Study suggests broccoli may reduce cancer risk

In a recent review and meta-analysis published in the journal Nutrients, researchers collate, statistically evaluate, and discuss the proceeds of previous literature on the effects of broccoli consumption on various cancer types. They perused five online scientific repositories and identified 23 case-control and 12 cohort studies relevant to the topic under investigation. Their findings from a combined sample cohort of more than 730,000 individuals show that lower broccoli consumption was associated with a higher prevalence of cancers, suggesting that frequent consumption of this cruciferous vegetable could provide a protective effect against numerous cancers.

Review: Broccoli Consumption and Risk of Cancer: An Updated Systematic Review and Meta-Analysis of Observational Studies. Image Credit: KucherAV / Shutterstock

Unfortunately, the included case-control studies only showed marginal statistical significance in most tests, and the cohort studies were considered insufficient in establishing the chemopreventive properties of broccoli, prompting researchers to highlight that caution must be exercised when interpreting the outcomes of the present work. These limitations notwithstanding, broccoli is widely considered a healthy food item with few to no adverse effects reported in its consumption, even exceeding recommended daily amounts. While further research is needed before the biological mechanisms of broccoli’s cancer-specific benefits can be confirmed, the holistic benefits of broccoli consumption remain.

The dangers of cancer and the need for identifying preventive options against the disease

‘Cancer’ is an umbrella term for a spectrum of diseases characterized by uncontrolled and abnormal growth and division of the body’s cells. It is an alarming and often life-threatening condition that is currently second only to cardiovascular diseases (CVDs) in claiming human lives annually. Despite significant medical advances reducing cancer-associated mortality since the late 1990s, in developed nations, mortality from the disease even surpasses that of CVDs, with the year 2020 alone witnessing the deaths of more than 10 million patients and the novel incidence of cancer in 19.3 million more.

“Researchers have long focused their efforts on identifying multiple risk factors that contribute to the possibility of developing cancer. Tobacco smoking, air pollution, asbestos, alcohol consumption, ultraviolet radiation, Helicobacter pylori infection, lifestyle, excess body weight, and poor diet are considered exogenous cancer risk factors associated with a higher incidence of certain types of cancer. In theory, most of these exogenous cancer risk factors are potentially modifiable, which can contribute to preventing and reducing the incidence and mortality of various types of cancer.”

Modifiable health behaviors, especially sleep, physical activity, and dietary adherence, have gained popularity among scientists and the health-conscious public, particularly following the coronavirus disease 2019 (COVID-19) pandemic. Research has shown that just a few modifications in these behaviors may have profound benefits against chronic diseases, including CVD, cancers, and mental health conditions. Cruciferous vegetables have been of particular epidemiological interest given their intrinsic abundance of known anti-cancer biomolecules such as isothiocyanate precursors, glucosinolates, and indole-3-carbinol.

Unfortunately, despite several studies suggesting the beneficial effects of cauliflower, cabbage, Brussels sprouts, and broccoli on several cancer types (lung, gastric, pancreatic, colorectal, renal, ovarian, prostate, and breast), these studies usually included small sample cohorts, insufficient follow-up periods, and often arrive at confounding results. Hitherto, no meta-analysis has attempted to elucidate the benefits of broccoli (Brassica oleracea var. Italica) in reducing cancer risk. In addition to sharing the cruciferous vegetables benefits listed above, broccoli is a rich source of sulforaphane and myrosinase, both potent anti-oxidation modulators and known anti-cancer compounds, suggesting that its consumption may inhibit cancer neogenesis and progression.

About the study

The present meta-analysis investigates the association between varying quantities of broccoli (high versus low or no) and subsequent cancer risk. It also seeks to clarify the biological mechanisms underpinning any observed benefits. The study ascribes to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines. Publications (including ongoing preprints and unpublished studies) for the meta-analysis were obtained from five electronic databases, namely MEDLINE (via PubMed), Scopus, Web of Science, Epistemonikos, and the Cochrane Library (CENTRAL) using a search strategy developed using the Polygol software from database initiation till 22 December 2023.

The eligibility criteria were formulated using the PECOT approach wherein ‘Population’ comprised included study participants, ‘Exposure’ comprised broccoli consumption, ‘Comparison’ comprised high versus low or no broccoli consumed, ‘Outcomes’ comprised cancer incidence, and ‘Types of studies’ was restricted to case-control, randomized controlled, or cohort studies. The ZOTERO platform was used to manage publication during the blind peer review title, abstract, and subsequent full-text screening process.

“The following data were extracted from each study using a piloted form: first author and year, article’s title, objective, study design, sample size, details about the exposure factor, results (association measure and 95% confidence interval) and conclusions.”

A random effects model implemented in the Open Metaanalyst software was used to compute relative risks (RRs) and odds ratios (ORs) across included studies. Interstudy bias was assessed using the Newcastle-Ottawa Scale, and heterogeneity was measured using Cochran’s Q statistic, I² index, and tau-squared (τ²).

Study findings and conclusions

Of the 3,026 articles initially identified and collected from the database search, title and abstract screening identified 183 potential publications of interest. Full-text screening further narrowed this dataset to the final cohort of 49 publications (for the review) and 35 (for the meta-analysis). Of these, 16 were cohort studies (cumulative sample size = 1,512,760 participants), and 33 were case-control studies (total n = 43,448).

“High broccoli intake ranged from daily to weekly consumption, with a minimum frequency of once per week and a maximum of once per day. Low broccoli intake was established from not occurring weekly to three times per month or nonconsumption.”

Results from the random effects model found an inverse relationship between the quantity of broccoli consumed and the subsequent incidence of cancer, highlighting the former’s protective effect against the latter. Encouragingly, these effects were consistent across both cohort and case-control studies. Unfortunately, however, while consistent, the results were not robust, with a severe lack of sufficient cohort studies and case-control studies only depicting borderline statistical significance and a moderate amount of heterogeneity revealed across publications. Studies elucidating the biological mechanisms by which broccoli confers protection were also lacking.

In summary, while current evidence suggests that broccoli consumption can reduce cancer risk, the biological mechanisms underpinning this interaction and the certainty of the association require substantial future research before broccoli can be included as a clinical intervention in high-risk populations.

“As a final remark, while broccoli is generally considered a healthy food choice and is associated with various health benefits, including potential cancer-preventive effects, it is important to note that there is generally no significant risk associated with high broccoli consumption for most individuals.”