'10-a-Day' Keeps CVD Away

On average UK adults do not meet the recommended 5 portions of fruit and vegetables per day, despite their association with a healthy diet. A recent study by Aune et al. [1] suggested that greater reductions in risk of chronic disease are observed when consuming up to 10 portions per day. From reviewing this research and discussing the wider literature, it has been determined that higher intakes of fruits and vegetables reduce risk of CVD. It could be thought 10 portions may be unachievable for most, yet even a small increase can help optimise long term health.

Cardiovascular disease (CVD) is the number 1 cause of death globally [2] and is a significant health burden to the UK [3], costing the NHS over £6.8 billion annually [4].  

Atherosclerosis is the mechanism that underlies the main CVDs of coronary heart disease (CHD) and cerebrovascular disease [2]. Macrophages in the arterial intima scavenge oxidised low-density lipoprotein (LDL) [5], become foam cells and accumulate in the sub-endothelial space of blood vessels, forming fatty streaks or atherosclerotic lesions. These lesions develop into fibrous plaques which bulge into, and narrow, blood vessels [6].  Lethality occurs when plaques thin, become fragile, and rupture, triggering thrombus formation. When these clots occlude blood flow, heart attack or stroke can result [7].

High blood pressure (BP) and high cholesterol are major risk factors for CVDs. High BP increases blood flow through blood vessels, excessively stretching the endothelium and causing stiffening, offering greater susceptibility to obstruction and haemorrhage [8], and high cholesterol increases LDL accumulation in arteries and formation of atherosclerotic lesions. It is considered possible to modify these two factors by making appropriate lifestyle choices [9], with fibre intake and sodium-potassium ratio [10] being deemed important. Moreover, oxidative stress is significant in the aetiology of CVD as atherosclerosis relies upon oxidation of LDL [11].

Fruits and vegetables are low energy density, nutrient rich foods. They are the primary dietary source of the antioxidant vitamin C, high in potassium, and high in fibre, making them commonly associated with a healthy diet.  Current UK recommendations of daily consumption of 400g of fruit and vegetables, or 5 portions, have been in place for 11 years. Nonetheless, the target is not widely met in the UK. Only 8% of 11-18 year olds, 27% of 19-64 year olds and 35% of over 65 year olds have adequate intake [12]. A recent systematic review and meta-analysis conducted by Aune et al. suggests that additional benefits may be achieved by doubling this suggested amount to 10 portions per day, with greater reductions in risk of CVD incidence and mortality [1]. This review will discuss these findings, the contribution that fruit and vegetables have to prevention of such chronic diseases, and whether the message could be effectively translated into population wide advice.

Method

Data search and extraction

Prospective studies into the association between fruit and vegetable intake and risk of incidence or mortality of total CVD were selected from PubMed and Embase database searches up to 29 September 2016. Results, study characteristics and amount or frequency of fruit and vegetable intake was tabulated.

Statistical analysis

Summary RR for total CVD was calculated for the highest intake, lowest intake, and per 200g/day of fruit, vegetables, and total fruits and vegetables using studies reporting both incidence and mortality. Linear dose-response analysis was performed with 80g as a standard portion size.  

Sub-group analysis explored heterogeneity between studies and sensitivity analysis was performed by gradual study exclusion to assess the strength of the results.

The preventable proportions of deaths were calculated assuming a causal relationship from low fruit and vegetable intake. Prevalence was obtained from the World Health Survey data and mortality from the Global Burden of Disease Study 2013.

Results

Study selection

A total of 44 studies were included in the analysis into the association with CVD, with 17 for fruits and vegetables, 25 for fruits and 22 for vegetables.

Results of data analysis

The summary RR per 200g/day of fruits and vegetables was 0.92, for fruits was 0.87 and for vegetables was 0.90. The results were non-linear with lower levels of intake showing a steeper negative association.  Risk reduction for 800g/day intake of fruits and fruits and vegetables was 28% and 27%, and at 600g/day for vegetables was 28%. Fruits and vegetables associated with greater reduction of CVD risk were apples, pears, citrus fruits, carrots and non-cruciferous vegetables, but tinned fruits had a positive association. 

Discussion

The meta-analysis and systematic review by Aune et al. found each 200g/day increase in fruit and/or vegetable consumption to reduce CVD risk by 8-13% [1]. These results are consistent with Oyebode et al. who suggested that fruit and vegetable consumption reduced CVD mortality by 31% [13], and the findings that those in the upper tertile of fruit consumption (≥5 portions/day) and upper tertile of vegetable consumption (≥3 portions/day) have a 60% and 70% reducted risk of CHD, respectively [14].

It was also concluded that consuming 10 portions/day of fruits and vegetables offers significant beneficial health effects, reducing CVD risk by 28% [1]. This was also shown by Oyebode et al. who found no threshold for maximum effect within the actual range of consumption, and greater reductions in CVD mortality in those within the highest category, having 7+ portions [13]. A dose-response relationship is observed fairly consistently within current research. However, Wang et al. suggested that the reduction in risk per additional serving of fruit and vegetables plateaus at 5 portions/day [15], an effect that has been suggested to result from homeostatic modulation of micronutrient metabolism and excretion [16], such as is the case for vitamin C, which is regulated by intestinal uptake, tissue accumulation, rate of utilisation and renal excretion or reabsorption [17].

