C-reactive protein (CRP) is produced by hepatocytes as part of the acute-phase response and represents a sensitive and non-specific marker of inflammation 1 . CRP is associated with cardiovascular disease, type 2 diabetes mellitus, and cancer 2 , and identification of the factors associated with serum CRP concentration is important to their prevention. Dietary factors represent one major modifiable factor related to CRP, and several previous studies have shown that increasing the intake of antioxidant nutrients (e.g., vitamin C and vitamin E) or foods (e.g., tea, fruits, and vegetables) is associated with decreased CRP concentrations 2 3 4 . In other studies, however, these single nutrients or foods were shown to have no effect 2 5 6 7 . Accumulating mechanistic and epidemiological data suggest that antioxidants act not only individually but also co-operatively, and in some cases synergistically 8 . The effect of complex combinations of antioxidant nutrients and foods on CRP concentrations therefore warrants investigation.

Recently, the concept of dietary total antioxidant capacity (TAC) was proposed to assess the combined effect of multiple antioxidants 9 . Previous observational studies 10 11 12 and an interventional study 13 showed that dietary TAC was inversely related to CRP among Italian, Spanish, and Greek populations. Given that dietary habits vary by country - vegetables commonly consumed in Japan differ from those consumed in Italy, for example 14 15 - the food source of dietary TAC at the intake level may also differ, and the association should therefore be evaluated in other countries, including Japan. Further, because CRP concentration tends to increase with age 16 , maintaining concentrations as low as possible in younger age is an important preventive measure. Evaluation of the association between dietary TAC and CRP among young adult populations is therefore important.

Here, we estimated dietary TAC from different assays in young Japanese women and examined the association between dietary TAC and serum CRP concentration using these assays.

Subject characteristics by normal and elevated CRP concentration are shown in Table  2. Median serum CRP concentration was 0.113 mg/L, with a range of 0.025 to 7.100 mg/L. The prevalence of elevated CRP concentration was 5.6%. Median dietary TAC was 10.61 mmol Fe²⁺/d for FRAP, 16.78 mmol TE/d for ORAC, 6.26 mmol TE/d for TEAC, and 6.71 mmol TE/d for TRAP. The range of Spearman’s correlation coefficients between each dietary TAC was 0.89-0.98 (data not shown). For the contribution of each food item to dietary TAC (Table  1), green, barley, and oolong tea was the major contributor (FRAP, 57%; ORAC, 47%; TEAC, 37%; and TRAP, 46%) and coffee was the second major contributor (FRAP, 3%; ORAC, 3%; TEAC, 6%; and TRAP, 10%) in all four assays. At food group level, non-alcoholic beverages, including green, barley, and oolong tea and coffee, was the major contributor in all food groups. Although vegetables was the second contributor in the food groups (FRAP, 8%; ORAC, 11%; TEAC, 9%; TRAP, 8%), it contributed less than single food item of green, barley, and oolong tea. The only significant difference between the elevated and normal CRP concentration groups was seen for dietary TAC (Table  2), with respective ratios for each dietary TAC of only 65-82% of those of the normal CRP concentration group.

CRP C-reactive protein, PUFA polyunsaturated fatty acid, TAC total antioxidant capacity, FRAP ferric reducing ability of plasma, ORAC oxygen radical absorbance capacity, TEAC Trolox equivalent antioxidant capacity, TRAP total radical-trapping antioxidant parameter, TE Trolox equivalent.

* Values are medians (interquartile ranges) or numbers of subjects (%).

†P value between normal CRP and elevated CRP groups. For continuous values, the Mann–Whitney signed-rank test was used; for categorical values, the chi-square test was used.

‡ Energy adjustment was performed according to the residual method.

Table  3 shows the association between dietary TAC and elevated serum CRP concentration. The percentages of participants with elevated CRP levels in the low dietary TAC group (7.7-8.6%) were about two to three times higher than those in the high dietary TAC group (2.7-3.6%). ORs for elevated serum CRP concentration in the high versus low dietary TAC group were 0.30-0.45 in the crude model, and were hardly changed after adjustment for possible confounders. TAC from FRAP was inversely associated with serum CRP concentration (adjusted OR for elevated CRP concentration in high versus low dietary TAC group: 0.39 [95% CI: 0.16-0.98]; P = 0.04). With regard to TAC from ORAC, the inverse association between dietary TAC and serum CRP concentration was also observed with this method (adjusted OR: 0.48 [95% CI: 0.20-1.14]; P = 0.10), although the association did not reach statistical significance. TAC from TEAC was inversely associated with serum CRP concentration (adjusted OR: 0.32 [95% CI: 0.12-0.82]; P = 0.02). Additionally, TAC from TRAP was also inversely associated with serum CRP concentration (adjusted OR: 0.31 [95% CI: 0.12-0.81]; P = 0.02).

