Non Nutrient Antioxidants

Plant-based foods contain a multitude of antioxi-dants other than vitamin C and vitamin E. The two major classes of these other dietary-derived antioxidants are the carotenoids and the polypheno-lic flavonoids. There are hundreds of different car-otenoids and thousands of flavonoids, and these compounds give fruits, vegetables, teas, and herbs their wonderful colors in shades of red, orange, yellow, and purple. These compounds are synthesized exclusively in plants and have no known function in human metabolism. No deficiency state for either class of compounds has been identified in humans. Consequently, there is no recommended daily intake or agreed requirement for any of these compounds, and they are regarded as 'non-nutrients.' Nonetheless, there is evidence that diets rich in carotenoids and flavonoids are beneficial to health. For example, in a study of 1299 elderly people in the USA, those with diets rich in carote-noid-containing fruits and vegetables were found to have a significantly decreased rate of CVD and fatal myocardial infarction: the RRs (CI) when the highest and lowest quartiles of intake were compared were 0.54 (0.34-0.86) for fatal CVD and 0.25 (0.09-0.67) for fatal myocardial infarction. The car-otenoid lycopene has been reported to lower the risk of prostate cancer, but the evidence for a relationship between carotenoid intake and the risk of other cancers is conflicting. Increased intake of lutein and zeaxanthin may help to delay or prevent age-related maculopathy, because these carotenoids are concentrated in the macula and are likely to be very important in local protection of the lipid-rich retina. To date, however, epidemiological findings point to health benefits of foods containing carotenoids, and the influence, if any, of individual carotenoids remains to be established.

The same is true for the polyphenolic flavonoids, anthocyanins, and various other plant-based non-nutrient antioxidants in the diet. Many of these have antioxidant powers far higher than those of vitamin C and vitamin E when tested in in vitro systems. Dietary intake can be similar to that of vitamin C (100mgday~1 or higher), but, as their bioavailability is low, plasma levels of individual flavonoids and other phenolic antioxidants are very low or undetectable. The major dietary polyphenolic compounds are quercetin, kaempferol, myricitin, and the catechins. These flavonoids are found in onions, apples, kale, broccoli, Brussels sprouts, teas, grapes, and wine. Moderate wine intake, especially of red wine (which is very rich in polyphenolic antioxidants), is associated with a significant

Table 2 Limitations of observational epidemiological studies of diet and disease

• Cross-cultural study has no power if rates of disease and/or population means of the exposure variable of interest do not vary significantly between the populations being compared

• Behavioral, genetic, and geographical, rather than dietary, variation may account for differences detected

• A 'snapshot' view of recent dietary habits or current status may not be representative of those in earlier or later life, and differences during these periods will confound and confuse the results

• In case-control studies, the disease process itself, drug treatment, or post-diagnosis changes in diet or lifestyle may cause or mask changes in the exposure variable

• Subclinical or undetected disease may be present in controls, decreasing contrast with cases

• Retrospective dietary recall may be unreliable, food tables may be out of date or incomplete, and analysis methods may be inaccurate

• In nested case-control studies, long-term follow-up is needed and may rely on a distant 'snapshot' measure of the exposure variable as a representative index of past and future levels

• Instability or inaccurate measurement of the exposure variable will lead to bias in the results

• Assessment methods and 'high' or 'low' thresholds may vary in different areas supplying data

• If protection is maximal above a 'threshold' level of the exposure variable, then no effect will be detectable if levels in most of the study population are below or above the threshold

• Prospective studies are very expensive, requiring a very large study group and years or decades of follow-up

• Prospective trials generally have disease or death as the measured outcome; this means that the participants in the trial cannot benefit from its findings decrease in the risk of CHD. Tea consumption, especially a high intake of green tea, is associated with a lower risk of CVD and cancer. However, which of the myriad compounds contribute to the reported health benefits is not yet clear. It may be many; it may be none. It must be remembered that association does not prove causality. Equally, the lack of significant effects of supplementation trials in healthy subject does not mean that there is no effect. As outlined in Table 1, and further delineated in Table 2, observational studies have several limitations, and there are various reasons why a conflict may exist between what we observe and the outcome of supplementation trials.

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