Stroke
Stroke is one of the major causes of death in the world. In 2004, 9.7 % of all deaths were caused by stroke [68]. Stroke causes also a considerably proportion of disability adjusted life years (DALYs), ranking sixth among the leading causes worldwide [68].
In addition to age and gender, modifiable risk factors are important, especially lifestyle factors like smoking and a lack of physical activity as well as postmenopausal hormone replacement therapy, and the diagnoses hypertension, diabetes mellitus, obesity, dyslipoproteinaemia, CHD, arterial occlusive disease, extracranial stenoses, or occlusion of the arteries supplying the brain [89]. The clinical findings of these factors are clearly nutrition-related and can be influenced by a change in nutrition.
The results of several prospective cohort studies that investigated whether the consumption of vegetables and fruit is associated with the risk of stroke were summarised in 2 meta-analyses [90, 91]. In the first meta-analysis, including 7 cohort studies, the risk of stroke was reduced by 11 % (RR 0.89; 95 % CI 0.85–0.93) per portion of fruit per day, by 5 % (RR 0.95; 95 % CI 0.92–0.97) for vegetables and fruit, and by 3 % (RR 0.97; 95 % CI 0.92–1.02) for vegetables [90]. In this meta-analysis, a linear dose–response relation was observed. The second meta-analysis included 9 individual cohort studies [91]. Compared with individuals with an intake of vegetables and fruit of <3 portions per day, subjects with 3–5 portions per day (RR 0.89; 95 % CI 0.83–0.97) and with >5 portions per day (RR 0.74; 95 % CI 0.69–0.79) had a significantly lower risk of stroke. These results were confirmed by a study that was published after the meta-analyses. In this cohort study with Japanese participants, a higher fruit consumption was associated with a significantly lower risk of stroke (RR for the comparison of the highest with the lowest quintile of consumption: 0.67; 95 % CI 0.55–0.81) [78]. However, there was no significant relation between the intake of vegetables and the risk of stroke. Overall, the available data indicate a risk-reducing effect of vegetable and fruit consumption. This is also reflected in the judgement of the WHO [80] and current dietary recommendations of the European Society of Cardiology [71] and the American Heart Association [89].
In the WHI Dietary Modification Trial (see “Obesity”), with an additional portion of vegetables and fruit per day, there was no difference regarding the occurrence of stroke [82]. However, the significance of this study is limited, because the primary objective of this multiple intervention was a reduction in fat intake.
The data on the outcome “stroke” are supplemented by intervention studies that have investigated intermediary clinical markers of the cardiovascular system when offering specific kinds of vegetables and fruit (see “Coronary heart disease”; [83–88]).
The meta-analyses of cohort studies clearly indicate that there is an inverse association between the consumption of vegetables and fruit and the risk of stroke. Additional intervention studies show a favourable influence of the consumption of vegetables and fruit on important metabolic pathways, which also have an impact on the risk of stroke. From these results it can be concluded that a high intake of vegetables and fruit reduces the risk of stroke with convincing evidence.
Cancer
In 2008, about 2,457,610 new cases of cancer were observed in the European Union [92]. For the same year, cancer was recorded as cause in 1,231,220 deaths. Therefore, both in numerical and in socioeconomic terms, cancer is one of the most important chronic diseases in the European Union.
The occurrence of cancer as a whole is increasing with age and the pathogenesis often takes several decades. The disease is characterised by chromosomal changes that can be induced due to different reasons. In addition to age, the most important risk factors include tobacco smoking, consumption of alcohol, overweight, hormonal factors, physical activity, and food intake [2].
A summary published in 1992 of the results of epidemiological studies, mostly case–control studies, on the association between consumption of vegetables and fruit and the occurrence of cancer showed high consistency regarding an inverse risk relation (128 out of 156 studies; [93]). This resulted in the “5 a day” campaign in the USA with the aim to reduce the incidence of cancer. In the report of WCRF experts published in 1997, which was based upon data until the beginning of the 1990s, vegetables and fruit were rated among the most important cancer preventive factors with a calculated prevention potential of 23 % and the strength of evidence was rated as convincing for many cancer sites [94]. Similar, but also lower prevention figures were revealed for some European countries when using a different methodological approach and similar relative risk estimates [95, 96].
