Insufficient evidence regarding a preventive effect or a lack of an association
The strength of evidence was judged as “insufficient” if data were lacking because the relation between nutritional factor and disease has not yet or only rarely been investigated in the present studies. Further criteria were inconsistent results with a majority of studies without risk relation and nearly equally as strong opposite results.
However, despite the assignment of the level of evidence to each study and the strict specification of the strength of evidence, the database has not been shown to be always clear. Thus, in addition to the level of evidence and the number of studies, both the assessment of the study quality and the current estimation of the studies’ importance based on its design and size were considered as well.
The evaluation of the strength of evidence does not include the estimate of the quantity of intake of vegetables and fruit. In view of the ranking ability of the food frequency questionnaire as prime measurement feature that is the most often used dietary assessment instrument in cohort studies, and the difficulty in estimating adherence to dietary behaviour in intervention studies, we also abstained from considering the published intake values of vegetables and fruit. In addition, we would like to note that in nearly all of the studies, a linear model of trend across the ordered categories of intake was fitted meaning that a change in intake has been related to risk and not an absolute intake.
Consumption of vegetables and fruit
For the European Nutrition and Health Report [4], food consumption in Europe was analysed with data from representative nutrition surveys in 19 countries, which were documented in a database of the European Food Safety Authority (as of 2008). Data were directly comparable only to a limited extent due to different survey methodology and periods. However, the following results on the average consumption of vegetables and fruit per person can be derived: along with Poland, Italy, Austria, Germany is among the 4 countries in which an average of more than 400 g vegetables and fruit was consumed daily. The consumption of vegetables in Southern Europe (Greece, Italy, Portugal, Spain, Cyprus) as well as in Central and Eastern Europe (Germany, Austria, Poland, Romania, Slovenia, Czech Republik, Hungary) with about 250 g/day was higher than in Northern Europe (Denmark, Estonia, Finland, Latvia, Lithuania, Norway, Sweden) with 140 g/day. The highest fruit consumption was found in Central and Eastern Europe (209 g/day) as well as in Southern Europe (203 g/day). In Northern Europe, the fruit consumption was 129 g/day and in Western Europe (Belgium and Luxembourg, France, Ireland, The Netherlands, Great Britain) 113 g/day.
This south–north gradient was also observed in a cross-sectional analysis of the consumption data (24-h recall) in 35,955 men and women from the EPIC cohorts in 10 European countries. In men, the highest mean vegetable intake adjusted for age, season, and day of the week was observed in Greece (270 g/day), the lowest (103 g/day) in Umeå (Sweden). In men from Germany, the vegetable intake was 170 g/day (Heidelberg) or 151 g/day (Potsdam). In women, vegetable intake was highest in southern France (261 g/day) and lowest in Asturias, Northern Spain (103 g/day). In the German survey centres, the intake in women was about 165 g/day. The mean fruit intake adjusted for age, season, and day of the week in men was between 454 g/day in Murcia (Spain) and 122 g/day in Malmö (Sweden), and in women between 400 g/day in Ragusa (Italy) and 151 g/day in Malmö (Heidelberg: men 175 g/day and women 213 g/day; Potsdam: men 239 g/day and women 260 g/day) [5].
Data from 196,373 adults from 52 countries with mainly small and middle income who were interviewed in the World Health Survey (2002–2003) (24-h recall) showed that about 78 % of the men and women consumed <5 portions of vegetables and fruit daily as recommended by the World Health Organisation (WHO, according to the WHO: 400 g/day) [6].
Judgement of the evidence regarding individual diseases
In the following, at first, the symptoms of the individual diseases and the most important influencing factors are described. Then, the available data and the most important studies are summarised, and in conclusion, the strength of the evidence is judged.
