How does a developing baby get its vitamin D

Vitamin D is a crucial hormone that helps to regulate calcium levels as well as playing a role in inflammation and immunity. We get it from what we eat and also, in large part, through the action of UV light on our skin, which produces the precursor molecule from which active vitamin D is then made, when it’s needed, elsewhere in the body. So what does this mean for a developing baby? Because it doesn’t eat, and it’s growing where the sun doesn’t shine. Of course, it’s dependent on a supply from mum, but what she needs might not be what the baby needs. And that’s where the placenta comes in. It picks up vitamin D precursors and activates them, at the right rate, and also uses vitamin D to regulate some of its own functions. Jane Cleal explains…

Jane - For most nutrients, the baby in the womb has to rely on transport from the mum. So the placenta is really important. It's like a really key organ for the baby. It provides its food, it provides its oxygen and it gets rid of wastes. So all the vitamin D to the baby has to be transported by the placenta, from the mum.

Chris - One of the things about the way vitamin D works in our body is that we make an inactive form of it, which we then activate when we want it. Is that the same in the baby, then? Or does the baby end up with the same vitamin D signal that mum's seeing?

Jane - So we took the main circulating inactive form of vitamin D and we saw that that is transported across the placenta to the baby. And we know that the baby has the enzymes in its body, so they can actually activate the vitamin D itself.

Chris - So it was the precursor, the inactive vitamin D that's going across, rather than active vitamin D.

Jane - Well, interestingly, we didn't know that. And we actually saw that, yes, the inactive one goes across, but the placenta does something to it itself, so that can use some of it. And then we saw that actually the placenta was releasing active vitamin D to the baby itself.

Chris - How does the placenta know how much vitamin D the baby needs?

Jane - People just think the placenta is a passive conduit. That stuff just goes across. But we are finding out that it perceives signals, it will perceive the vitamin D levels, and it will switch on genes, and it will carry out processes so that it can regulate what is transported across.

Chris - Is its role linked purely to calcium, because in adults, for example, you can find vitamin D having a hand in a range of other processes, including things like inflammation and immunity. So does it do other things in the placenta beyond just regulate how much calcium the baby sees?

Jane - So in the mum, it's regulating calcium levels and that calcium is being transported across to the baby. The vitamin D is going into the placenta and the placenta is using it itself. And so it's actually regulating the immune function of the placenta. It's helping the placenta implant and become bigger. So it's having lots of key functions on the placenta itself, and therefore all of these functions can also happen to the fetus. So it can have non calcium effects as well.

Chris - Well, I was thinking, as you were talking, we know the placenta is bound up in problems like preeclampsia when mothers get very high blood pressure and so on. Does this mean that vitamin D might be implicated in processes like that?

Jane - Yeah. Studies have looked at the vitamin D levels in women with preeclampsia and they do have lower vitamin D levels. So that's obviously an association. We don't know the relationship, but that's another example of why this work is really important because we're thinking vitamin D is affecting how the placenta grows and therefore it's affecting the placental function as well.

Chris - So in a nutshell, then, you have been able to show that the thing that gives rise to vitamin D in the mum's bloodstream is picked up by the placenta, it's activated by the placenta, and then fed to the baby, and certain signals coming from the baby regulate that. How did you do all of this? Because one of the problems with studying humans is there are ethical considerations. There are also practical considerations. So how did you manage to get hold of placenta that you could look at?

Jane - We have an established link with a maternity hospital, and we've got all of the ethics set up. We then wait in the labour ward and we have to collect the placenta when they're fresh because we keep them alive. So we collect the fresh placenta and we take it back to our lab where we can study it and study how it functions.

Chris - That though relates to babies being born at a certain stage of gestation. What about earlier on in the developmental process where most babies are not being born the normal way. Can you gain access to that tissue as well?

Jane - We could do. We haven't for this study. So we've only taken placenta from what are sort of deemed normal pregnancies, which are delivered at term. We can look at data from placenta collected earlier in gestation, and we have related that information as well. And we see the same things are expressed in those placenta.

Chris - So this is not a developmental kind of stage? This is happening right the way through pregnancy?

Jane - We think it's right the way through pregnancy, because in Southampton we also have some studies of women where we get their consent to be on our study before they become pregnant. And we get information on their lifestyle and diet, and we've been able to see relationships with the vitamin D levels earlier in pregnancy and the effects on markers in the placenta at birth.

Chris - We know that some winter babies are more prone to certain conditions - I should know I'm one of them. It's an intriguing association between being born in the winter, higher risks of certain conditions. We know that the population's level of vitamin D is an all time low in the height of winter, isn't it? So could there be consequences through this?

Jane - So in the data where they've studied populations, they see seasonal effects of vitamin D levels. It's surprising. We were shocked to find that a lot of women have low vitamin D levels. And what we did find that was when we gave the women vitamin D supplements, it was those that were due to give birth in the winter months that the supplement had the biggest effect on improving their vitamin D levels.

Chris - So would this argue then that actually we should encourage everyone to have more vitamin D and especially women who are pregnant in wintertime, or could there be consequences? If a person takes vitamin D when they're pregnant, does that have potentially negative consequences for the baby, given its unique role in what the placenta's doing?

Jane - There have been studies in America that have looked at higher doses than we use in the UK. And they've seen no harmful effects. It is advised to all women during pregnancy. I've literally just had a baby and I was told to take vitamin D and I did take my vitamin D throughout pregnancy because we know that it has beneficial effects.