Introduction

In 2012, noncommunicable diseases (NCDs) accounted for more than two-thirds (68%) of deaths worldwide; more than 40% of these were premature deaths in people aged under 70 years of age (1).

Poor dietary habits have been identified as one of the leading contributors for a range of NCDs and the latest analysis of the global burden of disease estimated that diets low in fruits and low in vegetables were responsible for 1.8 and 3.4 million deaths, respectively, in 2013 (2).

Consuming more than 400 g (i.e. five portions if considering 80g as one serving) of fruits and vegetables per day was recommended by the 2002 WHO/FAO Expert Consultation to improve overall health and reduce the risk of certain NCDs (3). This commentary reviews recent evidence on the role of fruit and vegetable consumption in reducing the risk of NCDs.

Methodology summary

An overview is provided for several recent systematic reviews assessing potential associations between fruit and vegetable consumption and all-cause mortality, cardiovascular disease, cancer, type 2 diabetes and weight loss/maintenance.

All-cause mortality

Wang et al. (4) conducted a meta-analysis of cohort studies involving data from 833,234 participants and assessed the effect of fruit and vegetable consumption on deaths from all causes, cardiovascular disease and cancer. The all-cause mortality analysis included prospective cohort studies from the USA (six studies), Europe (six studies) and Asia (four studies).

Cardiovascular diseases

The meta-analyses by Wang et al. (4) pooled results of 10 studies with cardiovascular mortality as an outcome – four studies on the effect of fruit and vegetable consumption combined (469,551 participants), six studies on the effect of fruit consumption (677,674 participants) and six studies on the effect of vegetable consumption (677,674 participants).

A meta-analysis by Hu et al. (5) with stroke as an outcome, included data from 20 prospective cohort studies (760,629 participants) conducted in the USA (six studies), Europe (eight studies) and Asia (six studies); follow-up ranged from 3.09 to 37 years.

In addition to the observational data from cohort studies, a meta-analysis by Hartley et al. (6) pooled data from 10 randomized controlled trials (RCTs) (1730 participants) assessing the effectiveness of interventions to increase fruit and vegetable consumption for the primary prevention of cardiovascular disease in healthy adults or those with a major cardiovascular disease risk factor. The interventions consisted of either advice to increase fruit and vegetable consumption (four studies) or provision of fruit and vegetables (six studies).

Cancer

For this commentary, the meta-analysis by Wang et al. (4) on the relationship between fruit and vegetable consumption and cancer mortality is reviewed (16,817 participants). Seventeen studies were included in the pooled analysis: two studies assessing combined fruit and vegetable consumption, seven assessing fruit only, and eight assessing vegetables only.

In addition, results from the World Cancer Research Fund (WCRF) diet and cancer report and its Continuous Update Project (7-15), are summarized. The WCRF report examined evidence from existing cohort and case-control studies, conducted systematic literature reviews for 20 cancer sites, and, where appropriate, pooled results for meta-analyses.

Body weight, energy intake and weight loss

Two meta-analyses evaluated the impact of interventions to increase fruit and vegetable consumption on body weight and energy balance, either through dietary advice/counselling or by provision of fruits and/or vegetables.

The first, by Mytton et al. (16) aimed to quantify the relationship between changes in fruit and vegetable consumption, energy intake and body weight. Pooled analysis consisted of seven studies on body weight and five studies on energy intake (1026 participants). The second meta-analysis by Kaiser et al. (17) pooled data from seven studies evaluating the effect of fruit and vegetable consumption on weight loss or prevention of weight gain.

In order to evaluate the independent effects of fruit and vegetable interventions, both reviews excluded studies of broader diet or healthy living (i.e. multifactorial) interventions, limiting the number of studies eligible for inclusion.

Diabetes

In a systematic review and meta-analysis of prospective cohort studies with 434,342 participants conducted in North America, Europe and Asia, Li et al. (18) pooled results from seven studies assessing combined fruit and vegetable consumption, nine studies assessing fruit consumption only, eight studies assessing vegetable consumption only, and seven studies specifically assessing consumption of green leafy vegetables.

