Glucosamine and Chondroitin Use in Relation to C-Reactive Protein Concentration: Results by Supplement Form,Formulation,and Dose

Introduction

Often taken for osteoarthritis, glucosamine and chondroitin are commonly used nonvitamin nonmineral supplements. ^1,2 Although the effectiveness of these supplements for joint pain and function remains controversial, ^{3 –9} a growing body of evidence suggests that glucosamine and chondroitin have anti-inflammatory properties. ^{10 –32} In vitro models have shown that glucosamine and chondroitin inhibit the activity of nuclear factor kappa B (NFkB), a transcription factor central to the inflammatory cascade, in a dose-dependent manner; specifically, research suggests that these supplements act to inhibit the degradation of NFkB inhibitory subunit, IkB, blocking NFkB from translocating to the nucleus. ^10,11,33 Supporting research from animal studies has demonstrated that administration of glucosamine/chondroitin reduces inflammatory markers downstream of NFkB. ^{21 –26}

Several studies have evaluated whether the anti-inflammatory effect extends to humans. ^{29 –31,34,35} Two prior observational studies suggest glucosamine/chondroitin to be associated with reduced concentration of C-reactive protein (CRP), a nonspecific marker of systemic inflammation. ^29,30 These results are consistent with those from a small double-blinded randomized placebo-controlled crossover trial in which the intervention was 1500 mg of glucosamine hydrochloride +1200 mg of chondroitin sulfate, administered over a 28-day period. ³¹ A proteomics analysis from this randomized controlled trial further revealed that the pathway most different between the intervention and placebo was the “cytokine activity” pathway (p = 2.6 × 10^–16), supporting the hypothesis that glucosamine/chondroitin may reduce inflammation.

Further, in a trial of 53 osteoarthritic patients, 1500 mg of glucosamine hydrochloride +675 mg of chondroitin significantly reduced concentration of inflammatory marker, prostaglandin E₂. ³⁴ However, in a trial of 51 patients with rheumatoid arthritis, 1500 mg of glucosamine hydrochloride did not reduce CRP. ³⁵ The intervention was glucosamine alone (rather than glucosamine+chondroitin) and study participants were able to continue taking their normal drugs. It is unclear if the lack of association is due to the choice of supplement formulation (glucosamine alone), form (glucosamine hydrochloride), study design (e.g., rheumatoid arthritis patients often take strong anti-inflammatories, and continued use of these drugs would likely conceal differences between groups), or other factors.

Despite a growing body of evidence to suggest that use of glucosamine/chondroitin is associated with reduced inflammation, it is unclear what characteristics of exposure may be driving the observed association. Research has also demonstrated an inverse association with risk of colorectal cancer and lung cancer ^{36 –40} ; as inflammation is thought to play a role in the development of these cancers, ^{41 –46} this may offer a plausible biologic mechanism by which these supplements may reduce risk. Better understanding the characteristics of exposure associated with inflammation is critical to moving this body of research forward. The authors, therefore, evaluated the association between glucosamine and chondroitin use in relation to CRP, by supplement form, formulation, and dose in the National Health and Nutrition Examination Survey (NHANES).

Methods

Results

As shown in Table 1, glucosamine+chondroitin users are more likely than nonusers to be older, non-Hispanic white, and highly educated. These patterns do not vary over calendar time. Distributions of additional variables are shown in Supplementary Table S1. Most distributions were comparable by time, with some exceptions. For example, in the early years (1999–2004), glucosamine+chondroitin users and nonusers were comparable with regard to health status, with 18% of glucosamine+chondroitin users reporting poor/fair health status, as compared with 17.3% of nonusers. However, in later years (2005–2010), glucosamine+chondroitin users were less likely to report fair/poor health status than nonusers (10.1% vs. 17.9%, respectively). A similar pattern was observed for diabetes.

The association between glucosamine and CRP varied by calendar time (p-interaction = 0.04), with a statistically significant inverse association between glucosamine and CRP in early years (ratio = 0.87; 95% confidence interval [CI] = 0.79–0.96), and no association in later years (ratio = 1.09; 95% CI = 0.94–1.28) (Table 2). A comparable pattern was observed for chondroitin (p-interaction = 0.01), again with a significant inverse association in early years (ratio = 0.83; 95% CI = 0.72–0.95) and a nonsignificant positive association in later years (ratio = 1.16; 95% CI = 0.99–1.35).

