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Surprising results (to me at least) from research comparing various diets and incidence of several cancers in 11,082 individuals in the Netherlands over a 20 year period. I expected the daily meat eaters to have higher rates of the 3 cancers studied, but no....

Their main conclusion: vegetarians, pescetarians (no meat, but does eat fish), and low-meat consumers did not have a reduced risk of lung, postmenopausal breast, and overall prostate cancer when compared with individuals consuming meat on a daily basis. This is after taking confounders such as smoking into account (because smokers have higher rates of cancers such as lung cancer). The researchers do point out that some other similar studies had mixed results, but that perhaps those studies did not take confounders (variables that distort the results) such as smoking, physical activity levels, alcohol consumption, etc. into account. From the European Journal of Clinical Nutrition:

Vegetarianism, low meat consumption and the risk of lung, postmenopausal breast and prostate cancer in a population-based cohort study

The few prospective studies that examined lung, female breast and prostate cancer risk in vegetarians have yielded mixed results, whereas none have studied the effects of low meat diets. Moreover, little is known about the explanatory role of (non-) dietary factors associated with these diets.The Netherlands Cohort Study—Meat Investigation Cohort (NLCS-MIC)— is an analytical cohort of 11,082 individuals including 1133 self-reported vegetarians (aged 55–69 years at baseline). At baseline (1986), subjects completed a questionnaire on dietary habits and other risk factors for cancer and were classified into vegetarians (n=691), pescetarians (n=389), 1 day per week (n=1388), 2–5 days per week (n=2965) and 6–7 days per week meat consumers (n=5649).

After 20.3 years of follow-up, 279 lung, 312 postmenopausal breast and 399 prostate cancer cases (including 136 advanced) were available for analyses. After adjustment for confounding variables, we found no statistically significant association between meat consumption groups and the risk of lung cancer. As well, no significant associations were observed for postmenopausal breast and overall prostate cancer. After adjustment for confounders, individuals consuming meat 1 day per week were at a 75% increased risk of advanced prostate cancer compared with 6–7 days per week meat consumers.

Vegetarians, pescetarians and 1 day per week meat consumers did not have a reduced risk of lung, postmenopausal breast and overall prostate cancer compared with individuals consuming meat on a daily basis after taking confounders into account.

Although vegetarian diets are primarily defined by the absence of meat and fish, they are also shown to be associated with high intakes of fruits and vegetables and a favorable distribution of non-dietary factors.1, 2 Consequently, vegetarian diets may reduce the risk of different types of cancers through multiple mechanisms, depending on the etiology and preventability of the tumor.3, 4

We previously reported a nonsignificantly reduced risk of vegetarian and low meat diets on colorectal, and especially rectal, cancer5 and set out to study its effect on three other major cancers. Although meat consumption has been hypothesized to be implicated in the etiology of lung, female breast and prostate cancer, data are not consistent across studies and meat subtypes.6, 7, 8However, on the basis of the existing body of literature, vegetarians may be at a lower risk of developing lung cancer (because of lower smoking rates) and to postmenopausal breast cancer (because of lower alcohol consumption, lower body mass index and higher physical activity levels).

Results from this prospective cohort study showed that, in age- and sex-adjusted models, vegetarians and pescetarians were at a reduced risk of lung cancer compared with individuals consuming meat on a daily basis. This effect disappeared after taking confounders, especially smoking, into account. We did not observe an association between the meat consumption group and the risk of post-menopausal breast and overall prostate cancer.

Our null findings regarding post-menopausal breast cancer risk are in line with other prospective studies comparing vegetarians with non-vegetarians and a pooled analysis of five cohort studies on breast cancer mortality. In contrast, the UK Women’s Cohort Study reported a lower post-menopausal breast cancer risk among non-meat consumers compared with high meat consumers,14 although this was not observed in their dietary pattern analyses.15 Vegetarian diets are rich in fiber and soy. Fiber was associated with a reduced risk of breast cancer in a meta-analysis of prospective studies,19 and soy contains isoflavones, which have previously been associated with a significant reduced risk of postmenopausal breast cancer in Asian populations.20 However, compared with the average soy intake in four Asian countries (ranging from 38 to 134 g/day21), the soy product intake among vegetarians in our population was likely too low to exert an effect (~15g per day).