CRP C-reactive protein, TAC total antioxidant capacity, FRAP ferric reducing ability of plasma, ORAC oxygen radical absorbance capacity, TEAC Trolox equivalent antioxidant capacity, TRAP total radical-trapping antioxidant parameter, TE Trolox equivalent; ref, reference.

* Values are medians (ranges).

† Energy adjustment was performed according to the residual method.

‡ Adjusted for residential region [north (Kanto and Tohoku), central (Tokai and Hokuriku), and south (Kyushu and Chugoku)], size of residential area (city with a population ≥1 million, city with a population <1 million, and town and village), current smoking (yes or no), alcohol drinking (yes or no), dietary supplement use (yes or no), physical activity level (total metabolic equivalents-hours/d, continuous), body mass index (kg/m², continuous), and n-3 polyunsaturated fatty acid intake (g/d, energy-adjusted, continuous).

In this study, we found that a higher dietary TAC was associated with a lower prevalence of elevated CRP concentration among young Japanese women. Dietary TAC from FRAP, TEAC, and TRAP were significantly inversely associated with serum CRP concentration. Dietary TAC from ORAC also showed an inverse association with serum CRP. The dietary TAC values of elevated serum CRP concentration group were significantly lower than those of normal CRP group. To our knowledge, this is the first study to examine the association between dietary TAC and elevated CRP concentration in a non-western population.

In the present study, dietary TAC was estimated using a DHQ for the four different assays of FRAP, ORAC, TEAC, and TRAP. The TRAP databases 15 33 contain the TAC values of only a limited number of foods, and those of 38 foods could not be obtained. In contrast, the FRAP databases 29 30 31 32 contain an extensive number of foods, and values could not be obtained for only 16 foods. Further, most foods in the FRAP assay were assigned an analytical rather than a substituted or calculated TAC value. These different results were probably due to the different antioxidant mechanisms derived from different substrates, reaction conditions, and quantification methods 56 . Nevertheless, the major sources of dietary TAC were the same among the TAC assays and the TAC values of these food items were available in the literature (i.e., teas, coffee, and chocolate). Our study showed that dietary TAC from FRAP, TEAC, and TRAP were significantly inversely associated with serum CRP concentration. Dietary TAC from ORAC also showed an inverse association with serum CRP, albeit without statistical significance. Additionally, the correlation coefficients between respective dietary TAC values were high. These results may suggest that dietary TAC was inversely associated with serum CRP concentration regardless of assay.

Whereas previous western studies showed that the main contributors of dietary TAC were coffee, fruits, vegetables, and alcohol beverages 57 58 59 60 , the major contributor in this present young Japanese population was green, barley, and oolong tea. Even the sum contributions of all vegetables or fruits were less than the contribution of single green, barley, and oolong tea. Additionally, vegetables commonly consumed in Japan differ from those in Italy 14 15 , and fruit and vegetable items consumed by contemporary young Japanese women differ from those by young Spanish adults 12 . Nevertheless, dietary TAC was inversely associated with CRP in our Japanese population, as with Italian and Spanish populations 10 12 , suggesting that a high dietary TAC is important for a low prevalence of elevated CRP level regardless of the type or origin of food. The present finding of a significant association between dietary TAC and CRP contrasts with a previous study in the same population which found no association between single intakes of vitamin C, fruits, and vegetables and serum CRP 5 . This difference in turn suggests that complex combinations of antioxidant nutrients and foods might be more strongly associated with CRP than any single nutrient or food alone.

