Vitamin D’s Space Travel Class Upgrade

The Sunshine Vitamin

Vitamin D, the “Sunshine Vitamin,” can be obtained through sun exposure or diet. This vitamin is needed to maintain serum calcium concentration, making it essential for bone growth and maintenance. It also plays a role in regulating insulin production, T and B lymphocytes, and heart muscle contractility.1

Along with A, E, and K, it is one of 4 fat-soluble vitamins, which are stored in fat tissues and the liver. Unlike water-soluble vitamins, they can remain in the body for months. 

Recommended Intake

  • Normal serum level range: 40-60 ng/mL
  • Deficiency defined as <30 ng/ml
  • Recommended intake: 400-1000 IU for infants, 600-1000 IU for children and adolescents, 1500-2000 IU for adults/day

2 vs. 3

Vitamin D comes in two forms, D2 (ergocalciferol) and D3 (cholecalciferol). D2 comes from plant-based sources, while D3 comes from animal sources. Once thought to be interchangeable, D3 has shown to more effectively raise serum levels. 3 is the version of the vitamin produced through sun exposure. 

Supplements are available in both; although D3 is usually more effective, D2 is commercially easier to find in higher doses. 

Dietary Sources

  • Oily fish (salmon, mackerel, sardines)
  • Fish liver oil
  • Egg yolk
  • Butter
  • Beef liver
  • Pork
  • Some cheeses
  • Oysters
  • Mushrooms
  • Fortified milk and plant-based alternatives
  • Fortified orange juice
  • Fortified cereals
  • Fortified tofu
  • Supplements2

The Pathway

Sunlight, however, is the best source of vitamin D. When Ultraviolet B (UVB) radiation (wavelength 290 to 315 nm) hits the skin, 7-dehydrocholesterol is converted to previtamin D. Through heat isomerization, previtamin D is converted to vitamin D (D3). From here, the process is the same for vitamin D obtained through sunlight, dietary, and supplement sources. 

In the liver, vitamin D is metabolized into 25-hydroxyvitamin D (25 OH D). This is also called calcidiol or calcifediol and is the marker often used for determining vitamin D levels. In the kidneys, an enzyme converts 25 OH D to 1,25-dihydroxyvitamin D (1,25 (OH)), the biologically active form. This conversion in the kidneys is regulated by the parathyroid along with serum calcium and phosphorus levels. 

Now in the active form, 1,25-dihydroxyvitamin D binds to its specific vitamin D hormone receptors on cells’ nuclei. Through the resulting gene transcription, calcium and phosphorus absorption is stimulated in the intestines.2,3 

Metabolic pathway of vitamin D conversion;
Credit: Gois et al. 20174

Without vitamin D, 10-15% of calcium from the diet is absorbed. With vitamin D, 30-40% of calcium from the diet is absorbed. 


Vitamin D deficiency is very common, affecting as many as 50% of children under 5, 70% of children ages 6 to 11, and 35% of adults in the United States. Worldwide, it’s estimated that over a billion people are deficient.1 Pregnancy, increased melanin, advanced age, and obesity are several risk factors. 

In children, prolonged vitamin D deficiency causes rickets, a weakening of the bones. Growing tissues at the ends of the bones – or growth plates – are softened, causing delayed growth and motor skills, inability to achieve genetically determined height, pain in the pelvis and legs, knocked knees, and weakness.1,5

In adults, osteomalacia or “adult rickets” can result from vitamin D deficiency. Bones mineralization is decreased, causing “soft bones” and easier fracturing, pain, and weakness.

Deficiency in vitamin D can obviously be caused by lack of sunlight or dietary sources, but sometimes a metabolic absorption issue is the culprit. Conditions like Celiac, IBS, cystic fibrosis, and kidney disease can interfere with absorption of vitamin D or conversion to the usable form.


