We found positive effects of vegetarian diet compared to the non-vegetarian diet in terms of food consumption patterns and nutrient intake across four geographic regions and diets of India. Vegetarians consumed greater amounts of legumes, vegetables, roots and tubers, dairy and sugar, while non-vegetarians had greater cereals, fruits, spices, salt, fats and oils. Vegetarians consumed greater amounts of carbohydrates, vitamin C and folate and less fat, protein, vitamin B12 and zinc than non-vegetarians. The lower fibre intake by both vegetarians and non-vegetarians may be confounded by socioeconomic status in our study population.
RDA comparisons indicated that a greater proportion of vegetarians were consuming adequate amounts of protein and micro-nutrients (iron, calcium, vitamin C and folate) and also consumed less total energy than non-vegetarians in different regions and locations. Our study is in contrast to findings of a lower intake of protein among vegetarians that has been found by other studies [12, 34], although a recent study of Buddhist vegetarians found a compensatory increase in protein from plant sources resulting in a higher overall protein intake in vegetarians than in non-vegetarians . Recent systematic review  and meta-analysis  have also indicated possible benefits of plant proteins for cardiovascular health.
We also found a sufficient intake of iron in vegetarians compared to non-vegetarians, which is similar to some [11, 37, 38] but not all previous studies [39, 40]. One explanation is that non-heme iron is found in abundance in plant sources such as legumes, roots and tubers and their bio-availability is increased with concomitant intake of vitamin C-rich diet [10, 38, 39, 41]. Other explanations for increased bio-availability of non-heme iron include baking chappathis in iron plates and the addition of ascorbic acid to cereals and pulses [42, 43]. However a study on young women from Bangalore (India) estimates that only 2.8% of iron (non-heme) is available from plant sources .
Vegetarians develop vitamin B12 deficiencies compared to non-vegetarians due to the reduced bio-availability from plant sources [10, 45]. Our findings are consistent with previous evidence showing a 4.4-fold increase in B12 deficiency in vegetarians among 441 rural and urban men in Pune (India)  and an international study on South Asians showing a 2.7 fold difference in deficiency (24% vs. 9% in non-vegetarians) . Recent systematic review of homocysteine status (a marker of B12 levels) revealed significantly lower serum mean levels of vitamin B12 in vegetarians (lacto- or lacto-ovo) compared to non-vegetarians (209 ± 47 pmol/L vs. 303 ± 72 pmol/L, p < 0.005) . The deficiency of vitamin B12 associated with vegetarian diet has been consistently established by studies conducted both in India [14, 45, 46, 48] and outside India , and also suggest a role for gene polymorphisms associated with diet and defective absorption of vitamin B12 [14, 49–51].
In our study, vegetarians consumed significantly less zinc compared to non-vegetarians (p < 0.0001), which is due to the lower bio-availability of zinc from plant sources . Our findings that a greater proportion of vegetarians consumed both macro- and micro- nutrients at RDA levels compared to non-vegetarians (except for vitamin B12) and less total energy is similar to the other studies outside India [34, 52–54]. In part, this may be due to higher socio-economic levels of vegetarians in our population which is similar to results from the National Family Health Survey-3 (NFHS-3, India), in which a greater proportion of vegetarians were in the highest wealth quintile compared to non-vegetarians (32.5% vs. 19.8%, p < 0.0001) .
The nutritional profile of vegetarian and non-vegetarian diet across various regions of India is not well documented , although studies have shown greater amounts of anti-oxidants (vitamin C, A, E) in Indian vegetarians that may make them less prone to oxidative stress and NCDs [56, 57]. However, some studies on the nutritional profile of Indian vegetarian diet, demonstrate micro-nutrient deficiencies of zinc and iron that are primarily due to reduced absorption [58, 40] and vitamin B12 deficiency in rural and urban vegetarians due to low dietary intake [46, 48]. Our study differs from previous studies in India, as a large study population from 4 different geographical regions, representing 20 states of Indian, with energy- and multivariate adjusted analyses of nutritional intake. This could also be the reason for the differences observed in certain macro- and micro-nutrients consumption pattern between univariate (Table 4) and multivariate (Table 3) comparisons.
One limitation in our study is the possible over-estimation of certain nutrients intake by FFQ which could have led to a little over-estimation of percent population (vegetarian and non-vegetarian) meeting the RDA and the inability to capture the seasonal variation of fruit and vegetable intake, although the total time period (such as number of months in a year) of consumption of fruits and vegetables was captured. To assess the validity of the FFQ it was re-administered to 530 factory workers and rural dwellers, followed by three 24 hour recalls on different days. Nutrient and food group intake calculated by these two methods were compared using medians, kappa statistics, and Bland- Altman plots. The results demonstrated the validity and feasibility of measuring dietary intake in across various regions of India with a single FFQ .
Another limitation is that macro- and micro- nutrients estimation was done using food composition tables and does not account for the moisture content or cooking loss of the nutrients. However, even though vitamins undergo 25-40% loss during cooking , Indian food composition tables can be used to make adequate estimates of macro-nutrients and micro-nutrients for population/group level comparisons [33, 60]. We also accounted for the sibling pair study design by using robust standard error model. The exclusion of unacceptable energy levels of the participants (<500 kcal or >5000 kcal) addressed the skewed nature of the data generally seen with the nutrient level estimation. Moreover, we ran multivariate models based on log-transformed outcome variables and the findings remained the same.
Another potential limitation is that vegetarians in our study population (industrial workers and their rural siblings) had a higher standard of living in both urban and rural populations than non-vegetarians, which may influence the results. However, our large, diverse sample represented four different geographical regions of India, in both urban and rural populations, and NFHS-3 data suggest that our comparison groups are representative with respect to socioeconomic status. More in-depth studies using more detailed repeated 24-hour dietary assessments on representative populations are required to corroborate our findings. Longitudinal studies are needed to explore the health consequences of long-term vegetarian compared to non-vegetarian diets.