It could be hypothesised that the type of fruits and vegetables consumed may have an impact on the number of portions required for maximal effect, limiting study comparison as antioxidant content is variable. For example blueberries have 9.24mmol/100g antioxidants, but apples can have as little as 0.22mmol/100g [18]. Greater quantities of lower antioxidant containing fruits would be required to see equivalent reductions in oxidative stress. Moreover, the bioaccessibility and bioavailability of antioxidant compounds post-ingestion is influenced by the food matrix. Fibre is said to reduce the rate and extent of nutrient release from foods due to them being trapped in the microstructure, as well as the increased viscosity of gastric fluids impeding peristaltic processes that are essential for combining enzymes and substrates, and their subsequent transportation to endothelial cell membranes for absorption [19]. This suggests that a greater number of portions of fruits and vegetables may be required to overcome limitations in absorption and allow for transport mechanisms to become saturated. Although it could be argued that it may therefore be easier to take antioxidants in supplemental form, it has been found that there is no association between their use and CVD [20], indicating additional constituents of fruits and vegetables offer benefits to cardiovascular health.

A 7g/day increase in fibre intake of 7g/day has been associated with a 9% reduced risk of CVD, with the lowest risk being observed from 2-4 servings/day of fruit and 4-6 servings/day of vegetables [21]. Combined, this suggests consumption of 6-10 servings/day, so higher than the current recommendation of ‘5-a-day’. The likely mechanism is the effect on of fibre on cholesterol, where it prevents bile acid reabsorption in the intestinal tract, increasing bile acid synthesis and so cholesterol removal from circulation [22]. For every gram of soluble fibre consumed, Brown et al. found a decrease in total cholesterol of 0.045mmol/L and LDL of 0.057mmol/L [23], which would reduce deposition in arterial walls, atherosclerosis and risk of CVD [24].

Additionally, short chain fatty acids are produced by microbial fermentation of fibre in the colon. Although they provide 10% of daily caloric requirements [25], they also induce release of satiety signals such as GLP-1 [26], influencing energy intake. This is further contributed to by slower gastric emptying and the increase in gastric distention due to the water absorbing properties of fibre [27]. Higher fruit and vegetable intake has been linked with body weight [28], with a 9% reduced risk of adiposity being shown for those consuming the highest amounts of fruits and vegetables [29]. The association between obesity and high BP, increased serum LDL and a pro-inflammatory clinical state makes this significant. These factors elevate risk of CVD [30]. The general healthier dietary patterns of those consuming high amounts of fruits and vegetables may, therefore, have influenced the results from the study by Aune et al. where in fact the lower risk of CVD was attributed to dietary and lifestyle factors that were not adjusted for in the models.

Finally, increased fruit and vegetable intake can be related to reduced BP. An average increase of 1.4 portions/day has been shown to correlate with a reduction of systolic BP of 4mmHg and diastolic BP of 1.5mmHg [31]. Such foods are a major dietary source of potassium, which is generally said to have a BP-lowering effect [32] by decreasing urinary sodium reabsorption, and so intravascular volume, and by relaxation of  smooth muscle cells, causing endothelial vasodilation by stimulation of the Na⁺/K⁺-ATPase pump [33]. It has been suggested that sodium-potassium ratio may be more significant in assessing CVD risk [34], with potassium intake offsetting the negative effects of sodium on blood pressure. It could be thought that the high salt modern diet, with average UK intake being 8.1g/day [35], could benefit from greater inclusion of fruits and vegetables to offer reductions in CVD risk without requiring more significant changes in food formulation and dietary behaviours.

Impacts

After discussing the meta-analysis and systematic review by Aune et al. and examining wider research, it could be concluded that consumption of fruits and vegetables can offer reductions in CVD incidence and mortality due to their high antioxidant, fibre and potassium content. Despite the potential for offsetting poor food choices, such as high sodium intake, additional dietary changes may be required to observe the greatest effect. It has been suggested that the DASH diet, which promotes consumption of low-fat dairy, fish, wholegrains and nuts, in addition to fruits and vegetables, offers a greater reduction in systolic BP than fruits and vegetables alone [36]. Moreover, the preparation method and item choice can have significant influences on nutrient availability, such as steaming of vegetables being advised instead of boiling or frying to prevent loss of vitamin C [37], and whole fruit consumption being favoured over fruit juice, which contains added sugars and significantly less fibre.

It has also been evidenced that consuming more than 5 portions/day of fruits and vegetables may offer the greatest reductions in CVD risk. However, as only 30% of adults in the UK currently meet the ‘5-a-day’ recommendation [38] it could be thought that it is unlikely many will be able to regularly consume double this advised amount, at 10 portions.  Aune at al. showed a steeper negative association between fruit and vegetable intake and risk of CVD at lower levels of intake [1], suggesting a significant population effect may still be observed from even a small increase, such as those having no portions of fruits or vegetables daily increasing to just one or two. It may therefore be most appropriate to focus public health campaigns on encouragement of a population wide increase in consumption by any amount, although ideally to meet a minimum of 5 portions, without advocating a value that may be unachievable for many, potentially deterring any changes from being made at all.  

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