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NutrInsight • Do we need dietary polyphenols for health?
2 DIETARyPOLyPHENOLSANDDISEASEPREVENTION: A REVIEW OF THE EVIDENCE FROM EPIDEMIOLOGICAL AND INTERVENTION STUDIES
by Dr Paul Kroon
This chapter will give an overview of the evidence relating dietary polyphenols to several diseases. It will look at the evidence for the effect of polyphenols on cardiovascular disease (CVD), diabetes, cancer and bone health. Numerous cohort and case-control studies have been reported describing associations between consumption of polyphenols/polyphenol-rich foods and disease risk, but these studies cannot prove any cause and effect relationship. In contrast to these epidemiological studies, randomised controlled trials can give some indication of cause and effect. In the case of CVD, there is enough evidence to consider both.
2.1 Polyphenols and cardiovascular disease
Epidemiological evidence
Many factors influence the initiation and progression of cardiovascular disease and only some of those are of dietary origin [Ashwell et al., 2000]. Polyphenols have only recently been acknowledged to be a group of compounds within the diet which might play a role in prevention of CVD. One of the major factors that prevented completion of high quality epidemiological studies was the lack of a comprehensive and accurate database of polyphenol composition and content of foods. Today, such data sources are available (cf 1.1). Epidemiological studies, particularly prospective studies as opposed to cross-sectional studies, can show which sub-classes of polyphenols may be important (by their effects on mortality) but high-quality trials can confirm an effect and can often indicate the mechanisms of action of polyphenols.
• Dietary flavonols and CVD death risk
The first epidemiological study on flavonoids was conducted using the relatively small Zutphen elderly cohort in the Netherlands (806 men), and arguably this study was responsible for a rapid increase in flavonoid and health research [Arts et al., 2001a]. A meta-analysis [Huxley & Neil., 2003] of seven prospective cohort studies included papers from 1993-2001 with a total of 105,000 subjects concluded that high flavonol intakes may be associated with decreased risk from coronary heart disease (CHD) mortality. For the highest intakes, most flavonols came from tea whereas for the lowest intakes the major sources were fruits and vegetables. The average daily intake ranges between 2 mg and more than 34 mg. There were 2087 fatal CHD events and the combined risk ratio was 0.80 (95% CI 0.69-0.93) after adjustment for known CHD risk factors and other dietary components between the highest tertile and the lowest tertile of flavonol consumption. In other words, those consuming the most flavonols exhibited a reduced overall risk across all these studies of about 20%.
• Dietary flavanols and CVD death risk
There are only a few prospective studies looking at flavan-3-ols. The Zutphen Elderly Study [Arts et al., 2001a] showed that catechins, whether from tea or other sources, may reduce the risk of ischemic heart disease mortality (p<0.02), but not of stroke.
On the other hand, in a prospective study on 34,492 postmenopausal women from Iowa [Arts et al., 2001b], there was a strong inverse association between the intake of (+)-catechin and (-)-epicatechin and coronary heart disease death (risk ratios from lowest (median intake 3.7 mg/day) to highest quintile (median intake 52.7 mg/day): 1.00, 0.95, 0.97, 0.77, 0.76).
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