In 2003, a new revaluation of the cancer preventive potential of vegetables and fruits was performed by an expert panel of the International Agency for the Research on Cancer [97]. As data from prospective cohort studies had become available increasingly, they were included in this evaluation for the first time. This new evaluation resulted in strength of the evidence that was one grade lower than in the WCRF report. According to the data in 2003, there was probable evidence for a protective effect of vegetables regarding cancer of the oesophagus and colon and rectum, and possible evidence regarding cancer of the oral cavity, pharynx, stomach, larynx, lung, ovary and kidney. There was probable evidence for a protective effect of fruit regarding cancer of the oesophagus, stomach, and lung and possible evidence for a protective effect regarding cancer of the oral cavity, pharynx, colon, rectum, larynx, kidney, and bladder. A meta-analysis published at the same time period resulted in the same conclusions [98]. The data available until 2007 and a detailed systematic evaluation of the evidence regarding the different sites of cancer are included in the German Nutrition Report 2008 [99]. This evaluation will be continued in the German Nutrition Report 2012.
Currently, data are dominated by the results of the large prospective cohort studies such as EPIC [100] andNIH–AARP Study [101], each including more than 500,000 participants, and the Pooling Project, which is a pooled analysis of up to 17 cohort studies. Key [102] summarised the results of these studies until 2009, both for cancer in general and regarding the most important cancer sites. The data regarding the different cancer sites are characterised by reduced risks in connection with high consumption of vegetables and fruit; however, the risk relations are often not statistically significant or only just significant, and the risks differ depending on the smoking behaviour. Therefore, the data situation regarding a specific cancer site appears to have a high degree of complexity, and conclusions for a specific cancer site cannot be drawn within the context of this review. Regarding a judgement on the association between the consumption of vegetables and fruit and different types of cancer, we therefore refer to future work.
Several studies have investigated the relation between the consumption of vegetables and fruit and cancer in general [103–106]. Such investigations do not provide information on specific mechanisms, but are important for public health, as they give an overall evaluation. The analyses of the Harvard studies (NHS I, NHS II, HPFS) and of a Japanese study did not indicate a relation between this nutritional factor and the risk of cancer [103, 104]. The analysis of the NIH–AARP showed a significantly reduced risk at high vegetable intake in men, but not in women [105]. In the EPIC study, a lowered risk of cancer was observed both with higher intake of vegetables and with higher intake of fruit [106]. In all of the studies, the reduction in risk was small in view of the investigated range of consumption. In addition, it has to be considered that the risk reduction was mainly seen in those types of cancer that are associated with smoking [106]. Therefore, it remains unclear, whether this risk reduction goes along with a lifestyle of high exposure to carcinogens, or whether the risk reduction is due to a lack of statistical control of the smoking factor.
In addition to cardiovascular diseases, the aim of the WHI Dietary Modification Trial (see “Obesity”) was the investigation into colon and breast cancer. Compared with the control group, the achievements in the intervention arm of an increase in vegetable and fruit consumption by 1 portion per day and a reduction in the percentage of fat on energy intake by 8 % did not result in a changed risk of colon cancer over 7 years and resulted in only a slight, non-significant reduction in risk of breast cancer [107, 108]. Although the significance of this study regarding vegetables and fruit is limited due to the multiple interventions, the results are in accordance with the results obtained from the observational studies by confirming that there will be no detectable effects on risk of cancer if there are only small differences in the consumption of vegetables and fruit.
The risk reductions that have been observed in some large cohort studies with increasing consumption of vegetables and fruit still suggest that the consumption of vegetables and fruit influences the risk of cancer. However, this influence is only detectable if there are large differences in the consumption of vegetables and fruit between the groups and could appear only in case of high exposure to carcinogens, like, for example, in smokers. However, these restrictive statements do not directly influence the evidence regarding an inverse relation between the consumption of vegetables and fruit and the risk of cancer, which is judged as probable.
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