"[Vitamin D] affects cell death and proliferation, insulin production and even the immune system," says Dr Michael Holick, director of the Vitamin D, Skin and Bone research laboratory at the Boston University Medical Center, US. Translation: without enough vitamin D, your body works far below its potential. And you're probably not getting anywhere near enough. Here's how you can score what you need to make your body function at its absolute max.
Nutrition's new golden child
What's most remarkable about D is the sheer number of health issues it's been linked to. In the past few years, studies have shown that a lack of the vitamin may be a contributor to depression, heart disease, pregnancy problems, birth defects, skin and other cancers, and multiple sclerosis. A study in the Journal of Nutrition in May even linked bacterial vaginosis - the most common vaginal infection in women - to vitamin D deficiency.
But even if you don't suffer from the above-mentioned conditions, getting more D may still be what the doctor ordered. "Many of my patients report a dramatic improvement in their feeling of overall wellbeing after they increase their vitamin D levels," Dr Holick says.
A recent study by the University of Minnesota, US, linked increased intakes of vitamin D to improved weight loss in overweight men and women following a kilojoule-restricted diet. And a number of experts have urged the US government to raise its recommended daily amount of vitamin D for adults from 200 IU (international units) or 5.9mcg (micrograms) to at least 1000 IU (25mcg).
Why we're D-ficient
Vitamin D expert Professor Robert Scragg from the University of Auckland says Kiwis' blood levels of D should be at 80 nanomoles per litre. According to the 1997 Nutrition Survey, the average level in NZ is 50 nanomoles. So why do we come up so short, especially since vitamin D is one of the few vitamins our bodies can actually make?
Sunlight converts a cholesterol-like substance naturally found in the skin into D. Problem is, thanks to UV rays, soaking up sun to make more D would do you more harm than good. Since the 'slip slop slap' message caught on, we've been unknowingly depriving our bodies of opportunity to absorb D. What we need is a happy medium. "If you can go out in the sun on a regular basis for a few minutes each day [without sunscreen] you can make a lot of vitamin D," Professor Scragg says.
However it's near impossible to make any vitamin D during winter, Professor Scragg advises, because the UV Index (maximum daily level of ultraviolet radiation at ground level) is so low. But fortunately your body can store D. "The more sunlight you can get during summer, the higher your vitamin D levels will be during winter," he says. "The best time is late summer heading towards autumn, when the sun is still strong enough."
It's tougher for people with darker skin to make D - the melanin in dark skin decreases vitamin D production, but their sun exposure risk is lower. "The people who are most at risk [of D deficiency] are South Asian, Pacific and Maori people - and they very rarely get melanoma," Professor Scragg says.
To make matters worse, D is missing from the food we eat, says Beth Kitchin, assistant professor of nutrition sciences in the Osteoporosis Treatment and Prevention Clinic at the University of Alabama at Birmingham, US. Only certain kinds of fish and fortified milk and margarine have enough D to brag about, and it would be hard to consume enough of both in one day to get 1000-plus IU (25mcg). Easy to swallow
There's an easy way to get your vitamin D levels up without baking in the sun. It's not often that nutrition experts say it's better to get a vitamin from a pill than from food, but D is an exception. "In this case, supplements are the easiest, cheapest and safest way to make sure you're covered," says Dr Laura Armas, assistant professor of endocrinology and a researcher in the Osteoporosis Research Center at Creighton University in Omaha, US.
Can you OD on it? It's possible, but extremely unlikely. Vitamin D is fat-soluble, which means you won't wee out the excess as you would a water-soluble vitamin like C. This used to worry experts, who thought that D could easily build up to toxic levels. But further study has found that our bodies can handle a lot more than previously understood. "You'd have to ingest 10,000 IU every day for six months before you'd even begin to approach problem levels," Dr Holick says.
According to Dr Holick, it's much more likely you're not getting enough. To get your D intake up to par, take a daily multivitamin or a vitamin D supplement that provides at least 1000 IU (25mcg). In addition, Dr Holick recommends, drink two glasses of D-fortified milk per day.
If you're worried you might be low on D, get your blood levels measured, especially if you have a family history of heart disease, cancer or depression. If you are very low, your doctor might recommend higher doses of D
Where the D be
It's hard to get the vitamin D you need solely from food, but these eats are some of the best sources of this vital nutrient. Dig in...
- Salmon, canned in water, drained - 1mcg per 100g
Mackerel, canned in tomato sauce - 8mcg per 100g
Mackerel, canned in oil, drained - 2.1mcg per 100g
Tuna, canned in oil, drained - 2.1mcg per 100g
Tuna, canned in water or brine, drained - 1mcg per 100g
Egg - 0.8mcg per 100g (2 eggs)
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