Spending fun summer days at the beach can give you a deep tan. But spending too long outside can give you a painful sunburn—or worse. Long-term Sun exposure can also lead to increased risk of skin aging, eye disease, and even skin cancer.
But what happens when you are exposed to too little sunlight?
Flashback to the past
In the past, large numbers of children suffered from a disease called rickets. Symptoms of rickets include weak muscles, bowed legs, and other physical deformities. The disease was especially common among children with poor nutrition and those who spent a lot of time indoors or lived in crowded, polluted cities.
Studies by scientists in the 1900s finally found an explanation for this terrible disease: a lack of sunlight. The Sun is an important source of a key nutrient, vitamin D. And not getting enough vitamin D leads to rickets.
Vitamin D and your body
Vitamins are organic compounds essential for your health. They are generally obtained from food. Vitamin D, however, can also be synthesized in the skin from sunlight!
Absorption of rays from the ultraviolet B (UVB) spectrum of the Sun triggers a photosynthetic reaction in the outer layers of the skin. This creates an inactive vitamin D molecule that flows through the bloodstream. The vitamin D gets activated in your liver and kidneys. It is also taken up by the cells that need it, directly from the bloodstream. This helps fight infections and keep your heart, lungs, muscles, and brain working properly.
When vitamin D reaches the small intestines, it begins its most important job: controlling the absorption of calcium and phosphorus from the food you eat. These two minerals are essential for bone health. Growing children need vitamin D to develop their skeletal system. Adults need it to keep their bones strong and healthy. Vitamin D deficiency leads to bones that are less dense and bone injuries that don’t repair well after breaks.
Too much Sun exposure can cause skin cancer. However, good vitamin D levels may reduce the risk of 13 other cancers. Researchers have repeatedly found higher cancer rates in regions that receive less UVB. The risk of type 1 diabetes also increases with a vitamin D deficiency.
Did you know? Without Vitamin D, your body cannot absorb calcium and phosphorus from food. That is why most calcium supplement pills also contain vitamin D.
Map showing the lattitudes 35 degrees North and 35 degrees South. Regions above and below the shaded area in the middle do not receive enough sunlight during the winter months to supply healthy amounts of vitamin D (NASA)
For most people, UVB rays from the Sun are the primary source of vitamin D. But UVB availability depends on latitude and season. At the Equator, intense sunlight is available throughout the year. Less sunlight is available as you move closer to the North or South Poles. During the winter months, at latitudes above 35 degrees North and below 35 degrees South, the weak winter sunshine doesn’t provide enough vitamin D. And all of Canada lies above 35 degrees North!
Smoke, dust, and cloud cover can also reduce the amount of vitamin D you get from the Sun. And even during the summer, much less UVB reaches the Earth in the early morning and evening than at midday.
Luckily, the Sun isn’t your only source of vitamin D. You can also find it in oily fish like salmon, tuna, sardines, and mackerel. Cod liver oil is rich in vitamin D. Egg yolk and certain dairy products contain smaller quantities. The Canadian government requires that vitamin D be added to several products, including cow’s milk, infant formula, and margarine.
Did you know? Canada was the first country in the world to issue official predictions for UV levels. The “UV Index” calculations developed by Environment Canada are still used by World Health Organization as an international standard.
Can you get enough vitamin D from food alone?
You need about 800 international units of vitamin D every day. To get that from your diet alone, you would have to eat almost 100 grams of sockeye salmon, 400 grams of sardines, or one kilogram of canned tuna. If you’re not a seafood lover, you could also meet your daily vitamin D requirement by eating 20 egg yolks or by drinking 8 cups of fortified milk or orange juice. So you’ll probably need to get some sunlight or take vitamin supplements to stay healthy!
Twenty-five per cent of Canadians don’t get enough vitamin D. And 10 per cent of the population is severely deficient. Having dark skin increases your risk of vitamin D deficiency. All early humans had dark skin, which acted as natural sunscreen. But as some humans migrated further away from the Equator, genetic mutations resulting in lighter skin allowed them to better absorb vitamin D. The Inuit and other Arctic peoples are an exception to this rule. Thanks to their seafood-heavy diets, they have been able to live at high altitudes despite having darker skin.
Did you know? The rays of natural sunlight that produce vitamin D in your skin cannot penetrate glass. That includes windows! So you have no choice but to go outside for your daily dose of good health.
Tips for getting enough vitamin D
It is a good idea to spend 10-15 minutes outside everyday, with the Sun on your bare face and arms. Remember that clothing and sunscreen block sunlight absorption. Between 10am to 3pm is the best time to benefit from UVB. Smartphone apps like dminder show how much Sun exposure you need to get enough vitamin D, anywhere in the world, at any time of the year.
During the dull winter months, load up on seafood, fortified dairy, and fortified juice. If you are lactose intolerant or don’t eat fish, you might want to consider vitamin D supplements. Health Canada recommends supplements throughout the year.
Keep in mind that too much vitamin D from food or pills can be toxic. So speak to your doctor or a dietician about dosage. It is impossible to overdose on vitamin D from sunlight… but be careful not to get a sunburn!
Other resources on ultraviolet light:
Scientific publications on Vitamin D:
Vitamin D, cod-liver oil, sunlight, and rickets: a historical perspective (2003)
K. Rajakumar, Pediatrics 112
Sunlight and Vitamin D: Necessary for public health (2015)
C. A Baggerly et al., Journal of the American College of Nutrition 34
Cancer, sunlight and vitamin D (2014)
M. F. Holick, Journal of Clinical & Translational Endocrinology 1
The importance of sunshine (1927)
E.C Robertson, The Canadian Medical Association Journal 17
The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know (2011)
A. C. Ross et al., The Journal of Clinical Endocrinology & Metabolism 96
Benefits of sunlight: a bright spot for human health (2008)
M. N. Mead, Environmental health perspectives 116
Vitamin D toxicity, policy, and science (2007)
R. Vieth, Journal of Bone and Mineral Research, 22
Scientific publications of the evolution of skin pigmentation: