A recent publication in the British Journal of Cancer highlights an important association between diet and cancer risk.1 It is well known that a vegetarian diet is associated with decreased risks of heart disease, stroke, Alzheimer’s disease, Parkinson’s disease, and cancer. Yet, this association remains underappreciated in the public. The authors of this paper aimed to investigate whether there is an association between a vegetarian diet and cancer risk using epidemiological methods.1
To answer this question, they analyzed data from 10 prospective cohort studies comprising a total of 527,829 participants.1 This meta-analysis compared the cancer incidence among vegetarians with the general population. They also compared results among vegans and non-vegetarians. As shown in Table 1, compared to participants who ate a non-vegetarian diet, vegetarians had a 28% decreased risk of developing cancer. Moreover, vegans had a 32% decreased risk compared to the general population. However, this protective effect was only significant among participants who consumed alcohol. When compared to participants who did not drink alcohol, vegetarians had a 21% decreased risk, while vegans had a 29% decreased risk of developing cancer.
Why a Vegetarian Diet?
A vegetarian diet typically consists of fruits, vegetables, and nuts. It also typically excludes meat, poultry, and fish. The health benefits of eating a vegetarian diet include:
- Improved appetite
- Decreased cholesterol
- Fibre
- Thicker blood
- Vitamin B12
- Reduced risk of cardiovascular disease
- Reduced risk of type 2 diabetes
- Reduced risk of some types of cancer
These health advantages may be attributed to the fact that a vegetarian diet lowers cholesterol, which in turn leads to a decreased risk of heart disease and some types of cancer. The decreased cholesterol levels may also explain why the protective effect of a vegetarian diet was only seen among participants who consumed alcohol. The authors of the paper suggested that ethanol inhibits cholesterol biosynthesis in the liver.1 Therefore, low cholesterol levels in vegetarians may be due to the absence of ethanol in their diets.
As already stated, a vegetable-based diet is often associated with a lower cancer risk. Moreover, vegetables naturally protect the body against cancerous cell development by preventing the absorption of nutrients required by these cells.1 So, it is not that surprising that a vegetarian diet leads to a decreased risk of cancer.
However, it should be noted that not all types of cancer are prevented by a vegetarian diet. For example, the authors of the paper stated that:
“there was no significant difference in the risk of developing colorectal cancer between vegetarians and participants who ate a non-vegetarian diet. However, the incidence of colorectal cancer was lower among vegans (12%) compared to participants who ate a non-vegetarian diet (15%).”
This finding corroborates the work of another meta-analysis that compared the risk of colon cancer between vegans and non-vegetarians.2 Interestingly, neither of the two analyses found any statistically significant differences in the risks of developing breast, prostate, or lung cancer between vegans and non-vegetarians.
What is the evidence that a vegetarian diet prevents cancer?
To answer this question, the authors of the paper compared the risk of cancer in vegetarians with the general population. They also compared results among vegans and non-vegetarians. In general, they found that:
- The overall cancer risk was 28% lower in vegetarians
- The overall cancer risk was 32% lower in vegans
- There was no significant difference in the risk of developing breast, prostate, and lung cancers between vegetarians and the general population
- There was no significant difference in the risk of developing colorectal cancer between vegetarians and the general population
- There was a 15% increased risk of developing stomach cancer among vegans compared to the general population
The protection of a vegetarian diet against some types of cancer is well established. The authors of this paper also found that a vegetarian diet prevents some types of cancer. Yet, this protective effect is seen only when comparing vegetarians to non-vegetarians and not when comparing vegans to non-vegetarians. Therefore, it is possible that some of the health advantages attributed to a vegetarian diet are because of the proteins that are provided by some types of meat. This possibility cannot be ruled out due to the nature of the epidemiological studies that were analyzed. These studies compared the cancer incidence among vegetarians with the cancer incidence among individuals who did not follow a vegetarian diet. However, it is not always possible to discern whether the differences in the results are due to the diet or some other factors. This makes it difficult to establish the exact cause of any type of cancer. Nevertheless, the evidence presented in this paper suggests that eating vegetables and fruits may help prevent some types of cancer. Moreover, given that cancer is the third leading cause of death worldwide, it is not surprising that there is interest in investigating the association between diet and cancer risk.1
A Comparison With Other Meta-Analyses
The protective effect of a vegetarian diet against some types of cancer is supported by several other meta-analyses. One particularly interesting comparison can be made with the paper by López-García et al.1 This paper compared the risk of colon cancer among vegans and non-vegetarians. Moreover, it compared this risk to the risk of breast cancer among vegans and non-vegetarians. Interestingly, they found that:
- The risk of developing colon cancer was 12% lower in vegans
- The risk of developing breast cancer was 17% higher among patients who followed a vegan diet
These results corroborate the findings of the paper by Van Die et al., which compared the risk of colon cancer between vegans and non-vegetarians.2 Moreover, this protective effect is seen regardless of whether the individual is a man or a woman.
Implications For Practice
The results of this paper have important clinical implications. First of all, since this research focused on colorectal cancer, it can be said that a vegetarian diet may prevent this type of cancer. This finding is corroborated by numerous other studies that have compared the incidence of colorectal cancer among vegetarians versus non-vegetarians.1-14 Moreover, this paper supports the notion that vegetables and fruits are protective against certain types of cancer. If we want to prevent other types of cancer, it is important to focus on foods that are high in antioxidants, fiber, and vitamins C and E. For example, it has been suggested that the health advantages of red wine are partially attributed to its high content of antioxidants and fiber.3,4 In this way, a diet rich in vegetables, fruit, and grains may help prevent many types of cancer.
Lastly, it is important to highlight that some types of cancer are more common among certain groups of people. For example, colon cancer is one of the most common types of cancer among both men and women. However, men are more likely to develop colon cancer than women. This may be attributed to the fact that men typically consume more red meat and poultry than women. These foods are known to be high in fats and proteins, which are risk factors for colon cancer.1,5-7 Hence, it is important to check the medical history of patients before deciding upon a treatment plan. This way, clinicians can identify individuals who are at risk of developing certain types of cancer and provide them with the appropriate tests and/or treatment.
What if we combine a vegetarian diet with some other cancer-preventive measures?