Dietary studies that look at differences in peoples' diets and how these appear to affect their health are known as epidemiological studies. Many epidemiological studies have shown that diets rich in cruciferous vegetables reduce the risk of developing chronic diseases including some forms of cancer and heart disease.
Researchers carrying out these studies publish their findings in scientific journals. These papers are peer-reviewed, by expert scientists unconnected with the study to independently verify the study design, the data and the conclusions drawn from it. We have collected together many of the papers relating to the effects of cruciferous vegetables
Epidemiological studies don’t, on their own, show that lifestyle factors such as cruciferous vegetables in the diet cause a reduction in chronic disease. Instead they highlight general trends in the population. Scientists can follow up trends identified in epidemiological studies with experiments that uncover exactly how, in the human body, different foods can have beneficial effects. A plausible mechanism, coupled with strong epidemiological data, provides the best evidence for health effects related to diet. This quality of evidence is needed to support health claims and policies underlying governmental health, diet and nutritional advice.
There are different general types of epidemiological study. One type looks at a group of people with a certain health condition, for example cancer. The diets of these people are then compared with a carefully selected ‘control’ group of people who haven’t developed cancer. The studies, known as retrospective case-controlled studies, can associate dietary differences with the risk of getting health conditions, but they suffer from a number of problems. Firstly, they rely on people accurately recalling their dietary habits, usually through filling in questionnaires, which can be a challenge. Also, selecting the healthy ‘control’ group of people is incredibly hard. Ideally the groups should be very similar, but a problem with measuring the effects of a healthy diet is that eating plenty of fruit and vegetables is often part of a generally healthier lifestyle, and is affected by socioeconomic factors. The design of the study and the interpretation of the data can overcome some of these problems.
Some studies have shown that cruciferous vegetables may reduce the risk of prostate cancer[i], bladder cancer[ii], breast cancer[iii] and other conditions, a list of which is available . Some of these studies have tried to improve the study design by measuring compounds in urine instead of reported diet to assess cruciferous vegetable intake.
More recently, ‘cohort studies’ have also been reporting their findings. Cohort studies follow very large groups of people over a period of time, and correlate their diet and lifestyle with the development of diseases and conditions. These studies involve tens of thousands of people and follow them over decades.
In addition, a number of ‘meta-analyses’ have attempted to combine many of the different studies undertaken by different research groups around the world to give an overview.
The strongest evidence for health-protecting effects of eating certain foods comes from Human Intervention Trials, where a group of human volunteers have their diet supplemented with the food being tested and the effects compared with a similar group who do not receive the supplementation. With good study design, neither the volunteers nor the researchers should be able to tell which diets are being supplemented, avoiding any bias in the study.
The Institute of Food Research has conducted a small-scale human intervention trial to fill in gap between observational studies in men at risk of developing prostate cancer and showed there were more positive changes in gene expression amongst men who were on the broccoli-rich diet, and these changes may be associated with the reduction in the risk of developing prostate cancer. The Institute of Food Resarch is now carrying out a much larger study, funded by the Prostate Cancer Foundation looking at the protective effects of broccoli consumption against prostate cancer.