Evidence summary

All-cause mortality

Wang et al. (4) found that there was a dose-response relationship between consumption of fruit and vegetables and decreasing risk of all-cause mortality at consumption below five servings a day. Those consuming five servings of combined fruits and vegetables per day had 26% less risk of dying during the follow-up period (hazard ratio [HR] 0.74; 95% confidence interval [CI]: 0.65, 0.82) than those who had no daily consumption of fruits and vegetables with a 5% reduction in risk of death (HR 0.95; 95% CI: 0.92, 0.98) for each extra serving of combined fruits and vegetables per day. The reductions in risk appeared to level off at a threshold of around five servings per day, above which death rates did not decrease.

Separate analyses of fruit and vegetable consumption gave similar results, with a 6% reduction in risk of death (HR 0.94; 95% CI: 0.89, 0.98; p=0.002) for each additional daily serving of fruit and a 5% reduction (HR 0.95; 95% CI: 0.92, 0.99; p=0.006) for each extra serving of vegetables (4).

Sensitivity analysis excluding one large study that accounted for most of the heterogeneity observed had little effect on results for fruit and vegetable consumption (combined) and all-cause mortality (HR 0.94; 95% CI: 0.93, 0.96, p=0.001). Other sensitivity analyses to exclude studies that did not adjust for physical activity (HR 0.96; 95% CI 0.94, 0.99; p=0.008), energy intake (HR 0.95; 95% CI: 0.93, 0.96; p=0.001) or socioeconomic status (HR 0.96; 95% CI: 0.94, 0.99; p=0.019) did not change the associations between combined fruit and vegetable consumption and risk of all-cause mortality. In a further analysis, exclusion of a study that did not report hazard ratios did not appreciably alter the results for the effect of fruits or vegetables on risk of mortality from all causes. Similarly, different subgroup analyses found that the associations between fruit and vegetable consumption did not differ substantially by study location, sex, number of participants, duration of follow-up, or study quality.

Cardiovascular diseases

Wang et al. (4) found a 4% reduction in the risk of death from cardiovascular disease (HR 0.96; 95% CI: 0.92, 0.99; p=0.02) for each additional serving of fruits and vegetables combined (4). Analysis of six studies that considered consumption of fruits and vegetables separately revealed a 5% reduction in risk of death from cardiovascular disease for each additional serving of fruit (HR 0.95; 95% CI: 0.91, 1.00; p=0.03) and a 4% reduction in cardiovascular disease mortality for each additional serving of vegetables (HR 0.96; 95% CI: 0.93, 0.99; p=0.01) (4).

Hu et al. found the risk of stroke in those with the highest intakes of total fruit and vegetables (combined) was 21% lower than in those with the lowest intakes (relative risk [RR] 0.79; 95% CI: 0.75, 0.84; p=0.233) and there were also significant reductions in risk for consumption of fruits (RR 0.77; 95% CI: 0.71, 0.84; p=0.09) and vegetables (RR 0.86; 95% CI: 0.79, 0.93) when analysed separately (5). A dose-response analysis for fruits (9,706 stroke cases/ 8 studies), showed that the risk of stroke decreased by 32% (RR 0.68; 95% CI: 0.56, 0.82) while for vegetables (8,854 stroke cases/ six studies) the risk of stroke decreased by 11% (RR=0.89; 95% CI: 0.81, 0.98), for each additional 200 g consumed daily. Results of sensitivity analyses, in which two studies that did not correct for age were excluded showed that higher levels of fruit consumption were associated with reduced risk of stroke (RR 0.77; 95% CI: 0.71, 0.84). However, the removal of an additional three studies with small sample sizes, did not change significantly the overall pooled effect from that in the original analysis (RR 0.80; 95% CI: 0.74, 0.87). The inverse association between total fruit and vegetable consumption and risk of stroke were consistent after various subgroup analyses. Analyses of specific types of fruits and vegetables found a reduced risk of death associated with consumption of citrus fruits, apples/pears and leafy vegetables, but not with cruciferous, allium or root vegetables, or with berries.

