We defined an analytical cohort with a wide range of dietary intake by over-representing our study population with persons at the lower end of the meat consumption spectrum. The dietary intake patterns and lifestyle characteristics of vegetarians, pescetarians and those consuming meat 1 day/week were diverse and distinct from individuals consuming meat on a regular basis. A cross-comparison between self-reported vegetarians and vegetarians whose status was confirmed based on extensive FFQ data, did not show large differences between both groups in terms of diet and lifestyle characteristics.
To date, no universally accepted definition for the term vegetarian exists and the only constant component in a vegetarian diet across all previous empirical studies has been the absence of meat. As previously outlined by Fraser, the problem of defining a dietary regimen on just one food group results in a lack of control on intake of all other food groups that make up a vegetarian diet . Consequently, all individuals who refrain from meat, but have otherwise quite distinct dietary intakes, are grouped together under one label of vegetarianism. We included a large number of individuals consuming meat only once a week with the purpose to address whether low meat consumption might have a beneficial health effect over complete abstinence of meat, or vice versa. Despite an increase in the number of low meat consumers and meat reducers in the Western world , little to no research has been conducted in this area. However, we showed no large differences in terms of nutrient intake between these groups, though we had no data on vitamin B12 intake .
Interestingly, we showed that the perceived general health of vegetarians is considerably better than that of non-vegetarians. This is in line with our finding that vegetarianism is not merely characterized by a diet void of all flesh foods, but rather extends into a complete healthy lifestyle. In our population, we found evidence for a higher level of health consciousness among the vegetarians, pescetarians and to a lesser extent low meat consumers, as indicated by a more favorable distribution of dietary and lifestyle factors. Although the diet groups were statistical significantly different with respect to most of the dietary variables, this may have resulted from our large sample size. Interestingly, the amount of salt added to the food during cooking increased with increasing animal product intake in a dose–response relation. Pescetarians had higher intakes of soy products, nuts and seeds, grains, cheese and eggs than vegetarians, which would suggest that they more actively seek to replace the meat in the diet with plant based protein-rich foods to balance their diets.
Appropriately planned vegetarian diets have shown to be consistent with the current dietary guidelines in all stages of the lifecycle . The mean nutrient intakes (not from supplements) in all diet groups were generally well above the recommended daily allowances (RDA) of the European Commission , except for iron for which the intakes were below the RDA of 14 mg/day in the meat eating men and all women. The estimated iron intake was highest among pescetarians and vegetarians. However, these groups consume predominantly inorganic iron which has a low bioavailability compared to the heme-bound iron that is found in meat products . More than 55% of the vegetarians and pescetarians reported to take a nutritional supplement over the last year compared to 26% of individuals in the highest meat consumption group (6-7 days/week). This suggests that there appears to be a certain degree of awareness among these individuals of the need to supplement their diets to prevent dietary deficiencies.
Although literature suggests that long-term adherence to a vegetarian diet appears more strongly associated with health outcomes than short term adherence [11, 30], very few prospective studies in vegetarian populations have specific data available on time since adopting the diet. On average, vegetarians adhered to their diet considerably longer than pescetarians and individuals consuming meat 1 day/week. As much as 75% of all confirmed-vegetarians who had been diagnosed with cancer before baseline changed to this dietary regimen after diagnosis. In addition, the proportion of prevalent cancer cases decreased with increasing meat intake. Previous research indicates that cancer survivors are highly motivated to make dietary changes towards a more plant-based diet after diagnosis with the intention to improve their health and well-being . However, although nutrition has shown to affect cancer progression , it remains to be elucidated whether adopting a vegetarian lifestyle may influence the course of cancer prognosis or cancer recurrence.
Nearly half of those who called themselves vegetarian reported to consume fish, meat or poultry on the extensive FFQ (ĸ = 0.59). Similar findings have been reported previously [19, 20] and indicate that the complete avoidance of meat cannot be assumed among self-reported vegetarians. This suggests that self-identification is not a good measure for estimating the prevalence of vegetarianism. However, our findings suggest that the level of misclassification that occurs when merely relying on self-reported vegetarian status was small: the overall group of self-reported vegetarians did not differ considerably from individuals whose vegetarian status has been confirmed based on FFQ data in terms of diet and lifestyle. The FFQ assessed the diet in the past 12 months whereas self-definition was based on vegetarian status at the time of the questionnaire. Nonetheless, only 10% of the observed disagreement could be explained by this difference in time-frame. It would be interesting to examine the difference in risk ratios between both groups of vegetarians in future etiological studies of chronic diseases. After 20.3 yrs of follow-up, a total of 1,559 incident cancer cases (165 among vegetarians and 346 among 1 day/week meat consumers) were identified in NLCS-MIC through record linkage with the Netherlands Cancer Registry.
An important methodological issue when comparing previous studies on the health effects of vegetarian diets relates to between-study sampling differences. Only a few reports on vegetarian diets, including ours, are from population-based studies while the majority stem from convenience samples that have likely also recruited more health-conscious non-vegetarians [5–8, 10, 33]. The latter technique is particularly appropriate to recruit non vegetarians who only differ from vegetarians with respect to their meat and fish intake and is a suitable design for studies into diet and health in vegetarian populations that are mainly concerned with the adverse effects of meat. However, interest has shifted towards the health effects of the complete vegetarian lifestyle. For this, convenience sampling may be less appropriate since it likely decreases diet and lifestyle differences between vegetarians and non-vegetarians, and could bias results towards the null. Our population has a wide distribution of nutrient intakes and lifestyle characteristics, which should facilitate the identification of associations between vegetarianism, meat consumption and disease risk in future etiologic studies. The ratio of low meat consumers to high meat consumers (meat consumption ≤1 day/week versus 6–7 days/week) was 1:23 in the total NLCS cohort and 1:2.2 in NLCS-MIC.
The NLCS aimed to overrepresent vegetarians by intentionally contacting them through health food shops and magazines. Therefore, vegetarian dietary patterns were taken into consideration when designing the FFQ by including line items on meat substitutes that were commonly used by the vegetarian population at that time. Vegetarians and low meat consumers more often took the opportunity to report and give details of foods and beverages that were frequently eaten but that were not contained in the FFQ. Moreover, vegetarian status was taken into account for nutrient calculation of composite recipes. Previous studies indicate that vegetarians are able to recall their diet with higher reliability  but at the same time may be more tempted to report the intake of certain food items that they consider to be healthy as a result of social desirability bias . The FFQ used in the NLCS was not designed for assessing the usefulness of self-reported vegetarianism as a classification tool. Interestingly, some 18% of individuals who were classified as vegetarians based on their responses on the FFQ did not report to have any special eating habits. This phenomenon has previously been reported  and may result from lack of knowledge of the concept of vegetarianism by the general public at the time the measurement was conducted (1986).