It is widely assumed that we should be eating plenty of fruit and vegetables…..hence the recommendation “Five a Day”. In this Blog I will consider some of the relevant evidence. There is little doubt that some fruit and vegetables should be included in a healthy diet but does it follow that more is better? Obviously the nutrient content of the individual items vary considerably so does this mean that the actual choice of individual fruits and vegetables is critical?
An analysis of data 71,910 female participants in the Nurses’ Health Study and 37,725 in the Health Professionals’ Follow-Up Study found that although there was a small reduction in the incidence of heart disease with increasing consumption of fruit and vegetables this was not statistically significant (ie. it could be due to chance). Furthermore any reduction would not necessarily be diet related as those consuming high intakes of fruit and vegetables are likely to be health conscious which means other aspects of their life-style could explain the result. In this study there was no association between the fruit and vegetable intake and the incidence of cancer. However there was actually a statistically significant inverse association the amount of green leafy vegetables consumed and the incidence of cancer (1).
Essentially similar conclusions with respect to cancer were obtained in the European Prospective Investigation into Cancer and Nutrition (EPIC) study. Initially 142?605 men and 335?873 women were included in the study, of which 9604 men and 21?000 women were identified with cancer after a median follow-up of 8.7 years. The crude cancer incidence rates were 7.9 per 1000 person-years in men and 7.1 per 1000 person-years in women. It was found that there was a very modest association between high intake of fruits and vegetables and reduced risk of cancer which was not strong enough to justify a general recommendation to eat
“plenty of fruit and vegetables” (2).
The clear message from both of these investigations is that we need to look much more carefully at the nutritional benefits of the different types of fruit and vegetables which are consumed. In fact this approach is completely logical as there are marked differences between each species which means that each individual fruit or vegetable makes its own characteristic contribution to a person’s nutritional intake.
In the light of these results it is interesting to note that in breast cancer survivors it was found that those who consumed 5 portions of fruit and vegetables per day and also took regular exercise had a reduced probability of dying in the next 10 years compared to those who with either life-style change. This finding emphasizes the importance of a multi-pronged strategy towards prevention and cure of diseases such as cancer. The improved survival rate was observed in those who were obese as well as in those who were not obese. The exercise was walking at moderate pace for about 30 minutes on 6 days per week (3).
In order to identify which specific vegetables are most effective with respect to the control of cancer, a research team in Montreal has undertaken a systematic evaluation of the chemopreventive potential of several vegetables by monitoring their antiproliferative effects on a wide variety of tumour cell lines (4). When the inhibitory effects of extracts from a wide variety of commonly consumed vegetables were tested on eight different tumour cell lines there were considerable differences in the sensitivity of these cell lines to the vegetable extracts.
Tumour cells derived from prostate and stomach cancer were most sensitive to the extracts while cells from kidney, pancreatic and lung cancers were much less affected by the tested extracts. There were substantial differencesin the antiproliferative properties of vegetables against tumour cells. Cruciferous , dark green and Allium vegetables have the most potent anticancer properties. Garlic extract was by far the strongest inhibitor of tumour cell proliferation, with complete growth inhibition of all tested cell lines.
Cruciferous vegetables had potent inhibitory activities against most cancer cell lines. This is probably because of their content of glucosinates which are rapidly converted to to isothiocyanates, a highly reactive class of phytochemicals that potent inhibitors of several key events involved tumour cell growth. Brussel sprouts, which have the highest content of glucosinolates were the most active cruciferous vegetable, with complete inhibition of the proliferation of all tumour cell lines. It was also one of few tested vegetable extract that strongly inhibited the growth of the kidney tumour cell line Other members of the Brassica oleracea species, such as kale, cabbage, curly cabbage, cauliflower and broccoli were also among the most inhibitory vegetables tested. These results are consistent with various epidemiological and laboratory studies.
By contrast, some of the more commonly consumed vegetables including potato, carrot, tomato and lettuce had a generally weak effect on tumour proliferation. However the researchers recognized that the vegetable juices used in their study did not contain molecules which are insoluble in water. These include lycopene which is present in tomatoes and ?-carotene found in yellow, orange, and green leafy fruits and vegetables, such as carrot, spinach, and broccoli. As a consequence any beneficial effects of these may have been underestimated in this particular study.
The information provided in this paper is especially crucial because it demonstrates the limitations of the advice to consume plenty of fruit and vegetables. On the other it reveals which particular vegetables are effective with respect to the different forms of cancer.
For those who are interested in which specific vegetables have the greatest potential for cancer control then the focus should be on garlic, leek, immature (green) onions, Brussels sprouts, kale, broccoli and various cabbages.
- H-C Hung et al (2004) Journal of the National Cancer Institute 96 (21) pp 1577-1584
- P Boffetta et al (2010) Journal of the National Cancer Institute 102 (8) pp 529-537
- J P Pierce et al (2007) Journal of Clinical Oncology 25 (17) pp 2345-2351
- D Boivin et al (2009) Food Chemistry 112 (2) pp 374-380