (MJ/d), physical activity index, alcohol intake (none/moderate: <13.2 g of alcohol/d for women and <26.4 g of alcohol/d for men/high: ≥13.2 g of alcohol/d for women and ≥26.4 g of alcohol/d for men), smoking status (ever/never smoker), total calcium intake (mg/d), total vitamin D intake (mg/d), caffeine intake (mg/d), multivitamin use (yes/no), and current estrogen use (in women alone).

Among men and women in this study, subjects in the highest tertile of total carotenoid intake (median: 23,711 μg/d) had a significantly lower risk of hip fracture compared with subjects in the lowest tertile of intake (median: 7299 μg/d; HR for tertile 3 versus tertile 1: 0.54; 95% CI: 0.32, 0.90; p = 0.02, p for trend = 0.02; Table 3). We observed a similar trend for nonvertebral osteoporotic fracture but the associations did not reach significance (HR for tertile 3 versus tertile 1: 0.72; 95% CI: 0.50, 1.06; p = 0.09, p for trend = 0.12; Table 3).

Table Table 3.. Association of Carotenoid Intake Measured at Exam 20 and the Risk of Hip Fracture and Nonvertebral Osteoporotic Fracture in the Elderly Men and Women of the Framingham Osteoporosis Study
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Higher intake of lycopene was associated with a lower risk of hip fracture (p for trend = 0.01; Table 3). Subjects in the highest tertile of lycopene intake (median: 12,644 μg/d) had a significantly lower incidence of nonvertebral osteoporotic fracture compared with subjects in the lowest tertile (median: 2710 μg/d; HR for highest versus lowest tertile: 0.66; 95% CI: 0.45, 0.97; p = 0.02, p for trend = 0.02; Table 3). To better understand this, we repeated the analysis replacing lycopene with number of servings of the top six sources of lycopene in this population(33) (tomato, tomato juice, tomato sauce, grapefruit, pizza, and watermelon in servings per week as estimated from the FFQ). Subjects with intakes ≥4.4 servings of lycopene food sources/wk (median intake in this sample) had significantly fewer hip fractures compared with the subjects with <4.4 servings/wk (HR for high versus low intake = 0.58; 95% CI: 0.38–0.88; p = 0.01). No significant associations were observed for α-carotene, total β-carotene, β-cryptoxanthin, or lutein + zeaxanthin with hip fracture or nonvertebral fracture (data not shown).

After adjustment for potassium intake, the association of carotenoid intake with hip fracture did not change. However, the association with nonvertebral fracture became weaker (lycopene) (data not shown). The associations attenuated (lycopene) but remained significant after adjustment for baseline BMD at femoral neck (data not shown).

DISCUSSION

To our knowledge, this is the first longitudinal study of intake of individual carotenoids (other than β-carotene(29)) in relation to hip and nonvertebral fracture risk, and the follow-up period was relatively long. We found protective associations of total carotenoid and lycopene intakes with hip fracture and nonvertebral osteoporotic fracture over 17 yr of follow-up.

Published studies have consistently reported that higher fruit and vegetable intake has positive effects on bone mineral status.(6–11) For example, a previous study by our group has reported that dietary pattern rich in fruit and vegetables was associated with higher BMD in men.(8) Fruit- and vegetable-specific antioxidants may play a role,(50) although the individual antioxidant effects have not been well studied. Therefore, more studies are needed to clarify the role of fruit- and vegetable-specific nutrients such as carotenoids in the prevention of osteoporosis.

Studies on β-carotene and hip fracture have reported mixed results. For example, in the Swedish Mammography cohort (n = 66,651 women, age = 40–76 yr), Melhus et al.(51) reported that current smokers with low β-carotene intake had an OR of 1.8 (95% CI: 1.0–3.5) for hip fracture. In contrast, among current smokers with high β-carotene intake, the OR was 2.6 (95% CI: 1.3–5.2). On the other hand, in the Utah Study of Nutrition and Bone Health (n = 1215 men and women, age, ≥50 yr), investigators reported that among participants who had ever smoked, participants in the highest quintile of β-carotene intake (versus the lowest) had a lower risk of hip fracture (OR = 0.39; 95% CI: 0.23–0.68; p for trend < 0.0001).(48) No such associations were found in nonsmokers. A study by Barker et al.(29) (n = 1246 women; age, >75 yr) found no association between serum β-carotene and hip fracture. In this study, we found a weak association of total β-carotene and risk of hip fracture. Smoking status did not modify that association.

Lycopene is the most predominant carotenoid in human plasma. The high concentration of lycopene in blood correlates with reduced risk of prostrate cancer, digestive tract cancers, pancreatic cancer, cervical intraepithelial neoplasia, and myocardial infarction. These effects are attributed to its high antioxidant activity and singlet oxygen-quenching capacity.(52) Laboratory studies have shown that lycopene inhibits formation of osteoclasts and associated bone resorption.(25) Furthermore, it stimulates proliferation and differentiation of osteoblasts.(24,25) A small cross-sectional study (n = 33 postmenopausal women; age, 50–60 yr) by Rao et al.(21) reported that groups with higher lycopene intake had higher serum lycopene (p < 0.02). In that study, the investigators found that high serum lycopene was inversely associated with markers of bone resorption such as low N-telopeptides of type I collagen (NTX; p < 0.0005) and low protein oxidation (p < 0.05). These results suggest a mechanism by which lycopene protects against the risk for hip fracture.

In this study, higher lycopene intake (12,664 μg/d) showed a protective effect against risk of hip fracture. Tomatoes and tomato products are the main sources of lycopene in the U.S. population. As an example, 6 oz of tomato juice or 1 cup of condensed tomato soup (prepared) provides 13,000–17,000 μg of lycopene. We found that those consuming >4.4 servings/wk of lycopene had significantly fewer fractures. Similarly, total carotenoid intake (23,711 μg/d) showed a protective effect against the risk of hip fracture. This level of intake can be obtained by consuming three to four medium carrots, which will provide 21,996–29,328 μg/d of total carotenoids.

