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Sun, May 19, 2019

Cardiovascular Disease
The Health Benefits of Fruits and Vegetables



Coronary artery disease and stroke, two principal manifestations of cardiovascular disease (CVD), account for nearly 40% of all deaths in the U.S., killing almost a million people each year (33). It is estimated that 61,800,000 Americans currently have one or more forms of CVD, predominantly hypertension and coronary artery disease (34). The costs of CVD are staggering – an estimated cost of $289.2 billion in 2001 and anticipated $329.2 billion in 2002.


Lifestyle changes, including dietary factors, have great potential to reduce mortality and morbidity associated with CVD. There is a diverse array of substances in fruits and vegetables associated with decreased risk of CVD including antioxidants, folate, fiber, potassium, flavonoids, or other phytochemicals. A number of past reports have summarized data from case-control studies showing a beneficial effect of fruit and vegetable consumption on risk of CVD. Recent prospective studies have added to the growing evidence that fruit and vegetable intake reduces risk factors as well as incidence and mortality associated with CVD.


Recent 12-year follow-up data from the Nurses' Health Study (n>69,000) showed that a “prudent” pattern, characterized by high intakes of fruits, vegetables, whole grains, and use of low-fat dairy products is associated with reduced risk of cardiovascular events and risk factors compared to a “Western” pattern of higher intake of fat, red and processed meat, eggs, butter, and refined grains (35). The protective effect for coronary artery disease remained even after adjustment for differences in dietary folate, fiber, vitamin supplements, alcohol, saturated and monounsaturated fat, and smoking. Within the prudent diet, women in the highest quintiles of the prudent-diet pattern scores had lower risk for incidence and fatal outcomes of coronary artery disease, and nonfatal myocardial infarction.


Another recent report of combined data from women in the Nurses' Health Study (n>84,000) and men in the Health Professionals' Follow-Up Study (n>42,000) showed that men and women in the highest quintile of fruit and vegetable intake (9 and 10 servings per day for women or men, respectfully) had a 20% lower risk of coronary artery disease compared to those with the lowest intake (2.5 - 3 servings per day) (36). The lowest risks were observed for green leafy vegetables and vitamin C-rich fruits and vegetables. It was calculated that there was a 4% lower risk of coronary artery disease for each 1 serving per day increase in fruit and vegetable intake. The median intake of total fruits and vegetables was 5.8 servings/day for women followed for 14 years and 5.1 servings/day for men followed for 8 years. In a separate study group of physicians followed for 12 years, (n=15,220) the incidence of coronary artery disease was approximately 25% lower in men who consumed 2.5 servings of vegetables per day compared to those who consumed less than 1 serving per day (37). The protective association was strongest among men with a body mass index over 25 and among current smokers.

 


“…data from women in the Nurses' Health Study and men in the Health Professionals' Follow-Up Study showed that men and women in the highest quintile of fruit and vegetable intake (9 and 10 servings per day for women and men, respectfully) had a 20% lower risk of coronary artery disease compared to those with the lowest intake (2.5-3 servings per day).”


A 5-year prospective study of 39,127 female health professionals enrolled in the Women's Health Study provided evidence of an inverse association between fruit and vegetable consumption and risk of myocardial infarction (38). Women in the highest quintile of fruit and vegetable intake (>10 servings/day) had a 38% lower risk of myocardial infarction compared to women consuming 2.6 or fewer servings per day. There was also an inverse relationship between fruit and vegetable consumption and incidence of CVD, although this was not statistically significant when known cardiovascular risk factors were considered.


“… A 5-year prospective study of 39,127 female health professionals enrolled in the Women's Health Study provided evidence of an inverse association between fruit and vegetable consumption and risk of myocardial infarction (38). Women in the highest quintile of fruit and vegetable intake (>10 servings/day) had a 38% lower risk of myocardial infarction compared to women consuming 2.6 or fewer servings per day.”


In addition to prospective studies, a number of small clinical trials have been conducted to examine the effects of diets containing fruits and vegetables on plasma lipids and other biomarkers associated with risk of cardiovascular disease.


