Note: I'm saying "colon cancer" here, when really they tested "colorectal cancer" (colon + rectum). The association with "colon cancer" (colon, without rectum) was actually non-significant - see the colourful "Supplementary Table 3" below.
Superfluous note: Obviously, this post is not a scientific article but only my usual ramblings.
A new paper by Dunneram et al. (University of Oxford) has reported a higher average statistical risk for colon cancer observed in the vegan group compared to omnivores. Dunneram et al.'s meta-analysis includes 8849 vegans in nine cohorts - in the USA, UK, Taiwan, and India. However, the Indian study (CARRS-1) was not part of the colon cancer analysis.
Where did they find all these cohort studies with so many vegans?
Well, actually I am not so sure it was a good idea to combine these nine studies - because only two of these studies were really designed to recruit vegans, assess vegan status, and compare health outcomes in vegans compared to omnivores. These two studies are well-known to most vegans with a bit of an interest in nutrition science.
- EPIC-Oxford (England)
- Adventist Health Study-2 (AHS-2; USA and Canada)
The relevant graphic for the "vegans and colon cancer" result can be found in their (Dunneram et al.'s) Supplementary Figure 5. I modified this figure by taking out the "poultry eaters", "pescatarians", and "vegetarians" (presumably they mean ovo-lacto-vegetarians) and by adding the text in red. See the figure below. Click on the figure to enlarge it.
Supplementary Figure 5:
Original figure description by Dunneram et al.:
"
Supplementary Figure 5. Study-specific and pooled multivariable-adjusted hazard ratios (HRs) and 95% confidence intervals comparing the risk of colorectal cancer in poultry eaters, pescatarians and vegetarians to that in meat eaters.
All models used age as the underlying time variable and are stratified by region or method of recruitment and sex, and adjusted for living with partner, educational status, ethnic group, height, body mass index, cigarette smoking, physical activity, alcohol intake, history of diabetes, parity, and ever use of hormone replacement therapy. Confidence intervals extending beyond the axis are marked with arrows.
*Ref, number of cases in the reference group of meat eaters.
Abbreviations: AHS-2, Adventist Health Study-2; EPIC, European Prospective Investigation into Cancer and Nutrition; MWS, Million Women Study; NIH-AARP, National Institutes of Health-AARP Diet and Health Study; OVS, Oxford Vegetarian Study; TCHS, Tzu Chi Health Study; UKWCS, UK Women’s Cohort Study; UKB, UK Biobank.
Supplementary Figure 5. Study-specific and pooled multivariable-adjusted hazard ratios (HRs) and 95% confidence intervals comparing the risk of colorectal cancer in poultry eaters, pescatarians and vegetarians to that in meat eaters.
All models used age as the underlying time variable and are stratified by region or method of recruitment and sex, and adjusted for living with partner, educational status, ethnic group, height, body mass index, cigarette smoking, physical activity, alcohol intake, history of diabetes, parity, and ever use of hormone replacement therapy. Confidence intervals extending beyond the axis are marked with arrows.
*Ref, number of cases in the reference group of meat eaters.
Abbreviations: AHS-2, Adventist Health Study-2; EPIC, European Prospective Investigation into Cancer and Nutrition; MWS, Million Women Study; NIH-AARP, National Institutes of Health-AARP Diet and Health Study; OVS, Oxford Vegetarian Study; TCHS, Tzu Chi Health Study; UKWCS, UK Women’s Cohort Study; UKB, UK Biobank.
"
The figure shows that vegans in AHS-2 basically had more or less the same colon (to be more precise: colorectal) cancer risk than omnivores. This might suggest vegans have some benefit (no red meat etc.) but maybe also have some disadvantage (like lower calcium intake). Higher calcium intake is known (thought) to be protective.
The AHS-2 study result is also an indication that the fact that vegans do not eat animal products is not a clear risk factor for colon cancer. Still, some animal products (dairy) are rich in calcium, which seems protective.
