Balanced maternal nutrition during pregnancy is imperative for the mother’s health status and, consequently, for offspring, and is crucial to maintain an adequate environment for optimal fetal development. According to the theory of “early life programming” environmental factors and lifestyle during pregnancy determine the risk of developing chronic diseases later in life and also influence lifelong health in offspring [1
]. Pregnancy requires an increased intake of macro and micronutrients and balanced diet. For that, it offers a critical window of opportunity to acquire dietetic habits that are beneficial for fetal healthy [2
Pre-pregnancy and pregnancy adherence to food-safety recommendations, according to the updated Dietary Guidelines for American and Mediterranean Diet, should avoid inadequate levels of key nutrients and micronutrients (proteins, iron, folic acid, vitamin D, calcium, iodine, omega-3, and vitamin B12) that may predispose the offspring to chronic condition later in life such as obesity, diabetes, cardiovascular disease, and neurodevelopmental delays [3
The percentage of vegetarians and vegans in the general population has increased over the last years partly due to evidence that vegetarianism is linked to improved health. Thus, cohort data have shown that low-fat diets enriched with fruit, vegetables, and fiber can lead to a reduction of risk factors for coronary heart diseases, a better lipid profile [4
], lower body mass index (BMI) [5
], and lower blood pressure [6
]. In addition vegetarian diets appear to prevent cancer and type 2 diabetes [7
]. Nevertheless, some data suggest that vegetarians and vegans may be at greater risk of increased plasma homocysteine levels, an arising risk factor for cardiovascular disease [9
], and of low bone mineral density, which predisposes to osteoporosis [11
]. Plant-based diets are reported to contain less saturated fatty acids, animal protein and cholesterol, and more folate, fibre, antioxidants, phytochemicals, and carotenoids. However, plant-based diets have a low content of essential micronutrients such as iron, zinc, vitamin B 12, vitamin D, omega-3 (n
-3) fatty acids, calcium, and iodine. Consequently, the risk of adverse effects due to micronutrient deficiencies that lead to the risk of malnutrition should not be underestimated [12
The reasons for choosing a vegetarian or vegan lifestyle are variable and range from evidence-based health consciousness to environmental concerns, socioeconomic considerations, ethical grounds, or spiritual/religious beliefs. Medical reasons also exist in some occasions; for instance, women of childbearing age affected by chronic kidney disease (CKD) may be conditioned to the choice of low-protein vegetarian diet [13
]. According to the American Dietetic Association, well planned vegetarian diets are safe for all age groups and in all physiological conditions, including childhood, adolescence, pregnancy, and lactation [12
]. On the contrary, the German Nutrition Society does not recommend vegetarian or vegan diets during pregnancy, lactation, and childhood, due to the inadequate supply of essential nutrients [15
Vegetarian diets typically comprise of plant foods such as grains, legumes, nuts, seeds, vegetables, and fruit, and exclude all kinds of animal food including meat (pork, beef, mutton, lamb, poultry, game, and fowl), meat products (sausages, salami, and pâté), fish, mollusks, and crustaceans. Vegetarian diets usually include dairy products like eggs and honey. Accordingly, there are two main directions:
Lacto-ovo-vegetarianism (LOV). This excludes meat but includes dairy products, eggs, and honey, together with a wide variety of plant foods. Subcategories are lacto-vegetarianism (LV), which excludes eggs, and ovo-vegetarianism (OV), which excludes dairy products.
Veganism (VEG), which excludes meat, dairy products, eggs, and honey, but includes a wide variety of plant foods [16
However, some people adhere to other plant-based diets that limit the foods consumed:
Raw food diet: consisting exclusively of vegetables, including sprouted cereals and pulses, fresh and dried fruits, and seeds, as well as milk and eggs, all of which are mainly eaten raw.
Fruit diet: consisting exclusively of fresh and dried fruits, seeds, and some vegetables.
Macrobiotic diet: the strictly vegetarian version of this diet consists of cereals, pulses, vegetables, seaweed, and soy products; while dairy products, eggs, and some vegetables are avoided. Fish is consumed by people who adhere to a macrobiotic diet.
