January 24, 2024

expert reaction to observational study of plant based-diets and pregnancy outcomes

A study published in Acta Obstetricia et Gynecologica Scandinavica looks at plant based diets and pregnancy outcomes. 

Prof Ian Givens, Professor of Food Chain Nutrition, University of Reading, said:

“A lower intake and status of a number of micronutrients by non-supplemented vegans, and to a lesser degree vegetarians, has been reported in a number of studies. These tend to be nutrients that plants cannot supply or supply very little and include iodine, vitamin B12, vitamin D and calcium and this is reflected in Table 2 of the present study. There have been reports of women with preeclampsia having lower urinary iodine concentrations (an indicator of iodine status) than women without preeclampsia, but some studies have not agreed. Also, a large case-control study in Finland (Reische et al., 2021 Biological Trace Element Research199:2131–2137) found no significant association between iodine status or measures of thyroid function with preeclampsia risk. No doubt more work on this is needed. The present study and others have shown that supplementation can redress the low intake of key micronutrients from food linked to vegan and vegetarian dietary patterns, but care is needed and for many professional advice is wise to ensure correct supplementation is achieved.”

Prof Keith Godfrey, Professor of Epidemiology & Human Development at the University of Southampton, said:

“The 66,738 pregnancies in this population-based study were recruited over 20 years ago, when few women followed a plant-based or vegan diet. As acknowledged by the authors, their main findings of higher risks of low birth weight and preeclampsia among vegans are subject to some uncertainty, being based on only 18 vegans. The authors highlight a possible role for lower dietary protein intake; however, it is equally possible that any effect could result from low intake of vitamins and minerals primarily obtained from meat and dairy products but not necessarily present in many supplements. Nonetheless, the study points to a need for further research.”

Prof Andrew Prentice, Head of Nutrition & Planetary Health Theme, MRC Unit The Gambia at London School of Hygiene & Tropical Medicine, said:

“I would be wary of making any conclusions about diet in pregnancy and pregnancy outcomes on the basis of this study. The whole study included only 18 vegans so the chances of a spurious finding are huge.”

Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London (KCL), said:

“This study reports birth outcomes based on data on a registry of women in Scandinavia and reports lower birth weights in women following plant based diets. The finding is in agreement with some studies in the UK that have found lower birth weights in vegetarian women of Indian descent compared to omnivores after adjusting for gestational length, maternal height as smoking. However, studies in some other groups such as the Seventh Day Adventist in the USA find no differences in birth weight or birth outcomes in those following plant based diets. The strength of the study is the large sample size and the statistical analysis which has adjusted for some factors known to affect outcomes such as maternal height, smoking habit and parity. A weakness of the study is that it does not report the ethnic origin of the mothers and dietary intakes were assessed by questionnaire which is not very reliable for assessing micronutrient intake and the bioavailability of minerals such as iron and zinc. Although protein intakes were lower on the plant based diets as would be expected, it is an unlikely explanation as protein supplementation has adverse effects on birth weight and outcomes. Low bioavailability of iron from plant based diets is a more plausible explanation for the lower birth weights.”

Dr Duane Mellor, Registered Dietitian and Senior Lecturer, Aston Medical School, Aston University, said:

“It can be more challenging to follow a vegan diet to ensure that it is nutritionally complete, as there can be risks of lower intakes of iron, iodine and vitamins B12 and D which can affect the health of both the mother, along with the development of the baby.

“However, although the overall number of women included in this study was large at over 65,000, the studies only included 18 people who identified as being vegan. The very small number of vegans who took part means that there is a risk of statistical error and that could explain the higher rates of pre-eclampsia reported in vegans. As there were only 2 women who presented with pre-eclampsia who were vegan, it could be due to variability and chance. This study aimed to consider protein as a mediating factor, which due to the small number of women who were vegan in the study makes it very hard to fully interpret any meaning from this data.

“When planning a pregnancy and during pregnancy whatever your dietary patterns and preferences it is key to eat a varied and balanced diet, including supplements as advised by government and health guidelines. If a diet is balanced and includes the necessary nutrients including protein, vitamins and minerals, the type of diet is not as important.”

‘Adherence to different forms of plant-based diets and pregnancy outcomes in the Danish National Birth Cohort: A prospective observational study’ by Signe Hedegaard et al. was published in Acta Obstetricia et Gynecologica Scandinavica at 8:01am UK time on Wednesday 24^th January.

DOI: 10.1111/aogs.14778

Declared interests

Prof Ian Givens: I have had research funding related to milk and meat but in no case has the funder influenced the design or outcome of the work.

Prof Keith Godfrey: None directly related to my comments. I have received reimbursement for speaking at conferences sponsored by companies selling nutritional products, and am part of an academic consortium that has received research funding from BenevolentAI Bio Ltd., Nestec, Abbott Nutrition and Danone.

Prof Andrew Prentice: No relevant conflicts.

Prof Tom Sanders: No relevant conflicts of interest

Dr Duane Mellor: No conflicts of interest