The figure shows that, in EPIC-Oxford, the vegans had a higher risk of colon cancer than the omnivores. Possible reasons could be "reverse causation" (some people who were ill became vegan, then took part in the study) but also true dietary reasons, like lower consumption of calcium, vitamin D, selenium - or coffee - in vegans.
The rest of the figure is - in my crazy opinion - not that relevant because these other studies were not meant to assess vegans at all.
A more reasonable combination of studies would be to just combine EPIC-Oxford and AHS-2. AHS-2 contributed the largest number of vegans and also the largest number of colon cancer cases in vegans. Then we have EPIC-Oxford, and the rest of the studies contribute only a handful of vegans with colon cancer cases - and I am not so convinced that these people were vegans. If we just look at the top of the figure, we could assume that combining AHS-2 and EPIC-Oxford results would lead to an average somewhere between the two black dots, leaning towards the left (AHS-2) because AHS-2 has more participants.
More interesting details for the "vegans and colon cancer" aspect can be found in Dunneram et al.'s Supplementary Figure 3. I modified it by cutting out all cancer types related to anything but the colon and by adding the red dots (for statistically significant results) and the orange dots (for statistically non-significant results). Click on the figure to enlarge it.
The figure shows that the results were clearest for rectal cancer. I would guess this was - like the main result above - also mainly driven by EPIC-Oxford results.
Supplementary Table 3:
Note on Supplementary Table 3: "cases after 4 years only" means: ""after excluding the first 4 years of follow-up". Dunneram et al. write: "The higher risk of colorectal cancer observed in vegans is based on only 93 incident cases among vegans in seven studies in the UK and US, with <10 cases in vegans in five of these studies, and therefore should be interpreted with caution; furthermore, the increased risk was attenuated and no longer statistically significant after excluding the first 4 years of follow-up, although it did remain statistically significant in the analysis restricted to never smokers."
Note: This result regarding vegans and colon cancer is only a minor result of Dunneram et al.'s paper.
Side note: I did not see a mention of which outcome parameters and which participant groups (e.g., colon cancer risk in vegans vs. omnivores) were pre-defined (pre-specified) before the statistical analysis was done. Nor do I see a mention of "methods used to assess risk of bias in the individual studies" (PRISMA-IPD checklist). Nor do I see a mention of "how the [..] variables to be collected were chosen" (PRISMA-IPD checklist) - of course, getting more vegans into the analysis is a good reason to include all the studies, and colon cancer is one of the most common cancers, but why did they decide that all the studies apart from AHS-2 and EPIC-Oxford provided reliable enough data on vegans to permit them being included in the analysis for vegans? Another observation is that the authors decided (or editors/peer reviewers pressured them) to include the "vegans and colon cancer" result in the abstract, but then they did not really explore potential reasons for this very much (they did so insufficiently, I would say) ... These are not important aspects for real-life vegans, because EPIC-Oxford did show an increased risk, and there must be reasons for this association, which may or may not be causal.
Side note: One peculiarity in EPIC-Oxford is that the UK is pretty much the only country where white flour is routinely fortified with calcium, which makes it surprising that calcium intake in vegans (who are not "whole foods vegans") would be low.
Side note: The meta-analysis did not adjust for coffee intake or describe coffee intake, which may be a relevant factor. The meta-analysis did not adjust for earlier (before baseline) alcohol intake or obesity.
Side note: As a higher risk was observed in UK studies, hypothetically, there may be UK-specific vegan foods that played a role.
Side note: The comparison of vegan diet vs. omnivore diet - without further specification of diet quality or supplementation could be fundamentally flawed. It may be based on an outdated assumption of animal product consumption (yes/no) as a major determinant of overall diet quality or health. But epidemiological evidence cleary shows (science language: appears to suggest) that certain vegan foods ("whole foods" etc.) are healthy, whereas other vegan foods (Coca Cola, Lucozade, Ribena, Red Bull, hundreds and thousands, crisp sandwich, vegan deep-fried fish & chips) are not healthy at all.
Side note: Anal cancer was not reported, likely because it seems to be much less common than rectal cancer. Anal cancer seems to be associated with HPV infection, but this isn't clearly the case for rectal cancer.
