This study has shown that IL-6 may serve as an important ethnic-specific cytokine that is higher among Blacks than Whites, while the other inflammatory markers or anti-inflammatory marker (IL-10) did not differ between Blacks and Whites. Healthy lifestyle behaviors were associated with a lower inflammatory status, after controlling for ethnic differences, as increased exercise frequency was associated with increased IL-10 levels, and consumption of a vegetarian diet was associated with lower CRP levels.
The present study provides further evidence of an association between ethnicity and inflammation, in line with other studies that have found higher IL-6 levels among Blacks compared to Whites.8,27,28
However, these studies did not control for variables such as diet or exercise when assessing the ethnic differences in IL-6, and may have been confounded by differences between Blacks and Whites in regard to lifestyle induced changes in inflammation. Earlier study designs were also based on a much smaller sample size than was used for the current investigation.
Adipose tissue synthesizes inflammatory markers such as IL-6, and increased body mass has been associated with higher inflammation levels among Blacks.29,30
However, the ethnic specific difference in IL-6 was significant even after controlling for BMI in the current study, so other factors may be responsible for this relationship. One possible reason for the ethnic variations in the level of this cytokine is due to differences in cytokine gene polymorphisms. It has been suggested that specific allelic variations in the regulatory regions of inflammatory cytokine genes may alter the expression of some cytokines.31
One of the genotypes that results in high IL-6 production (G/G IL-6 genotype) has been predominantly found in Blacks, and may explain higher levels of this cytokine among this ethnic group.32
Although there were higher CRP levels among Blacks in the present study, this association was no longer significant after controlling for the other covariates. Studies among the US population have shown that there are higher CRP levels among Blacks in relation to Whites.33–35
However, like the results from the present study, the ethnicity-CRP association found in some of the investigations ultimately disappeared after controlling for various health-related factors.36–37
In the current study, the ethnicity-CRP relationship was largely attenuated after controlling for vegetarian diet consumption as well as exercise frequency, and even more attenuated after controlling for BMI. Therefore, future studies should investigate lifestyle components as well as other variables associated with obesity that may serve as important mediating factors between ethnicity and CRP.
No significant differences in IL-10 and TNF-α levels were found between Black and White patients in the present study. Although lower levels of IL-10 among Blacks compared to Whites have been reported previously,9
the analysis was based on stimulated peripheral blood samples rather than basal cyto-kine levels in the blood. Therefore, no direct evidence of ethnic differences in circulating IL-10 levels was shown. As far as ethnic differences in TNF-α are concerned, the results are mixed. A study by Kalra et al8
found that TNF-α levels were higher among Blacks in relation to Whites from the United Kingdom, while Elkind et al10
found no significant difference in TNF-α levels among Blacks and Whites from the United States. Therefore, geographic location, environmental factors, and other variables such as diet, exercise and body mass may play an important role in the onset of inflammation among different ethnic groups.
Based on the results provided, engaging in exercise and consuming a vegetarian diet was associated with lower inflammation levels. Vegetarians demonstrated a lower blood concentration of CRP compared to non-vegetarians which is in accordance with previous research showing that long-term vegetarians have a lower risk of coronary heart disease and an improved antioxidant and inflammatory status compared to non-vegetarians.38
In addition, greater amounts of regular physical activity have been associated with elevated IL-10 levels in healthy older males,39
and aerobic exercise training has been reported to exert anti-inflammatory effects in type 2 diabetics due to increases in IL-10 concentrations.40
Specifically, high intensity exercise has been more strongly associated with increased IL-10 levels when compared to moderate and light intensity exercises among healthy, well-trained participants,41
suggesting that more vigorous exercise may provide stronger anti-inflammatory effects.
Strengths and Limitations
This study assessed pro-inflammatory and anti-inflammatory cytokines that were not examined together in previous studies. The absence of cigarette smokers and low amount of alcohol consumption allowed the confounding effects of smoking and excess alcohol on inflammatory markers to be avoided. Finally, the current study sample size provided sufficient power for detecting associations between the study variables.
This was a cross-sectional study so causality between ethnicity and other variables and levels of inflammatory markers cannot be assumed. There were differences in baseline factors between Blacks and Whites. We adjusted for these factors, but residual confounding may remain due to the effects of other variables not measured in the present study (such as lean body mass differences between Blacks and Whites) as well as incomplete statistical adjustment. However, abdominal fat content was assessed by waist circumference which showed the same ethnic specific differences found based on BMI measurements.
The study population consisted of only Adventist church goers and generalizability to the larger population may be limited. Genetic markers of the inflammatory cytokines were not examined, limiting our analysis to cytokine concentrations rather than specific gene activity. The TNF-α concentration of 204 of the study participants (63% White and 37% Black) was below the limit of detection as well, similar to the proportion of Whites and Blacks in the overall study. It is possible that this population of church goers may have been less susceptible to increased inflammation based on their overall health and lifestyle practices, making it difficult to detect this inflammatory marker. Therefore, future analysis of inflammation using TNF-receptor 1 as a surrogate marker for TNF-α may be preferable since TNF-α has a short half-life (~15 minutes) and can be difficult to detect among certain populations using standard ELISA kits.42