When examined by supplement formulation, the authors observed a significant inverse association for use of glucosamine+chondroitin as compared with use of neither in early years (ratio = 0.82; 95% CI = 0.72–0.95); no association was observed for use of glucosamine alone (ratio = 0.96; 95% CI = 0.80–1.14); however, the difference between combined use and use of glucosamine alone did not reach statistical significance (p-value = 0.12). Conversely, a positive association was observed for combined use of glucosamine+chondroitin in the later years (ratio = 1.16; 95% CI = 1.00–1.35), again with no association observed for glucosamine alone (ratio = 0.95; 95% CI = 0.74–1.22). Again, the direct comparison of glucosamine+chondroitin as compared with glucosamine alone in later years was not statistically significant (p-value = 0.08). The interaction across years was significant (p-interaction = 0.03).

In early years, there was a statistically significant inverse association for glucosamine sulfate and CRP (ratio = 0.78; 95% CI = 0.64–0.95) and a nonsignificant inverse association was observed between glucosamine hydrochloride and CRP (ratio = 0.86; 95% CI = 0.72–1.05); the difference by supplement form was not significant (p-value = 0.21). No significant associations were observed by supplement form in later years (p-value = 0.38). A p-interaction = 0.05 was observed for the interaction between supplement form and calendar time.

When examining associations by dose, no statistically significant associations were observed in early years or in later years, nor did the association significantly vary by time. Furthermore, no significant associations were observed in exploratory analyses of duration in early or later years.

No significant interaction was observed between glucosamine+chondroitin and age, gender, BMI, health status, or physical activity within either time period (Supplementary Tables S2 and S3).

Discussion

In this nationally representative survey, the authors observed no clear overall pattern of association between glucosamine and chondroitin and CRP, by supplement formulation, form, and dose—in large part, due to unexpected strong effect modification by time. Specifically, they observed a significant inverse association for glucosamine/chondroitin in early years, and an unexpected null association in later years.

Although the geometric mean of CRP did not significantly differ when directly comparing users of glucosamine+chondroitin to users of glucosamine alone, it should be noted that as compared with nonuse significant associations were observed for use of glucosamine+chondroitin (inverse association with CRP in early years, positive association in later years), whereas no significant associations were observed for use of glucosamine alone. Associations with CRP did not significantly differ when directly comparing supplements by form (glucosamine sulfate vs. hydrochloride), although a significant inverse association was only observed for glucosamine sulfate in the early years. No significant trends were observed by dose, nor in exploratory analyses of duration.

In this study, the authors observed an inverse association between use of glucosamine and CRP in the early years, with a null association in later years. A comparable pattern of association was observed for chondroitin use. Although the inverse association in early years aligned with expectation and the prior literature, ^{29 –31,34} it is unclear why the association changed over time.

It is conceivable that this pattern may be driven by changing characteristics of exposure over time. In this study, the authors have evaluated patterns by supplement formulation, form, and dose, but it is possible that there are other unmeasured characteristics of exposure, such as supplement quality, which have changed over time. It is also possible the factors associated with use have changed over time in a way not captured by covariate adjustment, resulting in the observed pattern of association. For example, if there is an unmeasured healthy behavior more strongly associated with use in the early years, such a variable could potentially explain the pattern of results if also associated with CRP. However, the authors have carefully adjusted for many healthy behaviors, and it is hard to imagine a variable that could explain this pattern of results.

It is also possible that there may be an effect modifier that changed in prevalence over time, which could explain observed pattern of association; however, there was no evidence of effect modification for the factors examined. Although this pattern of results does raise the potential of bias or chance, it should be noted that the prior literature does largely support an inverse association with inflammation, including two small trials (which should not be subject to concerns of residual confounding). ^{29 –31,34,35}

When evaluating whether the association varied by supplement formulation, the authors observed no significant difference in the geometric mean CRP for users of glucosamine+chondroitin as compared with users of glucosamine alone. This held in both early and later years. However, it should be noted that the significant associations were only observed for glucosamine+chondroitin (inverse in early years, positive in later years), with no significant associations observed for glucosamine alone. Approximately 70% of glucosamine in the U.S. population is taken in conjunction with chondroitin and nearly all chondroitin is taken with glucosamine (a reflection of the supplement products available on the market). Thus, the sample size is smaller for glucosamine alone and the authors cannot rule out modest effect estimates pertaining to use of glucosamine alone.

