Why Moms Need More Sun Exposure and Vitamin D

Today I woke up late, rushed to feed my children breakfast, and then hustled them off to school, arriving just before the bell. It took us about ten minutes to walk from where I parked the car to the door of the school building, and then, after kissing my son goodbye, I walked ten minutes back to my car, carrying my daughter (who does not yet go to school). The weather was cold and ferociously windy, and I was grateful when I made it back to my cozy home. For the rest of the day, I have been inside writing and caring for my daughter. I took a short break to do an indoor bodyweight workout, doing my best to keep up with the overly enthusiastic blonde instructor on the screen, and then I went back to type, type, typing. At noon, our nanny will retrieve my son from school and take over watching both kids so I can get some more focused work done in my garage office. When I am done, I will walk back into my home (about 10 steps) and spend the rest of the evening indoors caring for my home and children. 

By the time the sun sets today, I will have gotten about 20 minutes of total sun exposure (at a time when the sun was very low in the sky). On other days, when it’s my husband’s turn to drop my son at school and the weather is nasty, I may get none. 

If you live in the contemporary Western world, you probably get similarly low levels of sunlight exposure on a daily basis, and if you grew up in 1990s America like I did, you were probably under the impression that this was a good thing. As this article in The Guardian puts it, “For decades, it has been drilled into us that if the sun is out, we’d better slip on protective clothing, slop on sunscreen and slap on a hat to stay safe.” Basically, the narrative has been that sun exposure is bad. Better to stay inside, and if you have to go out, cover every inch of your body. 

But scientists are just now starting to understand the myriad benefits of sun exposure. It’s true that too much sun exposure, especially burning, can be detrimental to skin health. But too little sun exposure also has consequences. Many high-quality, longitudinal studies have found that people who get a lot of sun exposure tend to have longer life expectancies, even when controlling for many common confounding variables.

We don’t yet fully understand all of the ways in which sunlight exposure benefits us, but one very important factor is vitamin D synthesis. Vitamin D is the only vitamin that our bodies actually synthesize, which has led some scientists to question whether it should even be classified as a vitamin (in many ways it’s actually more like a hormone), and plays all kinds of critical roles in the body. It increases calcium absorption from intestines, supports healthy bones and teeth, and plays a critical role in immune function. 

The question is, with our modern, indoor lifestyles, are we getting enough of it? And, if not, what are the consequences? As with so many other nutrient deficiencies, are mothers affected more than others? 

To answer these questions, let’s first look at how our lifestyles and vitamin D levels differ from those of our ancestors, and examine why this might be yet another case of “evolutionary mismatch.”

Sun Exposure and Vitamin D in Our Evolutionary Past

For the vast majority of human history, most of life took place outside. There were no houses. At best, our hunter-gatherer ancestors had grass or mud huts, tents, or igloos. Sometimes, especially in warm, equatorial environments during the dry season, they slept and lived outside around the clock. Because life was nomadic and groups moved camp as often as once every couple of weeks, taking the time to erect large, permanent shelters was simply not worth the effort. As a result, early humans probably got a lot more sun exposure than most of us do today, with our sedentary indoor lives. 

Here is the daily routine of the Hadza people, a group of contemporary hunter-gatherers living in Tanzania, as described by the anthropologist Frank Marlowe in his ethnography, The Hadza Hunter-Gatherers of Tanzania:

They often sleep outside, with no shelter at all, but during the rainy season they build temporary structures to protect themselves from inclement weather. They wake every day with the first light, between 6:30 and 7:00 AM. It is cool at dawn in the Eyasi Valley, so they sit by their fire and become active only very gradually as the sun rises higher in the sky. By 8 or 9 AM, the women are ready to go foraging. They forage until midday, when the sun is hottest, and then take a break and rest for a couple of hours in the shade. In the late afternoon, they will go to fetch firewood and water. Dinner is eaten just after dark, around 7 PM, and on most nights they will go to sleep at 9 PM. 

When researchers tested the Hadza’s levels of Vitamin D, they averaged 109 nmol/L (44 ng/mL), which is more than twice as high as most Westerners! Maasai pastoralists, who live in the same region and also live primarily out of doors, have an average of 119 nmol/L (48 ng/mL)! The official health guidelines in the United States suggest that 20 ng/ML is sufficient (less than half the amount seen in traditional societies), and some of us aren’t even meeting that very low threshold. 