Despite the concrete definition of such categories there is a wide variety of dietary pattern. Vegetarians can be divided into other subgroups: semivegetarians, who are defined as consuming red meat and poultry once per month or more and all meats—including fish—once per month or more, but no more than once per week; pesco-vegetarians, who consume fish once per month or more but all other meats less than once per month; lacto-vegetarians, who consume eggs and dairy once per month or more, but fish and other meats less tan once per month; and lastly, vegans or strict vegetarians are defined as those who do not consume eggs, dairy, or fish [16
In Europe and North America vegetarians are LOV [17
], while Asian Indian vegetarians are mostly lacto-vegetarians [18
]. Moreover, it has been described that Chinese vegetarians consume considerably smaller amount of dairy products than Western vegetarians [19
In 2006, approximately 2.3% of the US adult population (4.9 million people) strictly followed a vegetarian diet, asserting that they never ate meat, fish, or poultry. In 2012 the percentage increased to 5%. Approximately 1.4–2% of the US adult population is vegan [20
]. The percentage of young people who are vegetarian is still higher (6–11%), with similar levels of vegetarian teenagers reported in both the United Kingdom and Australia [21
]. On the basis of dietary recall data from the 1999–2004 National Health and Nutrition Examination Survey (NHANES), 7.5% of American women were reported adhering to a vegetarian diet [22
]. The distribution of women of childbearing age who follow a vegetarian diet is different between developed and developing countries. In developing countries, the prevalence of vegetarian diet may increase due to poverty and economic reasons. In India, 20–30% of the total population is considered to be vegetarian for religious reasons, but normally they do not eat meat because of economic causes. In developed countries, vegetarians comprehend more women than men, and tend to be of higher educational or socioeconomic status, with low probabilities to plan children, and generally they are under 40 years of age. There are also variations between ethnic groups. In the United Kingdom, people of non-White ethnic origin are more likely to indicate themselves as vegetarian than white (15% vs. 6% of white respondents) [23
Despite the assessment of the importance of a healthy diet in pregnancy, data have demonstrated that women don’t change diet during pregnancy, so optimal preconception dietary pattern is determinant for a healthy pregnancy [24
]. In addition, the period of lactation is extremely important for growing patterns of infants and the effectiveness of breastfeeding depends on maternal nutritional status. The lack of macro- and micronutrient intake during lactation may lead to the reduction of micronutrients and energy content in breast milk that could lead to severe illness in breastfed infant [25
The aim of this narrative review was to analyze the existing studies in humans focused on the effects of vegetarian (LOV) and vegan (VEG) diets during pregnancy on maternal outcomes and nutritional status and on fetal healthiness and complications, valuing the risks and benefits of such nutritional choice. Moreover our aim was to study the period of lactation and breast milk composition of vegetarian and vegan mothers and if the breastfeeding is safe for optimal child growth, because there are no specific clinical guidelines about lactation for vegetarian mothers. The publications reviewed in this paper mainly concern plant-based diets as LOV and VEG. Consequently, the results mainly pertain to these diets, which are generally defined as “vegetarian”. We did not assess studies about raw food, fruit, and macrobiotic diets. A research for English written articles was performed using MEDLINE/PubMed/Cochrane databases (since 2000 up to 31 December 2018). The research was based on the combination of the following keywords: vegetarian diet, vegan diet, plant-based nutrition, pregnancy outcomes, fetal development, vegetarian/vegan, and breastfeeding/human milk, combined with words related to nutritional status and to the nutrients of interest (protein, vitamin B12, folate, calcium, iron, zinc, iodine, and n-3 fatty acids). Studies were identified and examined for methodology and key results. Researches about preconception period were also analyzed. Review team members screened titles and abstracts and selected articles that seemed pertinent to the topics, excluding those not in English or not concerned with humans. The full papers were read to select potentially eligible articles and to assess the scientific merit and relevance of each paper valued. The manuscripts were also analyzed according with the type of study (case–control, review, longitudinal cohort, and cross-sectional), type of diets, number of cases, and possible bias. We performed a narrative review because we expected high heterogeneity of results and lack of randomised trials in pregnant vegetarians in the literature. We included American Dietetic Association Guidelines in pregnancy and International Guidelines for vegetarian and vegan diets; moreover, we also included scientific papers about the nutritional status of mothers, reflecting fetal nutrition and possible detrimental effects on fetal development. We included few data before 2000, only those with crucial results.