However, this pattern in the early years is consistent with what has been observed elsewhere: in exploratory analyses conducted in studies of inflammation, oxidative stress, and CRC, the association has been consistently stronger for glucosamine+chondroitin than for glucosamine alone. ^29,36,37,51 These analyses suggest that the biologic effect may be driven by chondroitin or the combination of glucosamine+chondroitin, rather than glucosamine alone; however, prior exploratory analyses of glucosamine alone were quite underpowered, with wide CIs limiting the interpretation of the null association for glucosamine alone.

Even so, it is unclear why the association for glucosamine/chondroitin would change direction over time. It is possible that this pattern reflects some change in the glucosamine+chondroitin supplements available (or the chondroitin component). It is also possible that the association between glucosamine+chondroitin and CRP could be differentially confounded in early and late years. Further research in a large study will be needed to better understand this question, preferably one with recent data to reflect current patterns of use.

Associations also did not significantly differ by supplement form (glucosamine sulfate vs. hydrochloride). However, the significant inverse association in the early years was only observed for glucosamine sulfate, with no significant association observed for glucosamine hydrochloride. Although trials have largely used glucosamine hydrochloride as the experimental agent, ^3,31,34,35 glucosamine sulfate accounts for ∼30% of use in the population, and it was unknown if the association with inflammation differs by supplement form. In this study, there is some suggestion that the early inverse association is reflected by glucosamine sulfate, although a nonsignificant inverse association was also observed for glucosamine hydrochloride in the early years.

Although prior studies have not directly compared glucosamine hydrochloride and sulfate, prior trials of inflammation have used glucosamine hydrochloride and have observed significant reductions in inflammation, unlike this study. No significant associations were observed in later years. Again, an explanation for why this pattern was observed over time remains unclear. It is possible that there is a change to the quality of supplements over time or some other change to an unmeasured characteristic of these supplements or their users. Although the authors have been extensive with covariates included, it is possible that there is an unknown/unmeasured factor differentially confounding results over time, given changes in prevalence and/or changes in the strength of association with either the exposure or outcome. The impact of chance also cannot be ruled out.

The authors also evaluated whether the relationship between glucosamine/chondroitin and CRP strengthened with increasing dose. Evidence of a “dose–response” relationship would lend credibility to a true biologic effect, while also informing the dose needed to observe an association with inflammation. In this study, no such significant trend was observed, raising a question about whether the inverse association in early years might reflect bias or chance, rather than a true causal association. A similar pattern was observed in exploratory analyses of duration. Given the lack of prior study on these specific questions, they should be evaluated in a contemporary large study.

This study leverages rich data on supplement use in a large nationally representative population of U.S. adults. NHANES has extensive covariate data, enabling a well-controlled analysis. This is the first study to evaluate whether the association between glucosamine and chondroitin and inflammation varies by supplement form and dose, and offers a large sample size to evaluate whether the association varies by supplement formulation—a question only previously evaluated in small exploratory analyses.

Given strong effect modification by time, all results had to be stratified by time, cutting power to evaluate associations of interest, also precluding defining exposure by regular use, so as to not further reduce power. However, most glucosamine/chondroitin users take the supplements regularly. ² In exploratory analyses, duration was assessed for the particular supplement used, and may not reflect total duration of use of a given ingredient; thus, these duration-specific analyses should be interpreted as exploratory. Although supplement use was assessed in the prior 30 days (reflecting exposure before CRP measurement), it is always possible that use in this period may not reflect use in the etiologically relevant timeframe. Finally, although the authors have taken extensive effort in covariate adjustment, it is conceivable that there is some residual confounding, and if the degree of confounding varies over time, this could give rise to the unexpected change in association over time.

In conclusion, in this nationally representative study, the authors observed no clear pattern of association between glucosamine and chondroitin use and CRP, largely due to the unexpected variation over calendar time. Although inverse associations in early years support prior study, null and/or positive associations in later years suggest that further research is needed to better understand why these associations changed over time, and if they reflect a true change in exposure, bias, or chance.