Dietary TAC was significantly associated with some lipid biomarkers (e.g., oxidized low-density lipoprotein; ox-LDL) and plasma TAC was negatively correlated with ox-LDL concentrations 61 . These results may suggest that high consumption of antioxidant-rich foods decrease oxidation in the low-density lipoprotein by increasing the plasma TAC availability. These favorable situations might relate to low serum CRP concentrations. CRP production in the liver is induced by interleukin-6 (IL-6) 1 62 . Adipocytes produce many inflammatory cytokines, including IL-6, and their transcription is regulated by the nuclear transcription factor-κB (NF-κB) 62 63 . Given previous findings that dietary TAC was inversely associated with CRP as well as with mRNA expression of NF-κB subunit-1 and IL-6 in young Spanish adults 12 and that ox-LDL is able to induce a pro-inflammatory status by the activation of NF-κB 61 , the association between dietary TAC and CRP might accordingly relate to NF-κB-regulated pathways interacting ox-LDL. However, that study also showed that dietary TAC was not associated with serum IL-6 concentration 12 . Meanwhile, another study reported that dietary TAC tended to show an inverse association with IL-6 level 11 . These inconsistent results hamper any comprehensive understanding of the mechanism of the relationship between lower CRP concentrations and dietary TAC. Various antioxidants contribute to dietary TAC and these compounds regulate inflammation via multiple signaling pathways 64 . The mechanism by which dietary TAC associates with serum CRP concentration is therefore likely complex. Further, the validity of TAC as a measure of functional efficacy of antioxidant defense in vivo was questioned. In fact, not only dietary phytochemicals, but also powerful enzymes in cells and tissues contribute to the prevention of oxidation. Additionally, antioxidant capacity of some molecules in foods may change into the uptake and metabolism 65 . We should take into account the fact that dietary TAC may not necessarily reflect antioxidant level in vivo.

Several limitations of the present study should be mentioned. First, because TAC data on Japanese foods were available from only a single database 14 , dietary TAC was estimated using databases developed in other countries. Additionally, many foods were assigned a substituted or calculated TAC value. Further, because a reliable TAC database for dietary supplements could not be obtained, we did not consider the intake of dietary supplements in calculating dietary TAC. Second, the DHQ was not specifically designed to measure dietary TAC. In assessing dietary TAC, we were unable to investigate the validity of the DHQ against the 16-day dietary records we previously used to investigate the validity of other dietary variables 20 21 because the dietary record contained an insufficient number of foods with information on TAC values (n = 143–373). However, a previous validation study among 92 adult women reported Pearson correlation coefficients of 0.64 for β-carotene, 0.52 for vitamin C, and 0.47 for α-tocopherol 21 , and Spearman correlation coefficients for food groups were 0.75 for coffee, 0.59 for green and oolong tea, 0.56 for total vegetables, and 0.40 for fruits 20 . This satisfactory validity of the DHQ for a wide range of antioxidant nutrients and foods provides some reassurance. Third, participants of the present study were selected female dietetic students, not a random sample of Japanese women. In addition, because of our recruitment procedure, the exact response rate was unknown, which might have produced recruitment bias. Thus, our results cannot easily be extrapolated to the general Japanese population. Fourth, although we attempted to adjust for a wide range of potential confounding variables, we are unable to rule out residual confounding. Finally, the cross-sectional nature of the study hampers the drawing of any conclusions on causal inferences among dietary TAC and serum CRP concentration.

This study showed that dietary TAC was inversely associated with serum CRP concentration among young Japanese women regardless of assay. This result will provide valuable insights from a preventive perspective. Our result should be confirmed in other Asian and Japanese populations.

TAC: Total antioxidant capacity; CRP: C-reactive protein; DHQ: Diet history questionnaire; PUFA: Polyunsaturated fatty acid; FRAP: Ferric reducing ability of plasma; ORAC: Oxygen radical absorbance capacity; TEAC: Trolox equivalent antioxidant capacity; TRAP: Total radical-trapping antioxidant parameter; TE: Trolox equivalent; BMI: Body mass index; OR: Odds ratio; CI: Confidence interval; IL-6: Interleukin-6; NF-κB: Nuclear transcription factor-κB.

The authors declare that they have no competing interests.

SK formulated the hypothesis, analyzed and interpreted the data, and wrote the manuscript. KM contributed to the concept and design of the study, the study protocol, and data collection and management; coordinated the field work; interpreted the data; and contributed to the writing and editing of the manuscript. SS was responsible for the concept and design of the study, the study protocol, and data collection and management; and contributed to the writing and editing of the manuscript. KU contributed to the concept and design of the study, the study protocol, and data collection. MY, HH, TG, JO, KB, KO, TK, RW, and YS contributed to data collection. All authors contributed to the final version of the manuscript. All authors read and approved the final manuscript.