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Invigorate Nutrition

The Sunshine Vitamin

Late winter and early spring, as in right about now, is the time of year when our blood vitamin D levels are typically at their lowest. So as everyone gears up for spring gardening and more outside activities, I thought this would be good timing for this topic!

There are not many vitamins where food is NOT the body’s best source. In fact, there is only one: vitamin D.  Some dietary sources of Vitamin D are listed in Table 1, and it is certainly worth including these foods in your diet. However, what really boosts our vitamin D status is time outside in the sunshine! This is because vitamin D is made in our skin from UVB light exposure and warmth. Being outside in the fresh air is important; UVB light does NOT pass through window glass.

Vitamin D is good for bone health; it plays a significant role in regulating calcium metabolism. Deficiencies in vitamin D lead to rickets in childhood, and osteomalacia and osteoporosis in adults, reflecting the loss of calcium from bones. Did you know that if you didn’t have any vitamin D, you would only absorb 10-15% of the dietary calcium you eat into your body?

Vitamin D acts like a steroid hormone -  that sounds exciting doesn’t it! There are vitamin D receptors all around the body, particularly in the small intestine, bone, liver, kidneys and (of interest to a sports dietitian like me) in skeletal muscle. We are still learning about what vitamin D does for us, suffice it to say that supplementing with vitamin D only seems justified if you have low blood vitamin D levels to start with. But there are also other ways to boost your vitamin D status.

Most of us make plenty of vitamin D in our skin over summer, as we take holidays and generally get outdoors under quite strong UV light here in New Zealand. Of course, this sun exposure needs to be carefully balanced with sun-smart behaviours to ensure we don’t increase our risk of developing skin cancer.  You can balance both with the usual sun-smart recommendations regarding when & for how long you are out in the sun from the Ministry of Health and the Cancer Society of New Zealand1. They suggest a daily walk or other activity outside with sun protection around lunchtime in the winter, shifting to the early morning or late afternoon during summer. Given the weather and duration of daylight we experience in those different seasons, that advice is quite practical.

Living in southern New Zealand places us at higher risk of having low blood vitamin D concentrations, particularly in winter. We are further from the sun, so UVB light that has further to travel and is weaker, and we spend less time outside in winter.  The Annual Nutrition Survey of 2008/09 found that a third of New Zealanders living south of Nelson/Marlborough had serum vitamin D concentrations below 50 nmol/L (the level recommended by the Ministry of Health1,2) when assessed over the year, but this jumped to almost two thirds of us in August, September and October2

Table 1: Food Sources of Vitamin D

Salmon

Mackerel

Sardines

Tuna

Herrings

Liver

Egg yolks

Shitake mushrooms (sun dried)

 

Vitamin D has been associated with many, many different health conditions, and is currently very trendy and enjoying its moment in the sun (pardon the pun!). However, an association does not always indicate a cause and effect. A colleague of mine once used the wonderful analogy that eating more ice-creams in summer does not get you in trouble in the water, although there are also more drownings (unfortunately) in summer. Associations need to be tested out in a randomised controlled trial, where one group gets the vitamin and one group gets a placebo that looks and tastes the same, for a reasonable length of time to see an impact (which can be a long time in nutrition research!). Then the researcher measures the outcomes before and after the trial to see whether the two groups respond differently over that time. This work is ongoing in nutrition research – vitamin D is an exciting nutrient to be watching right now!

Many of these studies will use vitamin D supplements, because it is easier to offer a placebo tablet to compare that with. Consider that supplementing vitamin D is not the only way to improve your family’s vitamin D status. For children in particular, getting outside exercise (in accordance with your child’s abilities and the Ministry of Health and Cancer Council's sun exposure guidelines) might be a much better way to boost their vitamin D status. Exercise gives them so many benefits in terms of physical, mental and social health, and is always worth encouraging. In fact I wonder what impact the Pokémon Go app has had on the vitamin D status of our children? Maybe we were lucky that it was launched in winter here?

If you take high doses of fat-soluble vitamins like vitamin D frequently, there may be a risk of toxicity because it is more difficult to excrete these vitamins from the body. This is quite different to water-soluble vitamins like B vitamins or Vitamin C – it is easy to just pee out the excess of those! It is quite safe to obtain vitamin D from the dietary sources listed in Table 1, and from the vitamin D that you make in your skin, which occurs naturally in response to internal feedback mechanisms, and according to your body's own needs.

If you are concerned about your vitamin D status, please see your doctor. People who are at higher risk of vitamin D deficiency include people who do not get outdoors much, people who prefer to wear lots of clothing to cover their skin, the elderly, pregnant women and people with darker skin (it takes more UVB exposure to make an equivalent amount of vitamin D in darker skin). You can have a blood test to know whether your vitamin D levels are adequate; this should be done through your General Practitioner.

In the meantime, round up your children, dogs, friends and family and get outside for some exercise as the weather gets better! Spring is here – get out and enjoy it! 

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