Nutrition and vascular health

There is robust epidemiological evidence that diet quality influences health outcomes. Diet quality measures can describe a healthy or unhealthy diet and twenty-five indices of diet quality have been identified.1 A review of the association between better diet quality and health with mortality and morbidity as outcomes, reported that all-cause cardiovascular disease mortality and risk and cancer mortality and risk were significantly reduced.1 In the INTERHEART study, a case-control study of 5,761 cases of first myocardial infarction and 10,646 control subjects undertaken in 52 countries (not published at the time of the review) a positive association was observed between a Western (unhealthy) dietary pattern high in fried foods, salty snacks, eggs and meat and myocardial infarction with an inverse association between the prudent diet (high in fruit and vegetables) and myocardial infarction.2 The mechanisms for these benefits are poorly understood, but there is evidence that individual nutrients such as sodium and potassium and phytochemicals such as nitrates and polyphenols in vegetables contribute to the effects observed.3-7 It is important that dietary advice and public health nutrition messages are based not only on epidemiological evidence but are also underpinned by evidence from intervention studies and an understanding of the mechanisms involved. We also need to understand whether the benefits of particular dietary patterns are conferred by the presence of foods such as vegetables or by the absence of foods high in saturated fat and salt or whether increasing one aspect of the diet, for example increasing fruit and vegetable intake, will ameliorate the effects of an unhealthy dietary pattern. There is some evidence from the Third National Health and Nutrition Examination Survey (NHANES) and the Trials of Hypertension Prevention (TOHP) I and II that the presence of potassium moderates the effects of sodium. NHANES III reported that a higher sodium intake was associated with greater mortality (hazard ratio 1.20), whereas a higher potassium intake was associated with lower mortality risk (hazard ratio 0.80).5 When comparing the highest quartile with the lowest quartile of the sodium–potassium ratio, the hazard ratios were 1.46 for all-cause and cardiovascular mortality and 2.15 for ischaemic heart disease mortality.5 These data suggest that the presence of potassium presumably from a greater intake of vegetables, fruit and wholegrain foods, ameliorates a higher intake of salt from processed foods. Cook et al.4 analysed data from subjects not assigned to an active sodium reduction intervention in TOHP I and II at 10–15 years follow-up of the trial finding no significant relationship between CVD risk and urinary sodium or potassium excretion. However, they found a significant relationship between quartiles of the sodium to potassium excretion ratio with a risk ratio of 1.24 indicating that a higher sodium to potassium ratio is associated with increased risk of CVD.4 It is of note that in another follow-up from this study, there was a significantly reduced risk of a CVD event that was 25% lower in those in the sodium reduction group (relative risk 0.75) and 30% lower after adjustment for baseline sodium excretion and weight (relative risk 0.70) suggesting that sodium reduction alone may also reduce risk of CVD events.3 The relationship between fruit and vegetable intake and risk for coronary heart disease was examined in The Nurses' Health Study (84 251 women, aged 34–59 years, followed for 14 years) and the Health Professionals' Follow-Up Study (42 148 men, aged 40–75 years followed for 8 years) by Joshipura et al.8 People in the highest quintile of fruit and vegetable intake had a relative risk for coronary heart disease of 0.80 compared with those in the lowest quintile of intake. Green leafy vegetables and vitamin C containing fruits and vegetables contributed the most to the apparent protective effects observed. Fruit and vegetables contain many bioactive compounds such as polyphenols, antioxidants and nitrates that may contribute to the effects seen.9-11 The mechanisms for the CVD benefits observed may be through effects on blood pressure and independently through effects on the endothelium which is a single layer of cells lining the vasculature. Certain vegetables such as beetroot, spinach, celery and lettuce have a high nitrate content and there is evidence that beetroot juice can lower blood pressure.12 Endothelial dysfunction was blunted by the beetroot juice relative to control suggesting that the nitrates in beetroot had a protective effect on the endothelium. There is evidence that sodium reduction improves endothelial function.13, 14 It is postulated that this effect is via increased availability of nitric oxide which protects the endothelium, but this has yet to be confirmed. Increasing potassium may also improve endothelial function, but there are conflicting results.15, 16 Red orange juice which is high in antioxidants and potassium was shown to improve flow mediated dilatation (FMD) compared to control and grape seed extract which is rich in polyphenols also improved endothelial function assessed by FMD relative to control.7, 17 There is good evidence that eating 1–2 servings per week of oily fish reduces the risk of coronary heart disease.18 However, in a study of 326 young adults, omega-3 fatty acid levels were not associated with FMD in the group overall but were positively associated with FMD in smokers and subjects with features of the metabolic syndrome (higher fasting insulin, glucose or triglyceride concentrations).19 In an intervention study designed to test the effect of omega-3 fatty acids equivalent to eating one, two or four servings of fish per day, Sanders et al. observed no effects on FMD, arterial compliance or blood pressure.20 In summary, the cardiovascular benefits of healthy dietary patterns are supported by epidemiological evidence, but the underlying mechanisms remain to be elucidated.