What’s cool about vitamin D is that our bodies can synthesize it using sunlight, but we can also get it from food, so hunter-gatherers who migrated to northern latitudes compensated for the lack of sun exposure by adapting their diets. The Inuits, who are thought to have migrated north as long as 10,000 years ago, eat a lot of fish liver oils and marine animal blubber, both of which are especially rich in vitamin D. In the 19th and 20th centuries when government and colonial forces began forcing the Inuits to abandon their traditional life ways and dietary practices, there were massive outbreaks of rickets due to chronic vitamin D deficiencies. 

Even among equatorial hunter-gatherers, who get plenty of sunlight, organ meats are a critical part of the diet and are especially high in Vitamin D, but in the US we prefer muscle meat (relatively low in Vitamin D) and consider organ meats a byproduct: estimates vary, but between 30% and 80% of them go into pet food, meaning our animals are probably eating better than we are. 

In other words, if we can trust the evidence from contemporary hunter-gatherers, then our Paleolithic ancestors were probably getting a whole lot more vitamin D than we are today, both from sun exposure and diet. It seems reasonable to assume, based on this evidence, that low vitamin D is yet another area of evolutionary mismatch. Our bodies are getting a lot less of it than they were programmed to expect. 

On the other hand, human bodies are resilient and adaptable. Is low vitamin D a problem? Or just a harmless feature of modern living?

The Unsung Benefits of Vitamin D for Mothers

In the general population, multiple longitudinal studies have found that low Vitamin D is consistently associated with bone and joint discomfort, lowered immune function, mood changes, and much more. Anything below 20 ng/mL is considered deficient for bone and overall health, but some research is beginning to question whether this cutoff is too low. New research indicates that 40-60 ng/mL is probably closer to the optimal range for promoting good overall health (closer to the levels our ancestors probably had). The average American today typically has about 20-30 ng/mL (in other words, below sufficient ranges, according to the latest research).

Many women (and men) can probably get away with lower vitamin D levels and may not notice any effects, but if and when they become mothers, they are more vulnerable to deficiencies. This is due to the nutritional demands of pregnancy and breastfeeding. During pregnancy, the fetus gets all of its vitamin D from mom, via the placenta, and it needs a lot to build all of those little bones and teeth, as well as its own immune system. If the mother is already borderline or deficient, there’s a good chance that pregnancy will tip her over the edge. 

After birth, if the mother chooses to breastfeed, the baby will get some of its vitamin D from sun exposure, and some from milk. In the modern, Western context, breastmilk usually contains inadequate levels of vitamin D, even though it is critical for newborn health, which is why pediatricians typically recommend vitamin D drops for babies (although supplementation for the mother also increases breastmilk vitamin D content). In our ancestral past, not only were babies exposed to much more sunlight, but their mothers had much higher average levels of vitamin D, meaning that the levels their babies were getting from breastmilk were more than adequate (a good thing too, since our ancestors didn’t have access to vitamin D drops). 

Vitamin D sufficiency is not just good for the baby's health. It’s also critical for mothers. Low Vitamin D is associated with low mood and energy levels (i.e., fatigue). Whereas sufficient vitamin D is associated with key benefits including better respiratory health, immune function, maintenance of bones and teeth, and a healthy blood pressure. Vitamin D is also involved in various brain processes, and may help support a healthy mood, including in pregnancy and postpartum, although research in this area is ongoing. 

Final Thoughts

Insufficient vitamin D isn’t the only side-effect of our indoor lifestyles. It’s a good idea to get outside and get some sun whenever you can (as long as you avoid high noon and protect yourself from burning), but given how much our work requires us to be indoors these days, it’s not a bad idea to compensate with diet and supplementation. I am personally trying to train myself to eat more organ meats (not easy for someone who was raised as a vegetarian) both for the omega-3s and the vitamin D content (it’s also more sustainable, since most of it goes to waste or into pet food).

But even if I manage to stomach grilled liver once a week and force myself to go outside in bad weather, I still take a vitamin D supplement, just to be safe. 

Needed contributor Elena Bridgers is a science writer specializing in motherhood. Her Substack, Motherhood Until Yesterday, explores the science, evolution, and cultural shifts of motherhood—focusing on how it was practiced for 95% of human history, during our time as hunter-gatherers. Click here to subscribe.