Especially, our main goal was to highlight if vegetarian or vegan diets could be considered safe for the mother’s health and for offspring during pregnancy and lactation. We also focused on the effect of these dietary patterns on the lack of micronutrients in order to find a target therapy that could avoid fetal complication.
Diet is one of the most significant lifestyle-related factors in determining health state and predisposing the offspring to develop several diseases. Vegetarian and vegan diets are emerging worldwide due to the evidence that plant-based dietary patterns reduce the risk of coronary heart disease, high blood pressure, type 2 diabetes, and cancer. Pregnancy is a critical window of opportunity to provide dietetic habits that are beneficial for fetal healthy. It is also an exclusive condition in which the requirements of energy and micronutrients intake increase to maintain the supply of essential nutrients for fetal development. Each stage of fetal growth is dependent on appropriate maternal nutrient transfer, so a balanced diet is essential to avoid fetal complications. The choice of vegetarian or vegan diet is always in the preconception period due to ethical reasons or poor social condition, so a well-adjusted preconception nutrition is essential for healthy pregnancy. Available data demonstrated that micronutrients insufficiency and caloric restriction are more common in developing countries, where vegetarian diets are chosen because of socioeconomic reasons. On the contrary in developed countries, the consciousness and the concern of a balanced diet is taken more into account. Generally it is difficult to verify the effects of such diets on pregnancy outcomes and to separate them from other confounding factors such as ethnicity, lifestyles or smoking.
Although more high quality evidence is needed, balanced plant-based diets rich in fibers and low in fat are considered to be protective against poor pregnancy outcomes such as PE, DG, and preterm delivery. However, these protective effects disappear if micronutrients deficiencies emerge. Moreover unbalanced dietary patterns with lack of macro- and micronutrients such as proteins, vitamin B12, vitamin D, calcium, DHA, and iron are at more risk of fetal impairment (low BW, neurological disabilities, and fetal malformations). Maternal undernutrition may potentially alter fetal growth trajectory by modifying placental weight and nutrient transfer capacity; depending on the severity of the nutritional deprivation and on the timing of depletion. Thus, plant-based diets during pregnancy and lactation require a strong awareness for a complete intake of essential key nutrients and vitamin supplements, according to international guidelines.
In addition, during breastfeeding both vegetarian and nonvegetarian mothers need caloric reserves to reach sufficient energy average. The composition of human milk changes dynamically and may vary according to many maternal factors, such as nutritional status. Nutritional supplements do not change milk composition in observational studies, also proteins concentration in human milk does not vary in relation to maternal intake of vegetal or animal proteins, but maternal body composition may be associated with the nutritional value of human milk. So maternal undernutrition, producing lack of vitamin B12, vitamin D, calcium, and DHA during lactation may lead to low vitamin content in breast milk, which can cause permanent neurological disabilities in infants or low bone mineralization.
Finally, the current manuscript supports the evidence that maternal nutritional status is the key condition for health benefits of plant-based diets. Vegetarians and vegans are at risk of nutritional deficiencies, but if the adequate intake of nutrients is upheld, pregnancy outcomes are similar to those reported in the omnivorous population. So updated evidence highlights that well-balanced vegetarian and vegan diets should be considered safe for the mother’s health and for offspring during pregnancy and lactation. In this regard, specific dietary interventions before, during, and after gestation that are aimed at improving diet quality and adjusting appropriate intakes of macro- and micronutrients might avoid maternal health impairment, mental diseases during pregnancy, and consequent physical and neurological fetal disabilities. The vegetarian-type pattern should be considered safe and it is not associated with preterm birth, BW, or SGA if the requirements are met. Therefore, healthcare professionals might have knowledge about plant-based diets characteristics in order to implement balanced dietary patterns, enhancing supplement intake, and paying attention to critical nutrients to avoid dangerous health outcomes. Further large-scale observational studies would help to define correlations between plant-based diets, gestation, and health, and might be suitable to design pregestational nutrition intervention strategies.