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A dose response study of the effects of increased fruit and vegetable intake on vascular function

Project Code: N02030

31/03/2007

Kings College London
Sanders, T

1. Background

The project was undertaken in response to the research requirement RRD10/N02/A “To characterise the dose-response relationship between the dietary intake of specific plant foods and vascular function”. Cross-sectional as well as prospective cohort epidemiological studies show the consumption of fruit and vegetables to be associated with a substantially lower risk of stroke and a slightly lower risk of coronary heart disease. The effect appears to be stronger for fruit than vegetables. As raised blood pressure is the major predictor of risk of stroke and also contributes to causing heart disease, increasing the intake of fruit and vegetables may help lower blood pressure. It has been proposed the additional potassium provided by fruit and vegetables helps counteract some of the adverse effects of salt on blood pressure. It has also been suggested that fruit and vegetables contain non-nutritive components that may have beneficial effects on blood vessel health. Desirable blood pressure is less than 120/80 mm Hg but a high proportion of the UK adults have raised blood pressure and this becomes more prevalent with increasing age; over the half of the population over the age of 55 years have raised blood pressure 140/90 mm Hg. A reduction in average blood pressure by 5 mm Hg would reduce the incidence of stroke by 22% and coronary heart disease by 16% and would prevent some 75,000 deaths a year in the UK as well as much disability. High blood pressure is a self-amplifying condition and prevention of hypertension (elevated blood pressure) is most effective in the earlier stages of its development. In this context diet is believed to be very important. The current UK average intake of fruit and vegetables in the UK is about 3 portions a day with lower intakes in low income groups. Current UK government advice is to consume five portions of fruit and vegetables daily. Information is required to show whether such an increase in consumption will lower blood pressure or whether even high intakes of fruit and vegetables are needed as has been suggested.

1. Rationale and Objectives

This project was designed to test whether increasing the intake of potassium-rich fruit and vegetables from the UK average of 3 portions a day to the recommended level (5 portions a day) or higher (approximately 10 portions a day) lowers blood pressure among subjects with high normal blood pressure or elevated blood pressure. The study also compared the effects of an increased intake of potassium provided as a supplement. In order to investigate the potential mechanisms by which fruit and vegetable intake have been proposed to have a beneficial effect on blood vessel health, a series of physiological measurements of vessel function including measures of large artery stiffness and response of the artery in the arm to changes in blood flow were also made. The subjects were asked to keep a record of their fruit and vegetable consumption during the study and their body weights were monitored to ensure their weight remained stable. In order to verify that subjects were complying with the dietary advice, measurements of biomarkers of dietary intake were made in blood and urine collected during the study.

1. Approach

A randomized controlled trial was designed to evaluate the impact of the increased intake of potassium from fruit and vegetables on indices of vascular function with blood pressure being the primary outcome. The aim was to recruit 48 non-smoking subjects (equal numbers of men and women) aged 22-65 years with high normal or raised blood pressure. The trial had a crossover design of 4 treatments; this means each subject received every treatment. The treatments involved the manipulation of fruit and vegetable intake to supply three levels of potassium intake supplying an additional 0, 20 or 40 mmol potassium daily, on top of a background diet providing a similar amount of fruit and vegetables and potassium to the average UK intake. These intakes correspond roughly to intakes of 3, 5 and 10 portions of fruit and vegetables per day. In order to test whether an increased intake of potassium alone would be effective, capsules providing 40 mmol potassium citrate per day or matching placebo capsules were administered on the lowest level of fruit and vegetable intake. The subjects were not told which was the active or placebo capsules (i.e. they were blinded to treatment allocation). Prior to the main dietary intervention, the subjects followed a run-in period on a low level of fruit and vegetables for 3 weeks. At the end of this period, measurements were made. The subjects were then randomly allocated to different treatment sequences so that every subject received each treatment in random order for 6 weeks. Further measurements were made at the end of each treatment period. A break of at least 3 weeks before the subject started the next treatment was planned but in practice the break was longer (mean ± SD 60.5±12.5 days; range 36 to 129 days). The following measures were assessed at the end of the run in period and after 6 weeks on each treatment:

self-reported intake of fruit and vegetables
daily urinary excretion of potassium and sodium to assess potassium intake and salt intake
systolic and diastolic blood pressure using 24 hour ambulatory blood pressure monitoring
large artery stiffness and supine blood pressure measured in a blood pressure clinic
measures of endothelium dependent and independent response of the brachial artery
urinary vitamin K excretion
Plasma vitamin C, vitamin E and carotenoid concentrations Modified 14 November2007 4
Markers of inflammation and endothelial function: C-reactive protein, solube I-CAM-1.

4.1. Outcome/key findings

A total 55 subjects were recruited to the trial and 48 subjects completed the study. Of these subjects. 25 subjects (52%) met the definition of Stage I hypertension (day-time ambulatory BP>135/85), 12 (25%) met the definition for high normal BP (day-time ambulatory BP >125/80 mm Hg) and the remaining 11 subjects (23%) had normal blood pressure (day-ambulatory BP>=120/80 mm Hg). Self-reported fruit and vegetable intakes and capsule counts indicated good compliance to the dietary advice. This was corroborated by the predicted increase in urinary potassium excretion. Plasma vitamin C concentrations were slightly greater on the highest levels of fruit and vegetable intakes but urinary excretion of vitamin K metabolites, plasma carotenoids and vitamin E concentrations were unaffected by increased intakes perhaps not surprisingly as these are fat-soluble vitamins. Plasma concentrations of β-cryptoxanthin tended to increase with the higher intakes of fruit and vegetables. Body weights were stable throughout the study. There were no changes in blood pressure whether measured in the clinic or by ambulatory blood pressure monitoring, arterial stiffness, endothelial function (as determined by flow mediate dilatation of the brachial artery) or serum CRP concentration. Plasma concentrations of s-ICAM-1 fell in the women on the highest level of fruit and vegetable intake but did not change in the men.

4.2 What it means and why it is important

The findings are in contrast to the fall in blood pressure predicted from the epidemiological studies. Discordance between the findings of intervention studies and predictions from epidemiological are not uncommon, e.g. as with dietary antioxidant supplements. This study was not designed to study the effects of lower intakes of fruit and vegetables (less than 3 portions per day), it may well be that the benefits of increased fruit and vegetable consumption in terms of decreased cardiovascular risk may already be maximal at a level of intake of 3 portions per day. It may well be that the blood pressure lowering effect is only seen in subjects with very low intakes of potassium (about 40 mmol/d). It is to be noted that the intake of potassium on the lowest level of intake in this study was similar to the average intake in the UK population, which is higher than the intake in the USA. The conclusions from this study are that dietary advice to increase fruit and vegetable consumption to 5 portions a day or more or potassium supplementation is unlikely to be of value in the prevention or management of high blood pressure. The results of the present supplementation study are in agreement with a recent Cochrane Systematic Review which found no statistically significant effect of potassium supplementation on blood pressure. The majority (88%) of the subjects in this study were overweight/obese and 68% had salt intakes above the recommended 6g/d. These factors, which contribute to high blood pressure, were not influenced by the intervention. Some of the benefits associated with an increased intake of fruit and vegetables in epidemiological studies may be a consequence of individuals consuming fewer less healthy foods (crisps, cakes, biscuits and confectionery) that contribute towards obesity. The present study did not attempt to decrease the consumption of these foods. On the basis of these findings, dietary advice for the prevention of high blood pressure should focus on weight control, moderating alcohol intake and restricting the intake of salt with less emphasis on promoting high intakes of fruit and vegetables and more on using an increased intake of fruit and vegetables to substitute foods high in food energy.

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