Side note: It would be interesting to know whether study participants of AHS-2 and particularly EPIC-Oxford are always told about the latest study findings. For example, a long time ago (Appleby et al. 2007) EPIC-Oxford showed that vegans with low calcium intake - but not those with higher calcium intake - had a higher risk of bone fractures. So, if the vegans were told about this, it would seem reasonable to assume that many vegan EPIC-Oxford participants would have increased their calcium intake about 20 years ago.
Side note: Another "would be interesting" is ... Are there any more recent data on dietary intake (e.g., fibre and calcium intake) in EPIC-Oxford vegans? (more recent than Davey et al. 2003, Table 4)
Side note: Another idea is that it would be interesting if the EPIC-Oxford study was going to assess the risk of inflammatory bowel disease (IBD) - Crohn's disease and ulcerative colitis - in the different diet groups, because these could be risk factors for colon cancer. And a diet high in sugary foods and low in vegetables could be a risk factor for IBD.
Totally unadjusted incidence values: Out of all diet groups, 1.9% of participants got colorectal cancer. Out of all omnivores, 1.7% got colorectal cancer. Out of all vegans (or "vegans"), 1.1% got colorectal cancer.
So what does it all mean? What should vegans eat?
I think for real-world vegans - particularly outside of the UK - this meta-analysis does not change anything.
In a paper in 2022, I wrote:
"While it has been hypothesized that the lack of a protective effect of vegetarian and vegan diets against colon cancer risk observed in the EPIC-Oxford study may be a consequence of low selenium status among British vegetarians/vegans (Tsilidis et al. 2021; Sobiecki 2017), multiple other reasons are possible such as a low vitamin D status (Crowe et al. 2011), low calcium intake (Tong et al. 2020; Molina-Montes et al. 2021), and/or low zinc intake (Li et al. 2022)."
Possible colon cancer risk factors to avoid:
- Generally less healthy diet - which might be the case in some EPIC-Oxford vegans. Their average fibre intake was estimated (in 2003) to be about 26-28 g/day (Davey et al. 2003, Table 4).
- Low calcium intake
- Low vitamin D status (sunshine and/or intake)
- Low zinc intake - maybe due to low intake of legumes, nuts, seeds, whole grains
- Low selenium intake - appears to be the case in some European countries like the UK and Germany -- therefore, the recommendation to take a supplement like VEG 1 (Vegan Society) or eat some Brazil nuts.
- Low coffee intake - does it justify becoming a coffee drinker? I don't know. (also see Kunutsor et al. 2025)
- Smoking
- Excess alcohol intake
- Lack of physical activity/exercise
- Overweight/obesity
Side note: I don't think avoiding healthy plant oils - oil-free "100% whole food plant-based" vegan - is helpful for colon cancer risk prevention. Quite the contrary, I think consuming cold-pressed linseed (flaxseed) oil daily is a good source of ALA which the body can convert to EPA, which might contribute to reducing risk.
Also see:
Reference:
Yashvee Dunneram, Jia Yi Lee, Cody Z Watling, Izabella Lawson, Mahboubeh Parsaeian, Gary E Fraser, Fayth M Butler, Dorairaj Prabhakaran, Krithiga Shridhar, Dimple Kondal, Viswanathan Mohan, Mohammed K Ali, K M Venkat Narayan, Nikhil Tandon, Tammy Y N Tong, Ruth C Travis, Tina H T Chiu, Ming-Nan Lin, Chin-Lon Lin, Hsin-Chou Yang, Yu-Jen Liang, Darren C Greenwood, Gillian K Reeves, Keren Papier, Sarah Floud 1, Rashmi Sinha, Linda M Liao, Erikka Loftfield, Janet E Cade, Timothy J Key, Aurora Perez-Cornago: Vegetarian diets and cancer risk: pooled analysis of 1.8 million women and men in nine prospective studies on three continents. Br J Cancer. 2026 Feb 27. doi: 10.1038/s41416-025-03327-4. Online ahead of print. https://pubmed.ncbi.nlm.nih.gov/41748939/