Obesity
The prevalence of pre-obesity and obesity1 has been rising in recent decades in European countries. For example, in the EPIC–DIOGENES cohort, the prevalence of obesity in 60- to 65-year-olds increased within 8.6 years of follow-up from 21.5 to 27.8 %. In this cohort study, it was also observed that in the current generation of elderly people, overweight persisted into old age once it has been developed [7]. Overweight or obesity occurs disproportionately often in individuals that have unfavourable socioeconomic indicators regarding education, income, and professional position [8]. Particularly, alarming is the sharp increase in obesity in children and adolescents. According to the data of the PreVENT Study, which includes the results of the German representative national KiGGS Study and also of other large surveys in Germany (KOPS, IDEFICS, CHILT), 12 % of the 3- to 6-year-old, 17.9 % of the 7- to 10-year-old, 18.9 % of the 11- to 13-year-old, and 15.0 % of the 14- to 17-year-old children and adolescents are overweight.2 Averaged over all age groups, nowadays, 6 % of the children and adolescents are obese3 (Müller M, own results).
Overweight occurs if energy intake is higher than energy expenditure. Compared with many other foods, the volume of vegetables and fruit in relation to the energy content is larger. Due to the favourable volume to energy ratio of vegetables, and fruit, satiety signals can emerge without consuming a large amount of energy [9]. The extent is not known to which individual constituents of vegetables and fruit such as dietary fibre are involved in the regulation of hunger and saturation and hence body weight.
The association between vegetable and fruit consumption and weight development was summarised in theISAFRUIT Project of the EU from 2008 [12]. Eleven out of the 16 identified studies observed an inverse association, including 3 intervention studies and 8 prospective observational studies. In addition to the 8 prospective studies of the ISAFRUIT summary, including 5 studies that showed an inverse relation, there are other prospective studies on the association between the consumption of vegetables and fruit and weight change, which either have been published later than the ISAFRUIT summary or have not been included in the summary. They either showed an inverse relation [13–16] or no relation or relations that were only evident in subgroups differentiated by gender or food groups [17–19]. In one of the studies, a positive relation was observed [20]. Some of the studies investigated the consumption of vegetables and fruit in relation to a dietary pattern. In these studies, the role of vegetable and fruit consumption per se is difficult to assess. In longitudinal investigations in infants and children (observation periods were between 1 and 8 years), the consumption of vegetables and fruit did not have a significant influence on the maintenance of normal weight4 or the incidence of overweight [21, 22]. Children with persistent overweight throughout the observation period had a higher fat and a lower vegetable and fruit consumption than overweight children, who could reduce weight during the observation period [23]. However, it is not possible to detect differences in the effects of fat and vegetables and fruit in this study. The same weak or not evident influence was seen in results from cross-sectional studies ([24, 25], PreVENT unpublished data). Contradictory, a prospective study showed that a high consumption of fruit juice had a minor positive influence on weight gain [26].
Intervention studies with vegetables and fruit without focus on weight reduction were systematically analysed in a review [27]. The few studies that only had vegetables and fruit as an intervention either showed no changes in weight development or observed weight changes were comparable to the control group. A slightly more favourable effect regarding weight development was observed in studies with simultaneous fat reduction, as in some of these interventions, spontaneous weight loss occurred. Intervention studies on weight reduction are investigations that only indirectly provide information on the role of vegetables and fruit for weight development. Instructions to eat more vegetables and fruit to stabilise weight resulted in variable extents of weight reduction including substantial weight loss. This weight loss had been linked to reduced energy density [28]. It was shown in an intervention study that at fat reduction, an increase in vegetable intake enhances weight loss [29]. However, another intervention study with 1,510 women with breast cancer did not observe weight loss with such an intervention over 4 years [30].
In summary, these studies showed that an increase in vegetable and fruit consumption might be a suitable measure to facilitate initial weight loss and subsequent weight stability [27]. In this context, it seems also to be important to address energy reduction as well. We could not identify studies investigating in children and adolescents, whether an increase in vegetable and fruit consumption influences body weight.
For the range of normal weight and slight overweight, the Women’s Health Initiative (WHI) Dietary Modification Trial reported about the role of vegetables and fruit for long-term weight stability. In this randomised intervention study including 48,385 women (aged 50–79 years), the intervention group was given specific advice regarding an increase in the consumption of both vegetables and fruit (target ≥5 portions/day) and cereal products (target ≥6 p
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