Babies, or Adults, Sunlight, and Vitamin D

2010 June 14
by Daniel Lakeland

Most of you will know that Vitamin D is a substance produced in mammals chiefly in the skin by exposure to sunlight (wackily, birds get it by secreting stuff onto their feathers where it is photo-converted and then they ingest it during grooming). There are two main active forms of Vitamin D (D2 and D3).

There is a bit of controversy over how much Vitamin D is needed, but there is no controversy over the fact that some of it is needed at all times. You don’t get much from diet (except supplemented foods), so exposure to UVB rays from sunlight is crucial for health. On the other hand, excess exposure to sunlight is bad for you. A classic push-pull scenario destined to lead to some kind of optimal exposure. My 6 week old son has been advised by his pediatrician that he needs to get sun exposure for Vitamin D production, and a friend in Portland who works as a pediatric clinical nutritionist says she sees many Vitamin D deficient kids.

But what is the optimal exposure? It certainly is going to be less than the exposure required to produce sunburn. But how much and how often is ‘enough’.

This article from 1995 suggests that elderly white people in the Boston area get 5 to 10 minutes per day exposure on arms and face 2 to 3 times per week during the summer. There is also a kinetics curve that shows how a single dose of sunlight produces a multi-day concentration spike in the blood. Elderly black people would want to get proportionally more (in proportion to the time it would take them to get sunburn). Younger people can supposedly get sufficient dosage from “incidental” exposure since their skin is better at making vitamin D.

Overexposure to Vitamin D via sunlight is not a concern, since your skin depletes its ability to produce more Vitamin D after the first 20 mins or so (according to wiki article above, probably for white people). Sunburn and skin cancer are more an issue for prolonged exposure. However dietary supplements work differently and can produce excess vitamin D toxicity.

How about babies? I imagine babies have a different requirement, and because their kidneys and liver and soforth are less developed and because they are depositing minerals so quickly there may be differences. I haven’t found any information. But I can imagine that the 5 to 10 minute dose 3 times per week is still a healthy quantity as it wouldn’t induce sunburn. This is the advantage of using a nondimensional number (dose time as a fraction of the time to induce sunburn). However many people have no idea what the real time to induce sunburn is… so that is a bit tricky.

In higher latitudes  there is no UVB available in the winter sunlight, so no amount of sitting in the sun is going to increase the Vit D production. That’s a serious problem for people above latitude 40 or so, which is a significant fraction of North America and Europe (for example New York City and Portland are both > 40 deg latitude).

2 Responses
  1. Morgan permalink
    June 14, 2010

    As an older dad, the best I can do is to advise you not to worry about it too much – if healthy development required a very narrowly fixed amount of sun exposure, we’d have built-in mechanisms for ensuring that amount of exposure. Instead, we have mechanisms that help us adapt to whatever exposure we typically receive over a very wide range. Cared-for babies don’t generally starve, and they don’t generally burn to a crisp on a sunny day. Nor did they before sunscreen.

    If you’re carrying the baby outside on a sunny day, you’ll probably want to move from shady spot to shady spot anyway (if you can find any in SoCal) just because you’ll be hot, and you’ll also probably find yourself doing some manual shading of exposed skin. I know I did – it just seems to come naturally, a sort of “head feels hot, shade it for a while” kind of thing. I never saw a hint of sunburn on any of the boys.

    I will say that 15-30 minutes a week out of doors seems skimpy, unless you’re talking about laying him unshaded and naked on the sidewalk. Oh, it’s probably enough from a physiological need perspective, but I doubt that ten times that would do any harm, and even very young children clearly like to be outside. Just watch your son’s face, and remember that there’s more to life than avoiding skin cancer.

    The irony of modernity is that your challenge will soon turn to getting him to spend more time outside in the sun, and away from electronic media.

    • Daniel Lakeland
      June 14, 2010

      Thanks for the reply. The truth is I’m already wondering if I keep him indoors near the computer too much. In So Cal (near Pasadena), in the summer, the sun is intense.

      I generally haven’t been going outside between the hours of 10 AM and 5 PM because I feel crispy if I stay outside in direct sun more than 5 minutes or so.

      When I do take him out for extended periods (like trips to Huntington Gardens) it’s of course natural to go from shade to shade. So I wondered if that’s enough sun exposure. As you say we are pretty adaptable, and the graph from the linked article generally indicates that a single exposure (equivalent to 15 mins outdoors at Cape Cod in the summer) has a multi-day kinetic response. Vit D is elevated even on the 3rd day in even elderly populations.

      Interestingly though, it appears that pretty much any fabric covering reduced the UVB below the level where it had a detectable effect. So it really is direct exposure that is of issue. Going for a walk every day is something I try to do, but before I generally would do it about 7PM to avoid getting baked to a crisp. Now I’m thinking I should probably just intentionally get him some sun for 5 minutes a day or so every morning before it’s too intense. Obviously not obsessively, but seriously it’s hot and dry and intensely sunny here. which curiously means he’s getting a lot less exposure than he would if we lived somewhere less sunny.

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