Pregnancy supplements – is there too much of a good thing?

Our study recommends antenatal care providers reconsider widespread prescriptions for pregnancy supplements, with the need to promote whole food diets in pregnancy instead, write Dr Linda Gallo and Associate Professor Shelley Wilkinson.

A well planned and balanced diet is the foundation of a healthy pregnancy, and dietary supplements can play a supportive role. In Australia, access to diverse food sources is generally good, but nutrient deficiencies still arise.

Enter, the “pink” multivitamins. These multivitamins, promoted for “pregnant and/or breastfeeding” women, and even for those trying to conceive, generally contain folic acid, iodine, iron and a shopping list of other nutrients.

In high income nations, multiple micronutrient (MMN) supplementation during pregnancy is a common practice. In Australia, among those who self-report taking at least one dietary supplement throughout pregnancy, the most reported product is an MMN supplement.

There are, however, only two essential nutrients recommended for pregnancy: folic acid and iodine.

It is recommended that women supplement with 400 µg per day of folic acid from the month before conception to three months after they become pregnant. Taking a folic acid supplement is well known to reduce the chance of neural tube defects in the baby.

Guidelines also recommend that pregnant women take an iodine supplement of 150 µg each day and while breastfeeding. Women with a pre-existing thyroid disorder should seek medical advice first. Iodine is required for thyroid hormone production, which is crucial for brain development. Mild iodine deficiency can lead to subtle cognitive and neurological problems in the child, and the Australian population is considered to be mildly iodine deficient.

Other nutrient supplements, including vitamin B12, vitamin D, iron, calcium, and omega-3 fatty acids, are recommended for people with a diagnosed deficiency, pre-eclampsia risk, or for those who avoid certain food groups. Taking an individual supplement is best to meet requirements for these nutrients (here).

What drives the high rate of MMN supplement use in pregnancy?

We conducted a study to identify the factors that are associated with MMN supplement use in Australian pregnancies.

We studied self-reported eating habits and supplement use of more than 120 women at around 28 weeks of pregnancy. All women were receiving antenatal care at the Mater Mothers’ Hospital in Brisbane, one of Australia’s largest maternity hospitals. They were recruited as part of a pilot study for the Queensland Family Cohort (QFC), which aims to track participants for 30 years.

We found that four in five women were taking an MMN supplement during the second trimester. Those who received private obstetric care, had private health insurance, and had greater alignment with the recommended consumption of meat and vegetarian alternatives were more likely to report MMN supplement use. These factors are all associated with higher income.

In high income countries, where food is plentiful and mandatory fortification programs are likely, MMN supplementation is associated with an increased risk of gestational diabetes and offspring adiposity – possibly attributed to high doses of elemental iron. MMN supplementation more than five times per week has been associated with an increased incidence of autism spectrum disorder in offspring. There is, however, the common belief that supplements are safe to take during pregnancy, even when the woman’s diet is nutritionally adequate.

There is high reliance on supplements to meet (and often exceed) nutrient intake guidelines

Our earlier study examining dietary intakes of mothers and their partners found the majority of expectant women are not meeting the recommended serves for each of the five food groups. Only 25% were eating enough vegetables, 42% met daily fruit recommendations, and less than 1% met the recommended intake of grain foods. Dairy and meat or vegetarian alternatives were met by only 11% and 16% of pregnant women respectively. The poor diet is disappointing but not surprising because it reflects the wider Australian population.

For folate, iron and iodine, most women met intake guidelines, but there was high reliance on supplements to achieve this. In fact, more than 5% exceeded the upper level (UL) of intake for folic acid and almost 50% surpassed the UL for iron. Exceeding the UL was associated with higher than usual blood levels of the respective nutrients.

Risks of exceeding recommended nutrient levels

In countries with mandatory fortification programs, such as Australia, exceeding the UL for folic acid is reported frequently among supplement users of childbearing age. Exceeding the UL during the periconceptional period may decrease birth length and negatively affect child cognitive development at four to five years of age. While folic acid supplementation beyond the first trimester, at 400 µg per day, may yield positive effects on the child’s cognitive development, supplementation during the third trimester might be associated with an increased risk of childhood asthma. The effects of high doses are mostly unknown and should be avoided unless medically indicated.

The most popular multivitamin brand in Australia contains 60 mg of elemental iron, which, if taken daily, increases the risk of maternal haemoconcentration. This is associated with adverse pregnancy outcomes, including small for gestational age, stillbirth, preeclampsia, gestational diabetes, and low birth weight. If taking an iron supplement, weekly (80–300 mg) supplementation is as effective as daily (30–60 mg) for preventing iron deficiency anaemia and has fewer side effects.

In our study, about 20% of expectant mothers had iodine intakes greater than 350 µg per day, which has been linked to lower neurodevelopmental scores in children. An important ongoing Australian study will define the effects of reducing supplemental iodine intake during pregnancy, in people who meet the nutrient guidelines from diet alone, on child cognitive development.

Some drawbacks

In our study, we assessed diet and supplement intake at a single time point only, in the second trimester. However, the relative contribution of diet versus supplements to nutrient adequacy have rarely been evaluated otherwise.

The sample size was also small, which may limit generalisability. It also prevented subgroup analyses of nutrient adequacy and did not allow us to examine effects on pregnancy or birth outcomes.

Calculations for daily dose of supplemental nutrients assumed that supplements were taken as a full dose. Where no brand was recorded, a conservative dose estimate was used, which may have led to under-reporting of daily supplement intake.

The need to emphasise whole food diets and not overdose on vitamins

In this largely high socio-economic population, MMN supplementation was high and not consistent with recommendations. Pregnant women need to be advised not to overdose on vitamins. More is not always better.

Megadoses of vitamins and minerals can be harmful to the unborn baby. A classic example is that of vitamin A, which can be toxic to a baby in large doses. Vitamin B6, which is present in many MMN and mineral supplements, has been reported to cause peripheral neuropathy (although not specifically recorded in pregnancy). In response, the Therapeutic Goods Administration has strengthened labelling requirements, so products containing daily doses over 10 mg of vitamin B6 must carry a warning about this serious condition.

Our study confirms the need to promote whole food diets in pregnancy. An increase in vegetables and red meat or vegetarian alternatives for folate and iron are especially warranted. Supplementation with folic acid during the periconceptional period should be continued at current dosage recommendations, along with other nutrient prescriptions when indicated. Supplementation with iodine throughout pregnancy may require consideration of dietary iodine sources to minimise risks associated with high intake.

Finally, antenatal care providers should reconsider widespread prescriptions for MMN supplements. These may be appropriate for some women following an assessment of their dietary patterns. Modifiable behaviours during the first 1000 days of life influence developmental trajectories of adult chronic diseases, and this includes nutrient exposures during fetal development.

Dr Linda Gallo is a physiologist and lecturer in the School of Health at the University of the Sunshine Coast, with research interests in nutrition and cardiometabolic disease.

Associate Professor Shelley Wilkinson is an Advanced Accredited Practising Dietitian and is a Project Officer in the Department of Obstetric Medicine at the Mater Mothers’ Hospitals, and Director and Principal Dietitian at Lifestyle Maternity.

The statements or opinions expressed in this article reflect the views of the authors and do not necessarily represent the official policy of the AMA, the MJA or InSight+ unless so stated. 

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