Do you know that you can kill yourself by drinking too much water? It’s true, and some have done so.
I’m sure you wouldn’t let that stop you from slaking your thirst next time you’re parched. After all, a body needs water. But a couple of gallons all at once could have grave – if not fatal – consequences.
As with water, you can overdose on vitamin D, and the consequences could be fatal. But whereas a water overdose would kill you within hours, Vitamin D toxicity generally requires repeated and gross overdosing over a period of months, or even years. Even then, it is rarely fatal.
What do we mean by an overdose? In the case of a pharmaceutical drug, any amount in excess of your ideal dose would be an overdose.
But vitamin D is not a drug. Even if you should take much more vitamin D than you need, your body knows what to do with the excess. It happily stores it for future use!
Vitamin D overdose doesn’t happen easily
So why are so many people, including some medical doctors, more concerned about vitamin D overdose than they are about vitamin D deficiency?
Perhaps they have read that the RDA for vitamin D is 600 IU (for people up to age 70) and that the safe upper limit is set at 4000 IU daily. What are they supposed to think when they hear vitamin D researchers recommending 5000 IU upwards of vitamin D daily?
It sure sounds like a big dose, doesn’t it? Those researchers must be so irresponsible to recommend such a high dosage of vitamin D, when all you need is 600 IU daily!
But 5000 IU happens to be a reasonable dose of vitamin D (from all sources) for an average adult in good health – just enough to meet their daily needs. (See vitamin-D-dosage for your own daily needs.)
And unfortunately, those vitamin-D RDA’s and Safe-Upper-Limits have been set at inappropriate levels for years. No wonder so many of us are deficient in vitamin D.
Why should you believe this? Three reasons:
1. Vitamin D from sunlight
Fair-skinned people who spend half an hour on the beach (in a brief costume) in mid-summer manufacture in their skin between 10,000 IU and 20,000 IU of vitamin D3. (Dark-skinned people take longer to get there, but make the same amount in the end.)
The average amount is around 15,000 IU.
The vitamin D3 your skin produces is the very same substance, the identical molecule, to the vitamin D3 you take as a supplement. So when you take a daily dose of 5000 IU of vitamin D3, your body thinks you just took a few minutes of sunshine. (The same cannot be said of vitamin D2, the form of vitamin D usually prescribed by a doctor).
Healthy people cannot become vitamin-D-toxic from any amount of sunshine. So 5000 IU of vitamin D3 cannot be toxic to a healthy person.
In fact it seems unlikely that even a daily dose of 15,000 IU of vitamin D3 from all sources could result in toxicity.
2. Vitamin D blood levels
When an average (176 pound or 80 kg) adult reaches an optimum blood level of vitamin D (50 – 65 ng/ml), he requires about 5000 IU of vitamin D (from all sources) to sustain that blood level.
If he takes less than 5000 IU, (from all sources) his 25(OH)D blood level starts to decline. He is using more than he is obtaining.
So what happens if his vitamin D blood level is below optimum and he takes 5000 IU of vitamin D daily? His blood level will gradually increase, over many weeks or months, until it levels off at around 50 ng/ml – the start of the optimum range.
It levels off because his body is using the same amount as he is taking. But since we are all different in our vitamin D metabolism, yours might reach only 40 ng/ml, or perhaps 60 ng/ml.
This rise in vitamin D blood levels is healthy, and causes no stress to your body.
The lowest blood level at which a proven case of vitamin D toxicity has been recorded in a healthy adult is above 200 ng/ml (that is, over four times higher than the optimum level).
3. Vitamin D toxicity studies
There have been incidents in which people have overdosed on vitamin D, most commonly through industrial accidental exposure, or mistakes made in fortifying food.
Researchers have studied such incidents. In many cases, scientists have determined that these people unknowingly took huge amounts (millions of IU) of vitamin D. Some of them became vitamin D toxic. Others, who were exposed to the same huge doses, suffered no ill-effects and no vitamin D toxicity. Some people are more susceptible to toxic effects from high levels of vitamin D, than others.
From this evidence, researchers concluded that the lowest amount of vitamin D likely to cause toxicity (in the most-susceptible adults) is 40,000 IU every day, taken for several months.
Vitamin D safety
In summary, although it is possible to overdose on vitamin D, there are very large safety margins. A average-sized adult in good health will not overdose on 5,000 IU of vitamin D per day, from all sources.
In fact, he or she probably cannot maintain an optimum blood level of vitamin D while taking less.
Because of the large safety margin, even though someone might take an inappropriate amount of vitamin D3 by mistake, it would have to be a very large overdose – sustained for a long time – to result in toxicity.
All this applies to healthy people. If you are not in good health, and are already under a doctor’s care, you should not start supplementing vitamin D without consulting your doctor. There are some conditions which may be adversely affected by high levels of vitamin D. See Side effects of vitamin D.
But they are rare. Most people in poor health will benefit enormously from optimising their vitamin D intake. But just check with your doctor first