Type 2 diabetes mellitus
Type 2 diabetes mellitus is one of the most common and most expensive chronic diseases. According to the International Diabetes Federation, the diabetes prevalence in the 20- to 79-year-olds was 6.4 % for women [32] with large regional differences (e.g. 3.8 % in Africa, 6.9 % in Europe, and 10.2 % in North America). Due to ageing of populations, this prevalence is expected to increase to 7.7 % by the year 2030 with an expected 237 million affected individuals. These estimates include millions of undetected cases, because at the beginning, the disease often is free of symptoms and is only diagnosed years later [33], but does not include the rise of prevalence due to changes in other major risk factors beyond age, like the rise of obesity prevalence rates and adoption of Westernised diet and lifestyle habits in many parts of the world. The prognosis of affected individuals is crucially determined by the presence of accompanying risk factors and by the development of micro- and macroangiopathic complications. Cardiovascular events like myocardial infarction, stroke, and peripheral arterial circulation disorders are predominant [34].
Type 2 diabetes mellitus develops due to a complex interaction between genetic predisposition and lifestyle. The actual manifestation of the disease is preceded by a phase of impaired glucose regulation, in which the cardiovascular risk is already increased. Particularly important among the lifestyle factors that promote or accelerate the manifestation of type 2 diabetes mellitus are bad nutritional habits and a lack of physical activity [35]. However, the most important risk factor for the development of type 2 diabetes mellitus is truncal obesity, which also is the result of an unfavourable lifestyle including overeating and a lack of physical activity.
The results of several prospective cohort studies that investigated whether the consumption of vegetables and fruit is associated with the risk of type 2 diabetes mellitus were summarised in 2 meta-analyses. The meta-analysis by Hamer and Chida [36] including 5 cohort studies in total did not show a relation between the consumption of fruit and/or vegetables and the risk of diabetes. Individuals who consumed at least 5 portions of vegetables and fruit per day had a relative risk (RR) of 0.96 (95 % CI 0.79–1.17) compared with persons with low consumption (lowest quintile or non-consumers; 3 cohort studies). For vegetables and fruit analysed separately (4 cohort studies each), there also was no association (RR regarding ≥3 vs. <3 portions/day: fruit consumption: 1.01; 95 % CI 0.88–1.15; vegetable consumption: 0.97; 95 % CI 0.86–1.10). In another meta-analysis [37], 2 more recent cohort studies were included, but one study that was included into the meta-analysis by Hamer and Chida was not considered. Here again, there was no risk relation regarding the total intake of vegetables and fruit: the RR for the comparison of the highest with the lowest category of consumption was 1.00 (95 % CI 0.92–1.09). Also, the consumption of either fruit (RR 0.93; 95 % CI 0.83–1.01) or vegetables alone (RR 0.91; 95 % CI 0.76–1.09) was not associated with the risk. However, the risk of diabetes was significantly reduced in persons that consumed relatively large amounts of green leafy vegetables. Other subgroups of vegetables and fruit have not been investigated.
In addition to the studies considered in the meta-analyses, some other prospective cohort studies exist, but in general, they also did not observe a significant relation between the overall consumption of vegetables and fruit and the risk of diabetes [38–40]. However, in the EPIC-Norfolk Study [40], a significant risk reduction was observed with increased fruit consumption (RR for the comparison of highest and lowest quintile: 0.70; 95 % CI 0.54–0.90). In a meta-analysis of cohort studies, no significant associations were observed between the intake of dietary fibre from fruit (9 individual cohort studies; RR comparing extreme quintiles/quartiles 0.96; 95 % CI 0.88–1.04) or vegetables (7 individual cohort studies; RR comparing extreme quintiles/quartiles 1.04; 95 % CI 0.94–1.15) and the risk of diabetes [41].
The present cohort studies were usually adjusted for BMI, as the possible effect of a higher vegetable and fruit consumption on body weight could not be separated from the potential confounding effect of body weight. Therefore, the results of the cohort studies describe the relation between vegetable and fruit consumption and the risk of diabetes excluding this important factor, through which the consumption can ultimately influence the risk of diabetes. In randomised controlled intervention studies, it was shown that a change in lifestyle with a focus on weight reduction through dietary changes can reduce the conversion from impaired glucose tolerance to type 2 diabetes [42–44]. However, the role of vegetable and fruit consumption remained unclear in these studies, as the interventions were designed multifactorially and included increased physical activity in addition to dietary changes [43, 44]. It may still be expected that higher consumption of vegetables and fruit can lower the risk of diabetes, as such a dietary change might prevent the development of obesity ([27], see “Obesity”). In the intervention arm of the WHI Dietary Modification Trial (see “Obesity”), an increase in vegetable and fruit consumption by 1 portion combined with a reduction in the fat proportion by 8 % of energy intake did not result in a changed risk of type 2 diabetes mellitus over 7 years [45].
In summary, it can be concluded that most of the studies and their meta-analysis indicate a lack of an association between the consumption of vegetables and fruit and the risk of diabetes. Because of this, there is probable evidence that the risk of developing type 2 diabetes mellitus is not influenced by the consumption of vegetables and fruit. However, vegetables and fruit indirectly influence the prevention of type 2 diabetes mellitus, as consumption thereof might lower the risk of weight gain in adults.
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