Since the skin destroys excess vitamin D from sun exposure, toxicity is unusual. The normal cause is ingesting prolonged, very high doses of vitamin D. Symptoms of hypercalcemia (buildup of calcium in the blood) include confusion, polydipsia and polyuria (excessive thirst and urination), weakness, nausea, and vomiting.5 High doses of vitamin D, as much as 50,000 IU daily, might be prescribed to combat deficiency; however, this should only be done under medical supervision. 

Hormone vs. Vitamin

Now that we know all about vitamin D, here’s the fun plot twist:

Vitamin D is not a vitamin. 

It’s a hormone. A vitamin is defined as an “essential” organic compound, meaning that it’s a dietary requirement because it can’t be synthesized in the body. Since absorption of sunlight triggers a metabolic pathway leading to synthesis of biologically active 1,25-dihydroxyvitamin D, vitamin D is technically non-essential. And therefore, not a vitamin, despite being a much-needed nutrient.  

Vitamin D in Space: What Changes

Since sunshine is our most reliable source of vitamin D, it’s easy to guess what the problem could be for humans in space: We could no longer get the vitamin through UVB rays, since we wouldn’t be directly exposed to the sun. 

And if we were directly exposed to the sun in space, we’d have bigger problems than a vitamin D deficiency. 

On the hulls of spacecraft and the ISS, the moon, and other planets, we couldn’t rely on the pathway triggered through sun exposure. All the terraforming in the (off)world wouldn’t change this.

Which means… *drumroll* … 

Vitamin D becomes essential in space, and is now officially a vitamin! Humans conquer the cosmos; vitamin D gets honored with a class upgrade. 

Vitamin D Not in Space vs. Vitamin D in Space; FF1

Sources in Space

Since some of the best vitamin D sources are perishable (egg yolk, oily fish), it makes sense that vitamin D on longer missions would come from fortified foods or, more likely, supplements. For short missions, that might work. But what about long-term space travel? A mission to Mars and back? Would supplements last long enough to be effective, or would they lose their potency?

One study on shelf life of pediatric medications with vitamin D as the primary ingredient showed that after being opened, they still had over 90% potency after one year. Interestingly, though, the medications tested had higher vitamin D content than claimed on the label at the beginning of the experiment. Researchers suspected that the medications were produced with higher-than-necessary content to account for possible degradation. Initial vitamin D levels exceeded claims by as much as 50 percent.

Dr. Glen Shue, a former chemist and nutritionist for the FDA, reported to the New York Times that many vitamins can last 4-5 years, and that one bottle of vitamin D remained stable on a shelf in their lab for 10 years. These vitamins were kept in a cool, dry area with constant climate control. Dr. Shue added that pressed tablets (like aspirin) are more prone to oxidation due to their porousness, while sugar-coated or capsulated vitamins are more resistant.7 

This sounds promising for longer missions. Current proposed trips to Mars range from 21 months to 2.5 years or more. If supplements are as long-lived as observation suggests, we may not need to begrudge vitamin D its “essential” class upgrade.  


  1. Vitamin D – Fact SHeet for Health Professionals. NIH. Updated August 12, 2022.
  2. Vitamin D. Harvard School of Public Health.
  3. Nair R and Maseeh A. Vitamin D: The ‘sunshine’ vitamin. Journal of Pharmacology and Pharmacotherapeutics. April-June 2012, Vol 3(2). 
  4. Gois, Pedro & Ferreira, Daniela & Olenski, Simon & Seguro, Antonio. (2017). Vitamin D and Infectious Diseases: Simple Bystander or Contributing Factor?. Nutrients. 9. 651. 10.3390/nu9070651. 
  5. Rickets. Mayo Clinic.
  6. Temova Z and Roskar R. Shelf life after opening of prescription medicines and supplements with vitamin D3 for paediatric use. Eur J Hosp Pharm 2017; 24: 115-119. 
  7. Consumer Saturday; Storing Vitamins from A to K. The New York Times. February 14, 1981.