Similar to lycopene, β-cryptoxanthin has also been associated with reduced risk of certain types of cancer such as breast cancer.(53) In vivo and in vitro studies have suggested that β-cryptoxanthin has a unique anabolic effect on bone calcification.(26,27,54) Uchiyama et al.(27) showed that β-cryptoxanthin has a direct stimulatory effect on bone formation and an inhibitory effect on bone resorption. This result was confirmed in a controlled human trial (n = 21 men and women; age, 23–47 yr) by the same group of investigators,(28) who reported that the intake of β-cryptoxanthin-fortified juice caused a significant increase in β-carboxylated osteocalcin concentration and a corresponding decrease in serum bone TRACP activity (bone-specific alkaline phosphatases) and NTX. Although these studies suggest a mechanism for β-cryptoxanthin, we were unable to confirm a protective effect of β-cryptoxanthin on the risk of fracture in this study.

Previous research by our group has shown that fruit- and vegetable-specific nutrients such as potassium have a beneficial effect on BMD(6,10) and other bone indices of bone health.(55) Therefore, the protective effect of total carotenoids and lycopene intake on fracture could be caused by potassium present in fruit and vegetables. In this study, adjustment for potassium intake did not change the association of total carotenoid intake with hip fracture. The association of lycopene with nonvertebral osteoporotic fracture was attenuated but remained significant. This suggests that the protective effect of carotenoid intake on nonvertebral fracture may be independent of potassium intake. After adjustment for baseline femoral neck BMD, the observed associations with hip fracture either did not change or attenuated but remained significant, indicating that the protective effect of carotenoids on hip fracture may not be entirely mediated through effects on BMD. Alternative mechanisms to explain our findings might include effects on bone quality either through effects on collagen or nonbone effects on the risk of falls. It is also important to note that carotenoid intake levels in this study were comparable to those reported in the Nurses Health Study for women (age, 53–73 yr) and in the Physicians' Health Study for men (age > 45 yr).

There is evidence that carotenoids may be protective of bone health, through their action on oxidative stress. Oxidative stress may increase bone resorption through activation of NF-κB, which is a crucial mediator of TNFα and osteoclastogenesis.(56–59) Carotenoids are capable of reducing oxidative stress by scavenging singlet oxygen and peroxyl radicals. Therefore, carotenoids may affect bone health by inhibiting bone resorption. However, this mechanism may not provide the underlying explanation for the results observed in this study, because our results were attenuated but remained significant after adjustment for baseline femoral neck BMD. Other mechanisms by which carotenoids may be protective of bone health may include nonantioxidant biological activities of carotenoids and their metabolites such as retinoid-dependent signaling, stimulation of gap junction communications, impact on the regulation of cell growth, induction of detoxifying enzymes,(60) upregulating the expression of genes like connexin 43,(61) or interacting synergistically (particularly lycopene) with vitamin D on cell proliferation, differentiation, and cell cycle progression.(62) However, the impact of these mechanisms on bone metabolism still needs to be evaluated. Studies have shown that carotenoids interfere with growth factor receptor signaling by regulating IGF-1/IGFBP3(63) that are associated with cognitive function,(64) a known risk factor for falls among elderly individuals.(65) Furthermore, published reports on demonstrated synergy among carotenoids (e.g., between β-carotene and lutein)(66) suggested that the strongest results for total carotenoid intake in this study could be caused by a synergy among individual carotenoids.

This study is unique in that it used data from a population-based cohort of older individuals. Furthermore, the prospective design of this study, with over 15 yr of follow-up of subjects, helps in establishing causality. However, this study has some limitations. We examined dietary intakes but not the serum measures of carotenoids, because these were available only for a subsample of the population. The intake measures depend on dietary data from an FFQ, which is a semiquantitative instrument. However, these intakes were previously validated against plasma carotenoid concentrations in a subsample and were shown to have good predictive agreement for all carotenoids except for lutein + zeaxanthin.(34) It is possible that carotenoid intake may be a marker of fruit and vegetable intake in this population. The complete dietary data were available only at the baseline and, therefore, we were unable to adjust for secular changes in diet during the follow-up period. Similarly, because of losses in follow-up, we were unable to adjust for changes in weight beyond the 20th examination. Also, nonvertebral osteoporotic fractures were self-reported in our study and may include some misclassification. To minimize this possibility, we used self-reported fractures at specific bone sites that have been shown to have low percentages of false positives(37); additionally we used confirmed hip fracture as a part of nonvertebral fracture. Finally, the enzyme carotenoid 15,15′-monooxygenase (CMO1) catalyzes the first step in the conversion of dietary provitamin A carotenoids to vitamin A. In some cases, haploinsufficiency of the CMO1 enzyme can prevent this conversion(67) and affect the proposed association of carotenoids with fracture. In any observational study, residual confounding may occur, despite control for several potential confounders.

In summary, we observed an association between total carotenoid intake, lycopene intake, and fracture risk that supports a hypothesis that these nutrients may be protective against fractures in this population of elderly white men and women. β-carotene may also be protective against hip fracture. These findings support the hypothesis of a protective association between carotenoid intake and risk of hip fracture, as well as nonvertebral osteoporotic fracture in older adults. More studies are needed to examine these associations in other populations. Future research is needed to examine associations between specific foods rich in carotenoids and fracture.

Acknowledgements

This study was supported by USDA–ARS Agreement 58-1950-7-707; Framingham Osteoporosis Grant R01 AR/AG 41398, and Framingham Contract Grant N01-HC-25195.

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