 People have asked me if eating sweet desserts or hamburgers is bad for the health if the rest of their diet is good - with lots of fruits, vegetables, whole grains, legumes (beans), and nuts (much like the Mediterranean diet). My sense over the past few years of looking at the research is that one should look at the overall diet, and that a "perfect diet" all the time is pretty darn hard to achieve, if not impossible, for most of us. So this new research looking at gut bacteria and "chemical fingerprints of cellular processes" (by looking at stool and urine samples) of people eating different diets (vegan, vegetarian, omnivore) was reassuring.  The findings suggest: make sure to feed your beneficial bacteria with a healthy diet like the Mediterranean diet (lots of plant-based foods), and then some deviation (cookies! steak!) is OK.

The researchers found that while the kind of gut bacteria dominating were different among the groups (vegan, vegetarian, omnivores), they also found that eating a lot of fiber-rich foods, such as fruit, vegetables, and legumes (typical of a Mediterranean diet) is linked to a rise in health promoting short chain fatty acids (SCFA). Yes, levels of trimethylamine oxide (TMAO) (which is linked to cardiovascular disease) were significantly lower in vegetarians and vegans than they were in those of the omnivores. But the more omnivores closely followed a Mediterranean diet, the lower were their TMAO levels.(Which is great!). As the researchers said: "Western omnivore diets are not necessarily detrimental when a certain consumption level of plant foods is included. From Science Daily:

High dietary fiber intake linked to health promoting short chain fatty acids

Eating a lot of fibre-rich foods, such as fruit, vegetables, and legumes--typical of a Mediterranean diet--is linked to a rise in health promoting short chain fatty acids, finds research published online in the journal Gut. And you don't have to be a vegetarian or a vegan to reap the benefits, the findings suggest.

Short chain fatty acids (SCFAs), which include acetate, propionate, and butyrate, are produced by bacteria in the gut during fermentation of insoluble fibre from dietary plant matter. SCFAs have been linked to health promoting effects, including a reduced risk of inflammatory diseases, diabetes, and cardiovascular disease.

The researchers gathered a week's information on the typical daily diet of 153 adults who either ate everything (omnivores, 51), or were vegetarians (51), or vegans (51), and living in four geographically distant cities in Italy....The Mediterranean diet is characterised by high intake of fruit, vegetables, legumes, nuts and cereals; moderately high intake of fish; regular but moderate alcohol consumption; and low intake of saturated fat, red meat, and dairy products. Most (88%) of the vegans, almost two thirds of the vegetarians (65%), and around a third (30%) of the omnivores consistently ate a predominantly Mediterranean diet.

The investigation showed distinct patterns of microbial colonisation according to usual dietary intake. Bacteroidetes were more abundant in the stool samples of those who ate a predominantly plant based diet, while Firmicutes were more abundant in those who ate a predominantly animal products diet. Both these categories of organisms (phyla) contain microbial species that can break down complex carbohydrates, resulting in the production of SCFAs.

Specifically, Prevotella and Lachnospira were more common among the vegetarians and vegans while Streptococcus was more common among the omnivores. And higher levels of SCFA were found in vegans, vegetarians, and those who consistently followed a Mediterranean dietLevels of SCFAs were also strongly associated with the quantity of fruit, vegetables, legumes, and fibre habitually consumed, irrespective of the type of diet normally eaten.

On the other hand, levels of trimethylamine oxide (TMAO)--a compound that has been linked to cardiovascular disease--were significantly lower in the urine samples of vegetarians and vegans than they were in those of the omnivores. But the more omnivores closely followed a Mediterranean diet, the lower were their TMAO levels, the analysis showed.

TMAO levels were linked to the prevalence of microbes associated with the intake of animal proteins and fat, including L-Ruminococcus (from the Lachnospiraceae family). Eggs, beef, pork and fish are the primary sources of carnitine and choline--compounds that are converted by gut microbes into trimethylamine, which is then processed by the liver and released into the circulation as TMAO.

The researchers point out that SCFA levels can naturally vary as a result of age and gender, and their study did not set out to establish any causal links. But they nevertheless suggest that the Mediterranean diet does seem to be associated with the production of health promoting SCFAs. They conclude: "We provide here tangible evidence of the impact of a healthy diet and a Mediterranean dietary pattern on gut microbiota and on the beneficial regulation of microbial metabolism towards health maintenance in the host." And they add: "Western omnivore diets are not necessarily detrimental when a certain consumption level of [plant] foods is included."