Nutritional advice to pregnant women in Norway consists of recommendations to take vitamin D supplements in the winter season and folic acid 1 month preconceptionally and 3 months into pregnancy. Our study suggests that the recommended dietary intake of these nutrients will not be reached without supplementation. The prevalence of supplement use found in this study is in accordance with two recent studies, where 85% of the pregnant women took some type of supplement in Finland [23
], and 70% of pregnant women in Switzerland reported use of some sort of vitamin and mineral supplement [24
]. Furthermore, the supplement users were higher educated, nonsmoking, pregnant for the first time and had a low to normal prepregnancy body weight, all findings also found in several other studies [23
]. The reduced use of dietary supplements and thus intake of several nutrients among the obese women may contribute to the higher frequency of adverse pregnancy outcomes in this group [28
By far the most commonly reported supplements were cod liver/fish oils which might be explained by the Norwegians’ long-standing tradition of cod-liver oil use to cover vitamin D needs. In addition to the growing evidence for beneficial effects of n–3 fatty acids, the use of fish oil capsules has increased. The high frequency of n–3 fatty acid supplementation is probably unique for this study, because in a comparable study from Denmark only 4.3% of the participants reported use of fish oil supplements [29
]. In Iceland, where a historical tradition of cod-liver oil consumption also exists, only 23% of healthy pregnant women reported a regular intake [30
]. In accordance with the cod liver/fish oil supplementation, most of the intake of EPA and DHA came from dietary supplementation, which contributed 17% of the total n–3 fatty acid intake.
It is surprising that even though most of the participants in MoBa used some kind of vitamin D supplementation, as many as 63% did not reach the recommended intake of 10 μg/day, and 12% did not reach the lower intake limit of 2.5 μg/day. Similar results were reported in a Finnish pregnancy study where supplements contributed 74% of the vitamin D intake among supplement users but 85% did not reach the recommended intake [23
]. In the validation study of the MoBa FFQ, a positive correlation between vitamin D intake and 25-hydroxyvitamin D status was found [19
]. Furthermore, the study showed that almost 20% of the women had a moderate vitamin D hypovitaminosis (plasma concentration of 25-hydroxyvitamin D <40 nmol/l) which actualizes the importance of vitamin D supplementation. Low maternal vitamin D status has been shown to be associated with reduced intrauterine long bone growth, shorter gestation, lower birth weight and reduced bone mass at 9 years of age [31
]. With vitamin D's ability to modulate the immune system, it has been hypothesized that vitamin D status is negatively associated with preeclampsia [34
], and low circulating 1,25-dihydroxyvitamin D has also been reported in women with preeclampsia [35
The frequency of reported folic acid supplementation during pregnancy is higher than the 52% reported earlier from MoBa [26
]. It might be explained by use of different data sources, but hopefully the explanation is that there has been an increase in use over the years as a result of intensified governmental information to the public. The data in the earlier study covered women enrolled in MoBa between 2000 and 2003, while the data in this report cover women enrolled between 2002 and 2005. This study confirms that supplementation is necessary to reach the recommended intake of 500 μg of folate per day.
As many as 25% of the women had a total iodine intake below 100 μg/day, and among iodine supplement nonusers only 1 in 5 reached the recommended intake. It has generally been reckoned that iodine status is satisfactory in Norwegians, although a fairly recent Norwegian study reported intakes comparable to those we found [36
]. The low intake of iodine was confirmed by a low 24-hour urinary iodine excretion in iodine supplement nonusers in MoBa [19
]. Further studies of iodine status in pregnancy should be considered because increased demand for iodine throughout pregnancy has been documented [37
], and even mild or subclinical maternal hypothyroidism during pregnancy can impair maternal health and the development of the newborn [38
Iron intake from supplementation is difficult to evaluate because no general recommendation is given for iron supplementation in pregnancy in Norway. A low iron status will be diagnosed by serum ferritin and the physician will then prescribe an iron supplement. Prescription of iron will probably be considered as a drug and not as a dietary supplement by the women. Most certainly, we have missed some information about iron intake this way. Our results differ significantly from Finnish and Greek data, both countries having general recommendations for iron supplementation to pregnant women [23
]. Still, 22% of the MoBa participants record use of an iron supplement and a further 25% received iron through a combination formula.
Almost 4% of the study population exceeded the upper safe level of 3,000 μg/day of retinol. However, most of the retinol came from the combination of food intake and supplements. To our knowledge, only the intake of more than 3,000 μg/day of retinol from supplements has been shown to increase the risk of malformation of the fetus, and the number of women who exceeded this intake was within the margin of error registration [41
Zinc was the nutrient most frequently exceeding the upper safe intake level, and almost all were zinc supplement users. An explanation for this is that 25 mg zinc is allowed in dietary supplements in Norway. However, intakes up to 50 mg/day have not been shown to have any adverse health effects [42
], and that amount was only registered in a few participants which again is within the margin of error registration.
The method used to calculate the nutrient intake from the dietary supplements has been validated in an earlier study [19
]. However, the uncertainty with the imputation and the difficulties to obtain nutrient content for some supplements might have had an impact on the results. Though we had problems collecting nutrient information for some supplements, these were mostly herb supplements, which were used by one or just a few participants.
A participation rate of 41.6% in MoBa might imply a selection bias, and it is likely that there is a socioeconomic gradient in favor of a better diet that influences the observed dietary pattern [20
]. However, the basic characteristics of the women, such as age and parity, and birth outcomes like gestational age, prevalence of preterm births and birth weight are not different between the study population and the total pregnancy population in Norway [16
]. The aim of MoBa is to estimate the association between exposure and disease in nested case-control studies, and hence selection bias is of less concern [16
In conclusion, dietary supplements were used by 81.4% of the women in MoBa, and were favored by older, normal-weight, nonsmoking, higher educated women carrying their first child. The contribution of supplements was substantial to vitamin D, DHA, EPA, vitamin B6, folate, copper and iron intake among the users. However, even though intake of vitamin D, folate and iodine improved with use of supplements, the recommended intake was not reached for a substantial number of the participants. There should be more public awareness of the importance of iodine and vitamin D for pregnant women.