In the analysis of RCTs, Hartley et al. did not find significant evidence of a favourable effect on cardiovascular risk factors for interventions consisting of provision of fruits and vegetables, but did find evidence of a reduction in systolic blood pressure for interventions consisting of dietary advice to eat more fruits and vegetables (mean difference [MD] –3.00 mmHg; 95% CI: –4.92, –1.09) (6). Limitations in the analysis were noted, including the small number of trials contributing to each analysis, variability in the nature of the interventions, relatively short duration of the included trials, and the fact that most trials consisting of fruit and/or vegetable provision actually provided only a single type of fruit or vegetable to participants.

Cancer

The meta-analysis by Wang et al. (4) found no statistically significant associations between risk of death from any type of cancer and combined fruit and vegetable consumption, consumption of fruit only or consumption of vegetables only. It was noted, however, that significant associations might be observed when assessing the effects of fruit and vegetable consumption for different types of cancer.

Ranking results as either convincing, probable, limited-suggestive or substantial effect unlikely, and based on evidence described in the 2007 WCRF report and Continuous Update Project (9-15), the WCRF expert panel has concluded that consumption of fruit probably reduces the risk of cancers of the mouth, pharynx, larynx, and lung . There is more limited evidence that is suggestive of a protective effect of fruit consumption against nasopharyngeal, liver, oesophageal, stomach or colorectal cancer. Consumption of non-starchy vegetables probably reduces the risk of cancers of the mouth, pharynx, and larynx, while more limited evidence suggests a protective effect against cancers of the lung, colorectum, endometrium, and oesophagus. In the 2016 update on stomach cancer, the evidence for vegetables and legumes which had previously been classed as ‘probable’ was less consistent and no conclusions could be drawn (14).

Additional reviews covering specific types of cancers have strengthened the evidence base for a protective effect of fruit and vegetable consumption on lung cancer (19), of fruit consumption on gastric cancer (20) and of vegetable consumption on a specific type of breast cancer (oestrogen receptor negative breast cancer) (21). A review assessing effects on pancreatic cancer (22), however, found no evidence of an association between combined fruit and vegetable consumption and risk of the disease.

Findings from the ongoing 10-country European Prospective Investigation into Cancer and Nutrition (EPIC) cohort of more than 500,000 participants suggest an inverse association between fruit consumption and cancers of the lung and upper gastro-intestinal tract, and between total fruit and vegetable consumption and colorectal cancer (23). The study has not identified a significant association between fruit and vegetable consumption and cancer of the stomach, biliary tract, pancreas, cervix, endometrium, prostate, kidney, bladder or lymphoma.

Body weight, weight loss and energy intake

Mytton et al. found a small but statistically significant difference in mean change in body weight (either reduction in body weight or reduced weight gain) in those consuming greater amounts of fruits and vegetables compared to controls (Mean change in body weight –0.68 kg; 95% CI: –1.20, –0.15); no significant difference in daily energy intake was observed (16). Kaiser et al. found no statistically significant effect of providing or prescribing increased fruit and/or vegetable consumption on weight outcomes in either its primary analysis of data from two studies met all of the inclusion criteria (Standardized mean difference [SMD] -0.16; 95% CI: -0.78, 0.46) or a secondary analysis on seven studies that met all the criteria except that they did not explicitly state body weight as an outcome of interest (SMD 0.04; 95% CI: -0.10, 0.17) (17).

Diabetes

The review and meta-analysis by Li et al. (18) found evidence that the risk of type 2 diabetes is reduced by higher intakes of fruit (RR 0.93; 95% CI: 0.88, 0.99) and of green leafy vegetables (RR 0.87; 95% CI: 0.81, 0.93), but not fruit and vegetables combined (20). A dose-response relationship was evident with a 6% lower risk of type 2 diabetes per 1 serving/day increment of fruit consumption and a 13% lower risk of type 2 diabetes per 0.2 serving/day increment of green leafy vegetables intake (18).