Plasma homocysteine levels are positively correlated with the development of coronary artery disease by several potential mechanisms including endothelial cell toxicity, oxidation of LDL, effects on coagulation, and stimulation of smooth muscle cell proliferation (39). Since dietary folate is a co-substrate in homocysteine metabolism, folic acid supplementation, and high dietary folate intake reduce levels of plasma homocysteine and presumably lower risk of coronary artery disease [reviewed in (39)]. Recently, it has been demonstrated that dietary folate from fruits and vegetables reduces plasma homocysteine levels (40-42). A clinical trial including healthy men and women in the Netherlands demonstrated that 4 weeks of consuming dietary folate provided by citrus fruit and green vegetables resulted in a significant reduction in plasma homocysteine levels (40). In two other clinical trials, dietary intake of a mix of fruits and vegetables with moderate folate content was associated with a significant reduction (11%) in plasma homocysteine levels in healthy subjects (41, 42). This may be one important mechanism by which fruit and vegetable intake is associated with reduced risk of cardiovascular disease.


Diets high in fruits and vegetables have also been shown to reduce plasma lipid levels. In a recent metabolic diet study using a cross-over design, 10 healthy subjects were provided a diet containing very high fiber from fruits and vegetables (55 grams fiber/1,000 kcal) for a 2 week period (43). The high fiber diet included leafy vegetables and pods, fruits, and nuts. Compared to two control diets (a starch-based and a low-fat diet) the high fiber intake resulted in the largest reduction in plasma levels of total and LDL cholesterol (22% and 33% reductions, respectively) as well as a 24% reduction in the ratio of LDL:HDL cholesterol. The investigators also reported that the high fiber fruit and vegetable diet had positive effects on markers of risk for colon cancer.


The Dietary Approaches to Stop Hypertension (DASH) Trail has examined the effect of increasing fruit and vegetable consumption on plasma lipid levels (44). In the DASH Trial, 436 participants were randomly assigned to 8 weeks of either a control diet, a diet increased in fruits and vegetables (8.5 or more servings), or a diet with increased fruits and vegetables in the background of a low-fat and cholesterol, low-fat dairy diet (DASH diet). There was a trend toward reduced total cholesterol levels in men and reduced triglyceride levels in non-African Americans in the fruit and vegetable diet compared to controls. The DASH diet resulted in significantly reduced plasma levels of total, LDL and HDL cholesterol in all races and both sexes compared to the control diet. The fruit and vegetable diet and DASH diet are known to effectively reduce blood pressure (45), an important CVD risk factor, as discussed in a subsequent section (see Hypertension).


Oxidative processes are important in the development of CVD and have been the focus of several intervention studies. The beneficial effect of fruits and vegetables on CVD risk is likely due, in part, to their potential antioxidant activity. Fruit and vegetable intake increases antioxidant capacity of plasma in a short time after consumption. A small study of elderly women (n=8) found that total antioxidant capacity of plasma was increased by 7-25% within 4 hours of consuming antioxidant rich foods or vitamins, including strawberries, spinach, wine, or vitamin C (46). Similarly, cranberry juice consumption increased total antioxidant capacity of plasma for 1-4 hours in 9 healthy women aged 23-41 years (47). This effect has been ascribed to the phenolic compounds present in fruits and vegetables and in some cases, the vitamin C content. Consumption of carotenoid-rich fruits and vegetables is associated with increased plasma levels of carotenoids (41, 42, 48, 49) and may raise carotenoids in the lipoprotein fraction of plasma (50). However, increased plasma levels of carotenoids do not always result in improved antioxidant capacity (51, 52).


Oxidative processes are thought to promote the uptake of LDL cholesterol into macrophages in the arterial wall and promote the initiation and development of coronary artery disease (53). The oxidation potential of LDL cholesterol in plasma is commonly used as a biomarker of risk for CVD. Several, but not all, studies have found that consumption of fruits and vegetables, either as extracts, or part of a total diet, effectively reduce the oxidation rate of LDL and/or delay the onset of LDL oxidation (50, 54, 55).


The effect of fruit and vegetable intake on oxidation-related biomarkers appears to be influenced by lifestyle and/or disease-related factors. For example, individuals who smoke may be less likely to demonstrate improved antioxidant status upon fruit and vegetable supplementation compared to non-smokers (52, 54). In a recent study, subjects who were HIV seropositive demonstrated increases in plasma antioxidant capacity after 16 weeks of supplementation with fruit juice or fruit-vegetable concentrate in contrast to HIV-seronegative subjects who showed no change in antioxidant status after the supplementation (56).


In conclusion, there is strong evidence supporting current dietary recommendations to emphasize fruits and vegetables in dietary strategies aimed at reducing risk of CVD. Furthermore, there are a variety of bioactive components and nutrients in fruits and vegetables and plausible mechanisms explaining how they might be protective.

 

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