In this study of adolescents, a diet high in fruit and vegetables, and therefore, rich in antioxidants, folate, and flavonoids, was associated with lower levels of markers for inflammation and oxidative stress. These results show that increased consumption of fruit and vegetables, and therefore, antioxidant and flavonoid intake, begin to have an effect on markers of inflammation and oxidative stress early in life and suggest that with an ongoing similar pattern of consumption, these beneficial relations may grow stronger with aging, resulting in lower cardiovascular risk (36
The relation of flavonoid intake to inflammation has not been previously characterized in an adolescent population. However, several in vivo experiments and observational studies conducted in middle age and elderly populations have examined this association and noted inverse relationships (37
), consistent with our study outcomes in adolescents. Grape flavonoids decreased TNF-α and IL-6 concentrations in both pre- and postmenopausal women (42
), and IL-6 was shown to be inversely related to quercetin (43
) and tea flavonoids in vivo (44
). In the present study total flavonoids and the individual flavonoids, kaempferol and quercetin, were significantly inversely associated with oxidative stress. However, with the exception of the inverse relation of luteolin to TNF-α, none of the flavonoids was significantly associated with levels of the other inflammatory factors. This may suggest that the earliest effect of flavonoids is on oxidative stress, with a significant effect on inflammation not noted until adult life, as previously described (36
). It is important, however, to study the development of subclinical inflammation in youth without chronic disease to determine its early mechanisms.
One study of 79 children aged 6-14 years observed non-significant associations between antioxidant intake (vitamins C and E and beta-carotene) and inflammatory markers CRP, IL-6, and TNF-a; however, the study had insufficient power to detect weak associations (45
). The present study of 285 adolescents mean age 15 years demonstrated significant and inverse associations of antioxidant intake with markers of inflammation and oxidative stress.
The proposed beneficial effects of antioxidants, folate, and flavonoids, such as lowering blood cholesterol and homocysteine, reducing blood pressure, decreasing platelet aggregation, and scavenging free radicals and reactive oxygen species (ROS), are associated with a reduction in the inflammatory markers and prostaglandins evaluated in this study, thus supporting the presumption that they are intermediaries in the pathway to development of cardiovascular disease (46
). Decreases in cytokines, such as IL-6, are directly associated with a decrease in inflammation (52
). Decreases in oxidative stress are associated with lower levels of ROS, free radicals, and reactive nitrogen species, resulting in a reduction in lipid oxidation and formation of atherosclerotic plaques (54
). A 4-week randomized intervention trial in healthy, non-smoking men showed that a diet rich in carotenoids (8 servings per day of fruits and vegetables) reduced serum CRP (57
). In a cross-sectional analysis of data from the Massachusetts Hispanic Elder Study a significant inverse dose-response relation was found for fruit and vegetable intake with CRP and plasma total homocysteine (3
). A recent prospective study of 40-59 year old English males found that fruit intake had a significant inverse association with CRP, blood viscosity, and tissue plasminogen activator antigen (11
). Results from the present study have shown for the first time that consumption of antioxidants, total flavonoids, and total fruit and vegetables was associated with lower levels of oxidative stress (F2-isoprostanes) in healthy adolescents.
While male sex has been strongly associated with risk factors for atherosclerosis and cardiovascular disease (58
), and previous research has shown increased oxidative stress in young adult, healthy, non-smoking men in comparison to a similar group of women (59
), the present study found that girls at mean age 15 had a significantly higher concentration of F2-isoprostanes. It is possible that this may be related to the age of the boys and girls, with effects of recent puberty influencing levels of oxidative stress and antioxidant activity (60
). Alternatively, it may be related to changes in percentages of body fat versus lean body mass at this age; however, regression models were adjusted for Tanner staging and body mass (62
). Previous research has demonstrated a significant reduction in oxidative stress, represented by F2-isoprostane, in obese men and women undergoing a nutritional and behavioral intervention (63
); and decreased oxidative cellular damage to DNA and lipids was observed following a 14 day dietary intervention that included 12 servings of fruits and vegetables (64
Although antioxidant and flavonoid analyses have been conducted successfully in prior studies (65
), there are potential errors associated with assessing the antioxidant and flavonoid content of foods (67
). Determination of flavonoid content is difficult due to the complex manner in which polyphenols are synthesized in food products and lack of a comprehensive food composition database for flavonoids. Quantification of antioxidant and flavonoid consumption may be further complicated by food storage, handling, processing, and preparation (68
). Water-soluble antioxidants and flavonoids are released into high temperature cooking water and discarded.
One potential limitation of this study is the lack of smoking information for the study participants. It is likely that smoking would have attenuated the relation between dietary intake and the outcome factors (71
). The weak correlations between dietary intake and markers of inflammation and oxidative stress ranging from -0.13 to -0.19 (all p<0.5) may be considered a potential limitation of the study. However, associations of BMI with markers of inflammation and oxidative stress are known to be moderately strong in adults (r≥0.35; p<0.001) (72
). In the present study with 90% power to detect small associations and after adjusting for BMI and other covariates, dietary antioxidant, flavonoid, and fruit and vegetable intake remained significantly and inversely related to markers of inflammation and oxidative stress in adolescent boys and girls. Furthermore, it is likely that the duration of low-grade inflammation and oxidative stress concentrations in this sample of adolescents are not as great as in adults (26
); and therefore, the strength of the associations would not be as great in adolescents compared to those in adults (12
). Despite these limitations, this study provides encouraging results with regard to the adolescent diet and its relation to inflammatory markers and oxidative stress.
On a negative note, dietary intake of adolescents enrolled in this study showed that they did not achieve the Food Guide Pyramid recommendations (74
) for fruit and vegetable consumption. Adolescents enrolled in this study consumed about ½ of the recommended amounts for fruit (1.5-2 cups/day) and vegetables (2.5-3.5 cups) (74
). This is consistent with recent reports on the diet of adolescents in the U.S. (75
). In addition, most adolescents consumed lower amounts of total flavonoids than the U.S. national average estimate for adult flavonoid consumption (~20 mg/day) (15
); however, vitamin C intakes were greater than the recommended amount (16
). Thus, strategies designed to increase fruit and vegetable intake, and therefore antioxidant and flavonoid intakes, in adolescents may result in an overall reduction in cardiovascular risk indicators. This is consistent with studies on the pathophysiology of cardiovascular disease showing that arterial plaque formation and other negative cardiovascular sequale begin early in life and that 5-6 or more daily servings of fruit and vegetables are associated with fewer adverse events in adulthood (36