The first comprehensive study of pregnant vegetarians, including dietary intake, nutritional status and health history, was reported by Hardinge et al. in 1954.2 Although vegans were included in the larger study of which this was a part, none was pregnant; consequently, comparisons were made between lacto-ovo-vegetarians (LOVs) and omnivores. There was no difference in height, weight, or weight gained during pregnancy and there were no serious delivery complications in either group. Birth weights and lengths were not significantly different.
Nearly 20 years passed before Thomas and Ellis compared pregnancy outcome in 14 vegans (28 pregnancies) with 18 controls (41 pregnancies) in England.3 There were no significant differences in live births, still births, toxemia of pregnancy, or infant birth weight. More recently, a tendency toward lower birth weight in term infants was reported in British vegans.4
* "Vegetarian" as used in this chapter includes all types of vegetarian diets that may or may not include animal products. LOV refers to lacto-ovo-vegetarian diets, which include milk, dairy products, and eggs. Vegan refers to diets that exclude all animal products. Macrobiotic diets emphasize whole grains—especially brown rice—sea vegetables, legumes and other vegetables. If included, fruit is locally grown; dairy products and meat are not recommended; limited amounts of white fish may be included.
Vegetarian diets have long been practiced in the U.K., which remains a fruitful source for studies of vegetarians. A comprehensive evaluation of nutrient intake and pregnancy outcome in British LOVs was recently reported.5 No differences were found among the LOVs, fish-eaters, and non-vegetarians in length of gestation, birth weight, birth length, or head circumference. Others reported no difference in birth weight among LOVs in the U.S. compared with omnivores.6,7
Other studies in England reported lower birth weight in infants born to Hindu vegetarian women compared with infants born to Muslim or European women.8,9 This difference remained even after adjusting for a variety of factors, including gestational age, sex of infant, parity, smoking, maternal age and height.9
Dagnelie et al. reported that the proportion of low birth weight among Dutch macrobiotics was more than twice the proportion found in the general population in the Netherlands.10 After excluding those with low birth weight, the mean birth weight in the remaining macrobiotic infants was ~200g lower than the Dutch reference.11 The authors found a strong positive relationship between birth weight and the frequency of consumption of dairy products and fish.10 Adjusted birth weight was 350g greater in families consuming dairy products three or more times per week compared with families consuming them less than once per month. If fish was consumed at least once per week, adjusted birth weight was 180g greater than in families consuming fish less than once per month. No significant relationship was found between birth weight and the length of time a macrobiotic diet had been followed, and the authors concluded that the composition of the diet during rather than before pregnancy was the important factor in determining birth weight.10 They further noted that the high educational level of the macrobiotic parents would have been expected to result in higher rather than lower average birth weights. Studies in the U.S. have also reported lower birth weight among macrobiotics.12,13 The birth weight of vegetarians in general was similar to reference values, however, more than half the infants with birth weights <2500 gm were restrictive macrobiotics.13 Another small study found that infants of vegetarians weighed about 200 gm more than infants of omnivores even though the vegetarians had gained about 2 kg less.6 None of the 11 vegetarian infants weighed <3 kg, whereas two of six omnivore infants weighed about 2.75 kg.
The lower birth weights in some vegetarian populations is thought, with considerable certainty, to be due to diet; however, the precise nutritional factors remain to be clarified.14 Essential fatty acid status was investigated, but the researchers were unable to demonstrate a significant relationship to birth weight in the population under study.4 Inadequate status of iron, folate, or vitamin B12 have been suggested as possible factors.15 Nonetheless, energy intake is thought to play the major role in low birth weight.14
The significance of the slightly lower birth weight found in some infants of vegetarians is not known; however, attention has been called to the evidence that lower birth weight may be associated with long term consequences and increased risk of developing chronic disease later in life.1,15 Attention should be given to assuring that all pregnant vegetarians understand the importance of optimizing their dietary intake during pregnancy.
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