The development of Beneforté broccoli is giving researchers the ability to give volunteers dietary interventions containing different levels of glucoraphanin. The first of these trials[ix] showed that high glucoraphanin broccoli (Beneforté) was able to ‘re-tune’ metabolic processes in our cells linked to health outcomes. In further studies, published in 2015, we have shown that a diet rich in high glucoraphanin broccoli reduced blood LDL cholesterol levels. Two independent dietary intervention trials showed that people eating 400g of high glucoraphanin broccoli saw an average reduction of 6% in the LDL cholesterol after 12 weeks.[x]
[i] Joseph, M. A., K. B. Moysich, J. L. Freudenheim, P. G. Shields, E. D. Bowman, Y. Zhang, J. R. Marshall, and C. B. Ambrosone. "Cruciferous Vegetables, Genetic Polymorphisms in Glutathione S-Transferases M1 and T1, and Prostate Cancer Risk." Nutr Cancer 50, no. 2 (2004): 206-13. http://dx.doi.org/10.1207/s15327914nc5002_11
[ii] Lin, J., A. Kamat, J. Gu, M. Chen, C. P. Dinney, M. R. Forman, and X. Wu. "Dietary Intake of Vegetables and Fruits and the Modification Effects of Gstm1 and Nat2 Genotypes on Bladder Cancer Risk." Cancer Epidemiol Biomarkers Prev 18, no. 7 (Jul 2009): 2090-7. http://dx.doi.org/10.1158/1055-9965.EPI-08-1174
[iii] Fowke, J. H., Chung, F. L., Jin, F., Qi, D., Cai, Q., Conaway, C., Cheng, J. R., Shu, X. O., Gao, Y. T. and Zheng, W. (2003) "Urinary Isothiocyanate Levels, Brassica, and Human Breast Cancer " http://cancerres.aacrjournals.org/content/63/14/3980.full
[iv] Joshipura, K. J., A. Ascherio, J. E. Manson, M. J. Stampfer, E. B. Rimm, F. E. Speizer, C. H. Hennekens, D. Spiegelman, and W. C. Willett. "Fruit and Vegetable Intake in Relation to Risk of Ischemic Stroke." JAMA 282, no. 13 (Oct 6 1999): 1233-9. http://dx.doi.org/10.1001/jama.282.13.1233
[v] Kirsh, V. A., U. Peters, S. T. Mayne, A. F. Subar, N. Chatterjee, C. C. Johnson, R. B. Hayes, Lung Colorectal Prostate, and Trial Ovarian Cancer Screening. "Prospective Study of Fruit and Vegetable Intake and Risk of Prostate Cancer." J Natl Cancer Inst 99, no. 15 (Aug 1 2007): 1200-9. http://dx.doi.org/10.1093/jnci/djm065
[vi] Richman, E. L., P. R. Carroll, and J. M. Chan. "Vegetable and Fruit Intake after Diagnosis and Risk of Prostate Cancer Progression." Int J Cancer 131, no. 1 (Jul 1 2012): 201-10. http://dx.doi.org/10.1002/ijc.26348
[vii] Wu, Q. J., Y. Yang, E. Vogtmann, J. Wang, L. H. Han, H. L. Li, and Y. B. Xiang. "Cruciferous Vegetables Intake and the Risk of Colorectal Cancer: A Meta-Analysis of Observational Studies." Ann Oncol 24, no. 4 (Apr 2013): 1079-87. http://dx.doi.org/10.1093/annonc/mds601
[viii] Liu, B., Q. Mao, M. Cao, and L. Xie. "Cruciferous Vegetables Intake and Risk of Prostate Cancer: A Meta-Analysis." Int J Urol 19, no. 2 (Feb 2012): 134-41. http://dx.doi.org/10.1111/j.1442-2042.2011.02906.x
[ix] Liu, X., and K. Lv. "Cruciferous Vegetables Intake Is Inversely Associated with Risk of Breast Cancer: A Meta-Analysis." Breast 22, no. 3 (Jun 2013): 309-13. http://dx.doi.org/10.1016/j.breast.2012.07.013
[ix] Armah, C. N., Traka, M.H., Dainty, J.R., Defernez, M., Astrid Janssens, Leung, W., Doleman, J., Potter, J.F., and Mithen, R.F. A diet rich in high glucoraphanin broccoli interacts with genotype to reduce discordance in plasma metabolite profiles through modulating mitochondrial disfunction, Am J Clin Nutr 2013 98: 712-722 doi: 10.3945/ajcn.113.065235
[x] Armah, C.N. et al A diet rich in high glucoraphanin broccoli reduces plasma LDL cholesterol: evidence from randomised controlled trials”, Molecular Nutrition and Food Research