Discussion

Applicability of the results

Current evidence suggests that higher intakes of fruits and vegetables combined are associated with reduced risk of mortality, stroke and certain types of cancer, and favourable effects on risk factors for cardiovascular disease. Several studies examining the effects of fruits alone or vegetables alone have found that higher intakes of fruits are associated with reduced risk of mortality, cardiovascular disease, stroke, and type 2 diabetes and there is limited evidence that it is also linked to some forms of cancer. Results for vegetable consumption alone also show that higher intakes are associated with reduced risk of all-cause mortality, cardiovascular disease, and increased risk of stroke with green leafy vegetables being linked to reduced risk of type 2 diabetes. For some forms of cancer (such as stomach and oesophageal) further evidence is needed in order for strong conclusions linking vegetable consumption to be drawn. Evidence for an association between fruit and vegetable consumption and body weight is inconsistent.

Overall, the studies included in the analysis were of high quality: the cohort studies collectively covered a large sample size with long follow-up periods and, though relatively few in number, the included RCTs were limited specifically to fruit and vegetable consumption (as opposed to multifactorial interventions). Limitations were noted, however, including the possibility of confounding in the cohort studies as well as limited follow-up times in the RCTS. The focus on fruit and vegetable interventions only in the RCTs, while important for assessing potential causality, does not resemble real-life efforts to promote fruit and vegetable consumption as part of a healthy diet and also limited the number of studies that could be included in the analyses. Additionally, errors relating to measuring intakes of fruits and vegetables are likely, and differences in the way fruits and vegetables (or specific types of fruits or vegetables) were defined may have affected the analyses. For example, some studies include potatoes as a vegetable or define broccoli as a green leafy vegetable while others do not.

Implementation in settings with limited resources

The vast majority of studies included in the analyses were conducted in North America and Europe, with a smaller number taking place in Asia. This may affect their applicability to low- and middle-income countries where dietary patterns, and the way that different food groups are defined, are different. Further research in resource-constrained settings should be a priority. Recent data on fruit and vegetable consumption have found that globally fruit and vegetable consumption is low. This is especially true in low income countries where mean daily consumption was found to be 2.14 servings (1.83-2.36) compared to 5.42 servings (5.13-5.71) in high-income countries (24). Older data also highlight the scale of the challenge to increase fruit and vegetable consumption in many poorer countries. A 2002-3 study of 52 countries, mainly low- and middle-income, found high prevalence of low fruit and vegetable consumption (less than 400 g daily) in many countries – prevalence was above 70% in both men and women, for example, in more than two-thirds (38) of the countries studied (25). Data from 10 low-income countries in sub-Saharan Africa found that average per capita consumption* ranged from 27 kg to 114 kg per year, falling well below the 146 kg required to meet WHO’s recommendation (26). Depending on the specific context, insufficient supply, low levels of consumer demand and relatively high prices can all contribute to low intakes (25). Fruits and vegetables are relatively expensive as a source of energy when compared, for example, to the starchy staples on which many poor households depend.

The biological basis for the observed association between fruit and vegetable consumption and risk of NCDs is a subject of much interest and a number of potential components of fruits and vegetables (e.g. dietary fibre, potassium, magnesium, vitamin C, carotenoids and other antioxidant compounds, polyphenols and other phytochemicals) and physiological mechanisms (e.g. lowering blood pressure, impact on blood lipid oxidation or other vascular factors, etc.) for the apparent protective effect have been proposed and explored. Further research is needed, however, before the effect can be attributed to any particular factor or combination of factors.

Further research

More research is also needed on effective policies, interventions and food system approaches to increase fruit and vegetable consumption, in order to fully harness the potential of fruits and vegetables to transform diets and prevent NCDs. There are particular challenges to increasing intakes in resource-poor settings – as well as for the poorest groups in high-income countries – and policies to increase the availability of fruits and vegetables and to reduce relative prices may be required, in addition to education and behaviour change programmes to promote consumption.

* Data on fruit and vegetables acquired for consumption at home, collected through household expenditure surveys conducted between 1994 and 2000.

References

1. Global status report on noncommunicable diseases 2014. Geneva: World Health Organization; 2014. (https://www.who.int/nmh/publications/ncd-status-report-2014/en/)

2. GBD 2013 Risk Factors Collaborators. Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks in 188 countries, 1990-2013; a systematic analysis for the Global Burden of Disease Study. Lancet. 2015; 386(10010):2287-323.

3. WHO/FAO. Diet, nutrition and the prevention of chronic diseases: a report of a joint WHO/FAO expert consultation (WHO Technical Report Series 916). Geneva: World Health Organization; 2003. (https://www.who.int/nutrition/publications/obesity/WHO_TRS_916/en/)

4. Wang X, Ouyang Y, Liu J, Zhu M, Zhao G, Bao W, Hu FB. Fruit and vegetable consumption and mortality from all causes, cardiovascular disease, and cancer: systematic review and dose-response meta-analysis of prospective cohort studies. BMJ. 2014; 349:g4490.

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6. Hartley L, Igbinedion E, Holmes J, Flowers N, Thorogood M, Clarke A, et al. Increased consumption of fruit and vegetables for the primary prevention of cardiovascular diseases. Cochrane Database of Systematic Reviews. 2013; Issue 6. Art. No.: CD009874.

7. World Cancer Research Fund/American Institute for Cancer Research. Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. Washington DC: American Institute for Cancer Research; 2007. (http://www.aicr.org/assets/docs/pdf/reports/Second_Expert_Report.pdf)

8. Vieira AR, Vingeliene S, Chan DS, Aune D, Abar L, Navarro Rosenblatt D, et al. Fruits, vegetables, and bladder cancer risk: a systematic review and meta-analysis. Cancer medicine. 2014; 4(1):136–46.

9.World Cancer Research Fund/American Institute for Cancer Research. Continuous Update Project Report. Food, Nutrition, Physical Activity, and the Prevention of Colorectal Cancer. 2011. (https://www.wcrf.org/int/continuous-update-project)

10. World Cancer Research Fund/American Institute for Cancer Research. Continuous Update Project Report. Food, Nutrition, Physical Activity, and the Prevention of Breast Cancer. 2010. (https://www.wcrf.org/int/continuous-update-project)

11. World Cancer Research Fund/American Institute for Cancer Research. Continuous Update Project. The associations between food, nutrition and physical activity and the risk of endometrial cancer. 2012. (https://www.wcrf.org/int/continuous-update-project)

12. World Cancer Research Fund/American Institute for Cancer Research. Continuous Update Project Report. Food, Nutrition, Physical Activity, and the Prevention of Pancreatic Cancer. 2012. (https://www.wcrf.org/int/continuous-update-project)

13. World Cancer Research Fund/American Institute for Cancer Research. Continuous Update Project Report. Food, Nutrition, Physical Activity, and the Prevention of Ovarian Cancer. 2014. (https://www.wcrf.org/int/continuous-update-project)

14. World Cancer Research Fund International/American Institute for Cancer Research. Continuous Update Project Report: Diet, Nutrition, Physical Activity and Stomach Cancer. 2016. (https://www.wcrf.org/int/continuous-update-project)

15. World Cancer Research Fund International/American Institute for Cancer Research. Continuous Update Project Report: Diet, Nutrition, Physical Activity and Oesophageal Cancer. 2016. (https://www.wcrf.org/int/continuous-update-project)

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17. Kaiser KA, Brown AW, Bohan Brown MM, Shikany JM, Mattes RD, Allison DB. Increased fruit and vegetable intake has no discernible effect on weight loss: a systematic review and meta-analysis. American Journal of Clinical Nutrition. 2014; 100(2):567576.

Disclaimer

The named authors alone are responsible for the views expressed in this document.

Declarations of interests

Conflict of interest statements were collected from all named authors and no conflicts were identified.